The Magellanic Bridge The Nearest Purely Tidal Stellar Population

a r X i v :a s t r o -p h /0612107v 1 4 D e c 2006

Draft version February 5,2008

Preprint typeset using L A T E X style emulateapj v.10/09/06

THE MAGELLANIC BRIDGE:THE NEAREST PURELY TIDAL STELLAR POPULATION

Jason Harris

Steward Observatory and

933North Cherry Ave.,Tucson,AZ,85721

Draft version February 5,2008

ABSTRACT

We report on observations of the stellar populations in twelve ?elds spanning the region between the Magellanic Clouds,made with the Mosaic-II camera on the 4-meter telescope at the Cerro-Tololo Inter-American Observatory.The two main goals of the observations are to characterize the young stellar population (which presumably formed in situ in the Bridge and therefore represents the nearest stellar population formed from tidal debris),and to search for an older stellar component (which would have been stripped from either Cloud as stars,by the same tidal forces which formed the gaseous Bridge).We determine the star-formation history of the young inter-Cloud population,which provides a constraint on the timing of the gravitational interaction which formed the Bridge.We do not detect an older stellar population belonging to the Bridge in any of our ?elds,implying that the material that was stripped from the Clouds to form the Magellanic Bridge was very nearly a pure gas.Subject headings:galaxies:evolution —galaxies:stellar content —galaxies:Magellanic Clouds —

galaxies:interactions

1.INTRODUCTION

Interactions are known to be an important driver of galaxy evolution,but a detailed understanding of their in?uence remains elusive.The Magellanic Clouds are a particularly compelling target for investigating the ef-fects of minor so-called “harassment”interactions,due to their proximity to the Milky Way,their close associa-tion over at least the past several Gyr,and their abun-dant gas reservoirs,which allow for ongoing star for-mation.The strongest evidence that their interaction has played an important role in driving the evolution of the Clouds lies in the extra-tidal features of the Magel-lanic Stream and Magellanic Bridge.Unlike the Mag-ellanic Stream,which appears to be a pure-gas feature (Guhathakurta &Reitzel 1998),there is a known stellar population associated with the Magellanic Bridge,and by measuring the ages,chemical abundances,and kine-matics of these stars,we can obtain strong constraints on the evolution of the dynamical event which formed the Bridge,and study in detail how star formation proceeds in the wake of such an event.

The Magellanic Bridge was ?rst reported in H I ob-servations by Hindman et al.(1963),and a young stellar component (the “inter-Cloud population”)was discov-ered by Irwin et al.(1985),who estimated the age of the stars at about 108yr.Demers &Battinelli (1998)pro-vided the most comprehensive study of the inter-Cloud population to date;they observed ?ve ?elds in the west-ern Bridge,and found stars as young as 10–25Myr in both clusters and in a di?use ?eld component up to 9?from the SMC.The young inter-Cloud population prob-ably formed in situ in the Bridge,in the wake of the Bridge-forming event,making it the nearest example of a stellar population whose formation was unambiguously triggered by a tidal interaction.Yet surprisingly,no de-tailed analysis of the star formation history of these stars has been performed to date,and we do not even know

Electronic address:jharris@https://www.sodocs.net/doc/649745804.html,

the full extent of the population,since no ?elds along the ridgeline of the H I gas have been observed east of the midpoint between the Clouds.

Furthermore,no study to date has speci?cally searched for an older component of the inter-Cloud population,which would represent a population of stars that was stripped from the Clouds during the Bridge-forming event.Tidal forces during an interaction should a?ect both gas and stars,so the inter-Cloud population should have an old component,other things being equal.

The present study will address these open questions regarding the inter-Cloud population in the Magellanic Bridge.We present our observations and data reduction in Section 2.In Section 3.1,we trace the eastward extent of the young inter-Cloud population,and in Section 3.2,we search for tidally-stripped stars in our Bridge ?elds.We brie?y examine the outer structure of the LMC using our four ?elds nearest that galaxy in Section 3.3.Finally,we present the star-formation history of the young inter-Cloud population in Section 3.4,and summarize the re-sults in Section 4.

2.OBSERVATIONS AND DATA REDUCTION

2.1.The Observations

The data were obtained on the nights of January 4and 52006(UT),at the Cerro-Tololo Inter-American Observatory (CTIO)4-meter telescope.We used the Mosaic-II camera,which images a 36′×36′?eld onto a 8k ×8k CCD detector array,to obtain short and long ex-posures in Washington C ,Harris R ,and Cousins I ?lters at twelve ?eld positions spanning the inter-Cloud region,and at one o?set ?eld at a similar Galactic latitude,but to the west of the SMC (see Table 1and Figure 1).

The ?eld positions were selected to uniformly sam-ple the inter-Cloud region,approximately following the ridgeline of the H I gas which forms the Magellanic Bridge (Putman 2000).In addition,we selected two ?elds to lie o?the main ridgeline,but still in regions of abundant H I emission (?elds mb03and mb14).Our

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?elds include the eastern half of the inter-Cloud region, where there are very few known star clusters,and where Demers&Battinelli’s?elds mostly lie far from the H I ridgeline.

The exposure times listed in Table1were chosen in or-der to detect stars as faint as the ancient main-sequence turn-o?with S/N>10in all three?lters.At each?eld position and for each of the C,R,and I?lters,we obtained a pair of long exposures for cosmic-ray rejec-tion,and a short exposure to record the photometry of brighter stars that are saturated in the long exposures. Seeing was stable during both nights;the FWHM varied between0.7′′and1.0′′.We observed standard star?elds several times per night for photometric calibration(see Table2),twilight?ats were obtained during evening and morning twilight of both nights,and bias frames were obtained in the afternoon prior to both nights.

2.2.Data Reduction

The data reduction followed the procedure docu-mented by the NOAO Deep Wide Field Survey team (Jannuzi et al.2003),and utilized the mscred package in IRAF1.Before reducing the data,we obtained an up-dated world coordinate system database for the CTIO 4-meter telescope from the CTIO website,dated from May2004,and we also obtained an updated crosstalk-correction parameter?le.

We?rst processed the bias frames using ccdproc and zerocombine.We then used ccdproc to perform over-scan correction and trimming,bias subtraction,bad pixel masking,ampli?er merging and crosstalk correction on all data images.

We ran objmasks on each twilight-sky exposure to mask out any detected stars in the images,and then com-bined the exposures using sflatcombine.This yielded normalized C,R,and I twilight-sky?ats for each observ-ing night.We then divided each data frame by the appro-priate normalized?at,using ccdproc with the sflatcor parameter activated.To further?atten the data images, we also constructed night-sky?ats from the collection of data images,after running objmasks on each image to mask out detected objects.We then ran ccdproc once again to apply the night-sky?ats.

We empirically determined the world coordinate sys-tem(WCS)for each exposure using msccmatch,which uses an astrometric database to identify stars in the im-age and determine the WCS from their known coordi-nates.We chose to use the Guide Star Catalog,version 2for the astrometric database,which we found yielded ?t residuals that were smaller by about a factor of two compared to the USNO-A2catalog.msccmatch provides an interactive interface with which the?t can be im-proved by eliminating outlying mismatched points.We were able to get the?t residuals in each image below 0.25arcsec.

Next,we used mscstack to combine each pair of long exposures into a single,averaged image.We had initially activated the cosmic-ray rejection feature of mscstack, but we found that if the seeing changed between the two 1IRAF is the Image Reduction and Analysis Facility,distributed by the National Optical Astronomy Observatories,which are oper-ated by the Association of Universities for Research in Astronomy, Inc.,under cooperative agreement with the National Science Foun-dation.exposures,the stellar?ux in the averaged image would be signi?cantly clipped by the rejection algorithm.In-stead,we stacked the images without rejection,and used craverage to detect cosmic-ray hits in the combined im-age and replace the a?ected pixels with the average of the surrounding pixel values.We also ran mscstack on the short-exposure images,despite the fact that they did not have multiple exposures to combine,because mscstack also performs a pixel-value replacement for the pixels in the bad pixel masks.

The?nal step in the reduction pipeline is to run mscpixarea,which corrects each image for the variable pixel scale across the?eld.We then use mscsplit to separate the eight CCD images per?eld into separate FITS?les.We choose to analyze the CCD images sepa-rately,in order to properly account for slight di?erences between the CCDs.

2.3.Instrumental Photometry

We used the daophot package in IRAF to perform stel-lar photometry on our images,using the method of point-spread function(PSF)?tting.For each CCD image,we ran daofind to identify peaks,and phot to obtain pre-liminary relative photometry of the detected sources. Next,we used pstselect to select?fty bright,rela-tively isolated sources to form a high-quality sample from which an empirical PSF model will be built.Candidate PSF stars are rejected if they contain saturated pixels,or if their centers are within ten pixels of any pixel?agged as bad by our data reduction procedure.

The PSF?tting is performed in a fully automated way,without user intervention.However,we do examine residual images after PSF subtraction to ensure that the automatic model parameters are correct.All available forms for the PSF are explored to?nd the best-?tting model.Next,group and nstar are run to identify and photometer stars that are close neighbors of the PSF sample.Substar is used to subtract these close neigh-bors from the image.We then repeat the PSF?tting,this time using the neighbor-subtracted image to produce a more accurate model.

Once the second-pass PSF has been determined,we run allstar to perform iterative PSF photometry of all detected sources in the image.Then we run daofind on an image in which all known objects have been sub-tracted using the PSF model,in order to?nd fainter stars in the image.Then allstar is run on the subtracted im-age,using this new list of fainter stars2.The two allstar photometry lists are combined into a single photometry table for the frame,and the IRAF task wcsctran is used to convert the stars’X,Y pixel coordinates to right ascen-sion and declination,using the world coordinate solution we determined during the data reduction procedure. The above photometry procedure results in some spu-rious detections due to two artifacts:bleed columns,and scattered-light halos around extremely bright stars.The bleed columns are?agged as bad pixels by the data re-duction procedure,but this does not prevent daofind from identifying sources along the bleed columns,nor 2Alternatively,we could have performed the second-pass allstar on the original image using a concatenated list of all detected ob-jects.We tested both methods on an image from mb20,the most crowded?eld.The di?erences in the photometry are consistent with the uncertainties estimated by allstar

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does it prevent allstar from trying to photometer these false sources.We therefore remove objects from the pho-tometry table that are within10pixels of a?agged bad pixel.Extremely bright stars have large scattered-light halos in the images,resulting in circular concentrations of false detections centered on these stars due to the el-evated?ux levels.To clean the photometry tables of sources detected in the wings of extremely bright stars, we reject all sources with anomalously high estimated sky values.In the absence of these extremely bright stars, the sky levels are quite stable,making the sky levels an e?cient way to identify spurious objects in the wings of bright objects.

The allstar program provides very accurate relative photometry,but because the PSF models are uncertain at large radii,the allstar magnitudes are normalized to a relatively small aperture size of4pixels.We there-fore need to apply an aperture correction to convert the allstar magnitudes to true instrumental magnitudes that represent the total?ux recorded from each star.To de-termine the aperture correction,we select bright,isolated objects from our sample and perform concentric-aperture photometry using phot with a series of aperture sizes,up to17pixels.The per-star aperture correction is simply the di?erence between the star’s allstar magnitude and its large-aperture magnitude:(m als?m ap17).However, since individual objects su?er from measurement errors and contamination from neighboring objects,we need to statistically determine the characteristic aperture cor-rection for the entire frame.The distribution of per-star aperture-correction values is typically a Gaussian with an asymmetric tail to negative values.The Gaussian spread is due to measurement uncertainties,and the asymmet-ric tail is due to?ux contamination,which is never com-pletely mitigated by our selection of isolated objects. An accurate determination of the aperture correction’s value and uncertainty requires that we attempt to iso-late the underlying Gaussian shape from the asymmet-ric skew caused by contaminating?ux.To do this,we ?rst determine the approximate position of the distribu-tion’s peak,and then?t a Gaussian function to the points to the positive side of this peak value,thereby ignoring the negative half that may su?er from?ux contamina-tion.We adopt the central value of the?tted Gaussian as the frame’s aperture correction,and its width as the uncertainty in the frame’s aperture correction.The in-strumental magnitude of each star is simply its allstar magnitude plus the frame’s aperture correction;we also add the aperture correction uncertainty in quadrature to each star’s photometric uncertainty.

2.4.Standard Star Observations and Photometric

Calibration

The?nal step in our determination of the stellar pho-tometry is to place the instrumental magnitudes we have measured onto a standard photometric system,using standard-star observations.Standard star?elds were im-aged several times on both of the observing nights.One standard?eld(SA101)was observed at two separate visits on each night in order to measure the e?ect of at-mospheric extinction.

The standard star observations are presented in Ta-ble2.We selected well-known standard?elds,?rst mea-sured by Landolt(1973),expanded for wide-?eld CCD instruments by Stetson(2000),and calibrated for the Washington C?lter by Geisler(1996).The standard ?elds were reduced using the pipeline procedure de-scribed in Section2.2.We then identi?ed sources in each standard?eld with daofind,and performed concentric-aperture photometry on all sources with phot in IRAF’s daophot package.

We use the measured aperture photometry of the ob-served standard stars,together with their total photom-etry as published by Stetson(2000)and Geisler(1996), to solve the following photometric calibration equation for each?lter and each CCD in the Mosaic-II array,and independently for the two nights of observing:

M=m+A+B?(R?I)?C?X

where M is the published total magnitude,m is the observed instrumental aperture magnitude,A is the pho-tometric zeropoint,B is the color term,R?I is the star’s true color,C is the atmospheric extinction term,and X is the airmass.Once we have determined A,B and C for each CCD and?lter,we will be able to convert the observed photometry of any star to its total photometry, given its observed color and the airmass at which it was observed.

We?rst determine C,the atmospheric extinction term, by examining the photometry from SA101,the standard ?eld that was observed at di?erent airmasses during each night.The C parameter is independent of the color term and zeropoint,so we simply need to?t a linear regression through the observed stars’magnitudes as a function of the observed airmass.The slope of the linear regression is C,the atmospheric extinction term.Note that we need not restrict ourselves to the actual standard stars for this step;since we only need the relative photometry to determine the extinction term,all of the stars observed in?eld SA101can be employed.

Having determined C,we proceed to simultaneously determine the zeropoint and color term.The observed magnitudes of all standard stars are?rst corrected for atmospheric extinction(m x=m?C?X);we then construct the quantity M?m x,the di?erence between the the published total magnitude of the star and its extinction-corrected instrumental magnitude,and?t a linear regression through the M?m x values as a func-tion of the published R?I colors.The slope of this regression is the color term B,and its zeropoint is A,the photometric zeropoint correction.Table3presents the photometric calibration parameters for each CCD and ?lter,and for each of the two observing nights.

Note that in Table3,the parameters for the Washing-ton C?lter are the same in all eight CCDs.The reason for this is that there are too few standard stars calibrated for the C?lter to support an independent determination for each CCD(see Table2),so we were forced to deter-mine an average C calibration for the entire mosaic.

As noted in Table3,there were too few R and I stan-dards present in some of the CCDs to support an in-dependent determination of their color terms and zero-points.For these cases,we perform a bootstrap esti-mate of the parameter values from those published at the CTIO website3.We determine the mean o?set between 3https://www.sodocs.net/doc/649745804.html,/mosaic/ZeroPoints.html

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our determined values of the photometric calibration pa-rameters,and those published by CTIO,for the CCDs that we were able to analyze.We then apply this mean o?set to the published values of the remaining CCDs,as an estimate of what we would have measured if we had observed enough standard stars in those CCDs.For the uncertainty in these bootstrapped parameters,we simply adopt the standard deviation of the mean o?set between the observed and published values.

We simultaneously perform a positional match of the sources in the C,R and I photometry lists,and apply the above photometric calibration to produce catalogs with total CRI photometry.We then match objects between the CRI catalogs from the short and long exposures of each?eld.For the positional matching,we use a max-imum match radius of0.5′′;to be retained in the cata-log,an object must be detected in the R band,with a matching detection in either C or I.For objects which are matched between the short and long exposures,we adopt the weighted mean photometry in the?nal cata-log;objects present only in the short or long catalog are included as well.The?nal calibrated composite photom-etry catalogs for the twelve observed?elds in the Magel-lanic Bridge(plus the observed o?set?eld)are presented in Table4,and rendered as pairs of Hess diagrams in Figure2.A Hess diagram is a pixelized color-magnitude diagram(CMD)in which each pixel value is proportional to the number of stars in the region covered by that pixel. The CMD of the o?set?eld is presented separately in Figure3.

2.5.Statistical Subtraction of Foreground/Background

Contamination

It is clear from comparing Figures2and3that the stellar populations in many of the?elds are dominated by foreground Galactic(and background extragalactic) contamination.In order to study the underlying inter-Cloud populations,we need to?rst perform a statistical subtraction of the contaminant foreground/background population.In doing so,we will assume that the popu-lation observed in the o?set?eld is representative of the contaminant population in each Bridge?eld(a reason-able assumption,given the similar Galactic latitude of the o?set?eld).

We proceed by?rst determining a scaling factor for normalizing the number of objects in the o?set?eld to the number of contaminant objects in each Bridge?eld. This is necessary to account for variations in the e?ective area covered by each?eld(which arise from masking out regions contaminated by bad pixels,very bright stars and bleed trails).The normalization factor is simply the ratio of object counts in the target and o?set?elds,for a se-lected subregion of each CMD that is expected to contain only contaminant objects.For the C?R CMD,the nor-malization region is de?ned by the criteria C?R>2.4, R<22.4?(C?R),and R>23.8?2?(C?R);for the R?I CMD the criteria are R?I>1.0and18

The statistical subtraction was generally successful in removing a component from each?eld’s CMD that is consistent with the contaminant population in the o?-set?eld.However,there are some artifacts present that bear explanation.Speci?cally,the faint end of many of the R?I CMDs appear to show an oversubtracted con-taminant population.This is simply due to the fact that the o?set?eld’s R?I CMD has a fainter detection limit than that in most of the target?elds.

3.ANALYSIS

3.1.Extent of the Young Inter-Cloud Population Young(age<1Gyr)stars provide an unambigu-ous tracer of the inter-Cloud population,because no foreground or background contaminants are expected to share the bright,blue region of the CMD with these stars. From previous work on the young inter-Cloud popula-tion by Demers&Battinelli(1998),we expected to ob-serve a population of young stars near the SMC,coin-cident with the young cluster population cataloged by Bica&Schmitt(1995).However,we did not know how far the young population would extend toward the LMC along the H I ridgeline.We isolate stellar populations younger than1Gyr in the C?R CMD,by selecting those stars with R<20mag and C?R<0mag(see dashed lines in Figure2).Stars matching these criteria are ab-sent in all of our?elds east of mb09,which corresponds roughly to the eastern extent of the Bica&Schmitt clus-ters.Interestingly,?eld mb09is also near the posi-tion along the H I Bridge where the gas surface density drops to the critical threshold for star formation of3–4M⊙pc?2(Kennicutt1989),which corresponds to the 5×1021cm?2seen throughout the eastern Bridge in Fig-ure4a of Br¨u ns et al.(2005).West of?eld mb09,the gas density is sustained at a level three times higher,and this is where star formation has been active in the Bridge.It would seem that the same star formation threshold ob-served for disk galaxies holds for this tidal debris envi-ronment as well.

3.2.Searching for Tidally-Stripped Stars in the

Inter-Cloud Region

It is perhaps not surprising that the young inter-Cloud population is con?ned to those regions where the gas density is relatively high,if we accept the hypothesis that these stars formed in situ,following the formation of the gaseous Bridge by a recent gravitational encounter between the Clouds.However,the tidal forces that pre-sumably formed the Bridge should have stripped stars and gas with equal e?ciency,so we expect to observe a population of such tidally-stripped stars in the inter-Cloud region.Yoshizawa&Noguchi(2003)conducted detailed numerical modeling of the stars and gas in the SMC,as it orbits both the LMC and Milky Way,in an attempt to reproduce the broad physical parameters of the Magellanic system.In their best-?tting model,there is an abundant population of stars in the inter-Cloud re-gion which formed in the SMC,and had been ejected into the inter-Cloud region by a tidal interaction with the LMC.The tidally-stripped stars should have a simi-lar age distribution to the stars in the galaxy from which

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they were stripped(at least for ages prior to the Bridge-forming event when their histories diverged).Since the stellar populations in both Magellanic Clouds exhibit a prominent red giant branch and a“red clump”horizontal branch,these bright features serve as ideal tracers of a putative stellar population that had been stripped from either of the Clouds during the Bridge-forming event. While some of the?elds in Figure4do show red gi-ant branch and red clump features,these older popu-lations appear to be con?ned to the?elds nearest the SMC(?elds mb02and mb03)or the LMC(?elds mb16–mb20).Furthermore,the surface density of these tracer populations increases sharply as the galactocentric sepa-ration of the?eld decreases,consistent with populations that are bound to the LMC and SMC.In Section3.3,we will demonstrate that the red giant populations in?elds mb16–mb20are consistent with a plausible exponential disk distribution centered on the LMC.For now,we sim-ply conclude that the red giant populations in these six ?elds near the SMC and LMC are very likely composed of stars bound to each respective galaxy,and are not indicative of a tidally-stripped stellar population in the Magellanic Bridge.

The Hess diagrams of the remaining six?elds(mb06–mb14)show no red features that can be associated with an old inter-Cloud population.However,the strength of this non-detection is limited by the presence of the contaminant population.To enhance our sensitivity to a potentially sparse old stellar population,we construct a composite pair of Hess diagrams from these six“true Bridge”?elds,and perform a new statistical contaminant subtraction on the composite population(see left panels of Figure5).Even in this composite Hess diagram which covers more than two square degrees of the inter-Cloud region,there is no detectable trace of an underlying red giant branch or red clump feature.

We can place an upper limit on the surface density of red giant branch stars in these six“true Bridge”by adding an arti?cial old stellar population at the distance of the Magellanic system(m?M=18.7mag,interme-diate between the two Clouds)to the composite inter-Cloud population.The arti?cial old stellar population is drawn from a theoretical isochrone(Girardi et al.2002) with Z=0.002and log(age)=10.0,to which we add photometric errors consistent with the data.We modu-late the number of arti?cial stars added until a red giant branch is marginally detectable(Figure5).We conclude from this exercise that there are fewer than1000red giant branch stars at the distance of the Bridge and brighter than R=23mag in the observed composite population. By applying a stellar mass function(Kroupa2001),we can convert the upper limit on the number of observed red giants to an upper limit on the total stellar mass present in a putative old stellar population.However, the conversion factor depends on the assumed age of the stars,because the fraction of the total stellar population that is brighter than R=23mag varies with age.For a10Gyr population,the upper mass limit is14800M⊙, and for a2.5Gyr population,the upper mass limit is 5300M⊙.Thus,the stellar surface mass density in these six“true Bridge”?elds is≤0.009M⊙pc?2;this is more than400times smaller than the average surface mass density of H I in the Magellanic Bridge(4M⊙pc?2, converted from the characteristic column density in the Bridge,Br¨u ns et al.2005).There does not appear to be any trace of a tidally-stripped stellar population in the Magellanic Bridge,at least in these six?elds along the H I ridgeline.

One potential caveat in this analysis is that we have assumed that the putative tidally stripped stellar pop-ulation would be spatially coincident with the gaseous Bridge.This need not be the case;if ram-pressure from the Milky Way halo has played a signi?cant role in the evolution of the Magellanic system(Mastropietro et al. 2005),then it is possible that the gaseous Bridge is now displaced from the region occupied by tidally-stripped stars between the Clouds.We investigate this possibility using data from the2-Micron All-Sky Survey(2MASS, Skrutskie et al.2006).

Using the Gator web-based database query service4at the NASA/IPAC Infrared Science Archive,we obtained near-infrared JHK photometry from the2MASS All-Sky Point Source Catalog,in two regions(shown as dashed boxes in Figure1).The?rst2MASS region(the“full-bridge region”)was selected to cover all plausible lo-cations where a tidally-stripped inter-Cloud population might exist.We selected a range in right ascension be-tween2.5h and3.5h,because these limits are bracketed by?elds mb06and mb14,which de?ne the edges of the “pure bridge”section of our sample,uncontaminated by LMC or SMC stars.We selected a very large range in declination,from?77?to?69?,to cover all plausible tra-jectories of a putative tidally-stripped stellar population. The second2MASS region(the“SW-LMC region”)was selected as a comparison?eld that is known to contain an old stellar population at the distance of the Magellanic system.This region spans4.2h to5h in right ascension, and?75?to?74?in declination.It is coincident with our?elds mb18,mb19and mb20,in which we have ob-served an old stellar population associated with the LMC (Section3.2).While the full-bridge region covers a solid angle ten times larger than that of the SW-LMC region (35square degrees and3.2square degrees,respectively), the2MASS catalog contains about the same number of stars in both regions(94000stars in the full-bridge re-gion,and92000stars in the SW-LMC region),due to the larger stellar surface density of the SW-LMC region. The2MASS J?K CMDs for these two regions are shown in Figure 6.In the SW-LMC region,there is an abundant population of red objects consisting of old stars associated with the the LMC.Following Nikolaev&Weinberg(2000),we identify the various sub-populations of these red objects.The bulk of the pop-ulation extends in a narrow diagonal sequence from J?K=1mag,K=14mag to J?K=1.25mag,K=11mag. Along this sequence,there is a sharp drop in the density of stars around K=12.3mag;this is the tip of the red giant branch.The stars in this sequence brighter than K=12.3mag are oxygen-rich asymptotic giants,while the stars which extend redward of J?K=1.25mag are carbon-rich asymptotic giants.In the full-bridge region’s J?K CMD,there is a small number of stars whose pho-tometry is consistent with these features(notably the～6 red objects around K=11mag which may be Carbon stars at the Magellanic distance),but considering the very large solid angle covered by the full-bridge region, 4https://www.sodocs.net/doc/649745804.html,/applications/Gator

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we do not regard these objects as a signi?cant detection of an old inter-Cloud population.We can place an up-per limit on the number of red giants at the distance of the Magellanic system that can remain undetected in the2MASS CMD,using the same synthetic popula-tion analysis described above for our optical CMDs.A red giant population containing150stars brighter than K=14mag is easily detectable when added to the 2MASS CMD,which implies that any old inter-Cloud stellar population that may be present has a total stellar mass no greater than2×106M⊙.This is1%of the total H I mass in the Magellanic Bridge(Br¨u ns et al.2005). However,the area covered by our full-Bridge region is about three times smaller than the area used to de?ne the Bridge by Br¨u ns et al.,so the true limit from the2MASS data is closer to3%.Thus,even accounting for the possi-bility that a putative tidally-stripped stellar population may be displaced from the gaseous Magellanic Bridge, we can still conclude that the Bridge material was more than97%gas when the Bridge was formed.

3.3.The Outer Disk of the Large Magellanic Cloud In?elds mb16–mb20we observe old stellar populations that we conclude are bound members of the LMC,based on the sharp increase in their surface density with de-creasing angular separation from the LMC.Gallart et al. (2004)and others have found that the LMC’s stellar radial pro?le follows an exponential disk to projected radii beyond7kpc,with no sign of a break which might indicate the onset of a kinematic halo.Fields mb16–mb20have projected separations from the LMC of be-tween5kpc and8.5kpc;however,when the orienta-tion of the LMC disk(van der Marel&Cioni2001)is taken into account,the in-disk galactocentric distances of these?elds are between6kpc and10.5kpc.We use the number of stars in the red clump feature as a proxy for the stellar surface density,and plot the surface den-sity pro?le in Figure7.The solid curve represents the best-?t exponential-disk model,with a scale length of α=0.98kpc,and the dotted and dashed curves are ex-ponential disk models?t by previous authors,as noted in the?gure caption.The surface density pro?le of the red clump stars in?elds mb16–mb20are generally con-sistent with previous measurements of the LMC’s outer exponential disk,but the fact that the pro?le is some-what steeper in this southwestern quadrant is interest-ing.A comprehensive survey of the stellar populations in the outer LMC is currently underway;we will there-fore postpone further discussion of the LMC’s structure until this survey is completed,when more de?nitive con-clusions can be made.

3.4.Characterizing the Purely Tidal Stellar Population

in the Inter-Cloud Region

We have determined that the stars in the inter-Cloud region appear to be exclusively composed of a stellar pop-ulation that formed in situ,in the wake of the Bridge-forming event(modulo some contribution from stars still bound to the LMC and to the SMC,in the observed Bridge?elds nearest those galaxies).This isolation of a tidally-triggered stellar population provides an impor-tant opportunity to examine the nature and evolution of star formation processes in tidal debris.We measure the age distribution of the inter-Cloud population to deter-mine when the star formation occurred,and how long it lasted.Since these stars presumably formed in the wake of the Bridge-forming event,these measurements provide an important constraint on the timing of that event.We will also look for spatial structure in the age distribu-tion,which may provide insights into how star formation proceeds when triggered by a gravitational interaction. Previous studies of the inter-Cloud population have es-timated the age of the youngest stars present,through simple isochrone?tting(e.g.,Demers&Battinelli 1998).Here we will perform a more detailed anal-ysis,using the StarFISH star formation history?tter (Harris&Zaritsky2001).This analysis is motivated by the clear presence of composite stellar populations in some of our?elds,and by our goal to constrain the du-ration of star-formation activity in the Bridge. StarFISH constructs a library of synthetic CMDs,each of which represents a model of what the photometric ob-servations would yield,if the observed stellar population had a single age and a single metallicity.The model pho-tometry is derived from theoretical isochrones;in this case we chose the latest Padua isochrones(Girardi et al. 2002).In order to accurately predict the observed pho-tometric distribution in the CMDs,the models include a distance modulus,a distribution of extinction values, and a detailed model of the photometric errors.The dis-tance modulus was simply chosen to be that of the SMC, 18.9mag,because the young stellar populations in the Bridge are near the SMC on the sky.The distribution of extinction values is drawn from regions near the eastern edge of the MCPS SMC extinction map(Zaritsky et al. 2002).For the photometric errors,we employ an ana-lytic model that reproduces the error statistics in the ob-served?elds.While we usually advocate for an empirical model based on arti?cial stars tests,these tests are only strictly necessary when the data images are crowded.In the present case,even in our?eld with the highest stellar surface density(mb20),we have detected roughly109000 stars in81922pixels,corresponding to a mean separation between objects of almost14pixels.

The StarFISH model library provides synthetic CMDs for the range of ages and metallicities thought to be present in the observed population;in the present case we constructed synthetic CMDs for16age bins spanning ages10Myr to12Gyr,spaced uniformly in log(age), and for three metallicity bins,Z=0.001,Z=0.002, and Z=0.004.The best-?t SFH is found by determin-ing the combination of amplitudes modulating these syn-thetic CMDs which produces a composite model CMD that most closely matches the observed CMD.To take advantage of the full CRI photometric data set in de-termining the SFH,the?t is actually performed on the CMD pair:C?R vs.R and R?I vs.I.

Because the contaminant population dominates many of our observed?elds,it is important to account for con-taminants in the SFH?t.We could have run StarFISH on the statistically-cleaned data set,but we instead chose to use the observed data set,and simply include the con-taminant o?set-?eld population as an additional ampli-tude in the model,in addition to the normal set of syn-thetic CMDs.The code will then select a multiplicative amplitude factor that optimally accounts for the contam-inant population,just as it does for each of the synthetic

7

CMDs.

The star formation histories of our twelve observed Magellanic Bridge?elds are presented in Figure8,and the results are consistent with the qualitative analysis of the CMDs presented in Sections3.1and3.2.Recent star formation has occurred only in the?elds west of mb11, and old stellar populations are con?ned to the six?elds nearest the SMC(mb02and mb03)and LMC(mb16–mb20).In?elds mb11,mb13and mb14there is no trace of a stellar population associated with the Magellanic system.

Our StarFISH analysis shows that star formation in the Bridge began around200–300Myr ago,and this measurement provides an important constraint on the timing of the Bridge-forming event.In?eld mb02,the ?eld nearest the SMC,we see a prolonged star formation episode spanning ages80–300Myr,and in mb03we see a slightly shorter episode spanning ages100–200Myr. In the other?elds in which a young stellar population is present(mb06–mb09),the star formation rates are much lower,making ages and durations more di?cult to deter-mine reliably.To boost the signal,we construct a com-posite population from these three?elds,and determine the SFH of the composite population(see Figure9).In these more easterly?elds,we see evidence for two distinct episodes of star formation,160Myr and40Myr ago.It is interesting that while star formation was active through-out the western Bridge100–200Myr ago,the more recent episode40Myr ago was apparently con?ned to regions further from the SMC.We note that Demers&Battinelli (1998)also found that the youngest inter-Cloud popula-tions are to be found in?elds eastward of the SMC Wing. The StarFISH solutions indicate that no signi?cant star formation occurred in the Bridge more recently than 40Myr ago.This conclusion can be con?rmed by direct inspection of the CMDs in Figure2:in no region do we see a signi?cant number of main sequence stars brighter than R=15mag,corresponding to the main-sequence turn o?position of a40Myr isochrone at the distance of the Magellanic system.

The conclusion that star formation in the Bridge largely ceased around40Myr seems to be at odds with a variety of previous research that?nds ev-idence of stellar populations much younger than this.Demers&Battinelli(1998)used main-sequence isochrone?tting to conclude that the western Bridge contains stars as young as10–25Myr.Meaburn(1986) reported the discovery of DEM171,a large circular Hα?lament in the Bridge which is likely photoionized by one or more massive O stars.Bica&Schmitt(1995)found that some of the clusters in their Bridge catalog have as-sociated emission nebulae,again implying the presence of massive stars.Mizuno et al.(2006)detected cold molec-ular clouds in the Bridge,which demonstrates at least the potential for ongoing star formation.

This apparent contradiction can be partly reconciled by understanding that we are not claiming there are ab-solutely no stars in the Bridge younger than40Myr; we?nd that the star-formation rate dropped o?around 40Myr ago,and has remained consistent with zero since then.We also note that our?eld selection covers the HI ridgeline of the Magellanic Bridge uniformly(see Fig-ure1),except for the segment around RA=2h,where much of the evidence for more recent star formation is to be found.These explanations do not reconcile our result with the conclusions of Demers&Battinelli (1998),however.They reported the widespread presence of stars aged10–25Myr in a number of?elds east of RA=2h.This is based on an analysis of their Fig-ure7(lower panel),in which theoretical isochrones are overplotted on a composite CMD from four of their ob-served?elds.While the observed main sequence does appear to follow the shape of the10Myr isochrone,it is clearly truncated around M V=?3mag,whereas a 10Myr population should have a main sequence that ex-tends up to M V=?5mag.A main sequence turn-o?at M V=?3mag is consistent with the40Myr age that we have found for the youngest bulk population in the Bridge.

4.SUMMARY

We have observed stellar populations in twelve?elds uniformly spanning the region between the Magellanic Clouds.Our?elds were selected to follow the ridgeline of the H I gas that forms the Magellanic Bridge,in order to look for stars that formed in situ in the Bridge from gas that had already been removed from one of the Clouds, and also for stars that were stripped from either of the Clouds by the same tidal forces that presumably stripped the gas.

We observed the previously known young stellar popu-lation in the western half of the inter-Cloud region,most recently characterized by Demers&Battinelli(1998), and extend on previous analyses in two key ways.First, we determine that the eastward extent of these stars is truncated aroundα=3h,corresponding also to the eastward extent of the star clusters cataloged by Bica&Schmitt(1995),and to the point at which the H I surface density falls below the critical threshold for star formation as determined by Kennicutt(1989).Second, we use the StarFISH program to determine quantitative star formation histories of the young inter-Cloud pop-ulation,?nding that star formation in the Bridge com-menced about200–300Myr ago,and continued over an extended interval,until about40Myr ago.

We found no evidence for a population of tidally-stripped stars in the inter-Cloud region,and our non-detection allows us to conclude that the material stripped from the Clouds into the Bridge was very nearly a pure gas,with an upper limit on the mass fraction in stars of less than10?4if coincidence with the gaseous Bridge is assumed,and0.03otherwise.This can potentially be understood if the pre-collision SMC had an extended en-velope of gas,surrounding a more tightly bound stellar component.In this case,a weak tidal interaction might unbind the gas envelope while leaving the stellar compo-nent undisturbed.It is known that some dwarf galaxies have H I gas extending beyond2–3times the radii oc-cupied by their stellar populations(Salpeter&Ho?man 1996),so perhaps this scenario is plausible.In fact, the recent numerical simulation of the tidal history of the SMC by citeyn03included such an extended gas envelope,in order to produce a pure-gas Magellanic Stream.Nevertheless,the inter-Cloud region in their best model contains an abundant population of tidally-stripped stars.In addition,Figure4a of Br¨u ns et al. (2005)shows that the H I gas in the Magellanic Bridge

8

appears to be contiguous with the higher-density gas in the central regions of the SMC,which are currently abundantly populated with stars.If the gaseous Bridge formed via the tidal extraction of this high-density gas from the central regions of the SMC,then the question remains:where are the stars in the Bridge that should have felt these same tidal forces?

Future observations may be able to address this ques-tion.While some kinematic measurements exist for a handful of stars in the inter-Cloud region(Maurice et al. 1987;Kunkel et al.1997),radial-velocity kinematics of a truly representative sample of the young inter-Cloud population would help us to better understand the dy-namical evolution of the Magellanic Bridge.A much deeper understanding of the SMC’s complex three-dimensional structure and kinematics would certainly help as well.These measurements(along with our cur-rent understanding of the orbital motions of the Clouds and the Milky Way)could then be used to motivate new detailed numerical simulations speci?cally targeting the formation of the Magellanic Bridge as a pure-gas feature.We may then better understand how tidal features are formed during minor harassment interactions,and what role such interactions play in driving the evolution of the participant galaxies.

I am very grateful for extended discussions with Ed-ward Olszewski on the interpretation of the data pre-sented here,and I would also like to thank Kurtis Williams,Abhijit Saha,Knut Olsen,Tim Abbot and Armin Rest for their assistance with the reduction of the Mosaic-II images.Finally,I would like to grate-fully acknowledge the constructive comments made by the anonymous referee.These comments prompted the 2MASS analysis,and substantially improved the paper. JH is supported by NASA through Hubble Fellowship grant HF-01160.01-A awarded by the Space Telescope Science Institute,which is operated by the Association of Universities for Research in Astronomy,Inc.,under NASA contract NAS5-26555.

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Br¨u ns, C.,Kerp,J.,Staveley-Smith,L.,Mebold,U.,Putman, M.E.,Haynes,R.F.,Kalberla,P.M.W.,Muller,E.,&Filipovic, M.D.2005,A&A,432,45

Demers,S.&Battinelli,P.1998,AJ,115,154

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Harris,J.&Zaritsky,D.2001,ApJS,136,25

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Irwin,M.J.,Kunkel,W.E.,&Demers,S.1985,Nature,318,160 Jannuzi, B.T.,Claver,J.,&Valdes, F.2003,The NOAO Deep Wide-Field Survey MOSAIC Data Reductions, https://www.sodocs.net/doc/649745804.html,/noao/noaodeep/ReductionOpt/frames.html Kennicutt,Jr.,R.C.1989,ApJ,344,685

Kroupa,P.2001,MNRAS,322,231

Kunkel,W.E.,Irwin,M.J.,&Demers,S.1997,A&AS,122,463 Landolt,A.U.1973,AJ,78,959

Mastropietro,C.,Moore,B.,Mayer,L.,Wadsley,J.,&Stadel,J. 2005,MNRAS,363,509Maurice,E.,Andersen,J.,Ardeberg,A.,Bardin,C.,Imbert,M., Lindgren,H.,Martin,M.,Mayor,M.,Nordstrom,B.,Prevot,L., Rebeirot,E.,&Rousseau,J.1987,A&AS,67,423

Meaburn,J.1986,MNRAS,223,317

Mizuno,N.,Muller,E.,Maeda,H.,Kawamura,A.,Minamidani, T.,Onishi,T.,Mizuno,A.,&Fukui,Y.2006,ApJ,643,L107 Nikolaev,S.&Weinberg,M.D.2000,ApJ,542,804

Putman,M.E.2000,Publications of the Astronomical Society of Australia,17,1

Salpeter,E.E.&Ho?man,G.L.1996,ApJ,465,595 Skrutskie,M.F.,Cutri,R.M.,Stiening,R.,Weinberg,M.D., Schneider,S.,Carpenter,J.M.,Beichman, C.,Capps,R., Chester,T.,Elias,J.,Huchra,J.,Liebert,J.,Lonsdale, C., Monet,D.G.,Price,S.,Seitzer,P.,Jarrett,T.,Kirkpatrick, J.D.,Gizis,J.E.,Howard,E.,Evans,T.,Fowler,J.,Fullmer, L.,Hurt,R.,Light,R.,Kopan,E.L.,Marsh,K.A.,McCallon, H.L.,Tam,R.,Van Dyk,S.,&Wheelock,S.2006,AJ,131,1163 Stetson,P.B.2000,PASP,112,925

van der Marel,R.P.&Cioni,M.-R.L.2001,AJ,122,1807 Yoshizawa,A.M.&Noguchi,M.2003,MNRAS,339,1135 Zaritsky,D.,Harris,J.,Thompson,I.B.,&Grebel,E.K.2004, AJ,128,1606

Zaritsky,D.,Harris,J.,Thompson,I.B.,Grebel,E.K.,&Massey, P.2002,AJ,123,855

9

TABLE1

Mosaic-II Fields and Exposures

Field ID Right ascension Declination Filter t exp Observing time Airmass

[sec][UT]

O?set00h13m?79?59′C6002006-01-0401:19 1.68·········R3002006-01-0401:42 1.72·········I3002006-01-0401:56 1.74·········C202006-01-0501:09 1.67·········R102006-01-0501:11 1.68·········I102006-01-0501:13 1.68 mb0201h48m?74?30′C6002006-01-0402:11 1.50·········R3002006-01-0402:34 1.54·········I3002006-01-0402:48 1.56·········C202006-01-0403:02 1.59·········R102006-01-0403:04 1.59·········I102006-01-0403:07 1.60 mb0302h00m?73?00′C6002006-01-0501:16 1.40·········R3002006-01-0501:42 1.42·········I3002006-01-0501:57 1.44·········C202006-01-0501:40 1.42·········R102006-01-0501:55 1.44·········I102006-01-0502:10 1.46 mb0602h24m?73?54′C6002006-01-0403:47 1.60·········R3002006-01-0404:13 1.66·········I3002006-01-0404:28 1.70·········C202006-01-0404:11 1.65·········R102006-01-0404:26 1.69·········I102006-01-0404:42 1.73 mb0802h42m?73?30′C6002006-01-0502:13 1.42·········R3002006-01-0502:38 1.45·········I3002006-01-0502:53 1.47·········C202006-01-0502:36 1.45·········R102006-01-0502:51 1.47·········I102006-01-0503:07 1.49 mb0903h00m?73?30′C6002006-01-0404:43 1.64·········R3002006-01-0405:09 1.71·········I3002006-01-0405:24 1.75·········C202006-01-0405:07 1.70·········R102006-01-0405:22 1.74·········I102006-01-0405:38 1.79 mb1103h18m?74?00′C6002006-01-0503:11 1.46·········R3002006-01-0503:37 1.49·········I3002006-01-0503:52 1.51·········C202006-01-0503:35 1.49·········R102006-01-0503:50 1.51·········I102006-01-0504:06 1.54 mb1303h36m?74?30′C6002006-01-0405:52 1.74·········R3002006-01-0406:17 1.81·········I3002006-01-0406:33 1.86·········C202006-01-0406:15 1.80·········R102006-01-0406:31 1.85·········I102006-01-0406:46 1.91 mb1403h42m?73?18′C6002006-01-0504:20 1.51·········R3002006-01-0504:45 1.55·········I3002006-01-0505:00 1.59·········C202006-01-0504:43 1.55·········R102006-01-0504:58 1.58·········I102006-01-0505:14 1.62 mb1603h54m?75?00′C6002006-01-0505:17 1.62·········R3002006-01-0505:42 1.68·········I3002006-01-0505:57 1.73·········C202006-01-0505:40 1.68·········R102006-01-0505:56 1.71·········I102006-01-0506:18 1.77 mb1804h12m?75?00′C6002006-01-0406:48 1.80·········R3002006-01-0407:14 1.88·········I3002006-01-0407:29 1.93·········C202006-01-0407:12 1.87·········R102006-01-0407:27 1.93·········I102006-01-0407:42 1.98 mb1904h30m?75?00′C6002006-01-0506:32 1.72·········R3002006-01-0506:57 1.78·········I3002006-01-0507:13 1.83·········C202006-01-0506:55 1.78·········R102006-01-0507:11 1.82·········I102006-01-0507:26 1.88 mb2004h48m?74?30′C6002006-01-0507:28 1.82·········R3002006-01-0507:54 1.91·········I3002006-01-0508:09 1.97·········C202006-01-0507:52 1.90

10

TABLE2

Standard Field Observations

Field ID Right ascension Declination Filter t exp Observing time Airmass N standards

[sec]

SA9200h55m00?40′C202006-01-0400:53 1.385·········R102006-01-0400:55 1.3889·········I102006-01-0400:57 1.39132 SA9200h55m00?40′C202006-01-0403:13 3.016·········R102006-01-0403:15 3.0898·········I102006-01-0403:17 3.14147 SA10109h57m?00?20′C202006-01-0405:43 1.346·········R102006-01-0405:45 1.3351·········I102006-01-0405:47 1.3379 SA10109h57m?00?20′C202006-01-0407:47 1.156·········R102006-01-0407:50 1.1549·········I102006-01-0407:51 1.1582 SA9806h52m?00?24′C202006-01-0407:55 1.7315·········R102006-01-0407:57 1.75662·········I102006-01-0407:59 1.77888 NGC229806h49m?36?00′C202006-01-0408:02 1.36············R102006-01-0408:04 1.37515·········I102006-01-0408:06 1.3842 SA9200h55m00?40′C202006-01-0501:01 1.427·········R102006-01-0501:03 1.4397·········I102006-01-0501:05 1.44144 SA10109h57m?00?20′C202006-01-0504:11 1.916·········R102006-01-0504:13 1.8855·········I102006-01-0504:15 1.8680 SA10109h57m?00?20′C202006-01-0506:23 1.227·········R102006-01-0506:25 1.2252·········I102006-01-0506:27 1.2277

11

TABLE3

Photometric Calibration Parameters

Filter CCD Extincton Ext.Unc.ZP ZP Unc.Color Term Col.Term Unc.

Night1

C1–8a0.2750.00150.0740.0200-0.1000.0100 R1b0.0720.0010-0.6900.0200-0.0320.0130 R20.0720.0010-0.7060.0045-0.0240.0077 R30.0720.0010-0.6840.0023-0.0420.0039 R40.0720.0010-0.6700.0045-0.0490.0083 R5b0.0720.0010-0.6820.0200-0.0350.0130 R60.0720.0010-0.6800.0033-0.0440.0050 R70.0720.0010-0.6660.0027-0.0520.0042 R80.0720.0010-0.7030.0027-0.0300.0050 I10.0400.0009-0.0290.0185-0.0030.0140 I20.0400.00090.0090.00220.0070.0016 I30.0400.00090.0080.0020-0.0010.0016 I40.0400.00090.0140.00380.0050.0041 I5b0.0400.0009-0.0060.02300.0030.0090 I60.0400.0009-0.0110.00200.0120.0015 I70.0400.00090.0100.00150.0030.0013 I80.0400.0009-0.0030.00310.0100.0022

Night2

C1–8a0.2990.00120.0260.0200-0.0870.0100 R1b0.0900.0010-0.7310.0200-0.0210.0130 R20.0900.0010-0.7320.0078-0.0220.0140 R30.0900.0010-0.7070.0030-0.0320.0050 R40.0900.0010-0.7270.0088-0.0470.0176 R5b0.0900.0010-0.7230.0200-0.0240.0130 R60.0900.0010-0.7100.0055-0.0300.0082 R70.0900.0010-0.7270.0067-0.0220.0094 R80.0900.0010-0.7480.0043-0.0200.0079 I1b0.0610.0007-0.0380.02200.0070.0130 I20.0610.0007-0.0110.00250.0020.0017 I30.0610.0007-0.0300.00470.0040.0038 I40.0610.0007-0.0030.0111-0.0190.0109 I5b0.0610.0007-0.0330.02000.0010.0130 I60.0610.0007-0.0290.00260.0180.0021 I70.0610.0007-0.0210.0031-0.0020.0021 I80.0610.0007-0.0290.00280.0110.0020

a There were to few C standards to support an independent transformation solution for each CCD.

b There were too few standards in this CCD to support an independent transformation solution.The zeropoint and color term values for this CCD are bootstrapped from the values published at the CTIO website.

TABLE4

Photometry of Stars in Magellanic Bridge Fields

Object ID Right ascension Declination CσC RσR IσI

[mag][mag][mag][mag][mag][mag]

Region mb02

12

Fig. 1.—A schematic view of the Magellanic Bridge

region.The positions of the twelve observed CTIO-4m?elds are shown as numbered boxes.The inter-Cloud?elds previously observed by Demers&Battinelli(1998)are shown as gray boxes.Star clusters from Bica&Schmitt(1995)are shown as points.The gray line is an approximate trace of the HI ridgeline through the inter-Cloud region,from Figure1in Putman(2000).The hashed areas represent the portions of the regions covered by the Magellanic Clouds Photometric Survey (LMC at left,Zaritsky et al.2004;SMC at right,Zaritsky et al.2002).

22

20

18

16

14

R

,

I

mb02mb03mb06mb08

22

20

18

16

14

R

,

I

mb09mb11mb13mb14

02

C-R

22

20

18

16

14

R

,

I

mb16

01

R-I

02

C-R

mb18

01

R-I

02

C-R

mb19

01

R-I

02

C-R

mb20

01

R-I

10

20

Fig. 2.—Color-magnitude Hess diagrams of each of the twelve?elds observed in the Magellanic Bridge.Each pair of panels consists of the C?R,R CMD(left)and the R?I,I CMD(right).The?elds are numbered in order of increasing right ascension(see Figure1). Dashed lines indicate the region of the C?R CMDs where main sequence stars younger than1Gyr are expected.The pixel values indicate the number of stars present in each pixel,ranging from zero(white)to20(black),as indicated by the scale bar on the right.

13

Fig. 3.—The CMD pair for the o?set?eld,which has a similar Galactic latitude to the Bridge?elds,but lies to the west of the SMC. It therefore should represent a pure foreground Galactic and background extragalactic population.As in Figure2,the C?R,R CMD is shown at left,and the R?I,I CMD is shown at right.The normalization regions,which are expected to contain only contaminant objects, are outlined with dashed lines.

14

22

20

18

16

14

R

,

I

mb02mb03mb06mb08

22

20

18

16

14

R

,

I

mb09mb11mb13mb14

02

C-R

22

20

18

16

14

R

,

I

mb16

01

R-I

02

C-R

mb18

01

R-I

02

C-R

mb19

01

R-I

02

C-R

mb20

01

R-I

-5

5

10

Fig. 4.—Color-magnitude Hess diagrams of the twelve observed Bridge?elds,after statistically subtracting a contaminant component derived from the o?set?eld population(see Figure3).Darker pixels indicate a local excess of stars compared to the contaminant population, whereas lighter pixels indicate a local de?cit of stars.In a perfect subtraction,the pixels lighter than the medium gray of zero di?erence would be consistent with the noise of the subtraction.Many of the?elds show large de?cits of stars at the faint end of the R?I CMD;we believe this is due to a mismatch in the faint limit between these target?elds and the o?set?eld.The pixel values indicate the di?erence in star counts(N f ield?N of f set)for each pixel,ranging from-5(white)to+10(black),as indicated by the scale bar on the right.

15

22 20 18 16 14 R

observed 0 2 C-R

22

20 18 16 14 R

+ 500 RGB

0 2 C-R

+ 1000 RGB

0 2 C-R

+ 2000 RGB

0 2 C-R

+ 4000 RGB

0 2

C-R

Fig. 5.—The top left panel shows the C ?R Hess diagram for a composite population formed by combining the “true Bridge”?elds (mb06–mb14).We see no trace of a red giant branch feature which would indicate an older stellar population in these ?elds.We determine the upper limit on the number of red giants which could remain undetected in this diagram by adding a synthetic red giant branch composed of increasing numbers of stars.From left to right,starting with the second panel we show the same Hess diagram after having added 500,1000,2000,and 4000synthetic red giant stars.The bottom row of panels show the same Hess diagrams,after having statistically subtracted a contaminant population derived from the o?set ?eld.

16

Fig. 6.—Near-infrared J?K color-magnitude diagrams for two regions between the Magellanic Clouds.At left is the“full-bridge region”,which spans35square degrees and is designed to cover the full region between the Clouds that is not contaminated by stars bound to either galaxy.At right is the“SW-LMC region”,which covers3.2square degrees and is coincident with our?elds mb18,mb19,and mb20,in which we have observed an old stellar population associated with the LMC.The dotted lines in each panel indicate the region where the bright red giant and asymptotic giant branches are expected at the distance of the Magellanic system.While an abundant population of these stars is present in the SW-LMC region,the much larger full-bridge region contains hardly any such stars.

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Fig.7.—The number of red clump stars present in?elds mb16–mb20vs.the physical distance of each?eld from the center of the LMC.In determining these distances,we have assumed that the red clump stars are in the disk of the LMC,whose geometry is known. The best-?t exponential-disk model for these data is shown as the solid line,and it is similar to previous determinations of the LMC’s exponential disk by(Hardy1978,;dashed line)and(Gallart et al.2004,;dotted line).

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Fig.8.—StarFISH SFH solutions for the twelve observed?elds in the Magellanic Bridge.Each panel is divided into three subpanels, showing the star formation rate as a function of time for three metallicities(top to bottom:Z=0.004,Z=0.002,and Z=0.001).The best-?t history is plotted with a heavy line in each sub-panel,and the shaded gray regions indicate the estimated uncertainty of the solution.Fields are presented in order of increasing right ascension,starting with mb02(the?eld nearest the SMC)in the lower left,up to mb20(the?eld nearest the LMC)in the upper right.

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Fig.9.—The SFH solution for a composite population formed from?elds mb06–mb09.The solutions for these?elds presented in Figure8are inconclusive,due to the small number of stars in these?elds.By forming a composite population,we can better constrain the average history of these three regions.