KINEMATICS OF DWARF GALAXIES AND THEIR REMNANTS IN THE MILKY WAY HALO

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1 KINEMATICS OF DWARF GALAXIES AND THEIR REMNANTS IN THE MILKY WAY HALO Abstract Jeffrey L. Carlin Advisor: Steven R. Majewski Department of Astronomy, University of Virginia Dwarf spheroidal (dsph) galaxies represent the lowmass end of the dark matter halo distribution, making them key discriminators between models of cosmological structure formation in the universe. Recent efforts have uncovered a few low-luminosity dsphs, as well as numerous stellar tidal streams (remnants of accreted, tidally disrupted dwarf galaxies) in the Milky Way halo, helping to explain the perceived deficit of observed dsphs relative to model predictions. To explore the reasons some halos survive intact, while others are tidally disrupted, orbital information is needed. To date, most kinematic studies of the dwarfs themselves, as well as known tidal features, have been limited to radial velocity programs, providing only one dimension of the full space motions. In the work described here, we measure tangential velocities ( proper motions ) of individual stars in these systems, which can be combined with the radial component to derive orbits. The dynamical information of the dsphs and known tidal streams can then be used to look for orbital associations, as well as to constrain the shape of the Milky Way dark matter halo. We report here work in progress on the absolute proper motion of the Carina dsph, as well as a proper motion survey of, among others, the Sagittarius and Monoceros tidal streams. Background Most of the mass of our Galaxy is made up of dark matter, the nature of which is poorly understood. On large scales (> Mpc) ( parsec (pc) = 3.86 x 3 km = 3.26 light years), Cold Dark Matter + Lambda (ΛCDM) models have been successful in predicting many of the properties of the Universe (Bahcall et al. 999), but have fallen short of predicting the small-scale distribution of dark matter around the Milky Way (MW). In particular, the number of predicted low-mass dark matter haloes ( clumps of dark matter) is more than an order of magnitude higher than the number of observed MW satellite dwarf galaxies (Klypin et al. 999, Moore et al. 999). This may be because dark matter sub-haloes that formed galaxies at early epochs (which, in turn, had their own globular clusters (GCs, which are large, roughly spherical conglomerations of 5 to 6 stars) orbiting in their potential wells) have been torn apart in tidal interactions with each other or with the Milky Way potential, scattering their GCs into the outer MW halo, and con- Fig.. Series of still images from a numerical simulation of a satellite being torn apart via tidal interactions with the Milky Way halo over the course of a few billion years. The Milky Way is shown in blue at the center of each panel, while the satellite (having mass and number of stars similar to known Galactic dsphs) is shown initially in red at upper left. Each panel is a snapshot after a timestep of roughly.5 billion years, increasing from left to right. The gravitational pull from the Milky Way s dark matter halo strips stars from the satellite as it orbits, leaving both a leading and trailing stream of stars along the satellite s orbital path. (From Kathryn Johnston, kvj/mw.html.) tributing their stellar populations to the MW stellar halo. Models have shown that the stellar halo of our Galaxy can be built up by tidally stripped stars from the original population of dwarf galaxies (Figure ), which were initially larger and lost a significant amount of their mass in tidal interactions (Bullock et al. 2). It may also be the case that remnants of disrupted, formerly larger satellites Carlin

2 Fig. 2. Spatial density of SDSS stars with (g r) <.4 around the north Galactic cap in equatorial coordinates, binned.5 degrees. The color plot is an RGB composite with blue for the most nearby stars with 2. < r < 2.66, green for stars with 2.66 < r < 2.33, and red for the most distant stars with 2.33 < r < 22.. This area has been dubbed the Field of Streams due the the large number of spatially coherent stellar streams seen in this region of sky. This plot has been displayed in a way that emphasizes the Sagittarius stream, the large feature in the lower half of the plot. Further structure that is visible includes the Monoceros Ring at α 2 and a new stream, dubbed the Orphan stream (also found independently by Grillmair 26) at 5 α 6 and δ 3. (From Belokurov et al. 26.) survive the tidal interactions and retain the orbital characteristics imparted to them at the time of the break-up, allowing us to trace the origins of these dynamical families. Since our Galaxy is the only galaxy for which we can obtain detailed observations of these stellar systems, it provides the primary constraint on dark matter models (ΛCDM and others) at small scales. It has been noted (Lynden-Bell 982, Lynden-Bell 976, Kunkel 979, Kunkel & Demers 976) that some families of Galactic dsphs and GCs appear to lie in common planes in the MW halo. This has led some to propose that these planes represent distinct orbital alignments, which have their origins in the accretion or breakup of a formerly larger parent satellite (e.g., the postulated Greater Magellanic Galaxy of Lynden-Bell (976); outer halo GCs: van den Bergh & Mackey 24). Palma et al. (22) studied the distribution of orbital poles of MW satellites in search of dynamical families (planar alignments), but were limited to the few (4 GCs and 6 dwarf galaxies) having measured proper motions at that time. They found some evidence for dynamical families, but as they point out, the present sample of objects for which we know the full dynamics is small, and excludes many of the young halo clusters that are more likely to have arrived in the MW halo via tidal capture. With more complete information about the full space motions of Galactic halo satellites, we will be able to search for more planar alignments, and trace the orbits back to the parent (pre-merger) populations. In particular, we will find an orbit for the Carina dsph, a proposed member of the Magellanic plane, which is thought to consist of the Magellanic Clouds, and the Ursa Minor, Draco, and Carina dsphs. Additionally, we ll derive the full space motion of the Leo I and Leo II dsphs, purported members of the FL 2 S 2 stream (Majewski 994). Knowledge of the early dwarf galaxy and GC populations of the Local Group can then be used to constrain ΛCDM model predictions about the number, mass spectrum, and timeline for formation of dark matter subhaloes, as well as the frequency and magnitude of merger events (i.e. accretion of smaller CDM haloes) in the hierarchical galaxy formation process. Since it is thought that one of the main mechanisms for building up the Milky Way stellar halo is tidal stripping (and dissolution) of dwarf galaxies and globular clusters, knowledge of the orbits of these objects will tell us where to look for tidal streams left behind in these interactions. Many tidal streams are now known Carlin 2

3 Fig. 3. Spatial distribution (in equatorial sky coordinates) of the Kapteyn s Selected Areas (denoted by the numbered points) for which we have proper motion data. The path of the Sgr dsph orbit is shown by the dark solid line, and the regions where Mon debris has been detected are the hashed areas. Grey shaded areas denote the SDSS QSO catalog coverage, and the Galactic disk is shown by the dot-dashed line. in the MW halo (e.g. Sagittarius dsph: Majewski et al. 23; Monoceros: Newberg et al. 22, Ibata et al. 23), but likely many more lie undiscovered. Indeed, with the completion of large-scale photometric surveys such as the Sloan Digital Sky Survey (SDSS; AdelmanMcCarthy et al. 26), many streams have been discovered in recent years (e.g. Grillmair 26; Grillmair & Dionatos 26; Belokurov et al. 26, see Fig. 2). Since SDSS covers only about 2% of the sky, there are likely many more streams yet to be discovered. The best-known and only widely agreed-upon case of a dwarf spheroidal undergoing tidal disruption in the Milky Way halo is the Sagittarius (Sgr) dsph. This dwarf was first discovered by Ibata et al. 994 in a kinematic study of the outer Galactic bulge, with the first largescale mapping of its leading and trailing tidal arms done by Majewski et al. 23 using Two Micron All Sky Survey (2MASS) M-giant stars. Various studies have reported the discovery of stars plausibly associated with debris from Sgr, either trailing or leading it (see Fig. for an illustration of leading/trailing tidal debris) along its orbit (a comprehensive summary of the detections appears in Majewski et al. 23). Line-of-sight velocities (i.e. RVs) of Sgr members have been determined at a few positions along the stream (e.g. Kundu et al. 22, Dohm-Palmer et al. 2, Monaco et al. 27), Carlin providing, along with the spatial distribution, constraints on models of the Sgr-Milky Way interaction (e.g., Law et al. 25, Johnston et al. 995, Martı nez-delgado et al. 24). Constraints on these models provide both a means of understanding the internal dynamics of satellite galaxies under strong tidal influence, and a probe of the shape and strength of the Galactic gravitational potential due to which this disruption is occurring. Another poorly understood feature is the Monoceros (Mon) structure, a distinct, ring-like structure at the edge of the Galactic disk (e.g., Yanny et al. 23; Ibata et al. 23). The Monoceros feature is thought to be a tidal stream from a disrupted satellite based on its spatial distribution and radial velocities (Rocha-Pinto et al. 23; Crane et al. 23), but as yet its origin and full extent have not been well characterized. To date, no systematic survey has addressed the tangential velocities (proper motions) of the known Galactic tidal streams. Only a few studies (e.g. Dinescu et al. 22) have published any proper motion results in the known tidal streams, leaving dynamical models of tidal stream production and evolution poorly constrained. 3

4 Studying Tidal Streams Using Kapteyn s Selected Areas In an effort to detect and characterize halo substructures, we have been working on a project to obtain full phase-space information (positions and full 3-D space motions) for individual stars in Kapteyn s Selected Areas (SAs; see Casetti-Dinescu et al. 26 for an overview of this project). These are regions on the sky that were chosen by Jacobus Kapteyn in 96 to be evenly spaced throughout the sky (see Fig. 3), providing a uniform sample to explore structure in the Milky Way disk and halo. As can be seen in Fig. 3, many of the SA fields lie along the orbit of the Sagittarius (Sgr) dsph. Furthermore, a number of these fields are in the regions where the Monoceros (Mon) structure has been observed. Proper motions (µs) are derived from matched photographic plate pairs that span between 5 and 8 years. The plates were digitized and measured with the Yale PDS microdensitometer, and positions (as well as magnitudes) of stars in these fields measured using the Yale image centering routine. Final proper motions in the SA fields typically have a precision of mas yr (mas = milli-arcsecond) per well-measured star. Conversion to absolute proper motions uses known quasars and background galaxies in the field as the fixed reference frame. In Fig. 4, we show the color-magnitude diagram (CMD) for one of the Kapteyn fields, SA 93, in the SDSS filter system. The swath of stars at.2 < (g r) <.7 is made up of nearby Galactic dwarf stars, while the upper left region of this plot represents MW thin/thick disk main-sequence turnoff (MSTO) stars. Note the blob of stars at 9.5 < g < 2.5,.2 < (g r) <.6. This region of the CMD is typically sparsely populated by distant MW halo stars. The distinct overdensity seen here likely consists of MSTO stars from a distant, but distinct stellar population in this field of view. As seen in Fig. 3, SA 93 lies quite near the path of the Sgr dsph orbit, so it is our suggestion that the overdensity in this field represents tidal debris from the Sagittarius dwarf. To further explore this possibility, we separate the CMD into two regions by (g r) color, a blue population with. < (g r) <., and a red population with. < (g r) <.7. As noted above, this red grouping represents mostly nearby dwarf stars in the Galactic disk. The blue region should contain stars from the thin/thick disk and halo of the Milky Way, as well as the overdensity discussed in the previous paragraph. If both the red and blue stars sample the same population (i.e. the Milky Way disk and halo), one would expect their proper motion distributions to be similar. However, as seen in Fig. 5, which shows the proper motions for all stars in SA Fig. 4. SDSS (g r),g color magnitude diagram for SA 93, showing all stars for which we have measured proper motions. General features of this plot are discussed in the text. Note the distinct overdensity at 9.5 < g < 2.5,.2 < (g r) <.6. This feature is not present along a typical line of sight at similar Galactic latitude. Candidate Sgr stream stars, identified by their radial velocities (see Fig. 6), are shown as red points, and seem to trace a red giant branch. 93, the distributions of the blue and red stars are quite different. Most importantly, the blue stars show a much tighter, clumpier distribution, as one would expect from a coherent stellar population sharing a common motion through the Galactic halo like the Sgr stream. The fact that a large grouping of stars shows coherence in both color-magnitude and proper motion space is fairly strong evidence that we have indeed found the tidal debris from Sagittarius predicted by the Sgr orbit and by models of its orbital evolution. However, to constrain the tidal disruption models for Sgr, we need full phase-space information (3-D velocities, positions, and distances) for the stream members. We thus selected stars from the proper-motion clump, which also lie in the overdensity region of the CMD, for spectroscopic follow-up to obtain the radial velocity (RV) component, as well as a distance, for each star. We observed 7 stars in this field with the Hydra multi-object spectrograph on the WIYN Carlin 4

5 Fig. 6. Measured radial velocities of Sagittarius debris candidate stars in SA 93. Sgr debris is evident as the tight grouping of stars (red points) at v 55 km s, well separated from the Milky Way thin/thick disk distribution, which is centered at v km s. Fig. 5. Vector point diagram (VPD) of SA 93 stars shown in Fig. 4, showing proper motions along the x and y directions of measurement, corresponding to equatorial (α,δ) coordinates on the sky. The stars have been divided into a blue and a red sample using color cuts of. < (g r) <., and. < (g r) <.7, respectively. Note the much more tightly-clumped distribution among the blue stars, including (red points) those identified as Sgr debris from their radial velocities (Fig. 6). 3.5-meter telescope at Kitt Peak National Observatory, Arizona. The radial velocities are plotted as a function of SDSS g magnitude in Fig. 6. The upper part of this plot, with a roughly gaussian scatter about zero velocity, is as one expects from Milky Way disk stars. A tight grouping of stars is seen at v 55 km s, far from the MW disk locus, in a region where very few MW stars should lie. This velocity is consistent with those predicted for the Sgr stream by Law et al. (25) and Martínez-Delgado et al. (24), so we identify this common-velocity group as Sgr tidal debris. Furthermore, these common-velocity stars comprise a tight clump in the blue plot (shown as red points) of Fig. 5, meaning they share a common proper motion as well. Further adding to the case that these represent a distinct population in this field is their position in the CMD (red points in Fig. 4). The roughly linear swath delineated by the putative Sgr stream stars resembles a red giant branch for a distant population. All the above evidence leads to the conclusion that we ve detected the Sgr stream in the SA 93 field. Further work (ongoing) will involve deriving metal abundances for the individual stars flagged as Sgr members, in order to further bolster the case for a Sagittarius origin, as well as to explore the metallicity gradient along the Sgr stream found by Chou et al. 26. Once metallicities are known, we can derive accurate distances using model isochrones. At this point we will have full phase-space data (as well as abundances) for the Sgr stream stars along this line of sight, allowing comparisons to extant models of the Sagittarius tidal stream. Constraints on the models will be provided by the detection (or non-detection) and characterization of Sgr debris in the many fields for which we have photographic plate data along the Sgr orbit. Models for the Sagittarius stream rely on many assumptions (or incomplete knowledge, at least) about the Milky Way gravitational potential. Johnston et al. 25 showed that if the dark matter halo of the Milky Way is non-spherical, tidal streams should precess over Carlin 5

6 q=.25 q=.25 q=. q=. q= q= Fig. 7. Proper motions along Sgr s southern trailing tail as predicted by the Law et al. (25) models (colored points). The dark symbols with -σ error bars show our preliminary results in SA 93 (Λ = 3 ) and SA 94 (Λ = 6 ). The LSR velocity adopted for this model is 22 km s, while the flattening of the halo q varies as specified in each panel. The colored dots represent N-body model particles stripped from Sgr since its last apogalacticon, i.e. present orbit, (yellow symbols), during the previous orbit (magenta), and two orbits ago (cyan); this color scheme matches that used in Law et al. (25). time. Thus one of the parameters when modeling the Sgr stream is the flattening of the (triaxial) halo, q, which is eqaul to. for a spherical halo, larger for a prolate halo, and less than one for the oblate case. In Fig. 7, taken from Casetti-Dinescu et al. (26), we show the predictions for the proper motions of the Sgr stream from the Law et al. (25) models, which were based solely on the distribution and radial velocities of M giants from Majewski et al. (23, 24). Preliminary proper motion results for two of our Kapteyn fields, SAs 93 and 94, are overplotted, and show general agreement with the predictions. However, more Sgr stream members are needed to more tightly constrain the Sgr proper motions, in order to discriminate between different values for the halo flattening, q. Majewski et al. (26) showed that using measured proper motions of fortuitously placed Sgr tidal debris along the trailing tail, one can determine the local standard of rest (LSR) velocity, i.e. the rotation velocity of the Sun about the Galactic center (denoted Θ), which is Fig. 8. Similar to Figure 7. The flattening of the halo adopted for this model is.9, while the velocity of the LSR varies as specified in each panel. Color representation is as in Fig. 7 and Law et al. (25). currently poorly known. As seen in Fig. 8, different values for Θ produce different proper motion distributions for the Sgr debris, since our motion through the Galaxy is reflected in the measured proper motions. Again, our preliminary proper motion results agree with the model predictions, but aren t yet precise enough to differentiate the true value of Θ. In both cases, the identification of Sgr debris stars provides targets for NASA s Space Interferometry Mission (SIM), which will derive extremely precise proper motions, which can then be used to discriminate the true values of both q and Θ using the above techniques. Absolute Proper Motions of Milky Way Dwarf Spheroidals To explore the origins and orbital evolution of dsphs in the Galactic halo, we have undertaken a research project that employs an innovative technique for digitization of archived photographic plates, combined with data from NASA s Hubble Space Telescope (HST), to find absolute proper motions (transverse motions) of individual stars in a sample of MW satellites. Hubble s superior angular resolution over ground-based telescopes enables a very precise determination of stellar positions, but unfortunately Hubble data cover relatively tiny sky areas and short time baselines between observations compared to ground-based data. Indeed, Piatek et al. (22) have undertaken a program to determine dsph proper motions Carlin 6

7 Fig. 9. Image of one of the Carina dsph photographic plate scans, with the calibration grid overlaid. The crossshaped fiducials, with known positions, can be used to map out distortions introduced by the scanner. with HST baselines of only a few years, using fields that are each centered on a known QSO (QUASi-stellAR object, or quasar an extremely luminous distant galaxy nucleus, powered by accretion of material onto a supermassive black hole), which provides a single tie to a fixed extragalactic reference frame for reduction to absolute proper motions. However, Dinescu et al. 24 developed a method for using background galaxy images in addition to QSOs to fix the extragalactic frame more firmly. These authors also combine the Fornax HST data with archived ground-based photographic images, providing a much longer time baseline and larger areal coverage (providing more background QSOs and galaxies for reference), and thus a more reliable result. We have a unique and valuable set of hundreds of archived photographs that includes data on many dsphs and GCs taken with the Palomar 2-inch telescope (one of the oldest giant telescopes) as far back as the 95s. We will apply the Dinescu et al. 24 method to exploit this dataset in conjunction with archival HST data to derive absolute proper motions for the halo objects in the sample. For many targets, we have already obtained radial velocities (RVs) for the stars using the HYDRA multiobject spectrographs on both the WIYN 3.5-meter and CTIO Blanco 4-meter telescopes. When combined with the two transverse components of the motion (the proper motion), these RVs provide full space velocities of individual stellar sources. The ensemble of these motions will be used to find bulk-motion orbits for the MW satellites in our program. Traditionally, the digitization of astronomical photographic data has employed microdensitometers, which are based on 96s-era technology (built for analysis of satellite spy photographs) and which use a single element photomultiplier detector to build images from photographs one miniscule pixel at a time. The process is laborious, taking more than 24 hours to raster the single photo-detector element across a whole photograph in fine (e.g., micron) steps. As a result of this lengthy process, the spatial and density precision in the digitization is degraded unless painstaking efforts are made to minimize electrical and thermal variations during the scanning process. However, only recently has modern digital scanner technology become reliable enough to approach the delivered precision of the microdensitometer. We have have been investigating the astrometric stability of scans from a commercial flatbed scanner based on sweeping, linear CCD array technology that is both large enough (8 x 24 inches) and has high enough resolution (24 dpi =.583 micron pixels) to do the work of the densitometer, but much more efficiently and perhaps even more precisely. We have found that while it holds the promise of rapid digitization (a few minutes per large format plate) of photographic images with. micron accuracy, the device introduces some systematic errors that need to be accounted for. Fortunately, the systematic errors appear to be periodic, readily determinable and even reproducible, all factors which mean that it is likely they can be mapped and removed. The key is to make sure that stable, precisely positioned reference points are provided within the input data that can be used to calibrate the systematic errors in the final images. To do this we have obtained a precisely ruled glass calibration plate with appropriately periodic fiducials that we superpose on each photographic image during the scanning process (Fig. 9). The fiducial references imprinted in the final image allow us to correct for errors introduced by the scanner. Characterization of this calibration grid, and development of software to derive and apply the necessary distortion corrections, are ongoing. As yet, it is unclear whether the commercially-available scanner will provide the required measurement precision. To ensure that we obtain results, as well as to provide a comparison for the scanner results, I have used the U.S. Naval Observatory s StarScan measuring machine to measure all of the plates for the Carina dsph. This device not only produces a digital image of each photographic plate, but measures positions of all the stars in the field to high accuracy. The delivered measurement precision for stars in one of the Carina plates is seen in Fig.. Measurement of these plates has just recently been completed, so much work remains to derive proper motions from these data. Our photographic data archive includes plates going back as far as the 95s for three dsphs and seven GCs. The dsphs include Carina (d= kpc, Mateo 998), Leo II (d=25 kpc), and Leo I (d=25 kpc). The previously discussed result for Fornax (d=4 kpc) of Dinescu et al. Carlin 7

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