THE GRAVITATIONAL HARASSMENT OF OUR DWARF GALACTIC NEIGHBORS

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1 THE GRAVITATIONAL HARASSMENT OF OUR DWARF GALACTIC NEIGHBORS Rachael L. Beaton University of Virginia The Local Group of galaxies contains some 40 galaxies, most of which fall into two morphological classes: dwarf Irregulars (dirrs) and dwarf Spheroidals (dsphs). dsphs are preferentially located in the densest regions of the Local Group, where they are more likely to have experienced interactions with large galaxies. dirrs, however, are located far from large galaxies, where they are less likely to have seen recent interactions, which have the potential to strip gas and inhibit future star formation. Furthermore, in optical observations, dirrs are dominated by patches of young stars and regions of ongoing star formation, whereas dsphs are dominated by a smooth distribution of old stars with none having been observed with ongoing star formation, from this it is proposed that dirrs are the "preharassed" versions of dsphs. In this study, we analyze the distribution of old stars in two Local Group dirrs, Leo A and Sextans B. This is accomplished with a semblance of wide field, but resolution limited, ground based imaging and small area, but highly resolved, archival space based imaging from the Hubble Space Telescope. By combining the strengths of these two datasets, a population of old stars has been found beneath the patchy young populations, which is smoothly distributed much like those of dsph galaxies. I. Introduction dwarf galaxies are the most numerous galaxy in the Universe. In fact, within the past year more than a dozen dwarf galaxies have been discovered in the local group (see references in Majewski et al. 2007). The inherent variation in the basic properties of these galaxies (shape, luminosity, star formation history, dark matter fraction, etc), invoke the combined evolutionary effects of both nature, the birth properties of the galaxy, and nurture, the collective environmental interactions an individual galaxy has experienced. This observational portrait demands a thorough understanding of the relationship between dwarfs and their companions. The Local Group dwarfs are particularly important in the quest to understand the lives of smaller systems, since they represent the only available galaxies whose individual stars can be resolved, even with the most powerful modern telescopes. dwarf galaxies exhibit similar structures as large galaxies and can be classified in three main types: (i) dwarf Irregular galaxies (dirrs), (ii) dwarf Spheroidal galaxies (dsph) and (iii) dwarf elliptical galaxies (des). The Local Group contains only one de, but is abounding with dirrs and dsphs, as well as transition type galaxies whose properties are intermediate between dirrs and dsphs. dirrs are typically metal-poor with ongoing star formation, as characterized by observations of HII regions. The overall appearance of dirrs is dominated by these young, star forming regions, which are distributed somewhat randomly across the visible extent of the galaxy. These regions combine to make an overall blotchy appearance. H1 contributes from 7% to 50% to the total mass of a dirr, serving as exhaustive supplies of gas for future star formation (Mateo 1998). dirrs are predominantly discovered on the isolated edges of the Local Group (Mateo 1998). dsphs are quite different from dirrs. dsph galaxies are characterized by a smoothly distributed population of old stars and are largely void of gas. dsphs are preferentially discovered in close proximity of the large galaxies of the Local Group and many are found within the halos of these large galaxies. Of the two dozen known dsphs, only three are observed in isolation, the Tucana dwarf, Cetus and AndXIV (Majewski et al. 2007; Mateo 1998). Further, many dsphs are observed in the process of being tidally disrupted by their parent galaxy (e.g. Fellhauer 2006; Willman et al. 2006; Zucker et al. 2006; Belokurov et al. 2006). Continued discoveries of Local Group dwarf galaxies have contributed significantly to the observed morphological segregation. This observed distribution supports the idea that dwarf galaxy evolution includes a factor of gravitational harassment by larger galaxies (Mayer et al. 2001a,b; 2006). In this scenario, close proximity to large galaxies results in strong a morphological effect on the dwarf via the physical mechanisms of RAM pressure and tidal stripping and in extreme cases total tidal disruption, which results in losses of both stellar and gaseous components. Gas stripping is even more pronounced in dwarf systems due to their shallow potential wells, which make gas stripping more efficient. Beyond their spatial distribution, dirrs and dsphs represent opposing extremes amongst dwarf galaxy Beaton - 1 -

2 properties. There exist relationships between gas fractions, the age of stellar populations, metal content, and mass to light ratios (Mateo 1998). The relative placement of dirrs and dsphs in conjunction with their observed distribution suggest that dirrs and dsphs may be the end product of a single population of galaxies, which have evolved under different environmental circumstances (Mateo 1998; Grebel et al. 2003). There are, however, a number of ambiguities in this relationship that argue that dirrs and dsphs are different in ways that interaction alone can not reproduce, namely the removal of circular velocities observed in dirrs, and the details of their chemical enrichment histories, more specifically that dsphs tend to be more metal rich than dirrs of the same age (Grebel et al. 1999). Numerical simulations by Kravtsov, Gnedin and Klypin (2004) followed the evolution of the Local Group under ΛCDM conditions. Over the course of the simulation, many dark matter galaxies formed which had no observable stellar or gaseous component. These dark halos also contributed to the overall tidal effects of the dsphs, creating the same observational signatures as perturbation from a larger galaxy. Further, the overall properties of this model traced the circular velocity, radial distribution and morphological segregation observed in the Local Group, including the presence of some isolated dsphs (Kravtsov, Gnedin and Klypin 2004). In this study, we continue to explore the suggested evolutionary relationship between dirr and dsph galaxies. We use ground and space imaging to seek and characterize the old stellar populations in two distant dirr galaxies Leo A and Sextans B and compare these results to an isolated dsph galaxy, AndXIV and other Milky Way dsphs. II. The Data Ground data were obtained for the dirrs Leo A and Sextans B from the Mini-Mosaic imager on the WIYN 3.5 meter telescope at Kitt Peak National Observatory. The Mini-Mosaic imager consists of a 1 x 4 array of 1024 x 2096 pixel CCDs which combine for a 9 by 9 field of view. These data were obtained under photometric conditions using the Washington + DD051 photometry system (M, T2 & DD051 filters). Each galaxy fit within the field of view of the instrument. These data were reduced using the standard Mosaic reduction procedures in the MSCRED package of the Image Reduction and Analysis Facility (IRAF). Multiple exposures were co-added in Figure 1: POSS II image with HST WFPC2 fields superimposed for (a) Leo A and (b) Sextans B both from the LOGPHOT archive. order to reach fainter magnitudes. Crowded field photometry was performed using the point spread fitting capabilities of DAOPHOT (Stetson 1987). The photometric results were calibrated to a standard photometric system using the standard observations of Geisler (1990). Space based data were obtained from the Local Group Stellar Populations (LOGPHOT) archive from the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) 1 (Holtzman, Afonso and Dolphin, 2006). 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Beaton - 2 -

3 LOGPHOT 2 is a substantial database of Local Group galaxies with long exposures using WFPC2. Each member of the database has been reduced using the same procedure, which is described in depth in Holtzman, Afonso and Dolphin (2006). Data were available for both Leo A and Sextans B. The data for Leo A were contained in two pointings, marked in green in Figure 1a, covering three WFPC2 filters (F439W, F555W and F814W). The data for Sextans B consist of only one pointing, as indicated in green in Figure 1b, in two WFPC2 filters (F606W and F814W). Ground imaging was obtained for the dsph AndXIV from the Mosaic imager on the Kitt Peak National Observatory Mayall 4-meter telecope. The Mosaic imager consists of a 2 x 4 array of CCDs combining for a field of view of 36 on a side. These data were obtained under photometric conditions in the Washington + DD051 filter system. The data were reduced using standard Mosaic reduction procedures in the MSCRED package of IRAF. Photometry was obtained via DAOPHOT and calibrated (Stetson 1987; Geisler 1990l; see also Majewski et al. 2007). 1. Data Limitations The collected ground and space data represent complementary datasets in their ability to probe for old stellar populations. Leo A and Sextans B represent two of the most distant confirmed Local Group galaxies (Mateo 1998) and thus present a challenge to ground based studies. Ground based imaging is strongly limited by the conditions of the Earth s atmosphere, which creates a firm limit at which objects can no longer be resolved. This limit is exceptionally crippling for crowded structures, in which the small angular separation results in a blending of individual stellar images. At these extreme distances, Galactic dwarf stars, which are intrinsically faint, and distant giant stars, which are intrinsically bright, are measured on Earth at the same magnitude. The Washington + DD051 filter system, however, presents a clear means to isolate distant giant stars from nearby dwarf stars. The color in the M and T2 broad-band filters can be used to separate spectral type. The DD051 filter is an intermediate band filter centered on the MgH + MgB stellar absorption feature. The width of the feature, and thus the measured flux in this band, is strongly Research in Astronomy (AURA), Inc., under NASA contract NAS dependent on the surface gravity of the star. Dwarf stars are compact and thus have a high surface gravity, in stark comparison to puffed up giant stars that have a low surface gravity. The addition of the DD051 filter introduces a color-color space diagnostic in which extragalactic giant stars cleanly separate from nearby dwarf stars (for further details see Majewski et al. 2000). Iteration between these two diagnostics will create a clean sample of distant giant stars, which are representative of an old stellar population. The MiniMosaic data, however, experienced a number of problems resulting from the saturation of the CCD chip due to the presence of very bright, saturated stars in a long exposure. Further this saturation problem appeared in many contexts during data processing, which combine to create high photometric errors, which limit the detailed use of the giant-dwarf selection method. In contrast, space imaging is free from the limitations induced by the atmosphere of the Earth and thus has a far fainter magnitude limit. This imaging, however, has a far smaller field of view and lacks the effectiveness of the Washington + DD051 filter system to discriminate between dwarf and giant stars, but is free from the other problems plaguing the ground imaging. 2. The Sample Sextans B was first resolved by Sandage and Carlson (1985) in an effort to estimate its distance via Cepheid variable observations. This resulted in the currently accepted distance of 1.34 Mpc. Tosi et al. (1991) used isochrone fitting techniques to constrain the star formation history of this galaxy in two distinct regions. The resolved stellar populations were best fit by two long lived bursts of star formation superimposed on a constant star formation at a significantly lower rate. Both regions of the galaxy were best described by this star formation history, implying an overall spatially consistent distribution of stellar populations. Warm gas patches have been observed in Sextans B, but the lack of detection of a strong main sequence implies a paucity of recent or ongoing star formation (Stobel et al. 1991; Sakai et al. 1997). Early studies of Leo A incorrectly concluded it contained no traces of an old population due to poor distance contraints (Tolstoy et al. 1998). More recent imaging, however, confirmed the presence of RR Lyrae variable stars amongst a rich population of Cepheid variables (Dolphin et al. 2002). These observations provided a well measured distance of 0.8 Mpc, as well as tracing the relative distribution of young blue stars against older red stars as a function of radius. Dolphin et al. (2002) Beaton - 3 -

4 concluded that the density of red stars increases with radius as the density of blue stars decreases. In a similar fashion, Vansevicius et al. (2004), traced the evolution of patterns in color-magnitude space with radius, again finding that red stars dominate the outer regions. AndXIV is recently discovered Local Group dsph at a projected distance of 170kpc from the center of the Andromeda galaxy (Majewski et al. 2007). AndXIV presents an unusual test case for any theory attempting to connect the observed properties of dirrs and dsphs. The velocity of this galaxy is far too large for it to be bound to Andromeda at its projected distance and the most recent mass estimates of Andromeda (Majewski et al. 2007; Bullock et al. 2001). Under these assumptions, AndXIV represents a dsph passing through the dense regions of the Local Group for the first time. Yet, despite its lack of interactions with a large galaxy, AndXIV has no associated H1 gas, a very low central concentration, and an elongated elliptical shape (Braun and Thilker 2004; Majewski et al. 2007). AndXIV represents a galaxy that seems to have existed under similar conditions to the isolated dirrs Leo A and Sextans B, but has progressed along a dramatically different evolutionary path. III. Data Analysis The WFPC2 HST filter system can be adapted to more standard optical magnitudes as described by Dolphin (2000). The observations of both Leo A and Sextans B were translated into the V and I system as a standard aspect of the LOGPHOT reduction pipeline (Holtzman, Afonso and Dolphin 2006). Similarly, the Washington + DD051 system can be converted to V and I following the empirical relations in Majewski et al. (2000). Thus both sets of data can be placed on the same photometric system for comparison. Figure 2: (a) Color magnitude diagram of LOGPHOT data for Leo A. The giant selection is marked by a plus. (b) The analogous color magnitude diagram (converted to V,I) from MiniMosaic data with giants marked as a plus. The limitations of ground data are evident from the shallowness of the giant branch. (c) Color-color diagram in the Washington + DD051 photometry system used to select giant stars. The giants separate from the Milky Way dwarf pattern. The panels in Figure 2 and Figure 3 compare the color magnitude diagrams from LOGPHOT and MiniMosaic imaging, including the color-color diagnostic used in the Washington + DD051 photometry system. The degree to which the ground data is limited is evident when comparing panel b and c. The LOGPHOT data reaches V = 26, whereas the ground data is limited to V=XX. The color offsets are largely a function of astrometric errors in the Mini Mosaic data which result in a misestimating of location based data corrections. Once the giants were selected, the angular distance from the center of each galaxy was determined. The galaxy center for Leo A was adopted from Cotton et al. (1999) as (09:59:26.46, +30:44:47.0) and for Sex B from Falco et al. (1999) as (10:00:00.10, +05:19:56.0). The density of giant stars was then calculated for concentric apertures from the center of the galaxy. Radial profiles are Beaton - 4 -

5 a common diagnostic of dsph galaxies as they characterize the global structure of the dwarf. The radial profile for each galaxy is presented in Figure 4 along with the equivalent diagram for the isolated dsph AndXIV. Similar diagrams for Milky Way dsphs are presented in Figure 5 for comparison. Leo A and Sextans B were binned according to circular apertures, whereas the Milky Way dsphs and AndXIV were binned according to the best fit ellipses to the star density map at each successive radii. From comparing these data it is clear that the dirr radial distribution of giant stars is very similar to that of the Milky Way dsphs and AndXIV. Exact analysis requires more careful density calculation using the best fit ellipses, as was done for AndXIV and the Milky Way dsphs. As was displayed in Figure 1, the WFPC2 coverage of Leo A and Sextans B is less than ideal, with gaps at some radii. The imaging, does, however, extend beyond the optically visible extent of the galaxy on the Sextans B minor axis and on the Leo A minor and major axes. Figure 6 presents the MiniMosiac imaging with the HST WFPC2 fields overlaid. The giant star selection from ground based data is also overlaid, indicating that red giant stars exist far outside of the HST WFPC2 printings, and beyond the optical extent of the galaxy visible in the panels of Figure 1. These data indicate that the optical portrait of dirrs as only consisting of patchy, young star formation is quite incomplete. IV. Discussions & Future Work The optical appearances of dirrs and dsphs are dramatically different. Whereas dirrs appear as patchy distributions of young stars, dsphs appear as smooth distributions of older stars. This characterization, however, uses two distinct stellar populations, one recently formed and largely still populating the habitat of its parent gas cloud and another which has existed in the galaxy for a substantially longer period of time and moves in the overall gravitational potential of the galaxy. It is reasonable to expect that the stars which have had time to adjust to the potential of the galaxy would be distributed differently from those which have not. Thus, a characterization of dirrs and dsphs which compares stars with similar populations is required to compare their structures more accurately. Figure 4 compares the density profile for four Local Group dwarfs. The overall similarities of these profiles suggest that the overall structure of a dirr is dramatically similar to the overall structure of a traditional dsph and an isolated dsph. This old star Figure 3: (a) Color magnitude diagram of LOGPHOT data for Sex B. The giant selection is marked by a plus. (b) The analogous color magnitude diagram (converted to V,I) from MiniMosaic data with giants marked as a plus. The limitations of ground data are evident from the shallowness of the giant branch and the general scattering. (c) Color-color diagram in the Washington + DD051 photometry system used to select giant stars. The giants separate from the Milky Way dwarf pattern, which is hard to distinguish due to the scatter in the diagram. Beaton - 5 -

6 of the WFPC2 imaging and is not patchy like the distribution of young stars. Furthermore, the detection of red giant stars far outside of the optical extent of the galaxy implies there is some structure beyond the regions of star formation. It is reasonable to presume that if these dirrs were observed without their young stars and were stripped of their H1 gas stores, the galaxies would be considered dsphs by their radial density distributions. The existence of isolated dsph galaxies challenges the notion that dsphs are the result of environmental interactions between a dirr and a larger galaxy. AndXIV is an ideal example of a dsph which is unlikely to have seen recent interactions (Majewski et al. 2007). Further, its measured properties are consistent with having accumulated significant tidal effects. This dsph and two similar dsphs, Cetus and Tucana, are an interesting puzzle for currentobservations. The simulations of Kravtsov, Gnedin and Klypin (2004) may provide some insight into this paradox. Figure 4: Radial density profile for (a) the isolated dsph AndXIV (Majewski et al. 2007), (b) dirr Leo A and (c) dirr Sextans B.population is distributed smoothly across the extent The majority of the newly discovered dwarf galaxies in the Local Group are being found in close proximity of larger galaxies, often within the galaxy itself. These dsph galaxies can be used as probes of the extended structure of large galaxies (see Majewski et al. 2007). In this context it becomes crucial to understand the structure and evolution of these galaxies in interaction with the large galaxy. Thus, a continued effort into studying the structure of dwarfs and the potential role of galaxy harassment in their current morphology is necessary to piece together the structure of larger galaxy. V. Acknowledgements I would like to acknowledge assistance with this research both in data collection and data analysis from Steven R. Majewski, Richard J. Patterson and Ricardo Munoz at the University of Virginia. In addiation, I acknowledge financial support from the Virginia Space Grant Consortium Undergraduate Scholarship for and an Undergraduate Research Harrison Award from the University of Virginia Center for Undergraduate Excellence. Figure 5 Radial density profiles for several Milky Way dsphs. The solid line represents the best-fit King Profile for each galaxy. Beaton - 6 -

7 Majewski, S.R., Ostheimer, J.C., Kunkel, W.E., & Patterson, R.J. 2000, AJ, 120, 2550 Majewski et al., 2007 ApJL, submitted (astroph/ v1) Mayer, L., et al. 2001a, ApJ, 547, L123 Mayer, L., et al. 2001b, ApJ, 559, 754 Mayer, L., Mastropietro, C. Wadsley, J., & Moore, B. 2006, MNRAS, 369, 1021 Sakai, S., Madore, B.F., & Freedman, W.L. 1997, AJ, 480, 589 Sandage, A., & Carlson, G. 1985, AJ, 90, 1019 Stetson, P.B. 1987, PASP, 99, 191 Strobel, N.V., Hodge, P. & Kennicutt, R.C., Jr 1991, AJ, 383, 148 Tolstoy, E., et al. 1998, AJ, 116, 1244 Tosi, M., Greggio, L., Marconi, G. & Focardi, P. 1991, AJ, 102, 951 Vansevicius, V., et al. 2004, AJL, 611, L93 Willman, B., et al 2006, AJ, submitted (astro-ph/ ) Zucker, D. B., et al. 2006, ApJ, 650, L41 Figure 6: The distribution of detected stars in Leo A from MiniMosaic data (dots), with potential giant stars marked (plus) and the WFPC2 fields superimposed (grey) for Leo A (top panel) and Sex B (low panel). VI. References Cited Belokurov, V., et al. 2006, ApJ, 647, L111 Braun, R., & Thilker, D.A. 2004, A&A, 417,421 Bullock, J.S., et al MNRAS, 321, 559 Cotton, W.D., Condon, J.J., Arbizzani, E. 1999, ApJS, 125, 409 Dolphin, A.E., 2000, PASP, 112, 1383 Dolphin, A.E., et al., 2002 AJ, 123, 3154Falco, E.E., et al. 1999, PASP, 111, 438 Fellhauer, M., et al. 2006, MNRAS, in press (astroph/ ) Geisler, D. 1990, PASP, 102, 344 Grebel, E. K., Gallagher, J. S., III & Harbeck, D. 2003, AJ, 125, 1926 Holtzman, J.A., Afonso, C. & Dolphin, A. 2006, ApJS, 166, 534 Kravtsov, A.V., Gnedin, O.Y. & Klypin, A.A. 2004, AJ, 609, 482 Mateo, M.L. 1998, ARAA, 36,435 Beaton - 7 -