Milky Way Mass and Potential Recovery Using Tidal Streams in a Realistic Halo

We present a new method for determining the Galactic gravitational potential based on forward modeling of tidal stellar streams. We use this method to test the performance of smooth and static analytic potentials in representing realistic dark matter halos, which have substructure and are continually evolving by accretion. Our FAST-FORWARD method uses a Markov Chain Monte Carlo algorithm to compare, in 6D phase space, an "observed" stream to models created in trial analytic potentials. We analyze a large sample of streams evolved in the Via Lactea II (VL2) simulation, which represents a realistic Galactic halo potential. The recovered potential parameters are in agreement with the best fit to the global, present-day VL2 potential. However, merely assuming an analytic potential limits the dark matter halo mass measurement to an accuracy of 5 to 20%, depending on the choice of analytic parametrization. Collectively, mass estimates using streams from our sample reach this fundamental limit, but individually they can be highly biased. Individual streams can both under- and overestimate the mass, and the bias is progressively worse for those with smaller perigalacticons, motivating the search for tidal streams at galactocentric distances larger than 70 kpc. We estimate that the assumption of a static and smooth dark matter potential in modeling of the GD-1 and Pal5-like streams introduces an error of up to 50% in the Milky Way mass estimates.Comment: 12 pages, 6 figures, submitted to ApJ; more information on our stream sample and a movie of the potential recovery method used can be found at http://www.astro.yale.edu/abonaca/research/potential_recovery.htm

The SAGA Survey: I. Satellite Galaxy Populations Around Eight Milky Way Analogs

We present the survey strategy and early results of the "Satellites Around Galactic Analogs" (SAGA) Survey. The SAGA Survey's goal is to measure the distribution of satellite galaxies around 100 systems analogous to the Milky Way down to the luminosity of the Leo I dwarf galaxy ($M_r < -12.3$). We define a Milky Way analog based on $K$-band luminosity and local environment. Here, we present satellite luminosity functions for 8 Milky Way analog galaxies between 20 to 40 Mpc. These systems have nearly complete spectroscopic coverage of candidate satellites within the projected host virial radius down to $r_o < 20.75$ using low redshift $gri$ color criteria. We have discovered a total of 25 new satellite galaxies: 14 new satellite galaxies meet our formal criteria around our complete host systems, plus 11 additional satellites in either incompletely surveyed hosts or below our formal magnitude limit. Combined with 13 previously known satellites, there are a total of 27 satellites around 8 complete Milky Way analog hosts. We find a wide distribution in the number of satellites per host, from 1 to 9, in the luminosity range for which there are five Milky Way satellites. Standard abundance matching extrapolated from higher luminosities predicts less scatter between hosts and a steeper luminosity function slope than observed. We find that the majority of satellites (26 of 27) are star-forming. These early results indicate that the Milky Way has a different satellite population than typical in our sample, potentially changing the physical interpretation of measurements based only on the Milky Way's satellite galaxies.Comment: 22 pages, 19 figures, 2 tables. Updated to published version. Survey website: http://sagasurvey.org

The Globular Cluster System of the Virgo Dwarf Elliptical Galaxy VCC 1087

We have analysed the globular cluster (GC) system of the nucleated dwarf elliptical galaxy VCC 1087 in the Virgo cluster, based on Keck/LRIS spectroscopy and archival HST/ACS imaging. We estimate VCC 1087 hosts a total population of 77+/-19 GCs, which corresponds to a relatively high V-band specific frequency of 5.8+/-1.4. The g-z color distribution of the GCs shows a blue (metal-poor) peak with a tail of redder (metal-rich) clusters similar in color to those seen in luminous ellipticals. Spectroscopy of a subsample of 12 GCs suggests that the GC system is old and coeval (~10 Gyr), with a fairly broad metallicity distribution (-1.8<[m/H]<-0.8). In contrast, an integrated spectrum of the underlying galaxy starlight reveals that its optical luminosity is dominated by metal-rich, intermediate-aged stars. Radial velocities of the GCs suggest rotation close to the major axis of the galaxy, and this rotation is dynamically significant with (v/sigma)^* >1. A compilation of the kinematics of the GC systems of 9 early-type galaxies shows surprising diversity in the v/sigma parameter for GC systems. In this context, the GC system of VCC 1087 exhibits the most significant rotation to velocity dispersion signature. Modeling the velocity dispersion profile of the GCs and galaxy stars suggest fairly constant mass-to-light ratios of ~3 out to 6.5 kpc. The present observations can entertain both baryonic and non-baryonic solutions, and GC velocities at larger radii would be most valuable with regard to this issue. We discuss the evolution of VCC 1087 in terms of the galaxy ``harassment'' scenario, and conclude that this galaxy may well be the remains of a faded, tidally perturbed Sc spiral [abridged].Comment: 17 pages, 13 figures, to appear in the A

Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: II. Rotating Versus Non-Rotating Dwarfs

We present spatially-resolved internal kinematics and stellar chemical abundances for a sample of dwarf elliptical (dE) galaxies in the Virgo Cluster observed with Keck/ESI. We find that 4 out of 17 dEs have major axis rotation velocities consistent with rotational flattening, while the remaining dEs have no detectable major axis rotation. Despite this difference in internal kinematics, rotating and non-rotating dEs are remarkably similar in terms of their position in the Fundamental Plane, morphological structure, stellar populations, and local environment. We present evidence for faint underlying disks and/or weak substructure in a fraction of both rotating and non-rotating dEs, but a comparable number of counter-examples exist for both types which show no evidence of such structure. Absorption-line strengths were determined based on the Lick/IDS system (Hbeta, Mgb, Fe5270, Fe5335) for the central region of each galaxy. We find no difference in the line-strength indices, and hence stellar populations, between rotating and non-rotating dE galaxies. The best-fitting mean age and metallicity for our 17 dE sample are 5 Gyr and Fe/H = -0.3 dex, respectively, with rms spreads of 3 Gyr and 0.1 dex. The majority of dEs are consistent with solar alpha/Fe abundance ratios. By contrast, the stellar populations of classical elliptical galaxies are, on average, older, more metal rich, and alpha-enhanced relative to our dE sample. The local environments of both dEs types appear to be diverse in terms of their proximity to larger galaxies in real or velocity space within the Virgo Cluster. Thus, rotating and non-rotating dEs are remarkably similar in terms of their structure, stellar content, and local environments, presenting a significant challenge to theoretical models of their formation. (abridged)Comment: 33 pages, 12 figures. To appear in the October 2003 Astronomical Journal. See http://www.ucolick.org/~mgeha/geha_dE.ps.gz for version with high resolution figure

A SAURON study of dwarf elliptical galaxies in the Virgo Cluster: kinematics and stellar populations

Dwarf elliptical galaxies (dEs) are the most common galaxy type in nearby galaxy clusters; even so, many of their basic properties have yet to be quantified. Here we present the results of our study of 4 Virgo dwarf ellipticals obtained with the SAURON integral field unit on the William Herschel Telescope (La Palma, Spain). While traditional long-slit observations are likely to miss more complicated kinematic features, with SAURON we are able to study both kinematics and stellar populations in two dimensions, obtaining a much more detailed view of the mass distribution and star formation histories. What is visible even in such a small sample is that dEs are not a uniform group, not only morphologically, but also as far as their kinematic and stellar population properties are concerned. We find the presence of substructures, varying degrees of flattening and of rotation, as well as differences in age and metallicity gradients. We confirm that two of our galaxies are significantly flattened, yet non-rotating objects, which makes them likely triaxial systems. The comparison between the dwarf and the giant groups shows that dEs could be a low-mass extension of Es in the sense that they do seem to follow the same trends with mass. However, dEs as progenitors of Es seem less likely as we have seen that dEs have much lower abundance ratios.Comment: 8 pages, 6 figures; to appear in the proceedings of the JENAM 2010 Symposium on Dwarf Galaxies (Lisbon, September 9-10, 2010); minor edits and references adde

NGC 770: A Counter-Rotating Core in a Low-Luminosity Elliptical Galaxy

We present evidence for a counter-rotating core in the low-luminosity (M_B = -18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This counter-rotating core is unusual as NGC 770 is not the primary galaxy in the region and it lies in an environment with evidence of on-going tidal interactions. We discovered the counter-rotating core via single-slit Keck/ESI echelle spectroscopy; subsequent integral field spectroscopy was obtained with the Gemini/GMOS IFU. The counter-rotating region has a peak rotation velocity of 21 km/s as compared to the main galaxy's rotation speed of greater than 45 km/s in the opposite direction. The counter-rotating region extends to a radius of 4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy which is 5.3'' (0.8 kpc) and is confined to a disk whose scale height is less than 0.8'' (0.1 kpc). We compute an age and metallicity of the inner counter-rotating region of 3 +/- 0.5 Gyr and [Fe/H] = 0.2 +/- 0.2 dex, based on Lick absorption-line indices. The lack of other large galaxies in this region limits possible scenarios for the formation of the counter-rotating core. We discuss several scenarios and favor one in which NGC 770 accreted a small gas-rich dwarf galaxy during a very minor merging event. If this scenario is correct, it represents one of the few known examples of merging between two dwarf-sized galaxies.Comment: 26 pages, 9 figures. Accepted to AJ. See this http://www.ociw.edu/~mgeha/geha.ps.gz for version with high resolution figure

Blue Variable Stars from the MACHO database I: Photometry and Spectroscopy of the LMC sample

We present the photometric properties of 1279 blue variable stars within the LMC. Photometry is derived from the MACHO database. The lightcurves of the sample exhibit a variety of quasi-periodic and aperiodic outburst behavior. A characteristic feature of the photometric variation is that the objects are reddest when at maximum outburst. A subset of 102 objects were examined spectroscopically. Within this subset, 91% exhibited Balmer emission in at least one epoch, in some cases with spectacular spectral variability. The variability observed in the sample is consistent with the establishment and maintenance of the Be phenomenon.Comment: 19 pages, AJ accepte

Chemodynamics of Compact Stellar Systems in NGC 5128: How similar are Globular Clusters, Ultra-Compact Dwarfs, and Dwarf Galaxies?

Velocity dispersion measurements are presented for luminous GCs in NGC 5128 derived from high-res. UVES spectra. The measurements are made with the pPXF code that parametrically recovers line-of-sight velocity dispersions. Combining the measured velocity dispersions with surface photometry and structural parameter data from HST enables both dynamical masses and M/L ratios to be derived. The fundamental plane relations of these clusters are investigated in order to fill the apparent gap between the relations of Local Group GCs and more massive early-type galaxies. It is found that the properties of these massive stellar systems match those of nuclear clusters in dwarf elliptical galaxies and UCDs better than those of Local Group GCs, and that all objects share similarly old (>8 Gyr) ages, suggesting a possible link between the formation and evolution of dE,Ns, UCDs and massive GCs. We find a very steep correlation between dynamical (M/L) ratio and dynamical mass of the form (M/L)_dyn ~ M_dyn^(0.24+/-0.02) above M_dyn = 2x10^6 Msol. Formation scenarios are investigated with a chemical abundance analysis using absorption line strengths calibrated to the Lick/IDS index system. The results lend support to two scenarios contained within a single general formation scheme. Old, massive, super-solar [alpha/Fe] systems are formed on short (<100 Myr) timescales through the merging of single-collapse GCs which themselves are formed within single, giant molecular clouds. More intermediate- and old-aged (~3-10 Gyr), solar- to sub-solar [alpha/Fe] systems are formed on much longer (~Gyr) timescales through the stripping of dE,Ns in the 10^13-10^15 Msol potential wells of massive galaxies and galaxy clusters.Comment: 12 pages (ApJ style) with 11 figures and 7 tables, accepted for publication in Ap