arXiv subjects

P. Guhathakurta

Publications and source records attributed to P. Guhathakurta.

At least 55 records · Page 3Linked to original sources

arXiv2010

Extragalactic Background Light Inferred from AEGIS Galaxy SED-type Fractions

The extragalactic background light (EBL) is of fundamental importance both for understanding the entire process of galaxy evolution and for gamma-ray astronomy, but the overall spectrum of the EBL between 0.1-1000 microns has never been determined directly from galaxy spectral energy distribution (SED) observations over a wide redshift range. The evolving, overall spectrum of the EBL is derived here utilizing a novel method based on observations only. This is achieved from the observed evolution of the rest-frame K-band galaxy luminosity function up to redshift 4 (Cirasuolo et al. 2010), combined with a determination of galaxy SED-type fractions. These are based on fitting SWIRE templates to a multiwavelength sample of about 6000 galaxies in the redshift range from 0.2 to 1 from the All-wavelength Extended Groth Strip International Survey (AEGIS). The changing fractions of quiescent galaxies, star-forming galaxies, starburst galaxies and AGN galaxies in that redshift range are estimated, and two alternative extrapolations of SED-types to higher redshifts are considered. This allows calculation of the evolution of the luminosity densities from the UV to the IR, the evolving star formation rate density of the universe, the evolving contribution to the bolometric EBL from the different galaxy populations including AGN galaxies and the buildup of the EBL. Our EBL calculations are compared with those from a semi-analytic model, from another observationally-based model and observational data. The EBL uncertainties in our modeling based directly on the data are quantified, and their consequences for attenuation of very high energy gamma-rays due to pair production on the EBL are discussed. It is concluded that the EBL is well constrained from the UV to the mid-IR, but independent efforts from infrared and gamma-ray astronomy are needed in order to reduce the uncertainties in the far-IR.

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arXiv2010

Local Group Dwarf Elliptical Galaxies: II. Stellar Kinematics to Large Radii in NGC 147 and NGC 185

We present kinematic and metallicity profiles for the M31 dwarf elliptical (dE) satellite galaxies NGC 147 and NGC 185. The profiles represent the most extensive spectroscopic radial coverage for any dE galaxy, extending to a projected distance of eight half-light radii (8 r_eff = 14'). We achieve this coverage via Keck/DEIMOS multislit spectroscopic observations of 520 and 442 member red giant branch stars in NGC 147 and NGC 185, respectively. In contrast to previous studies, we find that both dEs have significant internal rotation. We measure a maximum rotational velocity of 17+/-2 km/s for NGC 147 and 15+/-5 km/s for NGC 185. The velocity dispersions decrease gently with radius with an average dispersion of 16+/-1 km/s for NGC 147 and 24+/-1 km/s for NGC 185. Both dEs have internal metallicity dispersions of 0.5 dex, but show no evidence for a radial metallicity gradient. We construct two-integral axisymmetric dynamical models and find that the observed kinematical profiles cannot be explained without modest amounts of non-baryonic dark matter. We measure central mass-to-light ratios of ML_V = 4.2+/-0.6 and ML_V = 4.6+/-0.6 for NGC 147 and NGC 185, respectively. Both dE galaxies are consistent with being primarily flattened by their rotational motions, although some anisotropic velocity dispersion is needed to fully explain their observed shapes. The velocity profiles of all three Local Group dEs (NGC 147, NGC 185 and NGC 205) suggest that rotation is more prevalent in the dE galaxy class than previously assumed, but is often manifest only at several times the effective radius. Since all dEs outside the Local Group have been probed to only inside the effective radius, this opens the door for formation mechanisms in which dEs are transformed or stripped versions of gas-rich rotating progenitor galaxies.

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arXiv2009

Structure and Population of the Andromeda Stellar Halo from a Subaru/Suprime-Cam Survey

We present a photometric survey of the stellar halo of the Andromeda galaxy, using Suprime-Cam on the Subaru Telescope. A detailed analysis of VI color-magnitude diagrams of the resolved stellar population is used to measure properties such as line-of-sight distance, surface brightness, metallicity, and age, and these are used to isolate and characterize different components of the M31 halo: (1) several substructures, and (2) the smooth halo. First, we study M31's halo substructure along the north-west/south-east minor axis out to R ~ 100 kpc and the south-west major axis region at R ~ 60 kpc. We confirm two substructures in the south-east halo reported by Ibata et al. (2007) and discover two overdense substructures in the north-west halo. We investigate the properties of these four substructures as well as other structures including the western shelf and find that differences in stellar populations among these systems, thereby suggesting each has a different origin. Our statistical analysis implies that the M31 halo as a whole may contain at least 16 substructures, each with a different origin. Second, we investigate the properties of an underlying, smooth and extended halo component out to R > 100 kpc. We find that the surface density of this smooth halo can be fitted to a Hernquist model of scale radius ~ 17 kpc or a power-law profile with ~ R^{-2.17 +/- 0.15}. In contrast to the relative smoothness of the halo density profile, its metallicity distribution appears to be spatially non-uniform with non-monotonic variations with radius, suggesting that the halo population has not had sufficient time to dynamically homogenize the accreted populations. Further implications for the formation of the M31 halo are discussed.

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arXiv2009

Mixed Populations in Globular Clusters: Et Tu, 47 Tuc?

We exploit the large number of archival HST images of 47 Tuc to examine its subgiant branch (SGB) and main sequence (MS) for signs of multiple populations. In the cluster core, we find that the cluster's SGB exhibits a clear spread in luminosity, with at least two distinct components: a brighter one with a spread that is real but not bimodal, and a second one about 0.05 mag fainter, containing about 10% of the stars. In a less crowded field 6 arcminutes from the center, we find that the MS is broadened much more than can be accounted for by photometric errors, and that this broadening increases at fainter magnitudes.

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arXiv2008

AEGIS-X: The Chandra Deep Survey of the Extended Groth Strip

We present the AEGIS-X survey, a series of deep Chandra ACIS-I observations of the Extended Groth Strip. The survey comprises pointings at 8 separate positions, each with nominal exposure 200ks, covering a total area of approximately 0.67 deg2 in a strip of length 2 degrees. We describe in detail an updated version of our data reduction and point source detection algorithms used to analyze these data. A total of 1325 band-merged sources have been found to a Poisson probability limit of 4e-6, with limiting fluxes of 5.3e-17 erg/cm2/s in the soft (0.5-2 keV) band and 3.8e-16 erg/cm2/s in the hard (2-10 keV) band. We present simulations verifying the validity of our source detection procedure and showing a very small, <1.5%, contamination rate from spurious sources. Optical/NIR counterparts have been identified from the DEEP2, CFHTLS, and Spitzer/IRAC surveys of the same region. Using a likelihood ratio method, we find optical counterparts for 76% of our sources, complete to R(AB)=24.1, and, of the 66% of the sources that have IRAC coverage, 94% have a counterpart to a limit of 0.9 microJy at 3.6 microns (m(AB)=23.8). After accounting for (small) positional offsets in the 8 Chandra fields, the astrometric accuracy of the Chandra positions is found to be 0.8 arcsec RMS, however this number depends both on the off-axis angle and the number of detected counts for a given source. All the data products described in this paper are made available via a public website.

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arXiv2008

Darwin Tames an Andromeda Dwarf: Unraveling the Orbit of NGC 205 Using a Genetic Algorithm

NGC 205, a close satellite of the M31 galaxy, is our nearest example of a dwarf elliptical galaxy. Photometric and kinematic observations suggest that NGC 205 is undergoing tidal distortion from its interaction with M31. Despite earlier attempts, the orbit and progenitor properties of NGC 205 are not well known. We perform an optimized search for these unknowns by combining a genetic algorithm with restricted N-body simulations of the interaction. This approach, coupled with photometric and kinematic observations as constraints, allows for an effective exploration of the parameter space. We represent NGC 205 as a static Hernquist potential with embedded massless test particles that serve as tracers of surface brightness. We explore 3 distinct, initially stable configurations of test particles: cold rotating disk, warm rotating disk, and hot, pressure-supported spheroid. Each model reproduces some, but not all, of the observed features of NGC 205, leading us to speculate that a rotating progenitor with substantial pressure support could match all of the observables. Furthermore, plausible combinations of mass and scale length for the pressure-supported spheroid progenitor model reproduce the observed velocity dispersion profile. For all 3 models, orbits that best match the observables place the satellite 11+/-9 kpc behind M31 moving at very large velocities: 300-500 km/s on primarily radial orbits. Given that the observed radial component is only 54 km/s, this implies a large tangential motion for NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC 205's velocity appears to be near or greater than its escape velocity, signifying that the satellite is likely on its first M31 passage.

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arXiv2007

The Evolution of Galaxy Mergers and Morphology at z<1.2 in the Extended Groth Strip

We present the quantitative rest-frame B morphological evolution and galaxy merger fractions at 0.2 < z < 1.2 as observed by the All-wavelength Extended Groth Strip International Survey (AEGIS). We use the Gini coefficent and M_20 to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4 L_B^*, assuming pure luminosity evolution of 1.3 M_B per unit redshift. We find that the merger fraction remains roughly constant at 10 +/- 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa increases from 21+/- 3% at z ~ 1.1 to 44 +/- 9% at z ~ 0.3, while the fraction of Sb-Ir decreases from 64 +/- 6% at z ~ 1.1 to 47 +/- 9% at z ~ 0.3. The majority of z < 1.2 Spitzer MIPS 24 micron sources with L(IR) > 10^11 L_sun are disk galaxies, and only ~ 15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z ~ 1.1, while E/S0/Sa makeup over 90% of the red sequence at z ~ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the galaxy merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star-formation rate density since z ~ 1 is a result of declining star-formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z ~ 1.2.

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arXiv2007

The Sextet Arcs: a Strongly Lensed Lyman Break Galaxy in the ACS Spectroscopic Galaxy Survey towards Abell 1689

We present results of the HST Advanced Camera for Surveys spectroscopic ground-based redshift survey in the field of A1689. We measure 98 redshifts, increasing the number of spectroscopically confirmed objects by sixfold. We present two spectra from this catalog of the Sextet Arcs, images which arise from a strongly-lensed Lyman Break Galaxy (LBG) at a redshift of z=3.038. Gravitational lensing by the cluster magnifies its flux by a factor of ~16 and produces six separate images with a total r-band magnitude of r_625=21.7. The two spectra, each of which represents emission from different regions of the LBG, show H I and interstellar metal absorption lines at the systemic redshift. Significant variations are seen in Ly-alpha profile across a single galaxy, ranging from strong absorption to a combination of emission plus absorption. A spectrum of a third image close to the brightest arc shows Ly-alpha emission at the same redshift as the LBG, arising from either another spatially distinct region of the galaxy, or from a companion galaxy close to the LBG. Taken as a group, the Ly-alpha equivalent width in these three spectra decreases with increasing equivalent width of the strongest interstellar absorption lines. We discuss how these variations can be used to understand the physical conditions in the LBG. Intrinsically, this LBG is faint, ~0.1L*, and forming stars at a modest rate, ~4 solar masses per year. We also detect absorption line systems toward the Sextet Arcs at z=2.873 and z=2.534. The latter system is seen across two of our spectra.

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arXiv2007

Star Formation in AEGIS Field Galaxies since z=1.1 : The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-Forming Galaxies

We analyze star formation (SF) as a function of stellar mass (M*) and redshift z in the All Wavelength Extended Groth Strip International Survey (AEGIS). For 2905 field galaxies, complete to 10^10(10^10.8) Msun at z<0.7(1), with Keck spectroscopic redshifts out to z=1.1, we compile SF rates (SFR) from emission lines, GALEX, and Spitzer MIPS 24 micron photometry, optical-NIR M* measurements, and HST morphologies. Galaxies with reliable signs of SF form a distinct "main sequence (MS)", with a limited range of SFR at a given M* and z (1 sigma < +-0.3 dex), and log(SFR) approximately proportional to log(M*). The range of log(SFR) remains constant to z>1, while the MS as a whole moves to higher SFR as z increases. The range of SFR along the MS constrains the amplitude of episodic variations of SF, and the effect of mergers on SFR. Typical galaxies spend ~67(95)% of their lifetime since z=1 within a factor of <~ 2(4) of their average SFR at a given M* and z. The dominant mode of the evolution of SF since z~1 is apparently a gradual decline of the average SFR in most individual galaxies, not a decreasing frequency of starburst episodes, or a decreasing factor by which SFR are enhanced in starbursts. LIRGs at z~1 seem to mostly reflect the high SFR typical for massive galaxies at that epoch. The smooth MS may reflect that the same set of few physical processes governs star formation prior to additional quenching processes. A gradual process like gas exhaustion may play a dominant role.

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arXiv2007

Exploring Halo Substructure with Giant Stars X. Extended Dark Matter or Tidal Disruption?: The Case for the Leo I Dwarf Spheroidal Galaxy

We present a wide-field (4.5 deg^2) photometric and spectroscopic survey of the Leo I dwarf spheroidal (dSph) galaxy to explore its extended morphology and dynamics. As in previous papers in this series, we take advantage of photometry in the M, T_2, and DDO51 filter system to select LeoI red giant branch star candidates, and, so far, this selection technique has proven 100% reliable in selecting actual Leo I members among more than 100 M < 21.5 Leo I giant candidates having previous or new Keck DEIMOS spectroscopy to a radius >1.3 times the limiting radius of the fitted, central King profile. The two-dimensional distribution of all similarly-selected Leo I giant candidates is well fitted by a central single-component King profile of limiting radius 13.3 arcmin, but many giant stars are found outside this newly derived King limiting radius. The density profile thus shows a break at a major axis radial distance of ~10 arcmin produced by an excess of stars at and beyond the King limiting radius (spectroscopically confirmed to be made of true Leo I members), and primarily along the major axis of the main body of the rather elongated satellite. This spatial configuration, a rather flat velocity dispersion profile and an asymmetric radial velocity (RV) distribution among the Leo I members at large radii together support a picture where Leo I has been tidally disrupted on at least one, but at most two, perigalactic passages of a massive Local Group member. (abridged)

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arXiv2007

The DEEP2 Galaxy Redshift Survey: Redshift Identification of Single-Line Emission Galaxies

We present two methods for determining spectroscopic redshifts of galaxies in the DEEP2 survey which display only one identifiable feature, an emission line, in the observed spectrum ("single-line galaxies"). First, we assume each single line is one of the four brightest lines accessible to DEEP2: Halpha, [OIII] 5007, Hbeta, or [OII] 3727. Then, we supplement spectral information with BRI photometry. The first method, parameter space proximity (PSP), calculates the distance of a single-line galaxy to galaxies of known redshift in (B-R), (R-I), R, observed wavelength parameter space. The second method is an artificial neural network (ANN). Prior information, such as allowable line widths and ratios, rules out one or more of the four lines for some galaxies in both methods. Based on analyses of evaluation sets, both methods are nearly perfect at identifying blended [OII] doublets. Of the lines identified as Halpha in the PSP and ANN methods, 91.4% and 94.2% respectively are accurate. Although the methods are not this accurate at discriminating between [OIII] and Hbeta, they can identify a single line as one of the two, and the ANN method in particular unambiguously identifies many [OIII] lines. From a sample of 640 single-line spectra, the methods determine the identities of 401 (62.7%) and 472 (73.8%) single lines, respectively, at accuracies similar to those found in the evaluation sets.

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arXiv2006

Investigating the Andromeda Stream: III. A Young Shell System in M31

Published maps of red giant stars in the halo region of M31 exhibit a giant stellar stream to the south of this galaxy, as well as a giant "shelf" to the northeast of M31's center. Using these maps, we find that there is a fainter shelf of comparable size on the western side as well. By choosing appropriate structural and orbital parameters for an accreting dwarf satellite within the accurate M31 potential model of Geehan et al. (2006), we produce a very similar structure in an N-body simulation. In this scenario, the tidal stream produced at pericenter of the satellite's orbit matches the observed southern stream, while the forward continuation of this tidal stream makes up two orbital loops, broadened into fan-like structures by successive pericentric passages; these loops correspond to the NE and W shelves. The tidal debris from the satellite also reproduces a previously-observed "stream" of counterrotating PNe and a related stream seen in red giant stars. The debris pattern in our simulation resembles the shell systems detected around many elliptical galaxies, though this is the first identification of a shell system in a spiral galaxy and the first in any galaxy close enough to allow measurements of stellar velocities and relative distances. We discuss the physics of these partial shells, highlighting the role played by spatial and velocity caustics in the observations. We show that kinematic surveys of the tidal debris will provide a sensitive measurement of M31's halo potential, while quantifying the surface density of debris in the shelves will let us reconstruct the original mass and time of disruption of the progenitor satellite.

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arXiv2006

AEGIS: Galaxy Spectral Energy Distributions from the X-Ray to Radio

The All-wavelength Extended Groth Strip International Survey (AEGIS) team presents broad-band spectral energy distributions (SEDs), from X-ray to radio wavelengths, for 71 galaxies spanning the redshift range 0.55-1.16 ( ~0.7). Galaxies with secure redshifts were selected from a small (22 arcminute-square) sub-section of the Keck/DEIMOS galaxy redshift survey in the Extended Groth Strip field that has also been targeted for deep panchromatic imaging by Chandra (X-ray), GALEX (ultraviolet), Canada-France-Hawaii Telescope (optical), Hubble Space Telescope (optical/near infrared), Palomar Observatory (near infrared), Spitzer (mid/far infrared), and the Very Large Array (radio.) The absolute magnitude of the typical galaxy in our sample is M_B=-19.82. The ultraviolet to mid-infrared portion of their spectral energy distributions (SEDs) are found to be bracketed by two stellar-only model SEDs: an early burst followed by passive evolution and a constant star-formation rate since early times; this suggests that few of these galaxies are undergoing major starbursts. Approximately half the galaxies show a mid- to far-infrared excess relative to the model SEDs, consistent with thermal emission from interstellar dust. Two objects have power-law SEDs, indicating that they are dominated by active galactic nuclei; both are detected in X-rays. The galaxies are grouped by rest-frame color,quantitative optical morphology, and [OII] emission line strength (possible indicator of star formation). On average, the panchromatic SEDs of the galaxies, from the ultraviolet to the infrared, follow expected trends: redder SEDs are associated with red U-B, early-type morphology, and low [OII] emission, and vice versa for blue SEDs.

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arXiv2006

The All-wavelength Extended Groth Strip International Survey (AEGIS) Data Sets

In this the first of a series of Letters, we present a description of the panchromatic data sets that have been acquired in the Extended Groth Strip region of the sky. Our survey, the All-wavelength Extended Groth strip International Survey (AEGIS), is intended to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray (0.5 - 10 keV), GALEX ultraviolet (1200 - 2500 Angstrom), CFHT/MegaCam Legacy Survey optical (3600 - 9000 Angstroms), CFHT/CFH12K optical (4500 - 9000 Angstroms), Hubble Space Telescope/ACS optical (4400 - 8500 Angstroms), Palomar/WIRC near-infrared (1.2 - 2.2 microns), Spitzer/IRAC mid-infrared (3.6 - 8.0 microns), Spitzer/MIPS far-infrared (24 - 70 microns), and VLA radio continuum (6 - 20 cm). In addition, this region of the sky has been targeted for extensive spectroscopy using the DEIMOS spectrograph on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

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arXiv2006

A Strong-Lens Survey in AEGIS: the influence of large scale structure

We report on the results of a visual search for galaxy-scale strong gravitational lenses over 650 arcmin^2 of HST/ACS imaging in the DEEP2-EGS field. In addition to a previously-known Einstein Cross (the "Cross," HST J141735+52264, with z_lens=0.8106 and a published z_source=3.40), we identify two new strong galaxy-galaxy lenses with multiple extended arcs. The first, HST J141820+52361 (the ``Dewdrop''; z_lens=0.5798, lenses two distinct extended sources into two pairs of arcs z_source=0.while), 9818 the second, HST J141833+52435 (the ``Anchor''; z_lens=0.4625), produces a single pair of arcs (source redshift not yet known). All three definite lenses are fit well by simple singular isothermal ellipsoid models including external shear. Using the three-dimensional line-of-sight (LOS) information on galaxies from the DEEP2 data, we calculate the convergence and shear contributions, assuming singular isothermal sphere halos truncated at 200 h^-1 kpc. These are also compared against three-dimensional local-density estimates. We find that even strong lenses in demonstrably underdense local environments may be considerably affected by LOS contributions, which in turn, may be underestimates of the effect of large scale structure.

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arXiv2006

On the Incidence of Strong MgII Absorbers Along GRB Sightlines

We report on a survey for strong (rest equivalent width W_r >= 1A), intervening MgII systems along the sightlines to long-duration gamma-ray bursts (GRBs). The GRB spectra which comprise the survey have a heterogeneous mix of resolution and wavelength coverage, but we implement a strict, uniform set of search criteria to derive a well-defined statistical sample. We identify 15 strong MgII absorbers along 14 GRB sightlines (nearly every sightline exhibits at least one absorber) with spectra covering a total pathlength Delta z = 15.5 at a mean redshift = 1.1. In contrast, the predicted incidence of such absorber systems along the same path length to quasar sightlines is only 3.8. The roughly four times higher incidence along GRB sightlines is inconsistent with a statistical fluctuation at greater than 99.9% c.l. Several effects could explain the result: (i) dust within the MgII absorbers obscures faint quasars giving a lower observed incidence along quasar sightlines; (ii) the gas is intrinsic to the GRB event; (iii) the GRB are gravitationally lensed by these absorbers. We present strong arguments against the first two effects and also consider lensing to be an unlikely explanation. The results suggest that at least one of our fundamental beliefs on absorption line research is flawed.

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arXiv2006

The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)

We present spectroscopic observations of red giant branch (RGB) stars over a large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10-m telescope. Using a combination of five photometric/spectroscopic diagnostics -- (1) radial velocity, (2) intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity sensitive), (4) position in the color-magnitude diagram, and (5) comparison between photometric and spectroscopic [Fe/H] estimates -- we isolate over 250 bona fide M31 bulge and halo RGB stars located in twelve fields ranging from R = 12-165kpc from the center of M31 (47 of these stars are halo members with R > 60 kpc). We derive the photometric and spectroscopic metallicity distribution function of M31 RGB stars in each of these fields. The mean of the resulting M31 spheroid (bulge and halo) metallicity distribution is found to be systematically more metal-poor with increasing radius, shifting from <[Fe/H]> = -0.47+/-0.03 (sigma = 0.39) at R < 20 kpc to <[Fe/H]> = -0.94+/-0.06 (sigma = 0.60) at R ~ 30 kpc to <[Fe/H]> = -1.26+/-0.10 (sigma = 0.72) at R > 60 kpc, assuming [alpha/Fe] = 0.0. These results indicate the presence of a metal-poor RGB population at large radial distances out to at least R = 160 kpc, thereby supporting our recent discovery of a stellar halo in M31: its halo and bulge (defined as the structural components with R^{-2} power law and de Vaucouleurs R^{1/4} law surface brightness profiles, respectively) are shown to have distinct metallicity distributions. If we assume an alpha-enhancement of [alpha/Fe] = +0.3 for M31's halo, we derive <[Fe/H]> = -1.5+/-0.1 (sigma = 0.7). Therefore, the mean metallicity and metallicity spread of this newly found remote M31 RGB population are similar to those of the Milky Way halo.

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arXiv2006

Investigating the Andromeda Stream: I. Simple Analytic Bulge-Disk-Halo Model for M31

This paper is the first in a series which studies interactions between M31 and its satellites, including the origin of the giant southern stream. We construct accurate yet simple analytic models for the potential of the M31 galaxy to provide an easy basis for calculation of orbits in M31's halo. We use an NFW dark halo, an exponential disk, a Hernquist bulge, and a central black hole point mass to describe the galaxy potential. We constrain the parameters of these functions by comparing to existing surface brightness, velocity dispersion, and rotation curve measurements of M31. Our description provides a good fit to the observations, and agrees well with more sophisticated modeling of M31. While in many respects the parameter set is well constrained, there is substantial uncertainty in the outer halo potential and a near-degeneracy between the disk and halo components, producing a large, nearly two-dimensional allowed region in parameter space. We limit the allowed region using theoretical expectations for the halo concentration, baryonic content, and stellar $M/L$ ratio, finding a smaller region where the parameters are physically plausible. Our proposed mass model for M31 has $M_{bulge} = 3.2 \times 10^{10} \Msun$, $M_{disk} = 7.2 \times 10^{10} \Msun$, and $M_{200} = 7.1\times 10^{11} \Msun$, with uncorrected (for internal and foreground extinction) mass-to-light ratios of $M/L_R = 3.9$ and 3.3 for the bulge and disk, respectively. We present some illustrative test particle orbits for the progenitor of the stellar stream in our galaxy potential, highlighting the effects of the remaining uncertainty in the disk and halo masses.

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