arXiv subjects

Benjamin F. Williams

Publications and source records attributed to Benjamin F. Williams.

At least 55 records · Page 3Linked to original sources

arXiv2024

A NuSTAR Census of the X-ray Binary Population of the M31 Disk

Using hard (E>10 keV) X-ray observations with NuSTAR, we are able to differentiate between accretion states, and thus compact object types, of neutron stars and black holes in X-ray binaries (XRBs) in M31, our nearest Milky Way-type neighbor. Using ten moderate-depth (20-50 ks) observations of the disk of M31 covering a total of ~0.45 deg$^{2}$, we detect 20 sources at 2$σ$ in the 4-25 keV band pass, 14 of which we consider to be XRB candidates. This complements an existing deeper (100-400 ks) survey covering ~0.2 deg$^{2}$ of the bulge and the northeastern disk. We make tentative classifications of 9 of these sources with the use of diagnostic color-intensity and color-color diagrams, which separate sources into various neutron star and black hole regimes, identifying 3 black holes and 6 neutron stars. In addition, we create X-ray luminosity functions for both the full (4-25 keV) and hard (12-25 keV) band, as well as sub-populations of the full band based on compact object type and association with globular clusters. Our best fit globular cluster XLF is shallower than the field XLF, and preliminary BH and NS XLFs suggest a difference in shape based on compact object type. We find that the cumulative disk XLFs in the full and hard band are best fit by power laws with indices of 1.32 and 1.28 respectively. This is consistent with models of the Milky Way XLF from Grimm et al. (2002), Voss & Ajello (2010), and Doroshenko et al. (2014).

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arXiv2024

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). VI. The High-Mass Stellar Initial Mass Function of M33

We measure the high-mass stellar initial mass function (IMF) from resolved stars in M33 young stellar clusters. Leveraging \textit{Hubble Space Telescope's} high resolving power, we fully model the IMF probabilistically. We first model the optical CMD of each cluster to constrain its power-law slope $Γ$, marginalized over other cluster parameters in the fit (e.g., cluster age, mass, and radius). We then probabilistically model the distribution of MF slopes for a highly strict cluster sample of 9 clusters more massive than log(Mass/M$_{\odot}$)=3.6; above this mass, all clusters have well-populated main sequences of massive stars and should have accurate recovery of their MF slopes, based on extensive tests with artificial clusters. We find the ensemble IMF is best described by a mean high-mass slope of $\overlineΓ = 1.49\pm0.18$, with an intrinsic scatter of $σ^{2}_Γ = 0.02^{+0.16}_{0.00}$, consistent with a universal IMF. We find no dependence of the IMF on environmental impacts such as the local star formation rate or galactocentric radius within M33, which serves as a proxy for metallicity. This $\overlineΓ$ measurement is consistent with similar measurements in M31, despite M33 having a much higher star formation rate intensity. While this measurement is formally consistent with the canonical Kroupa ($Γ= 1.30$) IMF, as well as the Salpeter ($Γ= 1.35)$) value, it is the second Local Group cluster sample to show evidence for a somewhat steeper high-mass IMF slope. We explore the impacts a steeper IMF slope has on a number of astronomical sub-fields.

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arXiv2024

A Panchromatic Study of the X-ray Binary Population in NGC 300 on Sub-Galactic Scales

The population-wide properties and demographics of extragalactic X-ray binaries (XRBs) correlate with the star formation rates (SFRs), stellar masses ($M_{\star}$), and environmental factors (such as metallicity, $Z$) of their host galaxy. Although there is evidence that XRB scaling relations ($L_X$/SFR for high mass XRBs [HMXBs] and $L_X$/$M_{\star}$ for low mass XRBs [LMXBs]) may depend on metallicity and stellar age across large samples of XRB-hosting galaxies, disentangling the effects of metallicity and stellar age from stochastic effects, particularly on subgalactic scales, remains a challenge. We use archival X-ray through IR observations of the nearby galaxy NGC 300 to self-consistently model the broadband spectral energy distribution and examine radial trends in its XRB population. We measure a current ($<$100 Myr) SFR of 0.18$\pm$0.08 $M_{\odot}$ yr$^{-1}$ and $M_{\star}$= $(2.15^{+0.26}_{-0.14})\times10^9$ $M_{\odot}$. Although we measure a metallicity gradient and radially resolved star formation histories that are consistent with the literature, there is a clear excess in the number of X-ray sources below $\sim10^{37}$ erg s$^{-1}$ that are likely a mix of variable XRBs and additional background AGN. When we compare the subgalactic $L_X$/SFR ratios as a function of $Z$ to the galaxy-integrated $L_X$-SFR-$Z$ relationships from the literature, we find that only the regions hosting the youngest ($\lesssim$30 Myr) HMXBs agree with predictions, hinting at time evolution of the $L_X$-SFR-$Z$ relationship.

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arXiv2024

The JWST Resolved Stellar Populations Early Release Science Program V. DOLPHOT Stellar Photometry for NIRCam and NIRISS

We present NIRCam and NIRISS modules for DOLPHOT, a widely-used crowded field stellar photometry package. We describe details of the modules including pixel masking, astrometric alignment, star finding, photometry, catalog creation, and artificial star tests (ASTs). We tested these modules using NIRCam and NIRISS images of M92 (a Milky Way globular cluster), Draco II (an ultra-faint dwarf galaxy), and WLM (a star-forming dwarf galaxy). DOLPHOT's photometry is highly precise and the color-magnitude diagrams are deeper and have better definition than anticipated during original program design in 2017. The primary systematic uncertainties in DOLPHOT's photometry arise from mismatches in the model and observed point spread functions (PSFs) and aperture corrections, each contributing $\lesssim0.01$ mag to the photometric error budget. Version 1.2 of WebbPSF models, which include charge diffusion and interpixel capacitance effects, significantly reduced PSF-related uncertainties. We also observed minor ($\lesssim0.05$ mag) chip-to-chip variations in NIRCam's zero points, which will be addressed by the JWST flux calibration program. Globular cluster observations are crucial for photometric calibration. Temporal variations in the photometry are generally $\lesssim0.01$ mag, although rare large misalignment events can introduce errors up to 0.08 mag. We provide recommended DOLPHOT parameters, guidelines for photometric reduction, and advice for improved observing strategies. Our ERS DOLPHOT data products are available on MAST, complemented by comprehensive online documentation and tutorials for using DOLPHOT with JWST imaging data.

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arXiv2024

The JWST Resolved Stellar Populations Early Release Science Program VI. Identifying Evolved Stars in Nearby Galaxies

We present an investigation of evolved stars in the nearby star-forming galaxy WLM, using NIRCam imaging from the JWST resolved stellar populations early-release science (ERS) program. We find that various combinations of the F090W, F150W, F250M, and F430M filters can effectively isolate red supergiants (RSGs) and thermally-pulsing asymptotic giant branch (TP-AGB) stars from one another, while also providing a reasonable separation of the primary TP-AGB subtypes: carbon-rich C-type stars and oxygen-rich M-type stars. The classification scheme we present here agrees very well with the well-established Hubble Space Telescope (HST) medium-band filter technique. The ratio of C to M-type stars (C/M) is 0.8$\pm$0.1 for both the new JWST and the HST classifications, which is within one sigma of empirical predictions from optical narrow-band CN and TiO filters. The evolved star colors show good agreement with the predictions from the PARSEC$+$COLIBRI stellar evolutionary models, and the models indicate a strong metallicity dependence that makes stellar identification even more effective at higher metallicity. However, the models also indicate that evolved star identification with NIRCam may be more difficult at lower metallicies. We test every combination of NIRCam filters using the models and present additional filters that are also useful for evolved star studies. We also find that $\approx$90\% of the dusty evolved stars are carbon-rich, suggesting that carbonaceous dust dominates the present-day dust production in WLM, similar to the findings in the Magellanic Clouds. These results demonstrate the usefulness of NIRCam in identifying and classifying dust-producing stars without the need for mid-infrared data.

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arXiv2023

JWST Reveals Star Formation Across a Spiral Arm in M33

Young stellar objects (YSOs) are the gold standard for tracing star formation in galaxies but have been unobservable beyond the Milky Way and Magellanic Clouds. But that all changed when the James Webb Space Telescope was launched, which we use to identify YSOs in the Local Group galaxy M33, marking the first time that individual YSOs have been identified at these large distances. We present MIRI imaging mosaics at 5.6 and 21 microns that cover a significant portion of one of M33's spiral arms that has existing panchromatic imaging from the Hubble Space Telescope and deep ALMA CO measurements. Using these MIRI and Hubble Space Telescope images, we identify point sources using the new DOLPHOT MIRI module. We identify 793 candidate YSOs from cuts based on colour, proximity to giant molecular clouds (GMCs), and visual inspection. Similar to Milky Way GMCs, we find that higher mass GMCs contain more YSOs and YSO emission, which further shows YSOs identify star formation better than most tracers that cannot capture this relationship at cloud scales. We find evidence of enhanced star formation efficiency in the southern spiral arm by comparing the YSOs to the molecular gas mass.

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arXiv2023

The JWST Resolved Stellar Populations Early Release Science Program IV: The Star Formation History of the Local Group Galaxy WLM

We present the first star formation history (SFH) and age-metallicity relation (AMR) derived from resolved stellar populations imaged with the JWST NIRCam instrument. The target is the Local Group star-forming galaxy WLM at 970 kpc. The depth of the color-magnitude diagram (CMD) reaches below the oldest main sequence turn-off with a SNR=10 at M_F090W=+4.6 mag; this is the deepest CMD for any galaxy that is not a satellite of the Milky Way. We use Hubble Space Telescope (HST) optical imaging that overlaps with the NIRCam observations to directly evaluate the SFHs derived based on data from the two great observatories. The JWST and HST-based SFHs are in excellent agreement. We use the metallicity distribution function measured from stellar spectra to confirm the trends in the AMRs based on the JWST data. Together, these results confirm the efficacy of recovering a SFH and AMR with the NIRCam F090W-F150W filter combination and provide validation of the sensitivity and accuracy of stellar evolution libraries in the near-infrared relative to the optical for SFH recovery work. From the JWST data, WLM shows an early onset to star formation, followed by an extended pause post-reionization before star formation re-ignites, which is qualitatively similar to what has been observed in the isolated galaxies Leo~A and Aquarius. Quantitatively, 15% of the stellar mass formed in the first Gyr, while only 10% formed over the next ~5 Gyr; the stellar mass then rapidly doubled in ~2.5 Gyr, followed by constant star formation over the last ~5 Gyr.

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arXiv2023

Monitoring the X-ray Variability of Bright X-ray Sources in M33

We present a new five-epoch Chandra X-ray Observatory monitoring survey of the nearby spiral galaxy M33 which probes X-ray variability with time sampling between two weeks and four months. We characterize the X-ray variability of 55 bright point sources outside of the nucleus, many of which are expected to be high-mass X-ray binaries (HMXBs). We detect eight new candidate transients not detected in previous X-ray catalogs of M33 and discuss their possible nature. The final catalog includes 26 known HMXB candidates identified in the literature. We extend the baseline of the X-ray light curves up to 21 years by including archival X-ray observations of these sources. We compare the detection and non-detection epochs of the sources to suites of simulated source duty cycles and infer that most of our detected sources have duty cycles > 30%. We find only four sources whose detection patterns are consistent with having duty cycles below 30%. This large fraction of sources with high duty cycles is unexpected for a population of HMXBs, thus more frequent X-ray monitoring will likely reveal many more low duty cycle HMXBs in M33.

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arXiv2023

TREX: Kinematic Characterisation of a High-Dispersion Intermediate-Age Stellar Component in M33

The dwarf galaxy Triangulum (M33) presents an interesting testbed for studying stellar halo formation: it is sufficiently massive so as to have likely accreted smaller satellites, but also lies within the regime where feedback and other "in-situ" formation mechanisms are expected to play a role. In this work, we analyse the line-of-sight kinematics of stars across M33 from the TREX survey with a view to understanding the origin of its halo. We split our sample into two broad populations of varying age, comprising 2032 "old" red giant branch (RGB) stars, and 671 "intermediate-age" asymptotic giant branch (AGB) and carbon stars. We find decisive evidence for two distinct kinematic components in both old and intermediate-age populations: a low-dispersion (~22 km/s) disk-like component co-rotating with M33's HI gas, and a significantly higher-dispersion component (~50-60 km/s) which does not rotate in the same plane as the gas and is thus interpreted as M33's stellar halo. While kinematically similar, the fraction of stars associated with the halo component differs significantly between the two populations: this is consistently ~10% for the intermediate age population, but decreases from ~34% to ~10% as a function of radius for the old population. We additionally find evidence that the intermediate-age halo population is systematically offset from the systemic velocity of M33 by ~25 km/s, with a preferred central LOS velocity of ~-155 km/s. This is the first detection and characterisation of an intermediate-age halo in M33, and suggests in-situ formation mechanisms, as well as potentially tidal interactions, have helped shaped it.

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arXiv2023

Considering the Single and Binary Origins of the Type IIP SN 2017eaw

Current population synthesis modeling suggests that 30-50% of Type II supernovae originate from binary progenitors, however, the identification of a binary progenitor is challenging. One indicator of a binary progenitor is that the surrounding stellar population is too old to contain a massive single star.Measurements of the progenitor mass of SN 2017eaw are starkly divided between observations made temporally close to core-collapse which show a progenitor mass of 13-15 solar masses (final helium core mass of 4.4 to 6.0 solar masses - which is a more informative property than initial mass) and those from the stellar population surrounding the SN which find M<10.8 solar masses (helium core mass <3.4 solar masses). In this paper, we reanalyze the surrounding stellar population with improved astrometry and photometry, finding a median age of 16.8 (+3.2, -1.0) Myr for all stars younger than 50 Myr (helium core mass of 4.7 solar masses) and 85.9 (+3.2, -6.5) Myr for stars younger than 150 Myr. 16.8 Myr is now consistent with the helium core mass range derived from the temporally near explosion observations for single stars. Applying the combined constraints to population synthesis models, we determine that the probability of the progenitor of SN 2017eaw being an initially single-star is 65% compared to 35% for prior binary interaction. 85.9 Myr is inconsistent with any formation scenarios. We demonstrate that combining progenitor age constraints with helium core mass estimates from red supergiant SED modeling, late-time spectra, and indirectly from light curve modeling can help to differentiate single and binary progenitor scenarios and provide a framework for the application of this technique to future observations.

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arXiv2023

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). V. The Structure of M33 in Resolved Stellar Populations

We present a detailed analysis of the the structure of the Local Group flocculent spiral galaxy M33, as measured using the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region (PHATTER) survey. Leveraging the multiwavelength coverage of PHATTER, we find that the oldest populations are dominated by a smooth exponential disk with two distinct spiral arms and a classical central bar $-$ completely distinct from what is seen in broadband optical imaging, and the first-ever confirmation of a bar in M33. We estimate a bar extent of $\sim$1 kpc. The two spiral arms are asymmetric in orientation and strength, and likely represent the innermost impact of the recent tidal interaction responsible for M33's warp at larger scales. The flocculent multi-armed morphology for which M33 is known is only visible in the young upper main sequence population, which closely tracks the morphology of the ISM. We investigate the stability of M33's disk, finding $Q{\sim}1$ over the majority of the disk. We fit multiple components to the old stellar density distribution and find that, when considering recent stellar kinematics, M33's bulk structure favors the inclusion of an accreted halo component, modeled as a broken power-law. The best-fit halo model has an outer power-law index of $-$3 and accurately describes observational evidence of M33's stellar halo from both resolved stellar spectroscopy in the disk and its stellar populations at large radius. Integrating this profile yields a total halo stellar mass of ${\sim}5{\times}10^8\ M_{\odot}$, giving a total stellar halo mass fraction of 16%, most of which resides in the innermost 2.5 kpc.

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arXiv2023

The Panchromatic Hubble Andromeda Treasury XXI. The Legacy Resolved Stellar Photometry Catalog

We present the final legacy version of stellar photometry for the Panchromatic Hubble Andromeda Treasury (PHAT) survey. We have reprocessed all of the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) near ultraviolet (F275W, F336W), optical (F475W, F814W), and near infrared (F110W, F160W) imaging from the PHAT survey using an improved method that optimized the survey depth and chip gap coverage by including all overlapping exposures in all bands in the photometry. An additional improvement was gained through the use of charge transfer efficiency (CTE) corrected input images, which provide more complete star finding as well as more reliable photometry for the NUV bands, which had no CTE correction in the previous version of the PHAT photometry. While this method requires significantly more computing resources and time than earlier versions where the photometry was performed on individual pointings, it results in smaller systematic instrumental completeness variations as demonstrated by cleaner maps in stellar density, and it results in optimal constraints on stellar fluxes in all bands from the survey data. Our resulting catalog has 138 million stars, 18% more than the previous catalog, with lower density regions gaining as much as 40% more stars. The new catalog produces nearly seamless population maps which show relatively well-mixed distributions for populations associated with ages older than 1-2 Gyr, and highly structured distributions for the younger populations.

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arXiv2023

Extragalactic Star Cluster Science with the Nancy Grace Roman Space Telescope's High Latitude Wide Area Survey and the Vera C. Rubin Observatory

The Nancy Grace Roman Telescope's High Latitude Wide Area Survey will have a number of synergies with the Vera Rubin Observatory's Legacy Survey of Space and Time (LSST), particularly for extragalactic star clusters. Understanding the nature of star clusters and star cluster systems are key topics in many areas of astronomy, chief among them stellar evolution, high energy astrophysics, galaxy assembly/dark matter, the extragalactic distance scale, and cosmology. One of the challenges will be disentangling the age/metallicity degeneracy because young ($\sim$Myr) metal-rich clusters have similar SEDs to old ($\sim$Gyr) metal-poor clusters. Rubin will provide homogeneous, $ugrizy$ photometric coverage, and measurements in the red Roman filters will help break the age-metallicity and age-extinction degeneracies, providing the first globular cluster samples that cover wide areas while essentially free of contamination from Milky Way stars. Roman's excellent spatial resolution will also allow measurements of cluster sizes. We advocate for observations of a large sample of galaxies with a range of properties and morphologies in the Rubin/LSST footprint matching the depth of the LSST Wide-Fast-Deep field $i$ band limit (26.3 mag), and recommend adding the F213 filter to the survey.

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arXiv2023

RomAndromeda: The Roman Survey of the Andromeda Halo

As our nearest large neighbor, the Andromeda Galaxy provides a unique laboratory for investigating galaxy formation and the distribution and substructure properties of dark matter in a Milky Way-like galaxy. Here, we propose an initial 2-epoch ($Δt\approx 5$yr), 2-band Roman survey of the entire halo of Andromeda, covering 500 square degrees, which will detect nearly every red giant star in the halo (10$σ$ detection in F146, F062 of 26.5, 26.1AB mag respectively) and yield proper motions to $\sim$25 microarcsec/year (i.e., $\sim$90 km/s) for all stars brighter than F146 $\approx 23.6$ AB mag (i.e., reaching the red clump stars in the Andromeda halo). This survey will yield (through averaging) high-fidelity proper motions for all satellites and compact substructures in the Andromeda halo and will enable statistical searches for clusters in chemo-dynamical space. Adding a third epoch during the extended mission will improve these proper motions by $\sim t^{-1.5}$, to $\approx 11$ km/s, but this requires obtaining the first epoch in Year 1 of Roman operations. In combination with ongoing and imminent spectroscopic campaigns with ground-based telescopes, this Roman survey has the potential to yield full 3-d space motions of $>$100,000 stars in the Andromeda halo, including (by combining individual measurements) robust space motions of its entire globular cluster and most of its dwarf galaxy satellite populations. It will also identify high-velocity stars in Andromeda, providing unique information on the processes that create this population. These data offer a unique opportunity to study the immigration history, halo formation, and underlying dark matter scaffolding of a galaxy other than our own.

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arXiv2023

Multiwavelength Characterization of the High Mass X-ray Binary Population of M33

We present multi-wavelength characterization of 65 high mass X-ray binary (HMXB) candidates in M33. We use the Chandra ACIS survey of M33 (ChASeM33) catalog to select hard X-ray point sources that are spatially coincident with UV-bright point source optical counterparts in the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) catalog, which covers the inner disk of M33 at near infrared, optical, and near ultraviolet wavelengths. We perform spectral energy distribution (SED) fitting on multi-band photometry for each point source optical counterpart to measure its physical properties including mass, temperature, luminosity, and radius. We find that the majority of the HMXB companion star candidates are likely B-type main sequence stars, suggesting that the HMXB population of M33 is dominated by Be-XRBs, as is seen in other Local Group galaxies. We use spatially-resolved recent star formation history (SFH) maps of M33 to measure the age distribution of the HMXB candidate sample and the HMXB production rate for M33. We find a bimodal distribution for the HMXB production rate over the last 80 Myr, with a peak at $\sim$10 Myr and $\sim$40 Myr, which match theoretical formation timescales for the most massive HMXBs and Be X-ray binaries (Be-XRBs), respectively. We measure an HMXB production rate of 107$-$136 HMXBs/(M$_{\odot}$ yr$^{-1}$) over the last 50 Myr and 150$-$199 HMXBs/(M$_{\odot}$ yr$^{-1}$) over the last 80 Myr. For sources with compact object classifications from overlapping NuSTAR observations, we find a preference for giant/supergiant companion stars in BH-HMXBs and main sequence companion stars in neutron star HMXBs (NS-HMXBs).

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arXiv2023

Clusters, Clouds, and Correlations: Relating Young Clusters to Giant Molecular Clouds in M33 and M31

We use young clusters and giant molecular clouds (GMCs) in the galaxies M33 and M31 to constrain temporal and spatial scales in the star formation process. In M33, we compare the PHATTER catalogue of 1214 clusters with ages measured via colour-magnitude diagram (CMD) fitting to 444 GMCs identified from a new 35 pc resolution ALMA $^{12}$CO(2-1) survey. In M31, we compare the PHAT catalogue of 1249 clusters to 251 GMCs measured from a CARMA $^{12}$CO(1-0) survey with 20 pc resolution. Through two-point correlation analysis, we find that young clusters have a high probability of being near other young clusters, but correlation between GMCs is suppressed by the cloud identification algorithm. By comparing the positions, we find that younger clusters are closer to GMCs than older clusters. Through cross-correlation analysis of the M33 cluster data, we find that clusters are statistically associated when they are $\leq$10 Myr old. Utilizing the high precision ages of the clusters, we find that clusters older than $\approx 18$ Myr are uncorrelated with the molecular ISM. Using the spatial coincidence of the youngest clusters and GMCs in M33, we estimate that clusters spend $\approx$4-6 Myr inside their parent GMC. Through similar analysis, we find that the GMCs in M33 have a total lifetime of $\approx 11$-15 Myr. We also develop a drift model and show that the above correlations can be explained if the clusters in M33 have a 5-10 km s$^{-1}$ velocity dispersion relative to the molecular ISM.

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arXiv2023

The Spatial Correlation of High Mass X-ray Binaries and Young Star Clusters in Nearby Star-Forming Galaxies

We present an analysis of the two-point spatial correlation functions of high-mass X-ray binary (HMXB) and young star cluster (YSC) populations in M31 and M33. We find evidence that HMXBs are spatially correlated with YSCs to a higher degree than would be expected from random chance in both galaxies. When supplemented with similar studies in the Milky Way, Small Magellanic Cloud, and NGC 4449, we find that the peak value of the spatial correlation function correlates strongly with the specific star formation rate of the host galaxy. We additionally perform an X-ray stacking analysis of 211 non-X-ray detected YSCs in M31 and 463 YSCs in M33. We do not detect excess X-ray emission at the stacked cluster locations down to 3$σ$ upper limits of $\sim10^{33}$ erg s$^{-1}$ (0.35-8 keV) in both galaxies, which strongly suggests that dynamical formation within YSCs is not a major HMXB formation channel. We interpret our results in the context of (1) the recent star formation histories of the galaxies, which may produce differences in the demographics of compact objects powering the HMXBs, and (2) the differences in natal kicks experienced by compact objects during formation, which can eject newly-formed HMXB from their birth clusters.

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arXiv2023

The Panchromatic Hubble Andromeda Treasury XX: The Disk of M31 is Thick

We present a new approach to measuring the thickness of a partially face-on stellar disk, using dust geometry. In a moderately-inclined disk galaxy, the fraction of reddened stars is expected to be 50% everywhere, assuming that dust lies in a thin midplane. In a thickened disk, however, a wide range of radii project onto the line of sight. Assuming stellar density declines with radius, this geometrical projection leads to differences in the numbers of stars on the near and far sides of the thin dust layer. The fraction of reddened stars will thus differ from the 50% prediction, with a deviation that becomes larger for puffier disks. We map the fraction of reddened red giant branch (RGB) stars across M31, which shows prominent dust lanes on only one side of the major axis. The fraction of reddened stars varies systematically from 20% to 80%, which requires that these stars have an exponential scale height h_z that is 0.14+/-0.015 times the exponential scale length (h_r~5.5kpc). M31's RGB stars must therefore have h_z=770+/-80pc, which is far thicker than the Milky Way's thin disk, but comparable to its thick disk. The lack of a significant thin disk in M31 is unexpected, but consistent with its interaction history and high disk velocity dispersion. We suggest that asymmetric reddening be used as a generic criteria for identifying ``thick disk'' dominated systems, and discuss prospects for future 3-dimensional tomographic mapping of the gas and stars in M31.

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