arXiv subjects

Anthony H. Gonzalez

Publications and source records attributed to Anthony H. Gonzalez.

At least 73 records · Page 4Linked to original sources

arXiv2016

Spectroscopic Confirmation of a Protocluster at z=3.786

We present new observations of the field containing the z=3.786 protocluster, PC217.96+32.3. We confirm that it is one of the largest and most overdense high-redshift structures known. Such structures are rare even in the largest cosmological simulations. We used the Mayall/MOSAIC1.1 imaging camera to image a 1.2x0.6 deg area (~150x75 comoving Mpc) surrounding the protocluster's core and discovered 165 candidate Lyman Alpha emitting galaxies (LAEs) and 788 candidate Lyman Break galaxies (LBGs). There are at least 2 overdense regions traced by the LAEs, the largest of which shows an areal overdensity in its core (i.e., within a radius of 2.5 comoving Mpc) of 14+/-7 relative to the average LAE spatial density in the imaged field. Further, the average LAE spatial density in the imaged field is twice that derived by other field LAE surveys. Spectroscopy with Keck/DEIMOS yielded redshifts for 164 galaxies (79 LAEs and 85 LBGs); 65 lie at a redshift of 3.785+/-0.010. The velocity dispersion of galaxies near the core is 350+/-40 km/s, a value robust to selection effects. The overdensities are likely to collapse into systems with present-day masses of >10^{15} solar masses and >6x10^{14} solar masses. The low velocity dispersion may suggest a dynamically young protocluster. We find a weak trend between narrow-band (Lyman Alpha) luminosity and environmental density: the Lyman Alpha luminosity is enhanced on average by 1.35X within the protocluster core. There is no evidence that the Lyman Alpha equivalent width depends on environment. These suggest that star-formation and/or AGN activity is enhanced in the higher density regions of the structure. PC217.96+32.3 is a Coma cluster analog, witnessed in the process of formation.

astro-ph.GA↗

arXiv2016

IDCS J1426.5+3508: Weak Lensing Analysis of a Massive Galaxy Cluster at $z=1.75$

We present a weak lensing study of the galaxy cluster IDCS J1426.5+3508 at $z=1.75$, which is the highest redshift strong lensing cluster known and the most distant cluster for which a weak lensing analysis has been undertaken. Using F160W, F814W, and F606W observations with the Hubble Space Telescope, we detect tangential shear at $2σ$ significance. Fitting a Navarro-Frenk-White mass profile to the shear with a theoretical median mass-concentration relation, we derive a mass $M_{200,\mathrm{crit}}=2.3^{+2.1}_{-1.4}\times10^{14}$ M$_{\odot}$. This mass is consistent with previous mass estimates from the Sunyaev-Zel'dovich (SZ) effect, X-ray, and strong lensing. The cluster lies on the local SZ-weak lensing mass scaling relation observed at low redshift, indicative of minimal evolution in this relation.

astro-ph.CO↗

arXiv2016

Spitzer UltRa Faint SUrvey Program (SURFS UP). II. IRAC-Detected Lyman-Break Galaxies at 6 < z < 10 Behind Strong-Lensing Clusters

We study the stellar population properties of the IRAC-detected $6 \lesssim z \lesssim 10$ galaxy candidates from the Spitzer UltRa Faint SUrvey Program (SURFS UP). Using the Lyman Break selection technique, we find a total of 16 new galaxy candidates at $6 \lesssim z \lesssim 10$ with $S/N \geq 3$ in at least one of the IRAC $3.6μ$m and $4.5μ$m bands. According to the best mass models available for the surveyed galaxy clusters, these IRAC-detected galaxy candidates are magnified by factors of $\sim 1.2$--$5.5$. We find that the IRAC-detected $6 \lesssim z \lesssim 10$ sample is likely not a homogeneous galaxy population: some are relatively massive (stellar mass as high as $4 \times 10^9\,M_{\odot}$) and evolved (age $\lesssim 500$ Myr) galaxies, while others are less massive ($M_{\text{stellar}}\sim 10^8\,M_{\odot}$) and very young ($\sim 10$ Myr) galaxies with strong nebular emission lines that boost their rest-frame optical fluxes. We identify two Ly$α$ emitters in our sample from the Keck DEIMOS spectra, one at $z_{\text{Ly}α}=6.76$ (in RXJ1347) and one at $z_{\text{Ly}α}=6.32$ (in MACS0454). We show that IRAC $[3.6]-[4.5]$ color, when combined with photometric redshift, can be used to identify galaxies likely with strong nebular emission lines within certain redshift windows.

astro-ph.GA↗

arXiv2015

IDCS J1426.5+3508: The Most Massive Galaxy Cluster at $z > 1.5$

We present a deep (100 ks) Chandra observation of IDCS J1426.5+3508, a spectroscopically confirmed, infrared-selected galaxy cluster at $z = 1.75$. This cluster is the most massive galaxy cluster currently known at $z > 1.5$, based on existing Sunyaev-Zel'dovich (SZ) and gravitational lensing detections. We confirm this high mass via a variety of X-ray scaling relations, including $T_X$-M, $f_g$-M, $Y_X$-M and $L_X$-M, finding a tight distribution of masses from these different methods, spanning M$_{500}$ = 2.3-3.3 $\times 10^{14}$ M$_{\odot}$, with the low-scatter $Y_X$-based mass $M_{500,Y_X} = 2.6^{+1.5}_{-0.5} \times 10^{14}$ M$_\odot$. IDCS J1426.5+3508 is currently the only cluster at $z > 1.5$ for which X-ray, SZ and gravitational lensing mass estimates exist, and these are in remarkably good agreement. We find a relatively tight distribution of the gas-to-total mass ratio, employing total masses from all of the aforementioned indicators, with values ranging from $f_{gas,500}$ = 0.087-0.12. We do not detect metals in the intracluster medium (ICM) of this system, placing a 2$σ$ upper limit of $Z(r < R_{500}) < 0.18 Z_{\odot}$. This upper limit on the metallicity suggests that this system may still be in the process of enriching its ICM. The cluster has a dense, low-entropy core, offset by $\sim$30 kpc from the X-ray centroid, which makes it one of the few "cool core" clusters discovered at $z > 1$, and the first known cool core cluster at $z > 1.2$. The offset of this core from the large-scale centroid suggests that this cluster has had a relatively recent ($\lesssim$500 Myr) merger/interaction with another massive system.

astro-ph.CO↗

arXiv2015

Stellar masses and star formation rates of lensed dusty star-forming galaxies from the SPT survey

To understand cosmic mass assembly in the Universe at early epochs, we primarily rely on measurements of stellar mass and star formation rate of distant galaxies. In this paper, we present stellar masses and star formation rates of six high-redshift ($2.8\leq z \leq 5.7$) dusty, star-forming galaxies (DSFGs) that are strongly gravitationally lensed by foreground galaxies. These sources were first discovered by the South Pole Telescope (SPT) at millimeter wavelengths and all have spectroscopic redshifts and robust lens models derived from ALMA observations. We have conducted follow-up observations, obtaining multi-wavelength imaging data, using {\it HST}, {\it Spitzer}, {\it Herschel} and the Atacama Pathfinder EXperiment (APEX). We use the high-resolution {\it HST}/WFC3 images to disentangle the background source from the foreground lens in {\it Spitzer}/IRAC data. The detections and upper limits provide important constraints on the spectral energy distributions (SEDs) for these DSFGs, yielding stellar masses, IR luminosities, and star formation rates (SFRs). The SED fits of six SPT sources show that the intrinsic stellar masses span a range more than one order of magnitude with a median value $\sim$ 5 $\times 10^{10}M_{\Sun}$. The intrinsic IR luminosities range from 4$\times 10^{12}L_{\Sun}$ to 4$\times 10^{13}L_{\Sun}$. They all have prodigious intrinsic star formation rates of 510 to 4800 $M_{\Sun} {\rm yr}^{-1}$. Compared to the star-forming main sequence (MS), these six DSFGs have specific SFRs that all lie above the MS, including two galaxies that are a factor of 10 higher than the MS. Our results suggest that we are witnessing the ongoing strong starburst events which may be driven by major mergers.

astro-ph.GA↗

arXiv2015

The Massive and Distant Clusters of WISE Survey: MOO J1142+1527, A 10$^{15}$ M$_\odot$ Galaxy Cluster at z=1.19

We present confirmation of the cluster MOO J1142+1527, a massive galaxy cluster discovered as part of the Massive and Distant Clusters of WISE Survey. The cluster is confirmed to lie at $z=1.19$, and using the Combined Array for Research in Millimeter-wave Astronomy we robustly detect the Sunyaev-Zel'dovich (SZ) decrement at 13.2$σ$. The SZ data imply a mass of $\mathrm{M}_{200m}=(1.1\pm0.2)\times10^{15}$ $\mathrm{M}_\odot$, making MOO J1142+1527 the most massive galaxy cluster known at $z>1.15$ and the second most massive cluster known at $z>1$. For a standard $Λ$CDM cosmology it is further expected to be one of the $\sim 5$ most massive clusters expected to exist at $z\ge1.19$ over the entire sky. Our ongoing Spitzer program targeting $\sim1750$ additional candidate clusters will identify comparably rich galaxy clusters over the full extragalactic sky.

astro-ph.CO↗

arXiv2015

The mass distribution in an assembling super galaxy group at $z=0.37$

We present a weak gravitational lensing analysis of supergroup SG1120$-$1202, consisting of four distinct X-ray-luminous groups, that will merge to form a cluster comparable in mass to Coma at $z=0$. These groups lie within a projected separation of 1 to 4 Mpc and within $Δv=550$ km s$^{-1}$ and form a unique protocluster to study the matter distribution in a coalescing system. Using high-resolution {\em HST}/ACS imaging, combined with an extensive spectroscopic and imaging data set, we study the weak gravitational distortion of background galaxy images by the matter distribution in the supergroup. We compare the reconstructed projected density field with the distribution of galaxies and hot X-ray emitting gas in the system and derive halo parameters for the individual density peaks. We show that the projected mass distribution closely follows the locations of the X-ray peaks and associated brightest group galaxies. One of the groups that lies at slightly lower redshift ($z\approx 0.35$) than the other three groups ($z\approx 0.37$) is X-ray luminous, but is barely detected in the gravitational lensing signal. The other three groups show a significant detection (up to $5 σ$ in mass), with velocity dispersions between $355^{+55}_{-70}$ and $530^{+45}_{-55}$ km s$^{-1}$ and masses between $0.8^{+0.4}_{-0.3} \times 10^{14}$ and $1.6^{+0.5}_{-0.4}\times 10^{14} h^{-1} M_{\odot}$, consistent with independent measurements. These groups are associated with peaks in the galaxy and gas density in a relatively straightforward manner. Since the groups show no visible signs of interaction, this supports the picture that we are catching the groups before they merge into a cluster.

astro-ph.CO↗

arXiv2015

RCS2 J232727.6-020437: An Efficient Cosmic Telescope at $z=0.6986$

We present a detailed gravitational lens model of the galaxy cluster RCS2 J232727.6-020437. Due to cosmological dimming of cluster members and ICL, its high redshift ($z=0.6986$) makes it ideal for studying background galaxies. Using new ACS and WFC3/IR HST data, we identify 16 multiple images. From MOSFIRE follow up, we identify a strong emission line in the spectrum of one multiple image, likely confirming the redshift of that system to $z=2.083$. With a highly magnified ($μ\gtrsim2$) source plane area of $\sim0.7$ arcmin$^2$ at $z=7$, RCS2 J232727.6-020437 has a lensing efficiency comparable to the Hubble Frontier Fields clusters. We discover four highly magnified $z\sim7$ candidate Lyman-break galaxies behind the cluster, one of which may be multiply-imaged. Correcting for magnification, we find that all four galaxies are fainter than $0.5 L_{\star}$. One candidate is detected at ${>10σ}$ in both Spitzer/IRAC [3.6] and [4.5] channels. A spectroscopic follow-up with MOSFIRE does not result in the detection of the Lyman-alpha emission line from any of the four candidates. From the MOSFIRE spectra we place median upper limits on the Lyman-alpha flux of $5-14 \times 10^{-19}\, \mathrm{erg \,\, s^{-1} cm^{-2}}$ ($5σ$).

astro-ph.GA↗

arXiv2014

Star Formation in High-Redshift Cluster Ellipticals

We measure the star formation rates (SFRs) of massive ($M_{\star}>10^{10.1}M_{\odot}$) early-type galaxies (ETGs) in a sample of 11 high-redshift ($1.0 < z < 1.5$) galaxy clusters drawn from the IRAC Shallow Cluster Survey (ISCS). We identify ETGs visually from Hubble Space Telescope imaging and select likely cluster members as having either an appropriate spectroscopic redshift or red sequence color. Mid-infrared SFRs are measured using Spitzer 24 $μ$m data for isolated cluster galaxies for which contamination by neighbors, and active galactic nuclei, can be ruled out. Cluster ETGs show enhanced specific star formation rates (sSFRs) compared to cluster galaxies in the local Universe, but have sSFRs more than four times lower than that of field ETGs at $1 < z < 1.5$. Relative to the late-type cluster population, isolated ETGs show substantially quenched mean SFRs, yet still contribute 12% of the overall star formation activity measured in $1 < z < 1.5$ clusters. We find that new ETGs are likely being formed in ISCS clusters; the fraction of cluster galaxies identified as ETGs increases from 34% to 56% from $z \sim 1.5 \rightarrow 1.25$. While the fraction of cluster ETGs that are highly star-forming ($\textrm{SFR}\geq26\ M_{\odot}$ yr$^{-1}$) drops from 27% to 10% over the same period, their sSFRs are roughly constant. All these factors taken together suggest that, particularly at $z\gtrsim1.25$, the events that created these distant cluster ETGs$-$likely mergers, at least among the most massive$-$were both recent and gas-rich.

astro-ph.GA↗

arXiv2014

The Spitzer South Pole Telescope Deep Field Survey: Linking galaxies and halos at z=1.5

We present an analysis of the clustering of high-redshift galaxies in the recently completed 94 deg$^2$ Spitzer-SPT Deep Field survey. Applying flux and color cuts to the mid-infrared photometry efficiently selects galaxies at $z\sim1.5$ in the stellar mass range $10^{10}-10^{11}M_\odot$, making this sample the largest used so far to study such a distant population. We measure the angular correlation function in different flux-limited samples at scales $>6^{\prime \prime}$ (corresponding to physical distances $>0.05$ Mpc) and thereby map the one- and two-halo contributions to the clustering. We fit halo occupation distributions and determine how the central galaxy's stellar mass and satellite occupation depend on the halo mass. We measure a prominent peak in the stellar-to-halo mass ratio at a halo mass of $\log(M_{\rm halo} / M_\odot) = 12.44\pm0.08$, 4.5 times higher than the $z=0$ value. This supports the idea of an evolving mass threshold above which star formation is quenched. We estimate the large-scale bias in the range $b_g=2-4$ and the satellite fraction to be $f_\mathrm{sat}\sim0.2$, showing a clear evolution compared to $z=0$. We also find that, above a given stellar mass limit, the fraction of galaxies that are in similar mass pairs is higher at $z=1.5$ than at $z=0$. In addition, we measure that this fraction mildly increases with the stellar mass limit at $z=1.5$, which is the opposite of the behavior seen at low-redshift.

astro-ph.GA↗

arXiv2014

The Massive and Distant Clusters of WISE Survey II: Initial Spectroscopic Confirmation of z ~ 1 Galaxy Clusters Selected from 10,000 Square Degrees

We present optical and infrared imaging and optical spectroscopy of galaxy clusters which were identified as part of an all-sky search for high-redshift galaxy clusters, the Massive and Distant Clusters of WISE Survey (MaDCoWS). The initial phase of MaDCoWS combined infrared data from the all-sky data release of the Wide-field Infrared Survey Explorer (WISE) with optical data from the Sloan Digital Sky Survey (SDSS) to select probable z ~ 1 clusters of galaxies over an area of 10,000 deg^2. Our spectroscopy confirms 19 new clusters at 0.7 < z < 1.3, half of which are at z > 1, demonstrating the viability of using WISE to identify high-redshift galaxy clusters. The next phase of MaDCoWS will use the greater depth of the AllWISE data release to identify even higher redshift cluster candidates.

astro-ph.CO↗

arXiv2014

Discovery of a Strong Lensing Galaxy Embedded in a Cluster at z = 1.62

We identify a strong lensing galaxy in the cluster IRC 0218 (also known as XMM-LSS J02182$-$05102) that is spectroscopically confirmed to be at $z=1.62$, making it the highest-redshift strong lens galaxy known. The lens is one of the two brightest cluster galaxies and lenses a background source galaxy into an arc and a counterimage. With Hubble Space Telescope (HST) grism and Keck/LRIS spectroscopy, we measure the source redshift to be $z_{\rm S}=2.26$. Using HST imaging in ACS/F475W, ACS/F814W, WFC3/F125W, and WFC3/F160W, we model the lens mass distribution with an elliptical power-law profile and account for the effects of the cluster halo and nearby galaxies. The Einstein radius is $θ_{\rm E}=0.38^{+0.02}_{-0.01}$" ($3.2_{-0.1}^{+0.2}$ kpc) and the total enclosed mass is M$_{\rm tot} (< θ_{\rm E})=1.8^{+0.2}_{-0.1}\times10^{11}~{\rm M}_{\odot}$. We estimate that the cluster environment contributes $\sim10$% of this total mass. Assuming a Chabrier IMF, the dark matter fraction within $θ_{\rm E}$ is $f_{\rm DM}^{\rm Chab} = 0.3_{-0.3}^{+0.1}$, while a Salpeter IMF is marginally inconsistent with the enclosed mass ($f_{\rm DM}^{\rm Salp} = -0.3_{-0.5}^{+0.2}$). The total magnification of the source is $μ_{\rm tot}=2.1_{-0.3}^{+0.4}$. The source has at least one bright compact region offset from the source center. Emission from Ly$α$ and [O III] are likely to probe different regions in the source.

astro-ph.GA↗

arXiv2014

Discovery of a Very Large Structure at Z=3.78

We report the discovery of a large-scale structure containing multiple protoclusters at z=3.78 in the Boötes field. The spectroscopic discovery of five galaxies at z=3.783+/-0.002 lying within 1 Mpc of one another led us to undertake a deep narrow- and broad-band imaging survey of the surrounding field. Within a comoving volume of 72x72x25 Mpc^3, we have identified 65 Lyman alpha emitter (LAE) candidates at z=3.795+/-0.015, and four additional galaxies at z_spec=3.730,3.753,3.780,3.835. The galaxy distribution within the field is highly non-uniform, exhibiting three large (~3-5x) overdensities separated by 8-14 Mpc (physical) and possibly connected by filamentary structures traced by LAEs. The observed number of LAEs in the entire field is nearly twice the average expected in field environments, based on estimates of the Lya luminosity function at these redshifts. We estimate that by z=0 the largest overdensity will grow into a cluster of mass 10^15 Msun; the two smaller overdensities will grow into clusters of mass (2-6)x10^14 Msun. The highest concentration of galaxies is located at the southern end of the image, suggesting that the current imaging may not map the true extent of the large scale structure. Finding three large protocluster candidates within a single 0.3 deg^2 field is highly unusual; expectations from theory suggest that such alignments should occur less than 2% of the time. Searching for and characterizing such structures and accurately measuring their volume space density can therefore place constraints on the theory of structure formation. Such regions can also serve as laboratories for the study of galaxy formation in dense environments.

astro-ph.CO↗

arXiv2014

Spitzer UltRa Faint SUrvey Program (SURFS UP) I: An Overview

SURFSUP is a joint Spitzer and HST Exploration Science program using 10 galaxy clusters as cosmic telescopes to study z >~ 7 galaxies at intrinsically lower luminosities, enabled by gravitational lensing, than blank field surveys of the same exposure time. Our main goal is to measure stellar masses and ages of these galaxies, which are the most likely sources of the ionizing photons that drive reionization. Accurate knowledge of the star formation density and star formation history at this epoch is necessary to determine whether these galaxies indeed reionized the universe. Determination of the stellar masses and ages requires measuring rest frame optical light, which only Spitzer can probe for sources at z >~ 7, for a large enough sample of typical galaxies. Our program consists of 550 hours of Spitzer/IRAC imaging covering 10 galaxy clusters with very well-known mass distributions, making them extremely precise cosmic telescopes. We combine our data with archival observations to obtain mosaics with ~30 hours exposure time in both 3.6$μ$m and 4.5$μ$m in the central 4 arcmin x 4 arcmin field and ~15 hours in the flanking fields. This results in 3-$σ$ sensitivity limits of ~26.6 and ~26.2AB magnitudes for the central field in the IRAC 3.6 and 4.5$μ$m bands, respectively. To illustrate the survey strategy and characteristics we introduce the sample, present the details of the data reduction and demonstrate that these data are sufficient for in-depth studies of z >~ 7 sources (using a z=9.5 galaxy behind MACSJ1149.5+2223 as an example). For the first cluster of the survey (the Bullet Cluster) we have released all high-level data mosaics and IRAC empirical PSF models. In the future we plan to release these data products for the entire survey.

astro-ph.CO↗

arXiv2014

The Galaxy Cluster Mid-Infrared Luminosity Function at 1.3<z<3.2

We present 4.5 μm luminosity functions for galaxies identified in 178 candidate galaxy clusters at 1.3 < z < 3.2. The clusters were identified as Spitzer/IRAC color-selected overdensities in the Clusters Around Radio-Loud AGN (CARLA) project, which imaged 421 powerful radio-loud AGN at z > 1.3. The luminosity functions are derived for different redshift and richness bins, and the IRAC imaging reaches depths of m*+2, allowing us to measure the faint end slopes of the luminosity functions. We find that α = -1 describes the luminosity function very well in all redshifts bins and does not evolve significantly. This provides evidence that the rate at which the low mass galaxy population grows through star formation, gets quenched and is replenished by in-falling field galaxies does not have a major net effect on the shape of the luminosity function. Our measurements for m* are consistent with passive evolution models and high formation redshifts z_f ~ 3. We find a slight trend towards fainter m* for the richest clusters, implying that the most massive clusters in our sample could contain older stellar populations, yet another example of cosmic downsizing. Modelling shows that a contribution of a star-forming population of up to 40% cannot be ruled out. This value, found from our targeted survey, is significantly lower than the values found for slightly lower redshift, z ~ 1, clusters found in wide-field surveys. The results are consistent with cosmic downsizing, as the clusters studied here were all found in the vicinity of radio-loud AGNs -- which have proven to be preferentially located in massive dark matter halos in the richest environments at high redshift -- they may therefore be older and more evolved systems than the general protocluster population.

astro-ph.CO↗

arXiv2014

A UV to Mid-IR Study of AGN Selection

We classify the spectral energy distributions (SEDs) of 431,038 sources in the 9 sq. deg Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). There are up to 17 bands of data available per source, including ultraviolet (GALEX), optical (NDWFS), near-IR (NEWFIRM), and mid-infrared (IRAC/MIPS) data, as well as spectroscopic redshifts for ~20,000 objects, primarily from the AGN and Galaxy Evolution Survey (AGES). We fit galaxy, AGN, stellar, and brown dwarf templates to the observed SEDs, which yield spectral classes for the Galactic sources and photometric redshifts and galaxy/AGN luminosities for the extragalactic sources. The photometric redshift precision of the galaxy and AGN samples are sigma/(1+z)=0.040 and sigma/(1+z)=0.169, respectively, with the worst 5% outliers excluded. Based on the reduced chi-squared of the SED fit for each SED model, we are able to distinguish between Galactic and extragalactic sources for sources brighter than I=23.5. We compare the SED fits for a galaxy-only model and a galaxy+AGN model. Using known X-ray and spectroscopic AGN samples, we confirm that SED fitting can be successfully used as a method to identify large populations of AGN, including spatially resolved AGN with significant contributions from the host galaxy and objects with the emission line ratios of "composite" spectra. We also use our results to compare to the X-ray, mid-IR, optical color and emission line ratio selection techniques. For an F-ratio threshold of F>10 we find 16,266 AGN candidates brighter than I=23.5 and a surface density of ~1900 AGN per deg^2.

astro-ph.GA↗

arXiv2014

Evidence for a constant IMF in early-type galaxies based on their X-ray binary populations

A number of recent studies have proposed that the stellar initial mass function (IMF) of early type galaxies varies systematically as a function of galaxy mass, with higher mass galaxies having bottom heavy IMFs. These bottom heavy IMFs have more low-mass stars relative to the number of high mass stars, and therefore naturally result in proportionally fewer neutron stars and black holes. In this paper, we specifically predict the variation in the number of black holes and neutron stars based on the power-law IMF variation required to reproduce the observed mass-to-light ratio trends with galaxy mass. We then test whether such variations are observed by studying the field low-mass X-ray binary populations (LMXBs) of nearby early-type galaxies. In these binaries, a neutron star or black hole accretes matter from a low-mass donor star. Their number is therefore expected to scale with the number of black holes and neutron stars present in a galaxy. We find that the number of LMXBs per K-band light is similar among the galaxies in our sample. These data therefore demonstrate the uniformity of the slope of the IMF from massive stars down to those now dominating the K-band light, and are consistent with an invariant IMF. Our results are inconsistent with an IMF which varies from a Kroupa/Chabrier like IMF for low mass galaxies to a steep power-law IMF (with slope $x$=2.8) for high mass galaxies. We discuss how these observations constrain the possible forms of the IMF variations and how future Chandra observations can enable sharper tests of the IMF.

astro-ph.GA↗

arXiv2013

PyGFit: A Tool for Extracting PSF Matched Photometry

We present PyGFit, a program designed to measure PSF-matched photometry from images with disparate pixel scales and PSF sizes. While PyGFit has a number of uses, its primary purpose is to extract robust spectral energy distributions (SEDs) from crowded images. It does this by fitting blended sources in crowded, low resolution images with models generated from a higher resolution image. This approach minimizes the impact of crowding and also yields consistently measured fluxes in different filters, minimizing systematic uncertainty in the final SEDs. We present an example of applying PyGFit to real data and perform simulations to test its fidelity. The uncertainty in the best-fit flux rises sharply as a function of nearest-neighbor distance for objects with a neighbor within 60% of the PSF size. Similarly, the uncertainty increases quickly for objects blended with a neighbor more than four times brighter. For all other objects the fidelity of PyGFit's results depends only on flux, and the uncertainty is primarily limited by sky noise.

astro-ph.IM↗