arXiv subjects

Louis Abramson

Publications and source records attributed to Louis Abramson.

12 recordsLinked to original sources

arXiv2021

High-resolution imaging follow-up of doubly imaged quasars

We report upon three years of follow-up and confirmation of doubly imaged quasar lenses through imaging campaigns from 2016-2018 with the Near-Infrared Camera2 (NIRC2) on the W. M. Keck Observatory. A sample of 57 quasar lens candidates are imaged in adaptive-optics-assisted or seeing-limited $K^\prime$-band observations. Out of these 57 candidates, 15 are confirmed as lenses. We form a sample of 20 lenses adding in a number of previously-known lenses that were imaged with NIRC2 in 2013-14 as part of a pilot study. By modelling these 20 lenses, we obtain $K^\prime$-band relative photometry and astrometry of the quasar images and the lens galaxy. We also provide the lens properties and predicted time delays to aid planning of follow-up observations necessary for various astrophysical applications, e.g., spectroscopic follow-up to obtain the deflector redshifts for the newly confirmed systems. We compare the departure of the observed flux ratios from the smooth-model predictions between doubly and quadruply imaged quasar systems. We find that the departure is consistent between these two types of lenses if the modelling uncertainty is comparable.

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arXiv2021

Dynamical Modeling of the CIV Broad Line Region of the $z=2.805$ Multiply Imaged Quasar SDSS J2222+2745

We present CIV BLR modeling results for the multiply imaged $z=2.805$ quasar SDSS J2222+2745. Using data covering a 5.3 year baseline after accounting for gravitational time delays, we find models that can reproduce the observed emission-line spectra and integrated CIV fluctuations. The models suggest a thick disk BLR that is inclined by $\sim$40 degrees to the observer's line of sight and with a emissivity weighted median radius of $r_{\rm median} = 33.0^{+2.4}_{-2.1}$ light days. The kinematics are dominated by near-circular Keplerian motion with the remainder inflowing. The rest-frame lag one would measure from the models is $τ_{\rm median} = 36.4^{+1.8}_{-1.8}$ days, which is consistent with measurements based on cross-correlation. We show a possible geometry and transfer function based on the model fits and find that the model-produced velocity-resolved lags are consistent with those from cross-correlation. We measure a black hole mass of $\log_{10}(M_{\rm BH}/M_\odot) = 8.31^{+0.07}_{-0.06}$, which requires a scale factor of $\log_{10}(f_{{\rm mean},σ}) = 0.20^{+0.09}_{-0.07}$.

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arXiv2020

The black hole mass of the $z=2.805$ multiply imaged quasar SDSS J2222+2745 from velocity-resolved time lags of the CIV emission line

We present the first results of a 4.5 year monitoring campaign of the three bright images of multiply imaged $z=2.805$ quasar SDSS J2222+2745 using the Gemini North Multi-Object Spectrograph (GMOS-N) and the Nordic Optical Telescope (NOT). We take advantage of gravitational time delays to construct light curves surpassing 6 years in duration and achieve average spectroscopic cadence of 10 days during the 8 months of visibility per season. Using multiple secondary calibrators and advanced reduction techniques, we achieve percent-level spectrophotometric precision and carry out an unprecedented reverberation mapping analysis, measuring both integrated and velocity-resolved time lags for CIV. The full line lags the continuum by $τ_{\rm cen} = 36.5^{+2.9}_{-3.9}$ rest-frame days. We combine our measurement with published CIV lags and derive the $r_{\rm BLR}-L$ relationship $\log_{10}( τ/ {\rm day}) = (1.00\pm 0.08) + (0.48\pm 0.04) \log_{10}[λL_λ(1350{Å})/10^{44}~{\rm erg ~s}^{-1}]$ with 0.32$\pm$0.06 dex intrinsic scatter. The velocity-resolved lags are consistent with circular Keplerian orbits, with $τ_{\rm cen} = 86.2^{+4.5}_{-5.0}$, $25^{+11}_{-15}$, and $7.5^{+4.2}_{-3.5}$ rest-frame days for the core, blue wing, and red wing, respectively. Using $σ_{\rm line}$ with the mean spectrum and assuming $\log_{10} (f_{{\rm mean},σ}) = 0.52 \pm 0.26$, we derive $\log_{10}(M_{\rm BH}/M_{\odot}) = 8.63 \pm 0.27$. Given the quality of the data, this system represents a unique benchmark for calibration of $M_{\rm BH}$ estimators at high redshift. Future work will present dynamical modeling of the data to constrain the virial factor $f$ and $M_{\rm BH}$.

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arXiv2019

Inferences on the Timeline of Reionization at z~8 From the KMOS Lens-Amplified Spectroscopic Survey

Detections and non-detections of Lyman alpha (Ly$α$) emission from $z>6$ galaxies ($<1$ Gyr after the Big Bang) can be used to measure the timeline of cosmic reionization. Of key interest to measuring reionization's mid-stages, but also increasing observational challenge, are observations at z > 7, where Ly$α$ redshifts to near infra-red wavelengths. Here we present a search for z > 7.2 Ly$α$ emission in 53 intrinsically faint Lyman Break Galaxy candidates, gravitationally lensed by massive galaxy clusters, in the KMOS Lens-Amplified Spectroscopic Survey (KLASS). With integration times of ~7-10 hours, we detect no Ly$α$ emission with S/N>5 in our sample. We determine our observations to be 80% complete for 5$σ$ spatially and spectrally unresolved emission lines with integrated line flux $>5.7\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$. We define a photometrically selected sub-sample of 29 targets at $z=7.9\pm0.6$, with a median 5$σ$ Ly$α$ EW limit of 58A. We perform a Bayesian inference of the average intergalactic medium (IGM) neutral hydrogen fraction using their spectra. Our inference accounts for the wavelength sensitivity and incomplete redshift coverage of our observations, and the photometric redshift probability distribution of each target. These observations, combined with samples from the literature, enable us to place a lower limit on the average IGM neutral hydrogen fraction of $> 0.76 \; (68\%), \; > 0.46 \; (95\%)$ at z ~ 8, providing further evidence of rapid reionization at z~6-8. We show that this is consistent with reionization history models extending the galaxy luminosity function to $M_\textrm{UV} \lesssim -12$, with low ionizing photon escape fractions, $f_\textrm{esc} \lesssim 15\%$.

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arXiv2017

The Grism lens-amplified survey from space (GLASS). VIII. The influence of the cluster properties on Halpha emitter galaxies at 0.3<z<0.7

Exploiting the data of the Grism Lens-Amplified Survey from Space (GLASS), we characterize the spatial distribution of star formation in 76 high star forming galaxies in 10 clusters at 0.3< z <0.7. All these galaxies are likely restricted to first infall. In a companion paper we contrast the properties of field and cluster galaxies, whereas here we correlate the properties of Hα emitters to a number of tracers of the cluster environment to investigate its role in driving galaxy transformations. Hα emitters are found in the clusters out to 0.5 virial radii, the maximum radius covered by GLASS. The peak of the Hα emission is offset with respect to the peak of the UV-continuum. We decompose this offsets into a radial and tangential component. The radial compo- nent points away from the cluster center in 60% of the cases, with 95% confidence. The decompositions agree with cosmological simulations, i.e. the Hα emission offset correlates with galaxy velocity and ram-pressure stripping signatures. Trends between Hα emitter properties and surface mass density distributions and X-ray emissions emerge only for unrelaxed clusters. The lack of strong correlations with the global environment does not allow us to identify a unique environmental effect originating from the cluster center. In contrast, correla- tions between Hα morphology and local number density emerge. We conclude that local effects, uncorrelated to the cluster-centric radius, play a more important role in shaping galaxy properties.

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arXiv2016

The Grism lens-amplified survey from space (GLASS). VII. The diversity of the distribution of star formation in cluster and field galaxies at 0.3<z<0.7

Exploiting the slitless spectroscopy taken as part of the Grism Lens-Amplified Survey from Space (GLASS), we present an extended analysis of the spatial distribution of star formation in 76 galaxies in 10 clusters at 0.3< z <0.7. We use 85 foreground and background galaxies in the same redshift range as a field sample. The samples are well matched in stellar mass (10^8-10^11 M_sun) and star formation rate (0.5-50 M_sun/yr). We visually classify galaxies in terms of broad-band morphology, Halpha morphology and likely physical process acting on the galaxy. Most Halpha emitters have a spiral morphology (41+/-8% in clusters, 51+/-8% in the field), followed by mergers/interactions (28+/-8%, 31+/-7%, respectively) and early-type galaxies (remarkably as high as 29+/-8% in clusters and 15+/-6% in the field). A diversity of Halpha morphologies is detected, suggesting a diversity of physical processes. In clusters, 30+/-8% of the galaxies present a regular morphology, mostly consistent with star formation diffused uniformly across the stellar population (mostly in the disk component, when present). The second most common morphology (28+/-8%) is asymmetric/jellyfish, consistent with ram pressure stripping or other non-gravitational processes in 18+/-8% of the cases. Ram pressure stripping appears significantly less prominent in the field (2+/-2%), where the most common morphology/mechanism appears to be consistent with minor gas rich mergers or clump accretion. This work demonstrates that while environment specific mechanisms affect galaxy evolution at this redshift, they are diverse and their effects subtle. A full understanding of this complexity requires larger samples and detailed and spatially resolved physical models.

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arXiv2015

The Grism Lens-Amplified Survey from Space (GLASS). V. Extent and spatial distribution of star formation in z~0.5 cluster galaxies

We present the first study of the spatial distribution of star formation in z~0.5 cluster galaxies. The analysis is based on data taken with the Wide Field Camera 3 as part of the Grism Lens-Amplified Survey from Space (GLASS). We illustrate the methodology by focusing on two clusters (MACS0717.5+3745 and MACS1423.8+2404) with different morphologies (one relaxed and one merging) and use foreground and background galaxies as field control sample. The cluster+field sample consists of 42 galaxies with stellar masses in the range 10^8-10^11 M_sun, and star formation rates in the range 1-20 M_sun/yr. Both in clusters and in the field, Hα is more extended than the rest-frame UV continuum in 60% of the cases, consistent with diffuse star formation and inside out growth. In ~20% of the cases, the Hα emission appears more extended in cluster galaxies than in the field, pointing perhaps to ionized gas being stripped and/or star formation being enhanced at large radii. The peak of the Hα emission and that of the continuum are offset by less than 1 kpc. We investigate trends with the hot gas density as traced by the X-ray emission, and with the surface mass density as inferred from gravitational lens models and find no conclusive results. The diversity of morphologies and sizes observed in H_alpha illustrates the complexity of the environmental process that regulate star formation. Upcoming analysis of the full GLASS dataset will increase our sample size by almost an order of magnitude, verifying and strengthening the inference from this initial dataset.

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arXiv2013

The IMACS Cluster Building Survey: II. Spectral Evolution of Galaxies in the Epoch of Cluster Assembly

The IMACS Cluster Building Survey (ICBS) provides spectra of ~2200 galaxies 0.31<z<0.54 in 5 rich clusters (R <= 5 Mpc) and the field. Infalling, dynamically cold groups with tens of members account for approximately half of the supercluster population, contributing to a growth in cluster mass of ~100% by today. The ICBS spectra distinguish non-starforming (PAS) and poststarburst (PSB) from starforming galaxies -- continuously starforming (CSF) or starbursts, (SBH or SBO), identified by anomalously strong H-delta absorption or [O II] emission. For the infalling cluster groups and similar field groups, we find a correlation between PAS+PSB fraction and group mass, indicating substantial "preprocessing" through quenching mechanisms that can turn starforming galaxies into passive galaxies without the unique environment of rich clusters. SBH + SBO starburst galaxies are common, and they maintain an approximately constant ratio (SBH+SBO)/CSF ~ 25% in all environments -- from field, to groups, to rich clusters. Similarly, while PSB galaxies strongly favor denser environments, PSB/PAS ~ 10-20% for all environments. This result, and their timescale tau < 500 Myr, indicates that starbursts are not signatures of a quenching mechanism that produces the majority of passive galaxies. We suggest instead that starbursts and poststarbursts signal minor mergers and accretions, in starforming and passive galaxies, respectively, and that the principal mechanisms for producing passive systems are (1) early major mergers, for elliptical galaxies, and (2) later, less violent processes -- such as starvation and tidal stripping, for S0 galaxies.

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arXiv2013

The IMACS Cluster Building Survey. III. The Star Formation Histories of Field Galaxies

Using data from the IMACS Cluster Building Survey and from nearby galaxy surveys, we examine the evolution of the rate of star formation in field galaxies from $ z = 0.60$ to the present. Fitting the luminosity function to a standard Schechter form, we find a rapid evolution of $M_B^*$ consistent with that found in other deep surveys; at the present epoch $M_B^*$ is evolving at the rate of $0.38 Gyr^{-1}$, several times faster than the predictions of simple models for the evolution of old, coeval galaxies. The evolution of the distribution of specific star formation rates (SSFR) is also too rapid to explain by such models. We demonstrate that starbursts cannot, even in principle, explain the evolution of the SSFR distribution. However, the rapid evolution of both $M_B^*$ and the SSFR distribution can be explained if some fraction of galaxies have star formation rates characterized by both short rise and fall times and by an epoch of peak star formation more recent than the majority of galaxies. Although galaxies of every stellar mass up to $1.4\times10^{11}\Msun$ show a range of epochs of peak star formation, the fraction of "younger" galaxies falls from about 40% at a mass of $4\times10^{10}\Msun$ to zero at a mass of $1.4\times10^{11}\Msun$. The incidence of younger galaxies appears to be insensitive to the density of the local environment; but does depend on group membership: relatively isolated galaxies are much more likely to be young than are group members.

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arXiv2013

The IMACS Cluster Building Survey: IV. The Log-normal Star Formation History of Galaxies

We present here a simple model for the star formation history of galaxies that is successful in describing both the star formation rate density over cosmic time, as well as the distribution of specific star formation rates of galaxies at the current epoch, and the evolution of this quantity in galaxy populations to a redshift of z=1. We show first that the cosmic star formation rate density is remarkably well described by a simple log-normal in time. We next postulate that this functional form for the ensemble is also a reasonable description for the star formation histories of individual galaxies. Using the measured specific star formation rates for galaxies at z~0 from Paper III in this series, we then construct a realisation of a universe populated by such galaxies in which the parameters of the log-normal star formation history of each galaxy are adjusted to match the specific star formation rates at z~0 as well as fitting, in ensemble, the cosmic star formation rate density from z=0 to z=8. This model predicts, with striking fidelity, the distribution of specific star formation rates in mass-limited galaxy samples to z=1; this match is not achieved by other models with a different functional form for the star formation histories of individual galaxies, but with the same number of degrees of freedom, suggesting that the log-normal form is well matched to the likely actual histories of individual galaxies. We also impose the specific star formation rate versus mass distributions at higher redshifts from Paper III as constraints on the model, and show that, as previously suggested, some galaxies in the field, particularly low mass galaxies, are quite young at intermediate redshifts. As emphasized in Paper III, starbursts are insufficient ...[abridged]

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arXiv2013

The galaxy stellar mass function and its evolution with time show no dependence on global environment

We present the analysis of the galaxy stellar mass function in different environments at intermediate redshift (0.3 10^(10.5) M_sun, to study cluster, group, and field galaxies at z=0.3-0.45, and the ESO Distant Cluster Survey (EDisCS), at masses M_ast > 10^(10.2) M_sun, to investigate cluster and group galaxies at z=0.4-0.8. Therefore, in our analysis we include galaxies that are slightly less massive than the Milky Way. Having excluded the brightest cluster galaxies, we show thatthe shape of the mass distribution does not seem to depend on global environment. Our two main results are: (1) Galaxies in the virialized regions of clusters, in groups, and in the field follow a similar mass distribution. (2) Comparing both ICBS and EDisCS mass functions to mass functions in the local Universe, we find evolution from z~0.4-0.6 to z~0.07. The population of low-mass galaxies has proportionally grown with time with respect to that of massive galaxies. This evolution is independent of environment -- the same for clusters and the field. Furthermore, considering only clusters, we find that no differences can be detected neither within the virialized regions, nor when we compare galaxies within and outside the virial radius. Subdividing galaxies in terms of color, in clusters, groups, and field red and blue galaxies are regulated by different mass functions, but comparing separately the blue and red mass functions in different environments, no differences are detected in their shape.

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arXiv2008

Where have all the bulges gone?

Several recent studies indicate that bulges are more complex than merely structureless relaxed stellar systems. We study the HST images of a sample of 130 nearby early type (S0-Sab) disc galaxies and detect pure structureless bulges with the Sersic index n > 2 for only 12% of the galaxies. Other galaxies show varied substructure in their inner regions (inner bars, inner spiral arms, inner rings) and sometimes contain no bulge at all. Inner substructure is more common for these galaxies, which also display structure at larger scales.

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