arXiv subjects

S. H. Cohen

Publications and source records attributed to S. H. Cohen.

At least 19 recordsLinked to original sources

arXiv2024

JWST's PEARLS: resolved study of the stellar and dust components in starburst galaxies at cosmic noon

Dusty star-forming galaxies (DSFGs) contribute significantly to the stellar buildup at cosmic noon. Major mergers and gas accretion are often invoked to explain DSFGs' prodigious star-formation rates (SFRs) and large stellar masses. We conducted a spatially-resolved morphological analysis of the rest-frame UV/NIR emission in three DSFGs at z~2.5. Initially discovered as CO emitters by NOEMA observations of a bright Herschel source, we observed them with the JWST/NIRCam as part of the PEARLS program. The NIRCam data reveal the galaxies' stellar populations and dust distributions on scales of 250 pc. Spatial variations in stellar mass, SFR, and dust extinction are determined in resolved maps obtained through pixel-based SED fitting. The CO emitters are massive, dusty starburst galaxies with SFRs=340-2500 Msun/yr, positioning them among the most active SFGs at 2<z<3. They belong to the ~1.5% of the entire JWST population with extremely red colors. Their morphologies are disk like, with radii of 2.0-4.4 kpc, and exhibit substructures such as clumps and spiral arms. The galaxies have dust extinctions up to Av=5-7 mag extending over several kpc with asymmetric distributions that include off-center regions resembling bent spiral arms and clumps. Their NIR dust-attenuation curve deviates from standard laws, possibly implying different dust-star geometries or dust grain properties than commonly assumed in starburst galaxies. The proximity of galaxies with consistent redshifts, strong color gradients, an overall disturbed appearance, asymmetric dust obscuration, and widespread star formation collectively favor interactions (minor mergers and flybys) as the mechanism driving the CO galaxies' exceptional SFRs. The galaxies' large masses and rich environment hint at membership in two proto-structures, as initially inferred from their association with a Planck-selected high-z source.

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arXiv2024

JWST Photometric Time-Delay and Magnification Measurements for the Triply-Imaged Type Ia "Supernova H0pe" at z = 1.78

Supernova (SN) H0pe is a gravitationally lensed, triply-imaged, Type Ia SN (SN Ia) discovered in James Webb Space Telescope imaging of the PLCK G165.7+67.0 cluster of galaxies. Well-observed multiply-imaged SNe provide a rare opportunity to constrain the Hubble constant ($H_0$), by measuring the relative time delay between the images and modeling the foreground mass distribution. SN H0pe is located at $z=1.783$, and is the first SN Ia with sufficient light curve sampling and long enough time delays for an $H_0$ inference. Here we present photometric time-delay measurements and SN properties of SN H0pe. Using JWST/NIRCam photometry we measure time delays of $Δt_{ab}=-116.6^{+10.8}_{-9.3}$ and $Δt_{cb}=-48.6^{+3.6}_{-4.0}$ observer-frame days relative to the last image to arrive (image 2b; all uncertainties are $1σ$), which corresponds to a $\sim5.6\%$ uncertainty contribution for $H_0$ assuming $70 \rm{km s^{-1} Mpc^{-1}}$. We also constrain the absolute magnification of each image to $μ_{a}=4.3^{+1.6}_{-1.8}$, $μ_{b}=7.6^{+3.6}_{-2.6}$, $μ_{c}=6.4^{+1.6}_{-1.5}$ by comparing the observed peak near-IR magnitude of SN H0pe to the non-lensed population of SNe Ia.

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arXiv2024

Lensed Type Ia Supernova "Encore" at z=2: The First Instance of Two Multiply-Imaged Supernovae in the Same Host Galaxy

A bright ($m_{\rm F150W,AB}$=24 mag), $z=1.95$ supernova (SN) candidate was discovered in JWST/NIRCam imaging acquired on 2023 November 17. The SN is quintuply-imaged as a result of strong gravitational lensing by a foreground galaxy cluster, detected in three locations, and remarkably is the second lensed SN found in the same host galaxy. The previous lensed SN was called "Requiem", and therefore the new SN is named "Encore". This makes the MACS J0138.0$-$2155 cluster the first known system to produce more than one multiply-imaged SN. Moreover, both SN Requiem and SN Encore are Type Ia SNe (SNe Ia), making this the most distant case of a galaxy hosting two SNe Ia. Using parametric host fitting, we determine the probability of detecting two SNe Ia in this host galaxy over a $\sim10$ year window to be $\approx3\%$. These observations have the potential to yield a Hubble Constant ($H_0$) measurement with $\sim10\%$ precision, only the third lensed SN capable of such a result, using the three visible images of the SN. Both SN Requiem and SN Encore have a fourth image that is expected to appear within a few years of $\sim2030$, providing an unprecedented baseline for time-delay cosmography.

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arXiv2023

PEARLS: JWST counterparts of micro-Jy radio sources in the Time Domain Field

The Time Domain Field (TDF) near the North Ecliptic Pole in JWST's continuous-viewing zone will become a premier "blank field" for extragalactic science. JWST/NIRCam data in a 16 arcmin$^2$ portion of the TDF identify 4.4 $μ$m counterparts for 62 of 63 3 GHz sources with S(3 GHz) > 5 μJy. The one unidentified radio source may be a lobe of a nearby Seyfert galaxy, or it may be an infrared-faint radio source. The bulk properties of the radio-host galaxies are consistent with those found by previous work: redshifts range from 0.14 to 4.4 with a median redshift of 1.33. The radio emission arises primarily from star formation in $\sim 2/3$ of the sample and from an active galactic nucleus in $\sim 1/3$, but just over half the sample shows evidence for an AGN either in the spectral energy distribution or by radio excess. All but three counterparts are brighter than magnitude 23 AB at 4.4 $μ$m, and the exquisite resolution of JWST identifies correct counterparts for sources for which observations with lower angular resolution would mis-identify a nearby bright source as the counterpart when the correct one is faint and red. Up to 11% of counterparts might have been unidentified or misidentified absent NIRCam observations.

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arXiv2023

Spectroscopy of the Supernova H0pe Host Galaxy at Redshift 1.78

Supernova (SN) H0pe was discovered as a new transient in James Webb Space Telescope (JWST) NIRCam images of the galaxy cluster PLCK G165.7+67.0 taken as part of the "Prime Extragalactic Areas for Reionization and Lensing Science" (PEARLS) JWST GTO program (# 1176) on 2023 March 30 (AstroNote 2023-96; Frye et al. 2023). The transient is a compact source associated with a background galaxy that is stretched and triply-imaged by the cluster's strong gravitational lensing. This paper reports spectra in the 950-1370 nm observer frame of two of the galaxy's images obtained with Large Binocular Telescope (LBT) Utility Camera in the Infrared (LUCI) in longslit mode two weeks after the \JWST\ observations. The individual average spectra show the [OII] doublet and the Balmer and 4000 Angstrom breaks at redshift z=1.783+/-0.002. The CIGALE best-fit model of the spectral energy distribution indicates that SN H0pe's host galaxy is massive (Mstar~6x10^10 Msun after correcting for a magnification factor ~7) with a predominant intermediate age (~2 Gyr) stellar population, moderate extinction, and a magnification-corrected star formation rate ~13 Msun/yr, consistent with being below the main sequence of star formation. These properties suggest that H0pe might be a type Ia SN. Additional observations of SN H0pe and its host recently carried out with JWST (JWST-DD-4446; PI: B. Frye) will be able to both determine the SN classification and confirm its association with the galaxy analyzed in this work.

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arXiv2021

The NuSTAR Extragalactic Survey of the James Webb Space Telescope North Ecliptic Pole Time-Domain Field

We present the $NuSTAR$ extragalactic survey of the $James$ $Webb$ $Space$ $Telescope$ ($JWST$) North Ecliptic Pole (NEP) Time-Domain Field. The survey covers a $\sim$0.16 deg$^2$ area with a total exposure of 681 ks acquired in a total of nine observations from three epochs. The survey sensitivities at 20% of the area are 2.39, 1.14, 2.76, 1.52, and 5.20 $\times$ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the 3-24, 3-8, 8-24, 8-16, and 16-24 keV bands, respectively. The NEP survey is one of the most sensitive extragalactic surveys with $NuSTAR$ so far. A total of 33 sources were detected above 95% reliability in at least one of the five bands. We present the number counts, log$N$-log$S$, measured in the hard X-ray 8-24 and 8-16 keV bands, uniquely accessible by $NuSTAR$ down to such faint fluxes. We performed source detection on the XMM-$Newton$ and $Chandra$ observations of the same field to search for soft X-ray counterparts of each $NuSTAR$ detection. The soft band positions were used to identify optical and infrared associations. We present the X-ray properties (hardness ratio and luminosity) and optical-to-X-ray properties of the detected sources. The measured fraction of candidate Compton-thick (N$\rm _H\ge10^{24} cm^{-2}$) active galactic nuclei, derived from the hardness ratio, is between 3% to 27%. As this survey was designed to have variability as its primary focus, we present preliminary results on multi-epoch flux variability in the 3-24 keV band.

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arXiv2017

The Lyman Continuum escape fraction of faint galaxies at z~3.3 in the CANDELS/GOODS-North, EGS, and COSMOS fields with LBC

The reionization of the Universe is one of the most important topics of present day astrophysical research. The most plausible candidates for the reionization process are star-forming galaxies, which according to the predictions of the majority of the theoretical and semi-analytical models should dominate the HI ionizing background at z~3. We aim at measuring the Lyman continuum escape fraction, which is one of the key parameters to compute the contribution of star-forming galaxies to the UV background. We have used ultra-deep U-band imaging (U=30.2mag at 1sigma) by LBC/LBT in the CANDELS/GOODS-North field, as well as deep imaging in COSMOS and EGS fields, in order to estimate the Lyman continuum escape fraction of 69 star-forming galaxies with secure spectroscopic redshifts at 3.27 L*), while for the faint population (L=0.2L*) the limit to the escape fraction is ~10%. We have computed the contribution of star-forming galaxies to the observed UV background at z~3 and we have found that it is not enough to keep the Universe ionized at these redshifts, unless their escape fraction increases significantly (>10%) at low luminosities (M1500>-19). We compare our results on the Lyman continuum escape fraction of high-z galaxies with recent estimates in the literature and discuss future prospects to shed light on the end of the Dark Ages. In the future, strong gravitational lensing will be fundamental to measure the Lyman continuum escape fraction down to faint magnitudes (M1500~-16) which are inaccessible with the present instrumentation on blank fields.

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arXiv2016

Do the Most Massive Black Holes at $z=2$ Grow via Major Mergers?

The most frequently proposed model for the origin of quasars holds that the high accretion rates seen in luminous active galactic nuclei are primarily triggered during major mergers between gas-rich galaxies. While plausible for decades, this model has only begun to be tested with statistical rigor in the past few years. Here we report on a Hubble Space Telescope study to test this hypothesis for $z=2$ quasars with high super-massive black hole masses ($M_\mathrm{BH}=10^9-10^{10}~M_\odot{}$), which dominate cosmic black hole growth at this redshift. We compare Wide Field Camera 3 $F160W$ (rest-frame $V$-band) imaging of 19 point source-subtracted quasar hosts to a matched sample of 84 inactive galaxies, testing whether the quasar hosts have greater evidence for strong gravitational interactions. Using an expert ranking procedure, we find that the quasar hosts are uniformly distributed within the merger sequence of inactive galaxies, with no preference for quasars in high-distortion hosts. Using a merger/non-merger cutoff approach, we recover distortion fractions of $f_\mathrm{m,qso}=0.39\pm{}0.11$ for quasar hosts and $f_\mathrm{m,gal}=0.30\pm{}0.05$ for inactive galaxies (distribution modes, 68% confidence intervals), with both measurements subjected to the same observational conditions and limitations. The slight enhancement in distorted fraction for quasar hosts over inactive galaxies is not significant, with a probability that the quasar fraction is higher of $P(f_\mathrm{m,qso}>f_\mathrm{m,gal}) = 0.78$ ($0.78\,σ$), in line with results for lower mass and lower $z$ AGN. We find no evidence that major mergers are the primary triggering mechanism for the massive quasars that dominate accretion at the peak of cosmic quasar activity.

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arXiv2014

Early-type galaxies at intermediate redshift observed with HST WFC3: perspectives on recent star-formation

We present an analysis of the stellar populations of 102 visually-selected early-type galaxies (ETGs) with spectroscopic redshifts (0.3 10.5) recently star-forming ETGs appear to have larger sizes. Furthermore, high-mass, quiescent ETGs identified with likely companions populate a distinct region in the size-mass parameter space, in comparison with the distribution of massive ETGs with evidence of RSF. We conclude that both mechanisms of the quenching of star-formation in disk-like ETGs and (gas-rich, minor) merger activity contribute to the formation of young stars and the size-mass evolution of intermediate redshift ETGs. The number of ETGs for which we have both HST WFC3 panchromatic (especially UV) imaging and spectroscopically-confirmed redshifts is relatively small, therefore a conclusion on the relative roles of both of these mechanisms remains an open question.

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arXiv2014

Measuring galaxy morphology at $z>1$. I - calibration of automated proxies

[abridged] New near-infrared surveys, using the HST, offer an unprecedented opportunity to study rest-frame optical galaxy morphologies at z>1 and to calibrate automated morphological parameters that will play a key role in classifying future massive datasets like EUCLID or LSST. We study automated parameters (e.g. CAS, Gini, M20) of massive galaxies at 1 1: bulge-dominated systems have systematically higher concentration and Gini coefficients and are less asymmetric and rounder than disk-dominated galaxies. However, at z>1, galaxies are, on average, 50% more asymmetric and have Gini and M20 values that are 10% higher and 20% lower respectively. In bulge-dominated galaxies, morphological parameters derived from the rest-frame UV and optical wavelengths are well correlated; however late-type galaxies exhibit higher asymmetry and clumpiness when measured in the rest-frame UV. We find that broad morphological classes (e.g. bulge vs. disk dominated) can be distinguished using parameters with high (80%) purity and completeness of 80%. In a similar vein, irregular disks and mergers can also be distinguished from bulges and regular disks with a contamination lower than 20%. However, mergers cannot be differentiated from the irregular morphological class using these parameters, due to increasingly asymmetry of non-interacting late-type galaxies at z>1. Our automated procedure is applied to the CANDELS GOODS-S field and compared with the visual classification recently released on the same area getting similar results.

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arXiv2013

Stellar Populations of Lyman Break Galaxies at z=1-3 in the HST/WFC3 Early Release Science Observations

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z=1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 sq. arcmin in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z=1-3 are selected using dropout selection criteria similar to high redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout selected LBGs is accurate to within few percent; (2) the UV spectral slope (beta) is redder than at high redshift (z>3), where LBGs are less dusty; (3) on average, LBGs at z=1-3 are massive, dustier and more highly star-forming, compared to LBGs at higher redshifts with similar luminosities (0.1L*<~L<~2.5L*), though their median values are similar within 1-sigma uncertainties. This could imply that identical dropout selection technique, at all redshifts, find physically similar galaxies; and (4) stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of ~0.46, and star-formation rates are proportional to their stellar masses with a logarithmic slope of ~0.90. These relations hold true --- within luminosities probed in this study --- for LBGs from z~1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z~2, but to avoid any selection biases, and for direct comparison with LBGs at z>3, a true Lyman break selection at z~2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties, and their evolution.

astro-ph.CO↗

arXiv2012

A Panchromatic Catalog of Early-Type Galaxies at Intermediate Redshift in the Hubble Space Telescope Wide Field Camera 3 Early Release Science Field

In the first of a series of forthcoming publications, we present a panchromatic catalog of 102 visually-selected early-type galaxies (ETGs) from observations in the Early Release Science (ERS) program with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) of the Great Observatories Origins Deep Survey-South (GOODS-S) field. Our ETGs span a large redshift range, 0.35 < z < 1.5, with each redshift spectroscopically-confirmed by previous published surveys of the ERS field. We combine our measured WFC3 ERS and ACS GOODS-S photometry to gain continuous sensitivity from the rest-frame far-UV to near-IR emission for each ETG. The superior spatial resolution of the HST over this panchromatic baseline allows us to classify the ETGs by their small-scale internal structures, as well as their local environment. By fitting stellar population spectral templates to the broad-band photometry of the ETGs, we determine that the average masses of the ETGs are comparable to the characteristic stellar mass of massive galaxies, 11< log(M [Solar]) < 12. By transforming the observed photometry into the GALEX FUV and NUV, Johnson V, and SDSS g' and r' bandpasses we identify a noteworthy diversity in the rest-frame UV-optical colors and find the mean rest-frame (FUV-V)=3.5 and (NUV-V)=3.3, with 1$σ$ standard deviations approximately equal to 1.0. The blue rest-frame UV-optical colors observed for most of the ETGs are evidence for star-formation during the preceding gigayear, but no systems exhibit UV-optical photometry consistent with major recent (<~50 Myr) starbursts. Future publications which address the diversity of stellar populations likely to be present in these ETGs, and the potential mechanisms by which recent star-formation episodes are activated, are discussed.

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arXiv2011

Hubble Space Telescope Observations of Field Ultracool Dwarfs at High Galactic Latitude

We present a sample of 17 newly discovered ultracool dwarf candidates later than ~M8, drawn from 231.90 arcmin2 of {\it Hubble Space Telescope} Wide Field Camera 3 infrared imaging. By comparing the observed number counts for 17.5<J_125<25.5 AB mag to an exponential disk model, we estimate a vertical scale height of z_scl=290 +- 25 (random) +- 30 (systematic) pc for a binarity fraction of f_b=0. While our estimate is roughly consistent with published results, we suggest that the differences can be attributed to sample properties, with the present sample containing far more substellar objects than previous work. We predict the object counts should peak at J_{125}~24 AB mag due to the exponentially-declining number density at the edge of the disc. We conclude by arguing that trend in scale height with spectral type may breakdown for brown dwarfs since they do not settle onto the main sequence.

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arXiv2010

Hubble Space Telescope WFC3 Early Release Science: Emission-Line Galaxies from Infrared Grism Observations

We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns from the Wide Field Camera 3 on the Hubble Space Telescope. These new infrared grism data augment previous optical Advanced Camera for Surveys G800L 0.6-0.95 micron grism data in GOODS-South from the PEARS program, extending the wavelength covereage well past the G800L red cutoff. The ERS grism field was observed at a depth of 2 orbits per grism, yielding spectra of hundreds of faint objects, a subset of which are presented here. ELGs are studied via the Ha, [OIII], and [OII] emission lines detected in the redshift ranges 0.2 2.

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arXiv2010

UV-dropout Galaxies in the GOODS-South Field from WFC3 Early Release Science Observations

We combine new high sensitivity ultraviolet (UV) imaging from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with existing deep HST/Advanced Camera for Surveys (ACS) optical images from the Great Observatories Origins Deep Survey (GOODS) program to identify UV-dropouts, which are Lyman break galaxy (LBG) candidates at z~1-3. These new HST/WFC3 observations were taken over 50 sq.arcmin in the GOODS-South field as a part of the Early Release Science program. The uniqueness of these new UV data is that they are observed in 3 UV/optical (WFC3 UVIS) channel filters (F225W, F275W and F336W), which allows us to identify three different sets of UV-dropout samples. We apply Lyman break dropout selection criteria to identify F225W-, F275W- and F336W-dropouts, which are z~1.7, 2.1 and 2.7 LBG candidates, respectively. Our results are as follows: (1) these WFC3 UVIS filters are very reliable in selecting LBGs with z~2.0, which helps to reduce the gap between the well studied z~>3 and z~0 regimes, (2) the combined number counts agrees very well with the observed change in the surface densities as a function of redshift when compared with the higher redshift LBG samples; and (3) the best-fit Schechter function parameters from the rest-frame UV luminosity functions at three different redshifts fit very well with the evolutionary trend of the characteristic absolute magnitude, and the faint-end slope, as a function of redshift. This is the first study to illustrate the usefulness of the WFC3 UVIS channel observations to select z<3 LBGs. The addition of the new WFC3 on the HST has made it possible to uniformly select LBGs from z~1 to z~9, and significantly enhance our understanding of these galaxies using HST sensitivity and resolution.

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arXiv2010

The Size Evolution of Passive Galaxies: Observations from the Wide Field Camera 3 Early Release Science Program

We present results on the size evolution of passively evolving galaxies at 1 1.5. We identify 30 galaxies in ~40 square arcmin to H<25 mag. We supplement spectroscopic redshifts from the literature with photometric redshifts determined from the 15-band photometry from 0.22-8 micron. We determine effective radii from Sersic profile fits to the H-band image using an empirical PSF. We find that size evolution is a strong function of stellar mass, with the most massive (M* ~ 10^11 Msol) galaxies undergoing the most rapid evolution from z~2 to the present. Parameterizing the size evolution as (1+z)^{-alpha}, we find a tentative scaling between alpha and stellar mass of alpha ~ -1.8+1.4 log(M*/10^9 Msol). We briefly discuss the implications of this result for our understanding of the dynamical evolution of the red galaxies.

astro-ph.CO↗

arXiv2010

Lifting the Veil of Dust from NGC 0959: The Importance of a Pixel-Based 2D Extinction Correction

We present the results of a study of the late-type spiral galaxy NGC 0959, before and after application of the pixel-based dust extinction correction described in Tamura et al. 2009 (Paper I). Galaxy Evolution Explorer (GALEX) far-UV (FUV) and near-UV (NUV), ground-based Vatican Advanced Technology Telescope (VATT) UBVR, and Spitzer/Infrared Array Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 micron images are studied through pixel Color-Magnitude Diagrams (pCMDs) and pixel Color-Color Diagrams (pCCDs). We define groups of pixels based on their distribution in a pCCD of (B - 3.6 micron) versus (FUV - U) colors after extinction correction. In the same pCCD, we trace their locations before the extinction correction was applied. This shows that selecting pixel groups is not meaningful when using colors uncorrected for dust. We also trace the distribution of the pixel groups on a pixel coordinate map of the galaxy. We find that the pixel-based (two-dimensional) extinction correction is crucial to reveal the spatial variations in the dominant stellar population, averaged over each resolution element. Different types and mixtures of stellar populations, and galaxy structures such as a previously unrecognized bar, become readily discernible in the extinction-corrected pCCD and as coherent spatial structures in the pixel coordinate map.

astro-ph.CO↗

arXiv2009

Emission-Line Galaxies from the HST PEARS Grism Survey I: The South Fields

We present results of a search for emission-line galaxies in the Southern Fields of the Hubble Space Telescope PEARS (Probing Evolution And Reionization Spectroscopically) grism survey. The PEARS South Fields consist of five ACS pointings (including the Hubble Ultra Deep Field) with the G800L grism for a total of 120 orbits, revealing thousands of faint object spectra in the GOODS-South region of the sky. Emission-line galaxies (ELGs) are one subset of objects that are prevalent among the grism spectra. Using a 2-dimensional detection and extraction procedure, we find 320 emission lines orginating from 226 galaxy "knots'' within 192 individual galaxies. Line identification results in 118 new grism-spectroscopic redshifts for galaxies in the GOODS-South Field. We measure emission line fluxes using standard Gaussian fitting techniques. At the resolution of the grism data, the H-beta and [OIII] doublet are blended. However, by fitting two Gaussian components to the H-beta and [OIII] features, we find that many of the PEARS ELGs have high [OIII]/H-beta ratios compared to other galaxy samples of comparable luminosities. The star-formation rates (SFRs) of the ELGs are presented, as well as a sample of distinct giant star-forming regions at z~0.1-0.5 across individual galaxies. We find that the radial distances of these HII regions in general reside near the galaxies' optical continuum half-light radii, similar to those of giant HII regions in local galaxies.

astro-ph.CO↗