arXiv subjects

A. J. Barger

Publications and source records attributed to A. J. Barger.

At least 19 recordsLinked to original sources

arXiv2026

A Deep ALMA Survey of the Redshift Distribution of Dusty Star-forming Galaxies

We present an Atacama Large Millimeter/submillimeter Array (ALMA) spectroscopic follow-up survey of an 870 $μ$m-selected sample of dusty star-forming galaxies (DSFGs) in the GOODS-S field. We use these linescans to identify or confirm spectroscopic redshifts (spec-zs) for 26 sources. Including spec-zs from the literature, there are now secure or tentative spec-zs for 54 out of 75 DSFGs (72%). At $f_{870}>2.5$ mJy, the sample is 97% spectroscopically complete, allowing us to model the full DSFG redshift distribution down to nearly the confusion limit for a 15-m telescope at 850 $μ$m. This is the highest completeness for an unbiased sample at this flux limit to date. We find that nearly all of the DSFGs in our sample that were targeted with JWST/NIRSpec were spectroscopically identified, without much dependence on near-infrared or submillimeter flux or redshift. However, only 29% of our sample have JWST spectroscopic coverage. We use the spec-zs to evaluate various photometric redshift (photo-z) estimates, finding that all methods exhibit an outlier fraction of at least $>20$%. Nearly all of the photo-z methods tend to overshoot the redshifts, leading to overestimates of the number of DSFGs at high redshift ($z>4$). Our results suggest that $\lesssim6$% of $f_{870} \geq 2.25$ mJy DSFGs lie at $z>4$ and $\lesssim1$% lie at $z>5$, reflecting a steep decline in the abundance of massive dusty galaxies in the first 1.5 Gyr.

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arXiv2026

Double Dots: Compact Pairs Mark Little Red Dots and High-Redshift Broad-line AGNs

We utilize JWST imaging of the massive lensing cluster field A2744 to find close pairs of compact sources with separations less than 0.25". A large fraction of these "Double Dots" correspond to Little Red Dots (LRDs) or high-redshift broad-line active galactic nuclei (BLAGNs). Our analysis of 31 identified pairs reveals a median separation of 0.15". Statistical comparison against a uniform background shows that these are mostly physical pairs. We find that at least 16 of the 24 previously published LRDs in this field (~67%) are such pairs, as are both of the high-redshift BLAGNs. We demonstrate that the presence of a companion can significantly contaminate the measured spectral energy distribution, potentially masking the characteristic ``v-shape" used for LRD classification. Furthermore, our 2D spectroscopic analysis of several pairs reveals that BLAGN activity is not confined to the redder member of the pair but can originate in either one. Since most LRDs contain broad emission lines, our findings suggest that close pairs are extremely effective markers of galaxies with broad lines at high redshift. We speculate on possible mechanisms, concluding that we are likely seeing merger-driven accretion.

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arXiv2026

A Redshift-based Red Selection of Dusty Star-forming Galaxies

We use JWST observations (1.5 micron to 4.44 micron), together with complete ALMA observations (870 micron and/or 1.2 mm), of the massive lensing cluster field A2744 to show that galaxies between z=1.5 and z=5.5 with rest-frame red colors f_J/f_V > 3 correspond to dusty star-forming galaxies (DSFGs), little red dots (LRDs), and quiescent galaxies. The color selection picks out 34 of the 41 >4.5-sigma ALMA sources in the field (83%). We find that the luminous red sources are generally extended, while the less luminous red sources are almost all compact and correspond to the LRD population. We also find that the great majority of the luminous, extended red sources are DSFGs based on the ALMA data, with a small admixture of quiescent galaxies at z<3-4 that we identify based on their location in the rest-frame U-V versus V-J diagram. We do not detect any LRDs or quiescent galaxies at the >3-sigma level in the ALMA images. Roughly 10% of the DSFGs have high rest-frame X-ray luminosities and must be AGN dominated. The DSFGs and quiescent galaxies nearly all have M_star>10^{10} solar masses. These massive galaxies become rare at z>5, paralleling the fall off in the number of detected DSFGs.

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arXiv2025

Spectroscopic Ultraluminous Ly-alpha Luminosity Functions at z = 5.7 and z = 6.6 from HEROES: Evidence for Ionized Bubbles

We present spectroscopic Ly$α$ luminosity functions (LFs) at z = 5.7 and z = 6.6 based on a large 209 source sample of Ly$α$ emitter (LAE) candidates identified in Subaru/Hyper Suprime-Cam narrowband imaging and confirmed with Keck II/DEIMOS spectroscopy over a multi-year observing campaign. After applying photometric and spectroscopic cuts to produce homogeneous samples, we use the resulting samples of 49 z = 5.7 and 56 z = 6.6 LAEs to compute spectroscopic LAE LFs at each redshift. We correct our LFs for incompleteness using a source-injection simulation. We find excellent agreement with current spectroscopic and photometric LAE LFs from the literature. We look for evolution over the redshift range z = 5.7-6.6. We find a strong convergence of the LFs at L(Ly$α$) > $10^{43.4}$ erg s$^{-1}$. This convergence (noted in previous literature) provides strong evidence that the most luminous LAEs at z = 6.6 form ionized bubbles around themselves, allowing for greater Ly$α$ transmission through the neutral intergalactic medium, which is measured as an increase in the bright end of the z = 6.6 LF over the expected evolution in the LF based on the faint end. We infer that ultraluminous LAEs may play a significant role in reionization.

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arXiv2024

A Spectral Atlas of Lyman Alpha Emitters at z = 5.7 and z = 6.6

We present two uniformly observed spectroscopic samples of Ly-alpha emitters (LAEs) (127 at z = 5.7 and 82 at z = 6.6), which we use to investigate the evolution of the LAE population at these redshifts. The observations cover a large field (44 sq. deg) in the North Ecliptic Pole (HEROES), as well as several smaller fields. We have a small number of exotic LAEs in the samples: double-peaked Ly-alpha profiles; very extended red wings; and one impressive lensed LAE cross. We also find three broad-line AGNs. We compare the Ly-alpha line width measurements at the two redshifts, finding that the lower-luminosity LAEs show a strong evolution of decreasing line width with increasing redshift, while the high-luminosity LAEs do not, with a transition luminosity of log L(Ly-alpha) = 43.25 erg s-1 . Thus, at z = 6.6, the high-luminosity LAEs may be producing large ionized bubbles themselves, or they may be residing in overdense galaxy sites that are producing such bubbles. In order to avoid losses in the red wing, the radius of the ionized bubble must be larger than 1 pMpc. The double-peaked LAEs also require transmission on the blue side. For the four at z = 6.6, we use models to estimate the proximity radii, Ra , where the ionizing flux of the galaxy is sufficient to make the surroundings have a low enough neutral fraction to pass the blue light. Since the required Ra are large, multiple ionizing sources in the vicinity may be needed.

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arXiv2024

A Full Resolution of the 450 μm Extragalactic Background Light

The extragalactic background light (EBL) is the cumulative radiation outside the Milky Way. The determination of its corresponding primary emitting sources as well as its total energy level across the entire electromagnetic spectrum has profound implications for both cosmology and galaxy formation. However, the detailed origin of the EBL at far-infrared wavelengths, particularly those close to the peak of the cosmic infrared background, remains unclear. Here we report the results of our ongoing SCUBA-2 450 $μ$m survey of 10 massive galaxy cluster fields. By exploiting the strong gravitational lensing offered by these clusters, we obtain significant counts down to an unprecedented depth of $\sim$0.1 mJy at this wavelength, about ten times deeper than that reached by any other previous survey. The cumulative energy density based on the counts is 138.1$^{+23.9}_{-19.3}$ Jy degree$^{-2}$, or 0.45$^{+0.08}_{-0.06}$ MJy sr$^{-1}$. Comparing our measurements to those made by the COBE and Planck satellites, we find that at this flux density level, the 450 $μ$m EBL is entirely resolved by our SCUBA-2 observations. Thus, we find for the first time that discrete sources produce fully to the 450 $μ$m EBL, and that about half of it comes from sources with sub-mJy flux densities. Our deep number counts provide strong constraints on galaxy formation models.

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arXiv2024

Comparing SCUBA-2 and ALMA Selections of Faint DSFGs in Abell 2744

We make a comparison of deep SCUBA-2 450 $μ$m and 850 $μ$m imaging on the massive lensing cluster field Abell 2744 with Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm data. Our primary goal is to assess how effective the wider-field SCUBA-2 sample, in combination with red JWST priors, is for finding faint dusty star-forming galaxies (DSFGs) compared to the much more expensive mosaicked ALMA observations. We cross-match our previously reported direct ($>5σ$) SCUBA-2 sample and red JWST NIRCam prior-selected ($>3σ$) SCUBA-2 sample to direct ALMA sources from the DUALZ survey. We find that roughly 95% are confirmed by ALMA. The red priors also allow us to probe deeper in the ALMA image. Next, by measuring the 450 $μ$m and 850 $μ$m properties of the full ALMA sample, we show that 46/69 of the ALMA sources are detected at 850 $μ$m and 24/69 are detected at 450 $μ$m in the SCUBA-2 images, with a total detection fraction of nearly 75%. All of the robust ($>5σ$) ALMA sources that are not detected in at least one SCUBA-2 band lie at 1.2 mm fluxes $\lesssim$ 0.6 mJy and are undetected primarily due to the higher SCUBA-2 flux limits. We also find that the SCUBA-2 detection fraction drops slightly beyond $z=3$, which we attribute to the increasing 1.2 mm to 850 $μ$m and 1.2 mm to 450 $μ$m flux ratios combined with the ALMA selection. The results emphasize the power of combining SCUBA-2 data with JWST colors to map the faint DSFG population.

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arXiv2023

Mapping the Decline with Redshift of Dusty Star-forming Galaxies Using JWST and SCUBA-2

We use JWST NIRCam observations of the massive lensing cluster field A2744 to develop a red galaxy selection of f(F444W) > 1 uJy and f(F444W)/f(F150W) > 3.5 that picks out all 9 >4.5-sigma ALMA 1.1 or 1.2 mm sources and 17 of the 19 >5-sigma SCUBA-2 850 micron sources in the covered areas. We show that by using the red galaxies as priors, we can probe deeper in the SCUBA-2 850 micron image. This gives a sample of 44 >3-sigma SCUBA-2 850 micron sources with accurate positions, photometric redshifts, and magnifications. To investigate why our red galaxy selection picks out the 850 micron sources, we next analyze an extended sample of 167 sources with f(F444W) >0.05uJy and f(F444W)/f(F150W) >3.5. We find that the fainter f(F444W) sources in this sample are too faint to be detected in the SCUBA-2 850 micron image. We also show that there is a strong drop between z<4 and z>4 (a factor of around 5) in the ratio of the far-infrared luminosity estimated from the 850 micron flux to the nuLnu(5000) at rest-frame 5000A. We argue that this result may be due to the high-redshift sources having less dust content than the lower redshift sources.

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arXiv2023

2mm Observations and the Search for High-Redshift Dusty Star-forming Galaxies

Finding high-redshift (z>>4) dusty star-forming galaxies is extremely challenging. It has recently been suggested that millimeter selections may be the best approach, since the negative K-correction makes galaxies at a given far-infrared (FIR) luminosity brighter at z>4 than those at z=2-3. Here we analyze this issue using a deep ALMA 2mm sample obtained by targeting ALMA 870um priors (these priors were the result of targeting SCUBA-2 850um sources) in the GOODS-S. We construct the prior-based 2mm galaxy number counts and compare them with published blank field-based 2mm counts, finding good agreement down to 0.2mJy. Only a fraction of the current 2mm extragalactic background light is resolved, and we estimate what observational depths may be needed to resolve it fully. By complementing the 2mm ALMA data with a deep SCUBA-2 450um sample in the GOODS-S, we exploit the steep gradient with redshift of the 2mm to 450um flux density ratio to estimate redshifts for these galaxies without spectroscopic or robust optical/near-infrared photometric redshifts. Our observations measure galaxies with star formation rates in excess of 250 solar masses per year. For these galaxies, the star formation rate densities fall by a factor of 9 from z=2-3 to z=5-6.

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arXiv2023

Dust Properties of 870 Micron Selected Galaxies in the GOODS-S

We analyze the dust properties of 57 870 $μ$m selected dusty star-forming galaxies in the GOODS-S using new deep ALMA 1.2 mm, 2 mm, and 3 mm continuum imaging together with other far-infrared through millimeter data. We fit the spectral energy distributions (SEDs) with optically thin modified blackbodies to constrain the emissivity indices and effective dust temperatures, finding a median emissivity index of $β= 1.78^{+0.43}_{-0.25}$ and a median temperature of $T_d = 33.6^{+12.1}_{-5.4}$ K. We observe a negative correlation between $β$ and $T_d$. By testing several SED models, we determine that the derived emissivity indices can be influenced by opacity assumptions. Our temperature measurements are consistent with no evolution in dust temperature with redshift.

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arXiv2023

HEROES: The Hawaii eROSITA Ecliptic Pole Survey Catalog

We present a seven band (g, r, i, z, y, NB816, NB921) catalog derived from a Subaru Hyper Suprime-Cam (HSC) imaging survey of the North Ecliptic Pole (NEP). The survey, known as HEROES, consists of 44 sq. deg of contiguous imaging reaching median 5-sigma depths of g: 26.5, r: 26.2, i: 25.7, z: 25.1, y: 23.9, NB816: 24.4, NB921: 24.4 mag. We reduced these data with the HSC pipeline software hscPipe, and produced a resulting multiband catalog containing over 25 million objects. We provide the catalog in three formats: (1) a collection of hscPipe format forced photometry catalogs, (2) a single combined catalog containing every object in that dataset with selected useful columns, and (3) a smaller variation of the combined catalog with only essential columns for basic analysis or low memory machines. The catalog uses all the available HSC data on the NEP and may serve as the primary optical catalog for current and future NEP deep fields from instruments and observatories such as SCUBA-2, eROSITA, Spitzer, Euclid, and JWST.

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arXiv2022

Metallicities of Five z > 5 Emission-Line Galaxies in SMACS 0723 Revealed by JWST

JWST's Early Release Observations of the lensing cluster SMACS J0723.3-7327 have given an unprecedented spectroscopic look into the high-redshift universe. These observations reveal five galaxies at z > 5. All five have detectable [OIII]4363 line emission, indicating that these galaxies have high temperatures and low metallicities and that they are highly star forming. In recent work, the metallicities of these five galaxies have been studied using various techniques. Here we summarize and compare these previous results, as well as perform our own measurements of the metallicities using improved methodologies that optimize the extraction of the emission lines. In particular, we use simultaneous line fitting and a fixed Balmer decrement correction, as well as a novel footprint measurement of the emission lines in the 2D spectra, to produce higher fidelity line ratios that are less sensitive to calibration and systematic effects. We then compare our metallicities to those of z < 1 galaxies with high rest-frame equivalent widths of H-beta, finding that they may be good analogs. Finally, we estimate that the JWST galaxies out to z ~ 8 are young compared to the age of the universe.

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arXiv2022

A Submillimeter Survey of Faint Galaxies Behind Ten Strong Lensing Clusters

We present deep SCUBA-2 450 micron and 850 micron imaging of ten strong lensing clusters. We provide a >4-sigma SCUBA-2 850 micron catalog of the 404 sources lying within a radius of 4.5' from the cluster centers. We also provide catalogs of the >4.5-sigma ALMA 870 micron detections in the clusters A370, MACSJ1149.5+2223, and MACSJ0717.5+3745 from our targeted ALMA observations, along with catalogs of all other >4.5-sigma ALMA (mostly 1.2 mm) detections in any of our cluster fields from archival ALMA observations. For the ALMA detections, we give spectroscopic or photometric redshifts, where available, from our own Keck observations or from the literature. We confirm the use of the 450 micron to 850 micron flux ratio for estimating redshifts. We use lens models to determine magnifications, most of which are in the 1.5-4 range. After supplementing the ALMA cluster sample with Chandra Deep Field (CDF) ALMA and SMA samples, we find no evidence for evolution in the redshift distribution of submillimeter galaxies down to demagnified 850 micron fluxes of 0.5 mJy. Given this result, we conclude that our observed trend of increasing F160W to 850 micron flux ratio from brighter to fainter demagnified 850 micron flux results from the fainter submillimeter galaxies having less extinction. However, there is wide spread in this relation, including the presence of some optical/NIR dark galaxies down to fluxes below 1 mJy. Finally, with insights from our ALMA analysis, we analyze our SCUBA-2 sample and present 55 850 micron-bright z>4 candidates.

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arXiv2022

The Evolution of Lyman Alpha Emitter Line Widths from z=5.7 to z=6.6

Recent evidence suggests that high-redshift Ly-alpha emitting galaxies (LAEs) with logL(Ly-alpha) > 43.5 erg/s, referred to as ultraluminous LAEs (ULLAEs), may show less evolution than lower-luminosity LAEs in the redshift range z=5.7-6.6. Here we explore the redshift evolution of the velocity widths of the Ly-alpha emission lines in LAEs over this redshift interval. We use new wide-field, narrowband observations from Subaru/Hyper Suprime-Cam to provide a sample of 24 z=6.6 and 12 z=5.7 LAEs with log L(Ly-alpha) > 43 erg/s, all of which have follow-up spectroscopy from Keck/DEIMOS. Combining with archival lower-luminosity data, we find a significant narrowing of the Ly-alpha lines in LAEs at logL(Ly-alpha) < 43.25 erg/s -- somewhat lower than the usual ULLAE definition -- at z = 6.6 relative to those at z = 5.7, but we do not see this in higher-luminosity LAEs. As we move to higher redshifts, the increasing neutrality of the intergalactic medium should increase the scattering of the Ly-alpha lines, making them narrower. The absence of this effect in the higher-luminosity LAEs suggests they may lie in more highly ionized regions, self-shielding from the scattering effects of the intergalactic medium.

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arXiv2022

A Submillimeter Perspective on the GOODS Fields (SUPER GOODS). V. Deep 450 Micron Imaging

We present deep SCUBA-2 450 micron imaging of the two GOODS fields, achieving a central rms of 1.14 mJy for the GOODS-N and 1.86 mJy for the GOODS-S. For each field, we give a catalog of >4-sigma detections (79 and 16 sources, respectively). We construct the 450 micron number counts, finding excellent agreement with others from the literature. We match the 450 micron sources to 20 cm data (both fields) and ALMA 870 micron data (GOODS-S) to gauge the accuracy of the 450 micron positions. We use the extensive redshift information available on the fields to test how well redshift can be estimated from simple flux ratios (450 micron/850 micron and 20 cm/850 micron), finding tight correlations. We provide a catalog of candidate high-redshift submillimeter galaxies. We look for evolution in dust temperature with redshift by fitting the spectral energy distributions of the sources, but we do not find any significant redshift evolution after accounting for the far-infrared luminosity. We do not find evidence for 450 micron selection picking out warmer sources than 850 micron selection. We find that a 450 micron selected sample only adds low-redshift (z<1.5) galaxies beyond an 850 micron sample.

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arXiv2020

A Submillimeter Perspective on the GOODS Fields (SUPER GOODS). IV. The Submillimeter Properties of X-ray Sources in the CDF-S

The CDF-S is the deepest X-ray image available and will remain so for the near future. We provide a spectroscopic (64.5%; 64% with spectral classifications) and photometric redshift catalog for the full 7 Ms sample, but much of our analysis focuses on the central (off-axis angles <5.7') region, which contains a large, faint ALMA sample of 75 >4.5-sigma 850 micron sources. We measure the 850 micron fluxes at the X-ray positions using the ALMA images, where available, or an ultradeep SCUBA-2 map. We find that the full X-ray sample produces ~10% of the 850 micron extragalactic background light. We separate the submillimeter detected X-ray sources into star-forming galaxies and AGNs using a star formation rate (SFR) versus X-ray luminosity calibration for high SFR galaxies. We confirm this separation using the X-ray photon indices. We measure the X-ray fluxes at the accurate positions of the 75 ALMA sources and detect 70% at >3-sigma in either the 0.5-2 or 2-7 keV bands. However, many of these may produce both their X-ray and submillimeter emission by star formation. Indeed, we find that only 20% of the ALMA sources have intermediate X-ray luminosities (rest-frame 8-28 keV luminosities of 10^42.5-10^44 erg/s), and none has a high X-ray luminosity (>10^44 erg/s). Conversely, after combining the CDF-S with the CDF-N, we find extreme star formation (SFR>300 solar masses per yr) in some intermediate X-ray luminosity sources but not in any high X-ray luminosity sources. We argue that the quenching of star formation in the most luminous AGNs may be a consequence of the clearing of gas in these sources.

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arXiv2020

On the Absence of High-Redshift AGNs: Little Growth in the Supermassive Black Hole Population at High Redshifts

We search for high-redshift (z>4.5) X-ray AGNs in the deep central (off-axis angle <5.7') region of the 7 Ms Chandra Deep Field-South X-ray image. We compile an initial candidate sample from direct X-ray detections. We then probe more deeply in the X-ray data by using pre-selected samples with high spatial resolution NIR/MIR (HST 1.6 micron and Spitzer 4.5 micron) and submillimeter (ALMA 850 micron) observations. The combination of the NIR/MIR and submillimeter pre-selections allows us to find X-ray sources with a wide range of dust properties and spectral energy distributions (SEDs). We use the SEDs from the optical to the submillimeter to determine if previous photometric redshifts were plausible. Only five possible z>5 X-ray AGNs are found, all of which might also lie at lower redshifts. If they do lie at high redshifts, then two are Compton-thick AGNs, and three are ALMA 850 micron sources. We find that (i) the number density of X-ray AGNs is dropping rapidly at high redshifts, (ii) the detected AGNs do not contribute significantly to the photoionization at z>5, and (iii) the measured X-ray light density over z=5-10 implies a very low black hole accretion density with very little growth in the black hole mass density in this redshift range.

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arXiv2018

z ~ 2.5 - 3 Ionizers in the GOODS-N Field

We use deep F275W imaging from the $Hubble$ Deep UV Legacy Survey (HDUV) and G280 grism spectroscopy from $HST$/WFC3, along with new and archival optical spectra from Keck/DEIMOS, to search for candidate ionizing sources in the GOODS-N field at z ~ 2.5 - 3. Spectroscopic identification of our UV-selected sources are 99% complete to F275W = 25.5 in the region of the UV imaging, and we identify 6 potential ionizing galaxies or AGNs at z ~ 3. By far the brightest of these is a z = 2.583 AGN that totally dominates the ionizing flux in the region, with a specific ionizing volume emissivity at $912~ Å$ of $ε_{912}$ = $8.3^{27}_{1.4} \times 10^{24}$ erg s$^{-1}$ Hz$^{-1}$ Mpc$^{-3}$. Based on our spectroscopic data, we find four candidates are contaminated by foreground galaxies at z ~ 0.5 - 0.7. At $ε_{912}$ = $2.2^{7.2}_{0.4} \times 10^{23}$ erg s$^{-1}$ Hz$^{-1}$ Mpc$^{-3}$, the remaining candidate galaxy's contribution to the ionizing background lies well below the flux required to ionize the intergalactic medium at z ~ 2.5 - 3, consistent with previous observations that show AGNs provide the bulk of the ionizing background at these redshifts.

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