We present the results from the 1.2 mm continuum image obtained as part of the Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field. The 1.2 mm continuum ...image has a size of 2.9 (4.2) arcmin2 within a primary beam response of 50% (10%) and an rms value of . We detect 35 sources at high significance (Fidelity ≥0.5); 32 have well-characterized near-infrared Hubble Space Telescope counterparts. We estimate the 1.2 mm number counts to flux levels of in two different ways: we first use the detected sources to constrain the number counts and find a significant flattening of the counts below S ∼ 0.1 mJy. In a second approach, we constrain the number counts using a probability of deflection statistics (P(D)) analysis. For this latter approach, we describe new methods to accurately measure the noise in interferometric imaging (employing jackknifing in the cube and in the visibility plane). This independent measurement confirms the flattening of the number counts. Our analysis of the differential number counts shows that we are detecting ∼93% (∼100% if we include the lower fidelity detections) of the total continuum dust emission associated with galaxies in the Hubble Ultra Deep Field. The ancillary data allow us to study the dependence of the 1.2 mm number counts on redshift (z = 0−4), galaxy dust mass ( ), stellar mass ( ), and star formation rate ( ). In an accompanying paper we show that the number counts are crucial to constrain galaxy evolution models and the understanding of star-forming galaxies at high redshift.
Changing-look quasars are a recently identified class of active galaxies in which the strong UV continuum and/or broad optical hydrogen emission lines associated with unobscured quasars either appear ...or disappear on time-scales of months to years. The physical processes responsible for this behaviour are still debated, but changes in the black hole accretion rate or accretion disc structure appear more likely than changes in obscuration. Here, we report on four epochs of spectroscopy of SDSS J110057.70-005304.5, a quasar at a redshift of z = 0.378 whose UV continuum and broad hydrogen emission lines have faded, and then returned over the past ≈20 yr. The change in this quasar was initially identified in the infrared, and an archival spectrum from 2010 shows an intermediate phase of the transition during which the flux below rest frame ≈3400 Å has decreased by close to an order of magnitude. This combination is unique compared to previously published examples of changing-look quasars, and is best explained by dramatic changes in the innermost regions of the accretion disc. The optical continuum has been rising since mid-2016, leading to a prediction of a rise in hydrogen emission-line flux in the next year. Increases in the infrared flux are beginning to follow, delayed by a ~3 yr observed time-scale. If our model is confirmed, the physics of changing-look quasars are governed by processes at the innermost stable circular orbit around the black hole, and the structure of the innermost disc. The easily identifiable and monitored changing-look quasars would then provide a new probe and laboratory of the nuclear central engine.
We report a new changing-look quasar, WISE J105203.55+151929.5 at z = 0.303, found by identifying highly mid-IR-variable quasars in the Wide-field Infrared Survey Explorer (WISE)/Near-Earth Object ...WISE Reactivation (NEOWISE) data stream. Compared to multiepoch mid-IR photometry of a large sample of SDSS-confirmed quasars, WISE J1052+1519 is an extreme photometric outlier, fading by more than a factor of two at 3.4 and 4.6 m since 2009. Swift target-of-opportunity observations in 2017 show even stronger fading in the soft X-rays compared to the ROSAT detection of this source in 1995, with at least a factor of 15 decrease. We obtained second-epoch spectroscopy with the Palomar telescope in 2017 that, when compared with the 2006 archival SDSS spectrum, reveals that the broad Hβ emission has vanished and that the quasar has become significantly redder. The two most likely interpretations for this dramatic change are source fading or obscuration, where the latter is strongly disfavored by the mid-IR data. We discuss various physical scenarios that could cause such changes in the quasar luminosity over this timescale, and favor changes in the innermost regions of the accretion disk that occur on the thermal and heating/cooling front timescales. We discuss possible physical triggers that could cause these changes, and predict the multiwavelength signatures that could distinguish these physical scenarios.
We present a new semiempirical model for the dust continuum number counts of galaxies at 1.1 mm and 850 m. Our approach couples an observationally motivated model for the stellar mass and star ...formation rate distribution of galaxies with empirical scaling relations to predict the dust continuum flux density of these galaxies. Without a need to tweak the IMF, the model reproduces the currently available observations of the 1.1 mm and 850 m number counts, including the observed flattening in the 1.1 mm number counts below 0.3 mJy and the number counts in discrete bins of different galaxy properties. Predictions of our work include the following: (1) the galaxies that dominate the number counts at flux densities below 1 mJy (3 mJy) at 1.1 mm (850 m) have redshifts between z = 1 and z = 2, stellar masses of ∼5 × 1010 M , and dust masses of ∼108 M ; (2) the flattening in the observed 1.1 mm number counts corresponds to the knee of the 1.1 mm luminosity function. A similar flattening is predicted for the number counts at 850 m; (3) the model reproduces the redshift distribution of current 1.1 mm detections; and (4) to efficiently detect large numbers of galaxies through their dust continuum, future surveys should scan large areas once reaching a 1.1 mm flux density of 0.1 mJy rather than integrating to fainter fluxes. Our modeling framework also suggests that the amount of information on galaxy physics that can be extracted from the 1.1 mm and 850 m number counts is almost exhausted.
We use the results from the ALMA large program ASPECS, the spectroscopic survey in the Hubble Ultra Deep Field (HUDF), to constrain CO luminosity functions of galaxies and the resulting redshift ...evolution of (H2). The broad frequency range covered enables us to identify CO emission lines of different rotational transitions in the HUDF at z > 1. We find strong evidence that the CO luminosity function evolves with redshift, with the knee of the CO luminosity function decreasing in luminosity by an order of magnitude from ∼2 to the local universe. Based on Schechter fits, we estimate that our observations recover the majority (up to ∼90%, depending on the assumptions on the faint end) of the total cosmic CO luminosity at z = 1.0-3.1. After correcting for CO excitation, and adopting a Galactic CO-to-H2 conversion factor, we constrain the evolution of the cosmic molecular gas density (H2): this cosmic gas density peaks at z ∼ 1.5 and drops by a factor of to the value measured locally. The observed evolution in (H2), therefore, closely matches the evolution of the cosmic star formation rate density SFR. We verify the robustness of our result with respect to assumptions on source inclusion and/or CO excitation. As the cosmic star formation history can be expressed as the product of the star formation efficiency and the cosmic density of molecular gas, the similar evolution of (H2) and SFR leaves only little room for a significant evolution of the average star formation efficiency in galaxies since z ∼ 3 (85% of cosmic history).
Abstract
During galaxy mergers, gas and dust are driven toward the centers of merging galaxies, triggering enhanced star formation and supermassive black hole (SMBH) growth. Theory predicts that this ...heightened activity peaks at SMBH separations <20 kpc; if sufficient material accretes onto one or both of the SMBHs for them to become observable as active galactic nuclei (AGNs) during this phase, they are known as offset and dual AGNs, respectively. To better study these systems, we have built the ACS-AGN Merger Catalog, a large catalog (
N
= 204) of uniformly selected offset and dual AGN observed by the Hubble Space Telescope at 0.2 <
z
< 2.5 with separations <20 kpc. Using this catalog, we answer many questions regarding SMBH−galaxy coevolution during mergers. First, we confirm predictions that the AGN fraction peaks at SMBH pair separations <10 kpc; specifically, we find that the fraction increases significantly at pair separations of <4 kpc. Second, we find that AGNs in mergers are preferentially found in major mergers and that the fraction of AGNs found in mergers follows a logarithmic relation, decreasing as merger mass ratio increases. Third, we do not find that mergers (nor the major or minor merger subpopulations) trigger the most luminous AGNs. Finally, we find that nuclear column density, AGN luminosity, and host galaxy star formation rate have no dependence on SMBH pair separation or merger mass ratio in these systems, nor do the distributions of these values differ significantly from that of the overall AGN population.
We analyze the interstellar medium properties of a sample of 16 bright CO line emitting galaxies identified in the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) Large Program. ...This CO−selected galaxy sample is complemented by two additional CO line emitters in the UDF that are identified based on their Multi-Unit Spectroscopic Explorer (MUSE) optical spectroscopic redshifts. The ASPECS CO−selected galaxies cover a larger range of star formation rates (SFRs) and stellar masses compared to literature CO emitting galaxies at z > 1 for which scaling relations have been established previously. Most of ASPECS CO-selected galaxies follow these established relations in terms of gas depletion timescales and gas fractions as a function of redshift, as well as the SFR-stellar mass relation ("galaxy main sequence"). However, we find that ∼30% of the galaxies (5 out of 16) are offset from the galaxy main sequence at their respective redshift, with ∼12% (2 out of 16) falling below this relationship. Some CO-rich galaxies exhibit low SFRs, and yet show substantial molecular gas reservoirs, yielding long gas depletion timescales. Capitalizing on the well-defined cosmic volume probed by our observations, we measure the contribution of galaxies above, below, and on the galaxy main sequence to the total cosmic molecular gas density at different lookback times. We conclude that main-sequence galaxies are the largest contributors to the molecular gas density at any redshift probed by our observations (z ∼ 1−3). The respective contribution by starburst galaxies above the main sequence decreases from z ∼ 2.5 to z ∼ 1, whereas we find tentative evidence for an increased contribution to the cosmic molecular gas density from the passive galaxies below the main sequence.
The Atacama Large Millimeter/submillimeter Array (ALMA) SPECtroscopic Survey in the Hubble Ultra Deep Field (HUDF) is an ALMA large program that obtained a frequency scan in the 3 mm band to detect ...emission lines from the molecular gas in distant galaxies. Here we present our search strategy for emission lines and continuum sources in the HUDF. We compare several line search algorithms used in the literature, and critically account for the line widths of the emission line candidates when assessing significance. We identify 16 emission lines at high fidelity in our search. Comparing these sources to multiwavelength data we find that all sources have optical/infrared counterparts. Our search also recovers candidates of lower significance that can be used statistically to derive, e.g., the CO luminosity function. We apply the same detection algorithm to obtain a sample of six 3 mm continuum sources. All of these are also detected in the 1.2 mm continuum with optical/near-infrared counterparts. We use the continuum sources to compute 3 mm number counts in the sub-millijansky regime, and find them to be higher by an order of magnitude than expected for synchrotron-dominated sources. However, the number counts are consistent with those derived at shorter wavelengths (0.85-1.3 mm) once extrapolating to 3 mm with a dust emissivity index of β = 1.5, dust temperature of 35 K, and an average redshift of z = 2.5. These results represent the best constraints to date on the faint end of the 3 mm number counts.
We present a characterization of the physical properties of a sample of 35 securely detected, dusty galaxies in the deep ALMA 1.2 mm image obtained as part of the ALMA Spectroscopic Survey in the ...Hubble Ultra Deep Field (ASPECS) Large Program. This sample is complemented by 26 additional sources identified via an optical/infrared source positional prior. Using their well-characterized spectral energy distributions, we derive median stellar masses and star formation rates (SFR) of and 30 M☉ yr−1, respectively, and interquartile ranges of (2.4-11.7) × 1010 M☉ and 20-50 M☉ yr−1. We derive a median spectroscopic redshift of 1.8 with an interquartile range 1.1-2.6, significantly lower than submillimeter galaxies detected in shallower, wide-field surveys. We find that 59% 13%, 6% 4%, and 34% 9% of our sources are within, above, and below 0.4 dex from the SFR-stellar-mass relation or main sequence (MS), respectively. The ASPECS galaxies closely follow the SFR-molecular gas mass relation and other previously established scaling relations, confirming a factor of five increase of the gas-to-stellar-mass ratio from z = 0.5 to 2.5 and a mild evolution of the gas depletion timescales with a typical value of 0.7 Gyr at z = 1-3. ASPECS galaxies located significantly below the MS, a poorly exploited parameter space, have low gas-to-stellar-mass ratios of ∼0.1-0.2 and long depletion timescales >1 Gyr. Galaxies along the MS dominate the cosmic density of molecular gas at all redshifts. Systems above the MS have an increasing contribution to the total gas reservoirs from z < 1 to z = 2.5, while the opposite is found for galaxies below the MS.
Using the deepest 1.2 mm continuum map to date in the Hubble Ultra Deep Field, which was obtained as part of the ALMA Spectroscopic Survey (ASPECS) large program, we measure the cosmic density of ...dust and implied gas (H2+H i) mass in galaxies as a function of look-back time. We do so by stacking the contribution from all H-band selected galaxies above a given stellar mass in distinct redshift bins, and . At all redshifts, and grow rapidly as M decreases down to 1010 M , but this growth slows down toward lower stellar masses. This flattening implies that at our stellar mass-completeness limits (108 M and 108.9 M at z ∼ 0.4 and z ∼ 3), both quantities converge toward the total cosmic dust and gas mass densities in galaxies. The cosmic dust and gas mass densities increase at early cosmic time, peak around z ∼ 2, and decrease by a factor ∼4 and 7, when compared to the density of dust and molecular gas in the local universe, respectively. The contribution of quiescent galaxies (i.e., with little on-going star formation) to the cosmic dust and gas mass densities is minor ( 10%). The redshift evolution of the cosmic gas mass density resembles that of the SFR density, as previously found by CO-based measurements. This confirms that galaxies have relatively constant star formation efficiencies (within a factor ∼2) across cosmic time. Our results also imply that by z ∼ 0, a large fraction (∼90%) of dust formed in galaxies across cosmic time has either been destroyed or ejected to the intergalactic medium.
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