ALMACAL: The Evolution of Gas and Dust in Galaxies Using ALMA Calibrator Observations

A fundamental question in astronomy is how galaxies form and evolve. How does gas flow into and out of galaxies? What physical processes drive the evolution of the star formation rate history? What is the role of dusty star formation at high redshift? To answer these questions we must understand the complex interplay between galaxies and the surrounding circum-galactic medium and we have to study the evolution of molecular gas and dust in galaxies. We present the ALMACAL survey utilizing ALMA calibration observations for science. We use this unique dataset to study the evolution of molecular gas and dust in galaxies with cosmic time. Using this survey, we select a sample of CO emission line detections in gas-rich galaxies first identified as intervening absorbers. From this parent sample we select the three galaxies detected in multiple CO emission lines for a further analysis and follow up observations. Ultimately we are aiming for a better understanding of the population of gas-rich galaxies. As a pilot study, we use VLT/MUSE to follow up one absorption-selected system at $z \sim 0.5$ detected in multiple CO transitions. We find in total four galaxies at the absorber redshift, one of which was detected in CO. This provides further evidence that the connection between absorber and host galaxy is more complex than a simple one-to-one relation. We find that most probably the absorbing gas is tracing intra-group medium. Next we focus on the multiple CO transitions and study for the first time the CO spectral line energy distribution of absorption-selected galaxies. We find evidence for more excited ISM conditions compared to the Milky Way. This indicates that previous studies of absorption-selected systems might overestimate the molecular gas mass in some galaxies. Furthermore, we suggest that absorption-selected galaxies may preferentially trace group environments. In addition to the local baryon cycle in single objects, we study the global baryon cycle over cosmic time. To understand the processes that drive the evolution of the star formation rate history, we trace the evolution of the molecular gas mass density over cosmic time using intervening molecular absorption. In the currently largest available dataset of quasar spectra in the submillimetre regime, ALMACAL, we do not detect intervening CO absorption. We place constraints on the evolution of the molecular gas mass density. This suggests, combined with complementary measurements from the literature, a strong evolution following that of the star formation rate history. Finally, half of the star formation activity in the Universe is expected to take place in dusty star-forming galaxies. We use our ALMACAL dataset to search for dusty star-forming galaxies observed at $680 \mu$m. We determine the first high-frequency number counts at $680 \mu$m free of source blending and cosmic variance effects. At this wavelength we find that we resolve the majority of the extragalactic background light

ALMACAL IX: Multiband ALMA survey for dusty star-forming galaxies and the resolved fractions of the cosmic infrared background

Wide, deep, blind continuum surveys at submillimetre/millimetre (submm/mm) wavelengths are required to provide a full inventory of the dusty, distant Universe. However, conducting such surveys to the necessary depth, with sub-arcsec angular resolution, is prohibitively time-consuming, even for the most advanced submm/mm telescopes. Here, we report the most recent results from the ALMACAL project, which exploits the ‘free’ calibration data from the Atacama Large Millimetre/submillimetre Array (ALMA) to map the lines of sight towards and beyond the ALMA calibrators. ALMACAL has now covered 1001 calibrators, with a total sky coverage around 0.3 deg2, distributed across the sky accessible from the Atacama desert, and has accumulated more than 1000 h of integration. The depth reached by combining multiple visits to each field makes ALMACAL capable of searching for faint, dusty, star-forming galaxies (DSFGs), with detections at multiple frequencies to constrain the emission mechanism. Based on the most up-to-date ALMACAL data base, we report the detection of 186 DSFGs with flux densities down to S870 µm ∼ 0.2 mJy, comparable with existing ALMA large surveys but less susceptible to cosmic variance. We report the number counts at five wavelengths between 870 μm and 3 mm, in ALMA bands 3, 4, 5, 6, and 7, providing a benchmark for models of galaxy formation and evolution. By integrating the observed number counts and the best-fitting functions, we also present the resolved fraction of the cosmic infrared background (CIB) and the CIB spectral shape. Combining existing surveys, ALMA has currently resolved about half of the CIB in the submm/mm regime

ALMACAL. XI. Over-densities as signposts to proto-clusters? A cautionary tale

It may be unsurprising that the most common approach to finding proto-clusters is to search for over-densities of galaxies. Upgrades to submillimetre (submm) interferometers and the advent of the James Webb Space Telescope will soon offer the opportunity to find more distant candidate proto-clusters in deep sky surveys without any spectroscopic confirmation. In this letter, we report the serendipitous discovery of an extremely dense region centred on the blazar, J0217-0820, at z=0.6 in the ALMACAL sky survey. Its density is eight times higher than that predicted by blind submm surveys. Among the seven submm-bright galaxies, three are as bright as conventional single-dish submm galaxies, with S_870um > 3mJy. The over-density is thus comparable to the densest known and confirmed proto-cluster cores. However, their spectra betray a wide range of redshifts. We investigate the likelihood of line-of-sight projection effects using light cones from cosmological simulations, finding that the deeper we search, the higher the chance that we will suffer from such projection effects. The extreme over-density around J0217-0820 demonstrates the strong cosmic variance we may encounter in the deep submm surveys. Thus, we should also question the fidelity of galaxy proto-cluster candidates selected via over-densities of galaxies, where the negative K correction eases the detection of dusty galaxies along an extraordinarily extended line of sight.Comment: 7 pages, 5 figures, update with the accepted versio

ALMACAL X: Constraints on molecular gas in the low-redshift circumgalactic medium

Despite its crucial role in galaxy evolution, the complex circumgalactic medium (CGM) remains underexplored. Although it is known to be multi-phase, the importance of the molecular gas phase to the total CGM mass budget is, to date, unconstrained. We present the first constraints on the molecular gas covering fraction in the CGM of low-redshift galaxies, using measurements of CO column densities along sightlines towards mm-bright background quasars with intervening galaxies. We do not detect molecular absorption against the background quasars. For the individual, low-redshift, ‘normal’ galaxy haloes probed here, we can therefore rule out the presence of an extremely molecular gas-rich CGM, as recently reported in high-redshift protoclusters and around luminous active galactic nuclei. We also set statistical limits on the volume filling factor of molecular material in the CGM as a whole, and as a function of radius. ISM-like molecular clouds of ∼30 pc in radius with column densities of N(CO) ≳ 1016 cm−2 have volume filling factors of less than 0.2 per cent. Large-scale smooth gas reservoirs are ruled out much more stringently. The development of this technique in the future will allow deeper constraining limits to be set on the importance (or unimportance) of molecular gas in the CGM

ALMACAL VI: Molecular gas mass density across cosmic time via a blind search for intervening molecular absorbers

We are just starting to understand the physical processes driving the dramatic change in cosmic star-formation rate between z ∼ 2 and the present day. A quantity directly linked to star formation is the molecular gas density, which should be measured through independent methods to explore variations due to cosmic variance and systematic uncertainties. We use intervening CO absorption lines in the spectra of mm-bright background sources to provide a census of the molecular gas mass density of the Universe. The data used in this work are taken from ALMACAL, a wide and deep survey utilizing the ALMA calibrator archive. While we report multiple Galactic absorption lines and one intrinsic absorber, no extragalactic intervening molecular absorbers are detected. However, thanks to the large redshift path surveyed (Δz = 182), we provide constraints on the molecular column density distribution function beyond z ∼ 0. In addition, we probe column densities of N(H2) > 1016 atoms cm−2, five orders of magnitude lower than in previous studies. We use the cosmological hydrodynamical simulation IllustrisTNG to show that our upper limits of ρ(H2) ≲ 108.3M⊙Mpc−3 at 0 < z ≤ 1.7 already provide new constraints on current theoretical predictions of the cold molecular phase of the gas. These results are in agreement with recent CO emission-line surveys and are complementary to those studies. The combined constraints indicate that the present decrease of the cosmic star-formation rate history is consistent with an increasing depletion of molecular gas in galaxies compared to z ∼ 2

MUSE-ALMA Haloes IX: Morphologies and Stellar Properties of Gas-rich Galaxies

Understanding how galaxies interact with the circumgalactic medium (CGM) requires determining how galaxies morphological and stellar properties correlate with their CGM properties. We report an analysis of 66 well-imaged galaxies detected in HST and VLT MUSE observations and determined to be within $\pm$500 km s$^{-1}$ of the redshifts of strong intervening quasar absorbers at $0.2 \lesssim z \lesssim 1.4$ with H I column densities $N_{\rm H I}$ $>$ $10^{18}$ $\rm cm^{-2}$. We present the geometrical properties (S\'ersic indices, effective radii, axis ratios, and position angles) of these galaxies determined using GALFIT. Using these properties along with star formation rates (SFRs, estimated using the H$\alpha$ or [O II] luminosity) and stellar masses ($M_{*}$ estimated from spectral energy distribution fits), we examine correlations among various stellar and CGM properties. Our main findings are as follows: (1) SFR correlates well with $M_{*}$, and most absorption-selected galaxies are consistent with the star formation main sequence (SFMS) of the global population. (2) More massive absorber counterparts are more centrally concentrated and are larger in size. (3) Galaxy sizes and normalized impact parameters correlate negatively with $N_{\rm H I}$, consistent with higher $N_{\rm H I}$ absorption arising in smaller galaxies, and closer to galaxy centers. (4) Absorption and emission metallicities correlate with $M_{*}$ and sSFR, implying metal-poor absorbers arise in galaxies with low past star formation and faster current gas consumption rates. (5) SFR surface densities of absorption-selected galaxies are higher than predicted by the Kennicutt-Schmidt relation for local galaxies, suggesting a higher star formation efficiency in the absorption-selected galaxies.Comment: Accepted for publication in MNRAS, 25 pages, 19 figure

A molecular view of the cosmic baryon cycle

Studying the multiphase circum-galactic medium and its connection to the baryons in the host galaxies through the cosmic baryon cycle is an important step towards a better understanding of the evolution of galaxies. Large steps forward have been made in detecting the multiphase circum-galactic medium through absorption line studies and connecting the CGM to its host galaxy. However, the link to the molecular gas phase from which the stars form is still missing. We have exploited ALMA calibrator observations to perform a novel (sub)mm survey, ALMACAL. We search for CO emission lines from the host galaxies of known Lyman alpha absorbers. We have detected three galaxies in multiple CO transitions indicating more excited ISMs than in normal star-forming galaxies. Furthermore, we find more evidence for intervening absorbers being connected to groups rather than isolated galaxies from our ALMA and follow-up MUSE observations. In addition to the local baryon cycle, we also study the global baryon cycle by combining the cosmic star formation history with the evolution of Omega HI and Omega H2. By conducting a blind search for molecular absorption lines seen in the spectra of radio-bright quasars in ALMACAL, we will measure the evolution of Omega H2. The resulting measurement of the cosmic molecular gas density will serve as an independent comparison to the results of CO emission line surveys such as ASPECS and COLDz

H_2 molecular gas absorption-selected systems trace CO molecular gas-rich galaxy overdensities

Absorption-selected galaxies offer an effective way to study low-mass galaxies at high redshift. However, the physical properties of the underlying galaxy population remain uncertain. In particular, the multiphase circumgalactic medium is thought to hold key information on gas flows into and out of galaxies that are vital for galaxy evolution models. Here, we present ALMA observations of CO molecular gas in host galaxies of H_2-bearing absorbers. In our sample of six absorbers, we detect molecular gas-rich galaxies in five absorber fields, although we did not target high-metallicity (>50 per cent solar) systems for which previous studies reported the highest detection rate. Surprisingly, we find that the majority of the absorbers are associated with multiple galaxies rather than single haloes. Together with the large impact parameters, these results suggest that the H_2-bearing gas seen in absorption is not part of an extended disc, but resides in dense gas pockets in the circumgalactic and intragroup medium

Spatially resolved star formation relation in two HI-rich galaxies with central post-starburst signature

Context. E+A galaxies are post-starburst systems that are identified based on their optical spectra. These galaxies contain a substantial young A-type stellar component but display no emission lines, which indicates only little ongoing star formation (SF). HI 21 cm line emission is found in approximately half of the nearby E+A galaxies, indicating that they contain a reservoir of gas that could fuel active SF. Aims. We study the distribution and kinematics of atomic and molecular gas in two HI-rich galaxies, which show a typical E+A spectrum at the centre and SF at larger radii. From our results we aim to infer whether the SF activity of these galaxies is consistent with the activity seen in disc galaxies, or if it indicates a transition towards another evolutionary phase. Methods. We present newly obtained high spatial resolution radio interferometric observations of the HI 21 cm emission line using the Karl Jansky Very Large Array (VLA) and of the CO(1–0) emission line using the Atacama Large Millimeter/submillimeter Array (ALMA). We combine these data sets to predict the star formation rate (SFR) using a pressure-based SF relation and show that it does not correlate well with the SFR derived from Hα on sub-kpc scales. We apply a recently developed statistical model for the small-scale behaviour of the SF relation to predict and interpret the observed scatter. Results. We find regularly rotating HI gas that is smoothly distributed across the entire disc. The CO(1–0) emission line is not detected for either of the two galaxies. The derived upper limit on the CO mass implies a molecular gas depletion time of tdepl ≲ 20 Myr. However, because of the low metallicity, the CO-to-H2 conversion factor is highly uncertain. In the relations between the Hα-based SFR and the HI mass, we observe a substantial scatter we demonstrate results from small-number statistics of independent SF regions on sub-kpc scales. Conclusions. We confirm the HI-richness of ESO534-G001 and 2dFRS S833Z022, and find that the scale dependence of the atomic SF relation in these galaxies is consistent with the predicted increase in the scatter towards small size scales. This is due to the incomplete sampling of independent HI clouds and SF regions. This finding adds to the existing literature, which has reported a scale dependence of the molecular SF relation, showing that the atomic and molecular phases are both susceptible to the evolutionary cycling of individual regions. This suggests that the atomic gas reservoirs host substantial substructure, which should be observable with future high-resolution observations