Laya Ghodsi

We perform one of the first spatially resolved studies of warm (>100 K) and cold (10–100 K) molecular gas in the circumgalactic medium (CGM), focusing on the brightest cluster galaxy (BCG) of a cool-core galaxy cluster, MACS 1931−26, at z = 0.35. This galaxy has a massive H ₂ reservoir and a radio-loud active galactic nucleus and is undergoing a starburst event. We present new JWST observations of this system, revealing warm H ₂ gas that is cospatial with the cold molecular gas traced by CO, extending over 30 kpc around the BCG in a tail-like structure reaching into the CGM of this galaxy. Analysis of the mid-infrared pure H ₂ rotational lines H ₂ S(1), H ₂ S(5), and H ₂ S(9) indicate warm gas temperatures of 515.6 ± 0.8 and 535.2 ± 1.9 K in the BCG and tail regions, respectively. We compare the cold gas traced by the CO(3–2) observed with the Atacama Large Millimeter/submillimeter Array to the warm gas traced by JWST. The warm-to-cold molecular gas fraction is comparable in the BCG (1.4% ± 0.2%) and the CGM tail (1.9% ± 0.3%). Our analysis suggests that the dissipation of the kinetic energy of the H ₂ -emitting gas is sufficient to lead to the formation of the CO gas. This observation provides new insights into the molecular gas distribution and its potential role in the interplay between the central galaxy and its circumgalactic environment.

We present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of a starburst galaxy at cosmic noon hosting a radio-loud active galactic nucleus: PKS 0529-549 at z = 2.57. To investigate the conditions of its cold interstellar medium, we use ALMA observations that spatially resolve the [C i] fine-structure lines, [C i] (2–1) and [C i] (1–0), CO rotational lines, CO (7–6) and CO (4–3), and the rest-frame continuum emission at 461 and 809 GHz. The four emission lines display different morphologies, suggesting spatial variation in the gas excitation conditions. The radio jets have just broken out of the molecular gas but not through the more extended ionized gas halo. The [C i] (2–1) emission is more extended (≈8 kpc × 5 kpc) than detected in previous shallower ALMA observations. The [C i] luminosity ratio implies an excitation temperature of 44 ± 16 K, similar to the dust temperature. Using the [C i] lines, CO (4–3), and 227 GHz dust continuum, we infer the mass of molecular gas M _mol using three independent approaches and typical assumptions in the literature. All approaches point to a massive molecular gas reservoir of about 10¹¹ M _☉, but the exact values differ by up to a factor of 4. Deep observations are critical in correctly characterizing the distribution of cold gas in high-redshift galaxies, and highlight the need to improve systematic uncertainties in inferring accurate molecular gas masses.

The evolution of galaxies is largely affected by exchanging material with their close environment, the circumgalactic medium (CGM). In this work, we investigate the CGM and the interstellar medium (ISM) of the bright central galaxy (BCG) of the galaxy cluster, MACS1931−26 at z ∼ 0.35. We detected [CI](2−1), ¹²CO(1−0), and ¹²CO(7−6) emission lines with the APEX 12-m and NRO 45-m telescopes. We complemented these single-dish observations with ¹²CO(1−0), ¹²CO(3−2), and ¹²CO(4−3) ALMA interferometric data and inferred the cold molecular hydrogen physical properties. Using a modified large velocity gradient (LVG) model, we modelled the CO and CI emission of the CGM and BCG to extract the gas thermodynamical properties, including the kinetic temperature, the density, and the virialisation factor. Our study shows that the gas in the BCG is highly excited, comparable to the gas in local ultra luminous infrared galaxies (ULIRGs), while the CGM is likely less excited, colder, less dense, and less bound compared to the ISM of the BCG. The molecular hydrogen mass of the whole system derived using [CI](2−1) is larger than the mass derived from ¹²CO(1−0) in literature, showing that part of the gas in this system is CO-poor. Additional spatially resolved CI observations in both transitions, [CI](1−0) and [CI](2−1), and the completion of the CO SLED with higher CO transitions are crucial to trace the different phases of the gas in such systems and constrain their properties.

Environmental effects on the formation and evolution of galaxies have been one of the leading questions in galaxy studies during the past few decades. In this work, we investigate the relationship between the star formation activity of galaxies and their environmental matter density using the cosmological hydrodynamic simulation Simba. The galactic star formation activity indicators that we explore include the star formation efficiency (SFE), specific star formation rate (sSFR) and molecular hydrogen mass fraction (⁠|$f^*_{\mathrm{H_2}}$|⁠), and the environment is considered as the large scale environmental matter density, calculated based on the stellar mass of nearby galaxies on a 1 Mpc/h grid using the cloud in cell (CIC) method. Our sample includes galaxies with |$9<\log \frac{M_*}{{\rm M}_{\odot }}$| at 0 < z < 4, divided into three stellar mass bins to disentangle the effects of stellar mass and environment on the star formation activity of galaxies. For low- to intermediate-mass galaxies at low-redshifts (z < 1.5), we find that the star formation efficiency of those in high-density regions are ∼0.3 dex lower than those in low-density regions. However, there is no significant environmental dependence of the star formation efficiency for massive galaxies over all our redshift range, and low- to intermediate-mass galaxies at high redshifts (z > 1.5). We present a scaling relation for the depletion time of cold molecular hydrogen (t_depl = 1/SFE) as a function of galaxy parameters including environmental density. Our findings provide a framework for quantifying the environmental effects on the star formation activities of galaxies as a function of stellar mass and redshift. The most significant environmental dependence is seen at later cosmic times (z < 1.5) and towards lower stellar masses (⁠|$9<\log \frac{M_*}{{\rm M}_{\odot }}<10$|⁠). Future large galaxy surveys can use this framework to look for the environmental dependence of the star formation activity and examine our predictions.