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We perform simulations of the capabilities of the next generation Very Large Array in the context of imaging low order CO emission from typical high redshift star forming galaxies at ~ 1 kpc resolution. We adopt as a spatial and dynamical template the CO 1-0 emission from M 51, scaled accordingly for redshift, transition, and total gas mass. The molecular gas masses investigated are factors of 1.4, 3.5, and 12.5 larger that of M 51, at z = 0.5, 2, and 4.2, respectively. The z = 2 galaxy gas mass is comparable to the lowest mass galaxies currently being discovered in the deepest ALMA and NOEMA cosmological CO line surveys, corresponding to galaxies with star formation rates ~ 10 to 100 $M_odot$ yr$^{-1}$. The ngVLA will perform quality imaging at 1kpc resolution of the gas distribution and dynamics over this disk. We recover the overall rotation curve, galaxy orientation properties, and molecular ISM internal velocity dispersion. The model at z = 4.2 corresponds to a massive star forming main sequence disk (SFR ~ 130 $M_odot$ yr$^{-1}$). The ngVLA can obtain 1kpc resolution images of such a system in a reasonable integration time, and recover the basic galaxy orientation parameters, and, asymptotically, the maximum rotation velocity. We compare the ngVLA results with capabilities of ALMA and the Jansky VLA. ALMA and the VLA can detect the integrated low order CO emission from these galaxies, but lack the sensitivity to perform the high resolution imaging to recover the dynamics at 1kpc scales. To do so would require of order 1000 hrs per galaxy with these current facilities. We investigate a minimal ngVLA configuration, removing the longest baselines and much of the very compact core, and find good imaging can still be performed at 1 kpc resolution.
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The Early Universe Molecular Emission Line Galaxies (EMGs) are a population of galaxies with only 36 examples that hold great promise for the study of galaxy formation and evolution at high redshift. The classification, luminosity of molecular line emission, molecular mass, far-infrared (FIR) luminosity, star formation efficiency, morphology, and dynamical mass of the currently known sample are presented and discussed. The star formation rates derived from the FIR luminosity range from about 300 to 5000 M(sun)per year and the molecular mass from 4 x 10^9 to 1 x 10^{11} M(sun). At the lower end, these star formation rates, gas masses, and diameters are similar to those of local ultraluminous infrared galaxies, and represent starbursts in centrally concentrated disks, sometimes, but not always, associated with active galactic nuclei. The evidence for large (> 5 kpc) molecular disks is limited. Morphology and several high angular resolution images suggest that some EMGs are mergers with a massive molecular interstellar medium in both components. A critical question is whether the EMGs, in particular those at the higher end of the gas mass and luminosity distribution, represent the formation of massive, giant elliptical galaxies in the early Universe. The sample size is expected to grow explosively in the era of the Atacama Large Millimeter Array (ALMA).
We have recently developed a post-processing framework to estimate the abundance of atomic and molecular hydrogen (HI and H2, respectively) in galaxies in large-volume cosmological simulations. Here we compare the HI and H2 content of IllustrisTNG galaxies to observations. We mostly restrict this comparison to $z approx 0$ and consider six observational metrics: the overall abundance of HI and H2, their mass functions, gas fractions as a function of stellar mass, the correlation between H2 and star formation rate, the spatial distribution of gas, and the correlation between gas content and morphology. We find generally good agreement between simulations and observations, particularly for the gas fractions and the HI mass-size relation. The H2 mass correlates with star formation rate as expected, revealing an almost constant depletion time that evolves up to z = 2 as observed. However, we also discover a number of tensions with varying degrees of significance, including an overestimate of the total neutral gas abundance at z = 0 by about a factor of two and a possible excess of satellites with no or very little neutral gas. These conclusions are robust to the modelling of the HI/H2 transition. In terms of their neutral gas properties, the IllustrisTNG simulations represent an enormous improvement over the original Illustris run. All data used in this paper are publicly available as part of the IllustrisTNG data release.
The largest galaxies in the Universe reside in galaxy clusters. Using sensitive observations of carbon-monoxide, we show that the Spiderweb Galaxy -a massive galaxy in a distant protocluster- is forming from a large reservoir of molecular gas. Most of this molecular gas lies between the protocluster galaxies and has low velocity dispersion, indicating that it is part of an enriched inter-galactic medium. This may constitute the reservoir of gas that fuels the widespread star formation seen in earlier ultraviolet observations of the Spiderweb Galaxy. Our results support the notion that giant galaxies in clusters formed from extended regions of recycled gas at high redshift.
We present observations of redshifted CO(1-0) and CO(2-1) in a field containing an overdensity of Lyman break galaxies (LBGs) at z=5.12. Our Australia Telescope Compact Array observations were centered between two spectroscopically-confirmed z=5.12 galaxies. We place upper limits on the molecular gas masses in these two galaxies of M(H_2) <1.7 x 10^10 M_sun and <2.9 x 10^9 M_sun (2 sigma), comparable to their stellar masses. We detect an optically-faint line emitter situated between the two LBGs which we identify as warm molecular gas at z=5.1245 +/- 0.0001. This source, detected in the CO(2-1) transition but undetected in CO(1-0), has an integrated line flux of 0.106 +/- 0.012 Jy km/s, yielding an inferred gas mass M(H_2)=(1.9 +/- 0.2) x 10^10 M_sun. Molecular line emitters without detectable counterparts at optical and infrared wavelengths may be crucial tracers of structure and mass at high redshift.
A new method is used to measure the physical conditions of the gas in damped Lyman-alpha systems (DLAs). Using high resolution absorption spectra of a sample of 80 DLAs, we are able to measure the ratio of the upper to lower fine-structure levels of the ground state of C II and Si II. These ratios are determined solely by the physical conditions of the gas. We explore the allowed physical parameter space using a Monte Carlo Markov Chain method to constrain simultaneously the temperature, neutral hydrogen density, and electron density of each DLA. The results indicate that at least 5 % of all DLAs have the bulk of their gas in a dense, cold phase with typical densities of ~100 cm-3 and temperatures below 500 K. We further find that the typical pressure of DLAs in our sample is log(P/k) = 3.4 [K cm-3], which is comparable to the pressure of the local interstellar medium (ISM), and that the components containing the bulk of the neutral gas can be quite small with absorption sizes as small as a few parsec. We show that the majority of the systems are consistent with having densities significantly higher than expected from a purely canonical WNM, indicating that significant quantities of dense gas (i.e. n_H > 0.1 cm-3) are required to match observations. Finally, we identify 8 systems with positive detections of Si II*. These systems have pressures (P/k) in excess of 20000 K cm-3, which suggest that these systems tag a highly turbulent ISM in young, star-forming galaxies.
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