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Register a new userScience with an ngVLA. Cold gas in High-z Galaxies: The molecular gas budget
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The goal of this science case is to accurately pin down the molecular gas content of high redshift galaxies. By targeting the CO ground transition, we circumvent uncertainties related to CO excitation. The ngVLA can observe the CO(1-0) line at virtually any $z>1.5$, thus exposing the evolution of gaseous reservoirs from the earliest epochs down to the peak of the cosmic history of star formation. The order-of-magnitude improvement in the number of CO detections with respect to state-of-the-art observational campaigns will provide a unique insight on the evolution of galaxies through cosmic time.
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The goal of this science case is to study physical conditions of the interstellar medium (ISM) in distant galaxies. In particular, its densest component is associated with the inner cores of clouds -- this is where star formation takes place. Carbon monoxide is usually used to trace molecular gas emission; however, its transitions are practically opaque, thus preventing astronomers from piercing through the clouds, into the deepest layers that are most intimately connected with the formation of stars. Other dense gas tracers are required, although they are typically too faint and/or at too low frequencies to be effectively observed in high redshift galaxies. The ngVLA will offer for the first time the sensitivity at radio frequencies that is needed to target [CI]$_{1-0}$ (at $z>5$), as well as the ground transitions of dense gas tracers of the ISM such as HCN, HNC, HCO+ (at various redshifts $z>1$), beyond the tip of the iceberg of the hyper-luminous sources that could be studied up to now. These new tools will critically contribute to our understanding of the intimate interplay between gas clouds and star formation in different environments and cosmic epochs.
The goal of this science case is to address the use of a ngVLA as a CO redshift machine for dust-obscured high-redshift galaxies which lack of clear counterparts at other wavelengths. Thanks to its unprecedentedly large simultaneous bandwidth and sensitivity, the ngVLA will be able to detect low--J CO transitions at virtually any $z>1$. In particular, at $z>4.76$ two CO transitions will be covered in a single frequency setting, thus ensuring unambiguous line identification. The ngVLA capabilities fill in a redshift range where other approaches (e.g., photometric redshifts, search for optical/radio counterparts, etc) typically fail due to the combination of intrinsically faint emission and increasing luminosity distance. This will allow us to explore the formation of massive galaxies in the early cosmic times.
The next generation Very Large Array (ngVLA) will revolutionize our understanding of the distant Universe via the detection of cold molecular gas in the first galaxies. Its impact on studies of galaxy characterization via detailed gas dynamics will provide crucial insight on dominant physical drivers for star-formation in high redshift galaxies, including the exchange of gas from scales of the circumgalactic medium down to resolved clouds on mass scales of $sim10^{5},M_odot$. In this study, we employ a series of high-resolution, cosmological, hydrodynamic zoom simulations from the MUFASA simulation suite and a CASA simulator to generate mock ngVLA observations of a $zsim4.5$ gas rich star-forming galaxy. Using the DESPOTIC radiative transfer code that encompasses simultaneous thermal, chemical, and statistical equilibrium in calculating the molecular and atomic level transitions of CO from ALMA for comparison. We find that observations of CO(1-0) are especially important for tracing the systemic redshift of the galaxy and the total mass of the well-shielded molecular gas reservoir, while even CO(2-1) can predominantly trace denser gas regions distinct from CO(1-0). The factor of 100 times improvement in mapping speed for the ngVLA beyond the Jansky VLA and the proposed ALMA Band 1 will make these detailed, high-resolution imaging and kinematic studies of CO(1-0) routine at $zsim2-5$.
Stars form in cold clouds of predominantly molecular (H2) gas. We are just beginning to understand how the formation, properties, and destruction of these clouds varies across the universe. In this chapter, we describe how the thermal line imaging capabilities of the proposed next generation Very Large Array (ngVLA) could make major contributions to this field. Looking at CO emission, the proposed ngVLA would be able to quickly survey the bulk properties of molecular clouds across the whole nearby galaxy population. This includes many unique very nearby northern targets (e.g., Andromeda) inaccessible to ALMA. Such surveys offer a main observational constraint on the formation, destruction, lifetime, and star formation properties of clouds. Targeting specific regions, the ngVLA will also be able to heavily resolve clouds in the nearest galaxies. This will allow detailed studies of the substructure and kinematics --- and so the internal physics --- of clouds across different chemical and dynamical environments.
Gas density is widely believed to play a governing role in star formation. However, the exact role of density in setting the star formation rate remains debated. We also lack a general theory that explains how the gas density distribution in galaxies is set. The primary factor preventing the resolution of these issues is the limited number of observations of the gas density distribution across diverse environments. Centimeter- and millimeter-wave spectroscopy offer the most promising way forward in this field, but the key density-sensitive transitions are faint compared to the capabilities of current telescopes. In this chapter, we describe how a next-generation Very Large Array (ngVLA) represents the natural next step forward in this sensitivity-limited field. Such a facility would provide a crucial link between the `Milky Way and `Extragalactic views of star formation and dramatically advance our understanding of the drive and role of gas density in galaxies, building on current results from ALMA, NOEMA, the Green Bank Telescope, and other current facilities working in this area.
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