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Register a new userProbing the Physical Conditions of Atomic Gas at High Redshift
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Marcel Neeleman
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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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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.
We present results on the properties of neon emission in $zsim2$ star-forming galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Doubly-ionized neon ([NeIII]3869) is detected at $geq3sigma$ in 61 galaxies, representing $sim$25% of the MOSDEF sample with H$alpha$, H$beta$, and [OIII]$5007$ detections at similar redshifts. We consider the neon emission-line properties of both individual galaxies with [NeIII]3869 detections and composite $zsim2$ spectra binned by stellar mass. With no requirement of [NeIII]3869 detection, the latter provide a more representative picture of neon emission-line properties in the MOSDEF sample. The [NeIII]3869/[OII]3727 ratio (Ne3O2) is anti-correlated with stellar mass in $zsim2$ galaxies, as expected based on the mass-metallicity relation. It is also positively correlated with the [OIII]$5007$/[OII]$3727$ ratio (O32), but $zsim2$ line ratios are offset towards higher Ne3O2 at fixed O32, compared with both local star-forming galaxies and individual H~II regions. Despite the offset towards higher Ne3O2 at fixed O32 at $zsim2$, biases in inferred Ne3O2-based metallicity are small. Accordingly, Ne3O2 may serve as an important metallicity indicator deep into the reionization epoch. Analyzing additional rest-optical line ratios including [NeIII]$3869$/[OIII]$5007$ (Ne3O3) and [OIII]$5007$/H$beta$ (O3H$beta$), we conclude that the nebular emission-line ratios of $zsim2$ star-forming galaxies suggest a harder ionizing spectrum (lower stellar metallicity, i.e., Fe/H) at fixed gas-phase oxygen abundance, compared to systems at $zsim0$. These new results based on neon lend support to the physical picture painted by oxygen, nitrogen, hydrogen, and sulfur emission, of an ionized ISM in high-redshift star-forming galaxies irradiated by chemically young, $alpha$-enhanced massive stars.
We report the first detailed measurement of the shape of the CO luminosity function at high redshift, based on $>$320 hr of the NSFs Karl G. Jansky Very Large Array (VLA) observations over an area of $sim$60 arcmin$^2$ taken as part of the CO Luminosity Density at High Redshift (COLDz) survey. COLDz blindly selects galaxies based on their cold gas content through CO($J$=1$to$0) emission at $z$$sim$2-3 and CO($J$=2$to$1) at $z$$sim$5-7 down to a CO luminosity limit of log($L_{rm CO}$/K km s$^{-1}$ pc$^2$)$simeq$9.5. We find that the characteristic luminosity and bright end of the CO luminosity function are substantially higher than predicted by semi-analytical models, but consistent with empirical estimates based on the infrared luminosity function at $z$$sim$2. We also present the currently most reliable measurement of the cosmic density of cold gas in galaxies at early epochs, i.e., the cold gas history of the universe, as determined over a large cosmic volume of $sim$375,000 Mpc$^3$. Our measurements are in agreement with an increase of the cold gas density from $z$$sim$0 to $z$$sim$2-3, followed by a possible decline towards $z$$sim$5-7. These findings are consistent with recent surveys based on higher-$J$ CO line measurements, upon which COLDz improves in terms of statistical uncertainties by probing $sim$50-100 times larger areas and in the reliability of total gas mass estimates by probing the low-$J$ CO lines accessible to the VLA. Our results thus appear to suggest that the cosmic star-formation rate density follows an increased cold molecular gas content in galaxies towards its peak about 10 billion years ago, and that its decline towards the earliest epochs is likely related to a lower overall amount of cold molecular gas (as traced by CO) bound in galaxies towards the first billion years after the Big Bang.
Following the first pioneering efforts in the 1990s that have focused on the detection of the molecular interstellar medium in high redshift galaxies, recent years have brought great advances in our understanding of the actual physical properties of the gas that set the conditions for star formation. Observations of the ground-state CO J=1-0 line have furnished crucial information on the total masses of the gas reservoirs, as well as reliable dynamical mass and size estimates. Detailed studies of rotational ladders of CO have provided insight on the temperature and density of the gas. Investigations of the very dense gas associated with actively star-forming regions in the interstellar medium, most prominently through HCN and HCO+, have enabled a better understanding of the nature of the extreme starbursts found in many high-redshift galaxies, which exceed the star formation rates of their most active present-day counterparts by an order of magnitude. Key progress in this area has been made through targeted studies of few, well-selected systems with current facilities. With the completion of the Karl G. Jansky Very Large Array and the Atacama Large (sub)Millimeter Array, it will become possible to develop a more general framework for the interpretation of these investigations based on unbiased studies of normal star-forming galaxies back to the earliest cosmic epochs.
We present here a three-dimesional hydrodynamical simulation for star formation. Our aim is to explore the effect of the metal-line cooling on the thermodynamics of the star-formation process. We explore the effect of changing the metallicty of the gas from $Z/Z_{odot}=10^{-4}$ to $Z/Z_{odot}=10^{-2}$. Furthermore, we explore the implications of using the observational abundance pattern of a CEMP-no star, which have been considered to be the missing second-generation stars, the so-called Pop. III.2 stars. In order to pursue our aim, we modelled the microphysics by employing the public astrochemistry package KROME, using a chemical network which includes sixteen chemical species (H, H$^{+}$, H$^{-}$, He, He$^{+}$, He$^{++}$, e$^{-}$, H$_{2}$, H$_{2}^{+}$, C, C$^{+}$, O, O$^{+}$, Si, Si$^{+}$, and Si$^{++}$). We couple KROME with the fully three-dimensional Smoothed-particle hydrodynamics (SPH) code GRADSPH. With this framework we investigate the collapse of a metal-enhanced cloud, exploring the fragmentation process and the formation of stars. We found that the metallicity has a clear impact on the thermodynamics of the collapse, allowing the cloud to reach the CMB temperature floor for a metallicity $Z/Z_{odot}=10^{-2}$, which is in agreement with previous work. Moreover, we found that adopting the abundance pattern given by the star SMSS J031300.36-670839.3 the thermodynamics behavior is very similar to simulations with a metallicity of $Z/Z_{odot}=10^{-2}$, due to the high carbon abundance. As long as only metal line cooling is considered, our results support the metallicity threshold proposed by previous works, which will very likely regulate the first episode of fragmentation and potentially determine the masses of the resulting star clusters.
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