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The HI-to-H2 transition in the high redshift Universe – HIH2

The HI-to-H2 transition in the distant Universe

Constraining the conversion from atomic to molecular gas at high redshift using absorption spectroscopy.

Understanding the HI-H2 transition in different environments

The conversion from atomic gas (HI) into molecular gas (H2) in galaxies is a fundamental process deeply linked not only to the onset of star-formation but also to the feeding of active galactic nuclei. This means that this process is key in the evolution of normal galaxies as well as those hosting an active nucleus. <br /><br />However, the processes that lead to the formation of molecular gas imply complex physics and depend strongly on the chemical enrichment (such as abundance and properties of dust) and the physical conditions in the clouds (such as temperature, density, UV and cosmic-ray flux) . We therefore expect that the HI-H2 transition behaves differently in different environments, in particular in extreme conditions as expected close to quasars.<br /> <br />While important progress have been made about the transition in nearby clouds, thanks to resolved observations and theoretical works, our knowledge about the transition remains very limited when it comes to the distant Universe. This is mostly due to the difficulty and lack of suitable observations. The goal of the project is to resolve these issues.

Absorption spectroscopy of bright background sources constitutes an excellent way to probe the different constituents of the interstellar medium (HI, H2, metals, dust) at physical scales where the transition occurs. However, the transition remains practically out of reach till now due to the very small absorption cross-section of the molecular gas. Current absorption studies are therefore mostly limited to very diffuse and warm phases.
The project proposes to overcome the challenge thanks to innovative and complementary selections of the absorbers in large spectroscopic surveys, together with a follow-up on large telescopes. This allows one to obtain a census of cold gas and derive the prevailing physical and chemical conditions from the abundance and excitation of atomic and molecular species, with the help of advanced numerical codes and theoretical descriptions of the transition.

We have developed several techniques extremely efficient to select transitioning gas and studied their complimentary. In particular, we observe a transition over a very wide range of metallicities, when local studies are limited to a small range. We show that the ratio of the intensity of the UV flux to the volume density is a key quantity driving the point of conversion, for a given metallicity, as predicted by theoretical studies.

We have also studied the gas at the heart of high-redshift galaxies thanks to gamma-ray burst on one hand, and thanks to intervening absorbers along quasar lines of sight passing at very small impact parameters from the galaxy centroid (< 1 kpc).

We have obtained a first measurement of the H2 distribution function at z~3, showing that the diffuse molecular phase accounts for about 15% of the total H2 content, and that the conversion from atomic to molecular gas typically occurs at column densities much higher than observed locally, under the typical chemical enrichment observed at those redshifts.

We have probe the gas over several scales thanks to the observation of a gravitationally-lensed quasar putting direct constraints on the filling factor of cold gas. Cold gas is found to be confined in sub-pc clouds, but these clouds are distributed over at least kpc-scales.

Finally, our project allowed us to detect an over-density of molecular gas close to the quasar. This opens up the study of quasar fuelling and feedback, as well as probing the gaz physics in harsh environments.

In addition to explore a variety of environments (in terms of selection, chemical enrichment, ambient UV field, etc.) and perform detailed studies of the same, we now also wish to interpret the large amount of collected data in a global manner, in order to understand the distribution of molecular gas at high redshift, as a function of the associated galaxies characteristics.
The project's opening to gas associated to the quasar themselves opens up a new research domain that brings unique constraints on the issue of active nuclei and host galaxy co-evolution.

Noterdaeme et al. 2018, A&A, 612, A58 : «Spotting high-z molecular absorbers using neutral carbon»

Balashev & Noterdaeme 2018, MNRAS 478, L7 : «Constraining the H2 column distribution at z~3 from composite DLA spectra»

Ranjan et al. 2018, A&A, 618, A184 : «Molecular gas and star-formation in an absorption-selected galaxy: hitting the bull’s eye at z=2.46«

Krogager et al. 2018, A&A, 619, A142 : « Dissecting cold gas in a high-redshift galaxy using a lensed background quasar »

Heintz et al. 2019, A&A, 621, A20 : «Cold gas in the early Universe. Survey for neutral atomic-carbon in GRB host galaxies at 1 < z < 6 from optical afterglow spectroscopy»

Krogager et al. 2019, MNRAS, 486, 4377 : « The effect of dust bias on the census of neutral gas and metals in the high-redshift Universe due to SDSS-II quasar colour selection »

Noterdaeme et al. 2019, A&A, 627, A32 : «Proximate molecular quasar absorbers: Excess of damped H2 systems at zabs˜zQSO in SDSS DR14 »

Heintz et al. 2019, A&A, 629, A131: «New constraints on the physical conditions in H 2 -bearing GRB-host damped Lyman-a absorbers«

Balashev et al. 2019, MNRAS, 490, 2668: «X-shooter observations of strong H 2 -bearing DLAs at high redshift«

Ranjan et al. 2020, A&A, 633, A125: «Chemical enrichment and host galaxies of extremely strong intervening DLAs towards quasars«

Krogager et al., 2020, MNRAS, 495, 3014: «High-redshift damped Ly-alpha absorbing galaxy model reproducing the N(HI) - Z distribution«

Balashev et al. 2020, MNRAS, 497, 1946: «Nature of the DLA towards Q 0528-250: High pressure and strong UV field revealed by excitation of CI, H2 , and Si II«

Zou et al. 2020, ApJ, 901, 105: «Carbon-enhanced Lyman Limit System: Signature of the First Generation of Stars?«

Krogager & Noterdaeme 2020, A&A, 644, L6: «Modelling the Statistics of Cold Neutral Medium in Absorption-selected High-redshift Galaxies«

Klimenko et al. 2020, AstL, 46, 715: «Estimation of the Cosmic Microwave Background Temperature from Atomic C I and Molecular CO Lines in the Interstellar Medium of Early Galaxies«

Noterdaeme et al. 2021, A&A, 646, A108: «Down-the-barrel observations of a multi-phase quasar outflow at high redshift«

Noterdaeme et al. 2021, A&A, 651, A78: «Sharpening quasar absorption lines with ESPRESSO«

Noterdaeme et al. 2021, A&A, 651, A17: «Remarkably high mass and velocity dispersion of molecular gas associated with a regular, absorption-selected type I quasar«

Kosenko et al. 2021, MNRAS, 505, 3810: «HD molecules at high redshift: cosmic-ray ionization rate in the diffuse interstellar medium«

Heintz et al. 2021, MNRAS, 507, 1434: «GRB host galaxies with strong H2 absorption: CO-dark molecular gas at the peak of cosmic star formation«

Balashev et al. 2022, MNRAS, 509, L26: «CII*/CII ratio in high-redshift DLAs: ISM phase separation drives the observed bimodality of [CII] cooling rates«

Telikova et al. 2022, MNRAS, 510, 5974: «Extremely strong DLAs at high redshift: Gas cooling and H2 formation«

The conversion of atomic (HI) to molecular (H2) gas in galaxies is a fundamental process which is deeply associated to the onset of star formation and consequently to the evolution of galaxies. Indeed, stars are known to form in molecular clouds. In addition, many resolved observations of nearby galaxies have shown that the star-formation rate (SFR) surface density correlates strongly with the surface density of molecular gas. However, the processes that determine how the H2 gas form out of atomic gas involve complex physics and depend sensitively on the chemical enrichment (such as the abundance and properties of dust) and the physical conditions of the gaseous clouds (such as density, temperature, incident radiation field and cosmic ray flux). The HI-H2 transition is therefore expected to behave differently in different environments.
Important progress has been made recently to understand the transition in nearby clouds thanks to resolved observations and detailed theoretical works. In turn, we still know very little about he conversion in the distant Universe. The goal of this ANR is to address this issue.
Absorption spectroscopy towards bright background sources presents in principle an excellent way to directly measure the different constituents of the inter-stellar medium (HI, H2, gas-phase metals, dust) at the small scales on which the transition occurs. However, the transition remains almost elusive so far due to the very small absorption cross section of molecular gas. The field has therefore been mostly limited to the study of very diffuse atomic phases.
We will overcome this challenge through innovative and complementary selections of the absorbers in massive spectroscopic surveys and follow-up on large telescopes. We have already collected a very significant amount of data with the Very Large Telescope, which is ready for analysis. We will derive the physical and chemical conditions from the abundances and excitation of atomic and molecular species with the help of advanced modelling codes as well as testing and extending the transition theories. Finally, we will use the transition theories to derive molecular maps from high-resolution galaxy simulations and compare the gaseous environments with our observations.
The PI has built a solid team with a very complementary set of expertise. Funding is required to support the research, in particular for post-doctoral manpower and close interaction between the team members, and to maximise the overall project impact.

Project coordination

Pasquier Noterdaeme (Institut d'Astrophysique de Paris)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


IAP Institut d'Astrophysique de Paris

Help of the ANR 258,836 euros
Beginning and duration of the scientific project: September 2017 - 48 Months

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