[Paper Review] Discovery of a Perseus-like cloud in the early Universe: H I-to-H 2 transition, carbon monoxide and small dust grains at z abs ≈ 2.53 towards the quasar J0000+0048

P. Noterdaeme, Jens-Kristian Krogager+11 more|arXiv (Cornell University)|Sep 6, 2016

Astrophysics and Star Formation Studies141 references41 citations

TL;DR

This study presents the discovery of a high-redshift molecular cloud at z ≈ 2.53 toward quasar J0000+0048, exhibiting a molecular fraction f ≈ 50%, the highest observed in a high-redshift intervening system. Using high-resolution UV and optical spectra from VLT/UVES and X-Shooter, combined with Cloudy modeling, the authors identify a cold, dense cloud (nH ≈ 80 cm⁻³, T ≈ 50 K) with super-solar metallicity and small dust grains, consistent with a steep extinction curve and 2175 Å bump, while deriving a CMB temperature of 9.6 K, in agreement with adiabatic cooling predictions.

ABSTRACT

We present the discovery of a molecular cloud at zabs=2.5255 along the line of sight to the quasar J0000+0048. We perform a detailed analysis of the absorption lines from ionic, neutral atomic and molecular species in different excitation levels, as well as the broad-band dust extinction. We find that the absorber classifies as a Damped Lyman-alpha system (DLA) with logN(HI)(cm^-2)=20.8+/-0.1. The DLA has super-Solar metallicity with a depletion pattern typical of cold gas and an overall molecular fraction ~50%. This is the highest f-value observed to date in a high-z intervening system. Most of the molecular hydrogen arises from a clearly identified narrow (b~0.7 km/s), cold component in which CO molecules are also found, with logN(CO)~15. We study the chemical and physical conditions in the cold gas. We find that the line of sight probes the gas deep after the HI-to-H2 transition in a ~4-5 pc-size cloud with volumic density nH~80 cm^-3 and temperature of only 50 K. Our model suggests that the presence of small dust grains (down to about 0.001 μm) and high cosmic ray ionisation rate (zeta_H a few times 10^-15 s^-1) are needed to explain the observed atomic and molecular abundances. The presence of small grains is also in agreement with the observed steep extinction curve that also features a 2175 A bump. The properties of this cloud are very similar to what is seen in diffuse molecular regions of the nearby Perseus complex. The high excitation temperature of CO rotational levels towards J0000+0048 betrays however the higher temperature of the cosmic microwave background. Using the derived physical conditions, we correct for a small contribution (0.3 K) of collisional excitation and obtain TCMB(z = 2.53)~9.6 K, in perfect agreement with the predicted adiabatic cooling of the Universe. [abridged]

Motivation & Objective

• To investigate the physical and chemical conditions of a high-redshift molecular cloud in the early Universe.
• To determine the molecular fraction and metallicity of the absorber, identifying it as a DLA with super-solar metallicity.
• To understand the role of small dust grains and cosmic ray ionization in enabling high CO abundance in cold, dense gas.
• To measure the cosmic microwave background temperature at z ≈ 2.53 using CO excitation, testing cosmological models.
• To compare the cloud's properties with those of nearby Perseus complex diffuse molecular regions.

Proposed method

• High-resolution UV and optical spectroscopy using VLT/UVES and X-Shooter to detect absorption lines from H I, H2, CO, C I, and other species.
• Spectral fitting with the vpfit code to measure column densities and kinematic structure of the absorbing components.
• Application of the Cloudy spectral synthesis code to model the physical conditions (density, temperature, UV field, cosmic ray ionization rate) in the cloud.
• Use of the CO rotational level excitation to infer the kinetic temperature and correct for collisional excitation effects.
• Derivation of the CMB temperature at z ≈ 2.53 by correcting for collisional excitation and comparing observed CO excitation to theoretical models.
• Analysis of dust extinction using the broad-band SED and comparison with the 2175 Å bump and depletion patterns.

Experimental results

Research questions

• RQ1 What are the physical and chemical conditions in this high-redshift molecular cloud, and how do they compare to local diffuse molecular clouds?
• RQ2 Why is the molecular fraction f ≈ 50% in this system, and what processes enable such a high value at high redshift?
• RQ3 What is the role of small dust grains and cosmic ray ionization in sustaining high CO abundance in this cloud?
• RQ4 Can the CMB temperature at z ≈ 2.53 be measured from CO excitation, and does it agree with cosmological predictions?
• RQ5 What observational indicators (e.g., C I, extinction curve) can predict detectable CO in high-redshift DLAs?

Key findings

• The absorber is a Damped Lyman-α system with log N(H I) = 20.8 ± 0.1 cm⁻² and super-solar metallicity (Z ≈ 2.5 Z⊙), with a depletion pattern typical of cold gas.
• The molecular fraction f ≈ 50% is the highest observed to date in a high-redshift intervening system, with H2 and CO detected in a narrow, cold component (b ≈ 0.7 km s⁻¹, T ≈ 50 K).
• Cloudy modeling indicates a dense, cold cloud with nH ≈ 80 cm⁻³, a moderate UV field, and a high cosmic ray ionization rate (ζH ≈ a few × 10⁻¹⁵ s⁻¹).
• The presence of small dust grains (down to ~0.001 µm) and a 2175 Å bump are required to reproduce the observed extinction curve and CO abundance.
• After correcting for collisional excitation, the derived CMB temperature is 9.6 K at z = 2.53, in perfect agreement with the adiabatic cooling prediction TCMB(z) = T₀(1 + z).
• The cloud's physical and chemical properties are remarkably similar to those of diffuse molecular regions in the nearby Perseus complex, despite forming when the Universe was only 2.5 Gyr old.