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Register a new userHD molecules at high redshift: The absorption system at z=2.3377 towards Q 1232+082
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We present a detailed analysis of the H_2 and HD absorption lines detected in the Damped Lyman-alpha (DLA) system at z_abs=2.3377 towards the quasar Q1232+082. We show that this intervening cloud has a covering factor smaller than unity and covers only part of the QSO broad emission line region. The zero flux level has to be corrected at the position of the saturated H_2 and optically thin HD lines by about 10%. We accurately determine the Doppler parameter for HD and CI lines (b = 1.86+/-0.20 km/s). We find a ratio N(HD)/N(H_2)=(7.1 +3.7 -2.2)x10^-5 that is significantly higher than what is observed in molecular clouds of the Galaxy. Chemical models suggest that in the physical conditions prevailing in the central part of molecular clouds, deuterium and hydrogen are mostly in their molecular forms. Assuming this is true, we derive D/H = (3.6 +1.9 -1.1)x10^-5. This implies that the corresponding baryon density of the Universe is Omega_b h^2 = (0.0182 +0.0047 -0.0042). This value coincides within 1sigma with that derived from observations of the CMBR as well as from observations of the D/H atomic ratio in low-metallicity QSO absorption line systems. The observation of HD at high redshift is therefore a promising independent method to constrain Omega_b. This observation indicates as well a low astration factor of deuterium. This can be interpreted as the consequence of an intense infall of primordial gas onto the associated galaxy.
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We have analyzed the spectrum of the quasar PKS 1232+082 obtained by Petitjean et al. (2000). HD molecular lines are identified in an absorption system at the redshift z=2.3377. The column density of HD molecules in the system is estimated, N(HD)=(1-4)x10^{14} cm^{-2}. The temperature of excitation of the first rotational level J=1 relative to the ground state J=0 is T_ex=70+-7 K. This is, to our knowledge, the first detection of HD molecules at high redshift.
We present final statistics from a survey for intervening MgII absorption towards 100 quasars with emission redshifts between $z=3.55$ and $z=7.08$. Using infrared spectra from Magellan/FIRE, we detect 279 cosmological MgII absorbers, and confirm that the incidence rate of $W_r>0.3 AA$ MgII absorption per comoving path length does not evolve measurably between $z=0.25$ and $z=7$. This is consistent with our detection of seven new MgII systems at $z>6$, a redshift range that was not covered in prior searches. Restricting to relatively strong MgII systems ($W_r>1$AA), there is significant evidence for redshift evolution. These systems roughly double in number density between $z=0$ and $z=2$-$3$, but decline by an order of magnitude from this peak by $zsim 6$. This evolution mirrors that of the global star formation rate density, which could reflect a connection between star formation feedback and strong MgII absorbers. We compared our results to the Illustris cosmological simulation at $z=2$-$4$ by assigning absorption to catalogued dark-matter halos and by direct extraction of spectra from the simulation volume. To reproduce our results using the halo catalogs, we require circumgalactic (CGM) MgII envelopes within halos of progressively smaller mass at earlier times. This occurs naturally if we define the lower integration cutoff using SFR rather than mass. MgII profiles calculated directly from the Illustris volume yield far too few strong absorbers. We argue that this arises from unresolved phase space structure of CGM gas, particularly from turbulent velocities on sub-mesh scales. The presence of CGM MgII at $z>6$-- just $sim 250$ Myr after the reionization redshift implied by Planck--suggests that enrichment of intra-halo gas may have begun before the presumed host galaxies stellar populations were mature and dynamically relaxed. [abridged]
We present a systematic study of deuterated molecular hydrogen (HD) at high redshift, detected in absorption in the spectra of quasars. We present four new identifications of HD lines associated with known $rm H_2$-bearing Damped Lyman-$alpha$ systems. In addition, we measure upper limits on the $rm HD$ column density in twelve recently identified $rm H_2$-bearing DLAs. We find that the new $rm HD$ detections have similar $N({rm HD})/N(rm H_2)$ ratios as previously found, further strengthening a marked difference with measurements through the Galaxy. This is likely due to differences in physical conditions and metallicity between the local and the high-redshift interstellar media. Using the measured $N({rm HD})/N({rm H_2})$ ratios together with priors on the UV flux ($chi$) and number densities ($n$), obtained from analysis of $rm H_2$ and associated CI lines, we are able to constrain the cosmic-ray ionization rate (CRIR, $zeta$) for the new $rm HD$ detections and for eight known HD-bearing systems where priors on $n$ and $chi$ are available. We find significant dispersion in $zeta$, from a few $times 10^{-18}$ s$^{-1}$ to a few $times 10^{-15}$ s$^{-1}$. We also find that $zeta$ strongly correlates with $chi$ -- showing almost quadratic dependence, slightly correlates with $Z$, and does not correlate with $n$, which probably reflects a physical connection between cosmic rays and star-forming regions.
Molecules dominate the cooling function of neutral metal-poor gas at high density. Observation of molecules at high redshift is thus an important tool toward understanding the physical conditions prevailing in collapsing gas. Up to now, detections are sparse because of small filling factor and/or sensitivity limitations. However, we are at an exciting time where new capabilities offer the propect of a systematic search either in absorption using the UV Lyman-Werner H2 bands or in emission using the CO emission lines redshifted in the sub-millimeter.
Massive galaxies in the distant Universe form stars at much higher rates than today. Although direct resolution of the star forming regions of these galaxies is still a challenge, recent molecular gas observations at the IRAM Plateau de Bure interferometer enable us to study the star formation efficiency on subgalactic scales around redshift z = 1.2. We present a method for obtaining the gas and star formation rate (SFR) surface densities of ensembles of clumps composing galaxies at this redshift, even though the corresponding scales are not resolved. This method is based on identifying these structures in position-velocity diagrams corresponding to slices within the galaxies. We use unique IRAM observations of the CO(3-2) rotational line and DEEP2 spectra of four massive star forming distant galaxies - EGS13003805, EGS13004291, EGS12007881, and EGS13019128 in the AEGIS terminology - to determine the gas and SFR surface densities of the identifiable ensembles of clumps that constitute them. The integrated CO line luminosity is assumed to be directly proportional to the total gas mass, and the SFR is deduced from the [OII] line. We identify the ensembles of clumps with the angular resolution available in both CO and [OII] spectroscopy; i.e., 1-1.5. SFR and gas surface densities are averaged in areas of this size, which is also the thickness of the DEEP2 slits and of the extracted IRAM slices, and we derive a spatially resolved Kennicutt-Schmidt (KS) relation on a scale of ~8 kpc. The data generally indicates an average depletion time of 1.9 Gyr, but with significant variations from point to point within the galaxies.
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