Katherine Rawlins
Intervening gas along the line-of-sight to a background quasar can be probed through absorption features seen in the quasar spectrum. Of all such absorbing clouds, damped Lyman-alpha absorbers (DLAs) have a very high content of neutral gas, similar to our own galaxy, the Milky Way. Further, gas that harbours molecules is likely to be associated with star formation. Investigating the structure and dynamics of DLAs will lead to enhanced comprehension of galaxy formation and evolution.
Numerical models are constructed for four H2-bearing DLAs using the spectral synthesis code CLOUDY. Of these, one is a unique DLA with multicomponent H2 absorption, and is studied in greater detail. Numerical models are constrained individually for each H2 component.
The most significant outcomes of these studies pertain to the nature of the radiation field and dust grains in these systems. All four DLAs are found to harbour dust grains smaller in size than those seen in the Galactic interstellar medium. Alternate scenarios to this are also explored, and suggest the possibility of the grains being ISM-sized and porous. DLAs are mainly illuminated by the metagalactic background, which constitutes ionizing radiation from the surrounding quasars and galaxies. However, the background radiation alone is insufficient to model three of the four systems studied here. In addition to the background, these three DLAs are irradiated by either harsh X-rays, or ionizing radiation arising from local star formation. The model results also highlight the need for improved understanding of the metagalactic background itself. These studies also enable insight into the density, temperature and pressure conditions, cosmic ray ionization rate, elemental abundances, as well as heating and cooling processes within these high-redshift DLA environments.
Damped Lyman-alpha absorbers (DLAs) are intergalactic reservoirs of neutral hydrogen (H I), and are relevant to galaxy formation and evolution. They are probed through their rest-frame ultraviolet absorption features in the spectrum of a background quasar. H2 is detected in about 10-15 percent of high-redshift DLAs. Such cool gas is likely associated with star formation.
A quasar line-of-sight passing though absorbing clouds on its way to Earth is schematically depicted. The DLAs investigated in the current study are at redshift 2.34, 2.63, 2.42 and 2.05, and are labelled DLA 1, 2, 3 and 4 respectively. They all harbour H2, with DLA 4 exhibiting H2 absorption in 7 of 14 components.
DLAs are modelled using the plane-parallel geometry of a photodissociation region, with constant pressure across the cloud. Microphysical calculations are performed using the spectral synthesis code CLOUDY. Several gas-phase species are considered, along with silicate and graphite dust. The observed H2 rotational levels and neutral carbon (C I) fine structure levels act as constraints for the models.
A photodissociation region with incident ultraviolet radiation is schematically represented, with the regions where H II/H I, H I/H2, C II/C I/CO and O I/O2 transitions occur.
Slide 2: Result 1 – Smaller or porous dust grains
Compact grains following the MRN power-law size distribution, with exponent -3.5, are considered. The observed dust abundance constrains grain sizes to be smaller than in the interstellar medium (ISM), with grain radii between 0.0025-0.125 microns. Smaller grains have been proposed earlier too for DLA 1.
Porosity is the vacuum fraction in the grain volume. ISM-sized grains with porosity 0.55 replicate the best-fitting results obtained using smaller compact grains. However, no robust conclusion is drawn, as interstellar grain porosity is not tightly constrained yet.
Species-wise model-to-observed column density ratios are plotted for DLAs 1 and 2 with different grain sizes. Optimum grain sizes are half those of ISM grains. The models with smaller compact grains and ISM-sized porous grains respectively are compared through a plot made for DLAs 3 and 4.
Slide 3: Result 2 – Nature of the radiation field
The metagalactic background radiation from quasars and galaxies is incident on all the DLAs. Additional ultraviolet photons from local star formation are required for the DLA 2 model. Power-law X-ray radiation is incorporated for DLAs 1 and 3, and suggests that they are high-redshift X-ray dominated regions. The need for X-rays could be an indication of the possible role of alternate processes like hydrodynamical heating.
DLA 4 is irradiated only by the metagalactic background. Column density predictions depend on the background model incorporated in the calculations too. The Haardt-Madau (HM) background over predicts H I and under predicts the C I fine structure levels, as compared to the Khaire-Srianand (KS) background. This is depicted through a plot. Thus, the metagalactic background needs to be understood better.
The spectral energy distribution of the HM and KS backgrounds is plotted, along with the enhanced X-rays incorporated in DLA 1.
Slide 4: Result 3 – Physical properties and processes
The fractional contribution of various heating and cooling processes within the extent of DLA 4 (component 8) is plotted. H I and He I photoionization contribute to the heating, along with cosmic rays. Cooling occurs through [Si II], [O I] and [C II] fine structure line emission, as also through collisional excitation in ground state H2. Physical properties such as hydrogen density, gas pressure and temperature, metallicity, dust abundance and cosmic ray ionization rate for the 4 DLAs are tabulated.
Results are based on:
Rawlins, Srianand, Shaw, et al., 2018, MNRAS, 481, 2083
Shaw, Rawlins & Srianand, 2016, MNRAS, 459, 3234