Probing The Circumgalactic Medium With Quasar Absorption Lines

Abstract

Quasars (QSOs), the most distant and intrinsically luminous objects in the Universe, provide a direct probe to the history of the Universe. The quasar absorption spectroscopy has been one of the most used sensitive tools to probe the physical conditions of several different astrophysical environments. In particular, absorption lines observed in the background distance quasar spectra are unique probes of intervening gas from very high density (𝑛H ∼ 105 cm−3) outflowing gas to the extremely low density (𝑛H ∼ 10−5 cm−3) intergalactic medium (IGM). Studying them in absorption at low and high redshifts reveals a plethora of information about the nature of the gaseous environments, chemical enrichment, formation, and evolution of galaxies. This thesis examines the absorbing gas traced by double ionized carbon (C III) metal line species using archival data of high resolution optical (groundbased) and ultraviolet (UV) (spacebased) quasar spectra. In the first part of the thesis, a detailed survey of high 𝑧 (𝑧 > 2) C III systems has been carried out using archival spectroscopic data of ground based observations made with UVVisual Echelle Spectrograph (UVES) onboard Very Large Telescope(VLT) and the high Resolution Echelle Spectrometer (HIRES) on Keck. Out of a plethora of C III systems identified in this survey, 53 optically thin C III systems in the redshift range, 2.1 < 𝑧 < 3.4 are analyzed. A detailed photoionization model is performed for all these well aligned optically thin C III absorption systems. Using the photoionization models, several physical parameters such as density (𝑛H), metallicity ([C/H]), total hydrogen column density, and lineofsight thickness (𝐿) in each C III component is estimated. The systematic errors in these quantities contributed by the allowed range of the quasar spectral index used in the ultraviolet background radiation calculations are also considered. The inferred 𝑛H and overdensity (Δ) are much higher than the typical IGM gas measurements available in the literature and favor the absorption originating from gas associated with the circumgalactic medium (CGM). The derived parameters 𝑛H, L and [C/H] of the C III absorbing gas show statistically significant redshift evolution. To some extent, this redshift evolution is driven by the appearance of compact, high 𝑛H and high [C/H] components only at lower redshift end (2 ≤ 𝑧 ≤ 2.5). In this study, a correlation with more than 5𝜎 confidence level was found between [C/H] and 𝐿, L and neutral hydrogen column density (𝑁(H I)), 𝑁(H I) and [C/H]. Using a simple toy cooling model, it is explained that the L versus [C/H] correlation can be well reproduced if L is governed by the product of gas cooling time and sound speed as expected in the case of cloud fragmentation under thermal instabilities. In this case, other observed correlations can be explained by simple photoionization considerations. Although the C III systems studied in this work show similar physical properties as CGM gas absorbers, no galaxy association was found for these absorbers. Hence, it is crucial to study these optically thin C III absorbers over a large 𝑧 range, and correlating their 𝑧 evolution with global star formation rate density evolution can shed light on the physics of cold clump formation and their evolution in the CGM. In the second part of this thesis, a detailed analysis of 99 optically thin C III absorption systems at redshift, 0.2 ≤ 𝑧 ≤ 0.9 associated with neutral hydrogen column densities in the range, 15 ≤ log 𝑁(H I) (cm−2) ≤ 16.2 is presented. Using the photoionization models, various physical parameters such as the number density (𝑛H), Cabundance ([C/H]) and lineofsight thickness (𝐿) of the absorption systems are estimated. The derived parameters are found to be in the ranges, −3.4 ≤ log 𝑛H (in cm−3) ≤ −1.6, −1.6 ≤ [𝐶/𝐻] ≤ 0.4, and 1.3 pc ≤ 𝐿 ≤ 10 kpc, respectively, with most of them having subkpc scale thickness. Using both the low 𝑧 and previously reported high 𝑧 (2.1 ≤ 𝑧 ≤ 3.3) optically thin C III systems, the redshift evolution and the correlations between the derived physical parameters are explored. Significant redshift evolutions in 𝑛H, [C/H] and 𝐿 are observed for the C III selected absorbers. The redshift evolution of metallicity in C III systems are then compared with various types of absorption systems (such as damped Ly𝛼 absorptions (DLAs), subDLAs, and Lyman limit systems (LLSs)), which are directly associated with galaxies. The slope of [C/H] vs. 𝑧 for C III absorbers is steeper than the redshift evolution of cosmic metallicity of the DLAs but consistent with that of subDLAs. A strong anticorrelation between 𝐿 and [C/H] with a high significance level is found for the combined sample. This anticorrelation could be explained using the simple “isobaric” toy model, which shows that the C III absorber is in equilibrium with a hot phase medium. In the combined C III sample, two distinct [C/H] branches of C III populations (low[C/H] branch with [C/H] ≤ −1.2 and high[ C/H] branch with [C/H] > −1.2) are found when they are divided appropriately in the 𝐿 vs. 𝑁(C III) plane. Further studies of C III absorbers in the redshift range, 1.0 ≤ 𝑧 ≤ 2.0 are important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium. Finally, for the future direction of this work, a new PYTHON code “Quasar AbsorptionGALaxy Survey (QAGALS) is developed to automatically search for photometric/spectroscopic data of galaxies within a userdefined impact parameter around the intervening absorbers. QAGALS have also implemented the SED fitting code for modeling galaxy spectra with spectroscopic and photometric observations to compute the observed galaxy properties such as redshift, age, mass, star formation rate (SFR) and specific star formation rate (sSFR). For demonstration, QAGALS is used to search for galaxies from the SDSS DR16 catalog and compute the galaxy properties associated with the low 𝑧 C III sample.