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Probing cool and warm circumgalactic gas in galaxies and clusters with large spectroscopic and imaging surveys
Probing cool and warm circumgalactic gas in galaxies and clusters with large spectroscopic and imaging surveys
The gaseous atmosphere that surrounds galaxies is known as the circumgalactic medium (CGM). The CGM is the interface between the physical processes happening at small and large scales. The complex balance between these processes drives the formation and evolution of galaxies. On the one hand, cool pristine gas is accreted from the intergalactic medium (IGM) or cosmic filaments onto galaxies. At the same time, galactic outflows produced by supernovae or the active galactic nucleus (AGN) expel a large amount of gas and metals into the CGM, some of which can even escape the halo while some recycle, mix and fall back into the galaxy. These flows play a critical role in regulating galaxy formation, such as determining the timescales of gas depletion and star formation, the processes responsible for preventing star formation in galaxies, and the distribution of the baryonic and metal budgets in the gas surrounding galaxies. The circumgalactic gas is usually studied in absorption (caused by hydrogen or metals) against a luminous background source such as a quasar. These absorbers can provide invaluable insights into the gas flows and their connection to the galactic properties and environment. In this thesis, I characterize the properties of the cool and warm circumgalactic gas of galaxies, clusters and quasars using quasar absorbers. To study the circumgalactic gas around galaxies, I first developed a fully automated pipeline to model the quasar continuum emission. Then, I employ a matched kernel convolution technique combined with an adaptive signal-to-noise (S/N) criteria to detect intervening metal absorbers in their spectra. The pipeline resulted in the most extensive metal absorber catalogue to date. It provided an opportunity to perform one of the most extensive absorption line studies of the galactic atmosphere around different types of galaxies. Then, I combine these absorbers with large spectroscopic samples of galaxies and investigate the nature of the cold circumgalactic medium at z ~ 0.5. Thanks to the large sample sizes, I could characterize the scales of cold gas absorption (traced by MgII, singly ionized magnesium) in the CGM of star-forming galaxies. I find that cold gas absorption is higher in their CGM is strongest within ~ 50 kpc from the galaxy, and the incidence rate of absorbers in the inner parts of the halo is 2-4 times higher for star-forming galaxies compared to quiescent galaxies at similar redshifts. However, beyond 50 kpc, I see a sharp decline in cold gas absorption strength and incidence rate for both types of galaxies, indicating a dichotomy of physical conditions in the gas. The inner regions of the CGM are regulated mainly by galactic outflows, while the outer CGM is tightly linked with the galaxy's dark matter halo. Galaxies with a higher star formation rate (SFR) have a higher incidence of cold gas, implying a strong connection to the stellar outflows. On the other hand, I find that motion of absorbing gas is sub-virial around passive galaxies, implying that gas is gravitationally bound and unlikely to have originated from galactic outflows. Their low star formation activity further supports this observation. Therefore, the origin of cool gas around passive galaxies is possibly due to accretion from the intergalactic medium (IGM) or stripping of the gas from satellite galaxies. This analysis suggests that the physical origin of cool circumgalactic gas for star-forming versus quiescent galaxies is very different and requires different frameworks to model them. Next, I extend the transverse absorption line technique to a more dense environment such as a galaxy cluster. I study the cool gas absorption in galaxy clusters by connecting quasar absorbers in both redshift and projected kpc space with galaxy clusters detected in the legacy imaging survey of the Dark Energy Spectroscopic Instrument (DESI). I find that clusters host a significant amount of cool gas in their haloes, despite the fact that intracluster medium (ICM) is very hot, as evidenced by the fact that the gas emits mainly in X-rays. The total MgII mass within the cluster halo is ~ 8-10 times higher than for passive galaxies at similar redshifts. To understand the possible connection of cold gas absorption in clusters with the stellar activity and the halo properties of its members, I also connect these absorbers with photo -z selected cluster member galaxies from DESI. I find a statistically significant connection between absorbers and cluster galaxies on projected scales. The median projected distance between MgII absorbers and the nearest cluster member is ~ 200 kpc compared to ~ 500 kpc in random mocks with the same galaxy density profiles. A substantial fraction of absorbers (~ 50 percent) are located within the dark matter halo of those galaxies. However, I do not see a correlation between cool gas absorption strength and the star formation rate of the closest cluster neighbour. This suggests that cool gas in clusters, as traced by these absorbers, is either associated with satellites galaxies that are too low in mass to be found in the DESI catalogue, or associated with cold gas clouds in the intracluster medium that may originate in part from gas stripped from these cluster satellites in the past. Finally, I employed our absorber detection pipeline to search for highly ionized absorbers (CIV, triply ionized carbon) in the spectra of background quasars with MgII detections. By combining metal absorbers in low and high ionization states, I studied the spatial distribution of cool and warm gas in the CGM at z ~ 2 using the largest spectroscopic sample of quasars and absorbers. This analysis reveals large cool and warm gas reservoirs in quasar haloes, indicating that quasar haloes are highly metal-enriched. In addition to characterizing the overall spatial distribution of cool and warm gas around quasars, I also study the relative kinematics between metal absorbers and quasars. This analysis shows that the velocity dispersion of absorbing clouds is 3-4 times larger than star-forming or quiescent galaxies and similar to the motion of cool gas in galaxy clusters. Since quasars reside in much smaller haloes than clusters, we speculate that this may indicate a scenario where metal-enriched gas in their haloes is associated with outflows.
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Anand, Abhijeet
2022
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Anand, Abhijeet (2022): Probing cool and warm circumgalactic gas in galaxies and clusters with large spectroscopic and imaging surveys. Dissertation, LMU München: Faculty of Physics
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Abstract

The gaseous atmosphere that surrounds galaxies is known as the circumgalactic medium (CGM). The CGM is the interface between the physical processes happening at small and large scales. The complex balance between these processes drives the formation and evolution of galaxies. On the one hand, cool pristine gas is accreted from the intergalactic medium (IGM) or cosmic filaments onto galaxies. At the same time, galactic outflows produced by supernovae or the active galactic nucleus (AGN) expel a large amount of gas and metals into the CGM, some of which can even escape the halo while some recycle, mix and fall back into the galaxy. These flows play a critical role in regulating galaxy formation, such as determining the timescales of gas depletion and star formation, the processes responsible for preventing star formation in galaxies, and the distribution of the baryonic and metal budgets in the gas surrounding galaxies. The circumgalactic gas is usually studied in absorption (caused by hydrogen or metals) against a luminous background source such as a quasar. These absorbers can provide invaluable insights into the gas flows and their connection to the galactic properties and environment. In this thesis, I characterize the properties of the cool and warm circumgalactic gas of galaxies, clusters and quasars using quasar absorbers. To study the circumgalactic gas around galaxies, I first developed a fully automated pipeline to model the quasar continuum emission. Then, I employ a matched kernel convolution technique combined with an adaptive signal-to-noise (S/N) criteria to detect intervening metal absorbers in their spectra. The pipeline resulted in the most extensive metal absorber catalogue to date. It provided an opportunity to perform one of the most extensive absorption line studies of the galactic atmosphere around different types of galaxies. Then, I combine these absorbers with large spectroscopic samples of galaxies and investigate the nature of the cold circumgalactic medium at z ~ 0.5. Thanks to the large sample sizes, I could characterize the scales of cold gas absorption (traced by MgII, singly ionized magnesium) in the CGM of star-forming galaxies. I find that cold gas absorption is higher in their CGM is strongest within ~ 50 kpc from the galaxy, and the incidence rate of absorbers in the inner parts of the halo is 2-4 times higher for star-forming galaxies compared to quiescent galaxies at similar redshifts. However, beyond 50 kpc, I see a sharp decline in cold gas absorption strength and incidence rate for both types of galaxies, indicating a dichotomy of physical conditions in the gas. The inner regions of the CGM are regulated mainly by galactic outflows, while the outer CGM is tightly linked with the galaxy's dark matter halo. Galaxies with a higher star formation rate (SFR) have a higher incidence of cold gas, implying a strong connection to the stellar outflows. On the other hand, I find that motion of absorbing gas is sub-virial around passive galaxies, implying that gas is gravitationally bound and unlikely to have originated from galactic outflows. Their low star formation activity further supports this observation. Therefore, the origin of cool gas around passive galaxies is possibly due to accretion from the intergalactic medium (IGM) or stripping of the gas from satellite galaxies. This analysis suggests that the physical origin of cool circumgalactic gas for star-forming versus quiescent galaxies is very different and requires different frameworks to model them. Next, I extend the transverse absorption line technique to a more dense environment such as a galaxy cluster. I study the cool gas absorption in galaxy clusters by connecting quasar absorbers in both redshift and projected kpc space with galaxy clusters detected in the legacy imaging survey of the Dark Energy Spectroscopic Instrument (DESI). I find that clusters host a significant amount of cool gas in their haloes, despite the fact that intracluster medium (ICM) is very hot, as evidenced by the fact that the gas emits mainly in X-rays. The total MgII mass within the cluster halo is ~ 8-10 times higher than for passive galaxies at similar redshifts. To understand the possible connection of cold gas absorption in clusters with the stellar activity and the halo properties of its members, I also connect these absorbers with photo -z selected cluster member galaxies from DESI. I find a statistically significant connection between absorbers and cluster galaxies on projected scales. The median projected distance between MgII absorbers and the nearest cluster member is ~ 200 kpc compared to ~ 500 kpc in random mocks with the same galaxy density profiles. A substantial fraction of absorbers (~ 50 percent) are located within the dark matter halo of those galaxies. However, I do not see a correlation between cool gas absorption strength and the star formation rate of the closest cluster neighbour. This suggests that cool gas in clusters, as traced by these absorbers, is either associated with satellites galaxies that are too low in mass to be found in the DESI catalogue, or associated with cold gas clouds in the intracluster medium that may originate in part from gas stripped from these cluster satellites in the past. Finally, I employed our absorber detection pipeline to search for highly ionized absorbers (CIV, triply ionized carbon) in the spectra of background quasars with MgII detections. By combining metal absorbers in low and high ionization states, I studied the spatial distribution of cool and warm gas in the CGM at z ~ 2 using the largest spectroscopic sample of quasars and absorbers. This analysis reveals large cool and warm gas reservoirs in quasar haloes, indicating that quasar haloes are highly metal-enriched. In addition to characterizing the overall spatial distribution of cool and warm gas around quasars, I also study the relative kinematics between metal absorbers and quasars. This analysis shows that the velocity dispersion of absorbing clouds is 3-4 times larger than star-forming or quiescent galaxies and similar to the motion of cool gas in galaxy clusters. Since quasars reside in much smaller haloes than clusters, we speculate that this may indicate a scenario where metal-enriched gas in their haloes is associated with outflows.