Utilizing Damped Lyman Alpha Absorbers to Trace Dark Matter Halos

Abstract

This dissertation consists of three main chapters, all of which use the FIRE-2 cosmological hydrodynamic simulations of Milky Way-mass dark matter halos and their galaxy-scale subhalos to investigate how well absorption line systems are associated with and can be used to infer dark matter halo mass for redshifts $z=0$ to 3. Chapter 1 focuses on the relationship between the dark matter, stellar, and gas distributions in the primary host halos themselves, Chapter 2 on how well stars and damped \lya (DLA) systems trace the subhalo mass function over $10^{8}$-$10^{12}$ solar masses, and Chapter 3 asks what we can learn about the host halo(s) mass from observing a DLA sightline. We plan to submit Chapters 2 and 3 for publication to MNRAS. In Chapter 1, we examine sightlines through the five primary host halos growing into MW-mass halos by $z=0$ that exceed the neutral gas surface density threshold for Lyman Limit Systems (LLS; $17 \le \log N_{HI} \le 20.3$ cm$^{-2}$) and DLAs ($\log N_{HI} \ge 20.3$ cm$^{-2}$). Both the DLA and LLS area covering fractions decrease from $z=3$ to $0$ in all five realizations. The declines follow a power law: $f_{cov}=A (1+z)^{B},$ where $A=(8.47\pm0.46) \times 10^{-4}$ and $B=3.29\pm0.03$ for DLAs and $A=(2.05\pm0.06) \times 10^{-3}$ and $B=3.15\pm0.03$ for LLSs. Due to baryonic cooling and subsequent star formation from $z=3$ to $0$, the stellar mass fractions increase from $\sim 10^{-2}$ to $10^{-1}$, the total gas fractions decrease from $\sim 10^{-1}$ to $7 \times 10^{-2}$, and neutral HI gas fractions decrease from $\sim 5 \times 10^{-2}$ to $10^{-2}$. The DLA and LLS mass fractions decline as well, from a few $\times 10^{-3}$ to $<10^{-3}$ and from $\sim 5 \times 10^{-3}$ to $\le 10^{-3}$, respectively. In Chapter 2, we explore a new approach to constraining the mass function of galaxy-scale dark matter subhalos: using neutral absorbing hydrogen gas in the form of damped \lya (DLA) systems, i.e., at projected gas densities larger than $\log N_{HI}=20.3$ \cms. We consider five primary host halos growing into Milky Way-mass objects by $z=0$ and their subhalos from the FIRE-2 cosmological zoom-in simulations, comparing the fractions of subhalos traced by galaxies with those intersecting DLA sightlines. Our analysis extends down to subhalo masses of $10^{8}$ \Msun\ and stellar masses of $\sim 10^{5}$ \Msun from $z = 3$ to 0. Within the $\pm 1\sigma$ errors, all subhalos with instantaneous masses $\gtrsim 10^{10}$ \Msun intersect DLA sightlines. At $z \ge 2$, roughly 50\% of $10^{8.6}-10^{9.2}$ \Msun subhalos intersect DLAs, dropping to 10-20\% at lower $z$. The number of subhalos intersecting a DLA beam exceeds those hosting an observable galaxy at all subhalo masses at $z \ge 2$ and for $\le 10^{9}$ \Msun at any redshift. Most generally, low mass, high redshift subhalos are more associated with DLAs than with galaxies due to increases in the DLA area covering fraction and galaxy cosmological dimming with redshift. Some subhalos that host a galaxy do not intersect a DLA sightline. Thus, observing both gas absorbers and galaxies would improve subhalo mass function constraints from either method alone. Subhalos are clearly connected with DLA sightlines: 1) the number of DLAs rises with the number of subhalos across the five FIRE-2 realizations of the MW system, 2) that the total number of subhalos intersecting DLA sightlines exceeds that of the randomly-drawn sample of non-DLA beams, and 3) more subhalos at every mass intersect DLA beams than randomly-drawn sample of non-DLA beams. In Chapter 3, we use FIRE-2 cosmological zoom-in simulations of five Milky Way-mass systems to assess what observations of damped \lya (DLA) systems could reveal about the mass of galaxy-scale dark matter halos intersecting the DLA sightline. We consider ``(sub)halos'' down to $10^{8}$ \Msun, a population that includes subhalos within the Milky Way-mass host halo and halos at larger radii. At all redshifts, DLA beams have a larger fraction of higher mass to lower mass subhalos relative to random beams. The mass of the most massive (sub)halo in each DLA sightline is significantly higher (by 0.7-1.4 dex) than in random beams, with a mean ranging from $10^{9.4}$ to $10^{9.9}$ \Msun from high to low redshift. If we consider the additional constraint imposed by ``observing" the DLA column density, the most massive (sub)halo in a DLA sightline at $z=3$ is $10^{9.3}$-$10^{9.5}$\Msun at $\log N_{HI} = 20.3$-20.6 cm$^{-2}$ and $10^{9.1}$-$10^{9.5}$ \Msun at $\log N_{HI} = 20.6$-20.9 cm$^{-2}$, respectively. At $z=1$, the most massive (sub)halo is $10^{9.7}$-$10^{10.3}$ \Msun for $\log N_{HI} = 20.3$-20.6 cm$^{-2}$. For the first time, we predict the DLA column density function, an analog to the galaxy mass function that depends on both the $\Lambda$CDM subhalo mass function and the physical properties of the gas in the sightlines intersected by the subhalos.