In Chapter 2 we presented the results of a survey for Ca ii λλ3934, 3969 absorption-line systems culled from ∼ 95, 000 Sloan Digital Sky Survey (SDSS) Data Release 7 and Data Release 9 quasar spectra. With 435 doublets identified in the catalog, this list is the largest Ca ii catalog compiled to date, spanning redshifts z < 1.34, which corresponds to the most re-
cent 8.5 Gyrs of the history of the Universe. We derived statistics on the Ca ii rest equivalent width distribution and incidence. We found that the λ3934 rest equivalent width (Wλ3934
0 )
distribution cannot be described by a single exponential function. A double exponential function is required to produce a satisfactory description. The function can be written as a sum of weak and strong components: ∂n/∂Wλ3934
0 = (Nwk? /Wwk? )exp(−W0λ3934/Wwk? ) + (N?
str/Wstr? )exp(−W0λ3934/Wstr? ). A maximum likelihood fit to the unbinned data indicates: Nwk? = 0.140±0.029, W?
wk = 0.165±0.020 ˚A, Nstr? = 0.024±0.020, and Wstr? = 0.427±0.101 ˚A. This suggests that the Ca ii absorbers are composed of at least two distinct populations. The incidence of the overall Ca ii absorber population does not show evidence for evolution in the standard cosmology. The normalization of the no-evolution curve, i.e., the value of the Ca ii incidence extrapolated to redshift z = 0, for W0λ3934 ≥ 0.3 ˚A, is n0 = 0.017± 0.001. In comparison to Mg ii surveys, we found that only 3% of Mg ii systems in the SDSS have Ca ii, confirming that Ca ii systems are rare. We also reported on some preliminary investigations of the nature of the two populations of Ca ii absorbers, and showed that they can likely be distinguished using their Mg ii properties.
In Chapter 3 we presented measurements of element abundance ratios and dust in Ca ii absorbers identified in SDSS DR7+DR9. As described in Chapter 2, we formed a statistical sample of 435 Ca ii absorbers and postulated that their statistical properties might be representative of at least two populations of absorbers. Here in Chapter 3, we showed that if the absorbers are roughly divided into two subsamples with Ca ii rest equiv- alent widths larger and smaller than Wλ3934
0 = 0.7 ˚A, they are then representative of two physically different populations. Comparisons of abundance ratios between the two Ca ii absorber populations indicate that the weaker Wλ3934
0 absorbers have properties consistent with halo-type gas, while the stronger absorbers have properties intermediate between halo- and disk-type gas. We also showed that, on average, the dust extinction properties of the overall sample is consistent with a LMC or SMC dust law, and the stronger absorbers are nearly 6 times more reddened than their weaker counterparts. The absorbed-to-unabsorbed composite flux ratio at λrest = 2200 ˚A is R ≈ 0.73 and E(B − V ) ≈ 0.046 for the stronger Ca ii absorbers (Wλ3934
0 ≥ 0.7 ˚A), and R ≈ 0.95 and E(B − V ) ≈ 0.011 for the weaker Ca ii absorbers (Wλ3934
In Chapter4we used SDSS imaging data to study galaxies associated with Ca ii absorbers in the redshift interval zabs = [0.02, 0.65]. The Ca ii absorbers were identified in our survey for Ca ii λλ3934, 3969 absorption-line systems in SDSS DR7+DR9 quasar spectra. First, we discussed four cases where we identified galaxies with SDSS spectroscopic redshifts that match that of the Ca ii absorbers. The four galaxies, three of which are star-forming, have redshifts between 0.04 − 0.24, impact parameters between 5 − 25 kpc, and luminosities between ∼ 0.1-1 L?
r. We then specifically considered the question of whether a sample of Ca ii absorbers separated above and below Wλ3934
0 = 0.7 ˚A showed different associated galaxy properties. This rest equivalent width value was motivated by Ca ii λ3934 absorption-line properties described in Chapters 2 and 3. This was investigated statistically by stacking and forming composite images. We then inferred associated galaxy properties by assuming that any excess galaxy light above the background is due to associated galaxies at z = zabs. We found that galaxies associated with Ca ii absorbers above and below Wλ39340 = 0.7 ˚
A have significantly different statistical properties. The stronger Ca ii absorbers have a more concentrated and steeper light profile than the weaker Ca ii absorbers, with average luminosity-weighted impact parameters of≈ 26 kpc and ≈ 48 kpc, respectively. The results from our overall study of Ca ii absorbers in the SDSS give rise to a consistent picture, with the various derived observables (impact parameters, metal abundance ratios, dust) qualitatively correlated as might be expected. However, the fact that Ca ii absorbers, which are often known to contain significant columns of neutral and molecular gas, can exist at large impact parameters from any associated galaxy needs to be theoretically explained. The results most likely provide important clues about inflows, outflows, and chemical enrichment in the extended circumgalactic regions surrounding both luminous and under-luminous galaxies.
In Chapter5, we presented Hubble Space Telescope - Cosmic Origins Spectrograph spectra of ten quasars located behind M31, selected to investigate the properties of gas associated with its extended disk, halo, and HVCs. The sightlines have impact parameters ranging between b = 13 kpc and 112 kpc. No absorption is detected in the four sightlines selected to sample any extended disk (or halo) gas that might be present in the outer regions of M31 beyond an impact parameter of b > 57 kpc. Of the six remaining sightlines, all of which lie at b < 32 kpc and within the NHI = 2× 1018 cm−2 boundary of the H i disk
of M31, we detected low-ionization absorption at M31 velocities along four of them (three of which include Mg ii absorption). We also detected Mg ii absorption from a HVC. This HVC sightline does not pass through the 21 cm disk of M31, but we detected additional Mg ii absorption at velocities distinct from the HVC that presumably arises in the halo. We found that along sightlines where both are detected, the velocity location of the low-ion gas tracks the peak in 21 cm emission. High-ionization absorption is detected along the three inner sightlines, but not along the three outer sightlines for which C iv data exist. As inferred from high-resolution 21 cm emission line maps of M31’s disk and extended regions, only one of the sightlines may be capable of harboring a damped Lyα system, i.e., with NHI ≥ 2 × 1020 cm−2. This sightline has impact parameter b = 17.5 kpc, and we detected both low- and high-ion absorption lines associated with it. The impact parameters of our observed sightlines through M31 are similar to the impact parameters of galaxies identified with Mg ii absorbers at redshifts 0.1 < z < 1.0 in a 2011 study by Rao et al. However, even if we only count cases where absorption due to M31 is detected, the Mg ii λ2796 rest equivalent width values are significantly smaller. In comparison, moderate-to-strong Mg ii absorption from Milky Way gas is detected along all ten sightlines. Thus, this study indicates that M31 does not present itself as an absorbing galaxy which is typical of higher-redshift galaxies inferred to give rise to moderate-strength quasar absorption lines. M31 also appears not to possess an extensive large gaseous cross section, at least not along the direction of its major axis.