This is the third in a series of papers investigating the properties of Ca ii absorbers identified in moderate-resolution SDSS quasar spectra. The redshift range over which Ca ii absorbers can be studied in SDSS optical spectra (0 < z < 1.34) allows us to study them over the most recent 8.5 Gyrs of our cosmic history. The ability to study Ca ii absorbers at the lowest redshifts makes them unique, as the rest wavelengths of more common QAL transitions lie in the UV. For example, in SDSS spectra, Mg ii can only be studied down to redshift z = 0.4. However, the incidence of Ca ii absorption in quasar spectra is much more rare in comparison to Mg ii, which makes performing a survey to discover a large number of them challenging.
In Paper I (Chapter 2 of this thesis) we described our SDSS survey for Ca ii, which resulted in the identification of 435 Ca ii absorbers, and we characterized their statistical properties, which included evidence for the existence of at least two populations of ab- sorbers, e.g., based on the need for a two-component fit to model their Wλ3934
0 distribution. Among the various possibilities we considered, the simplest was the proposal that “weak Ca ii absorbers” with Wλ3934
0 < 0.7 ˚A might have different astrophysical properties than “strong Ca ii absorbers” with Wλ3934
0 ≥ 0.7 ˚A. Owing to the nature of our evidence, if two populations do exist, one would expect the populations to be mixed near Wλ3934
0 ≈ 0.7 ˚A. In Paper II (Chapter 3of this thesis) we investigated element abundance ratios and dust in the weak and strong absorbers using their composite spectra. We found that the weak Ca ii absorbers had element abundance ratios typical of halo-type gas, whereas the strong Ca ii absorbers had element abundance ratios typical of a mix of halo- and disk-type gas. Moreover, while both populations of Ca ii absorbers showed evidence for dust in the form of element depletions and the reddening they caused, the strong Ca ii absorbers showed nearly six times more reddening than the weak Ca ii absorbers. The reddening was found to be consistent with either LMC- or SMC-type extinction laws, but a MW-type extinction law was ruled out.
In this Chapter, we investigated the galaxy populations associated with the weak and strong Ca ii absorbers using SDSS imaging data. We identified four galaxies along quasar sightlines that had SDSS spectroscopic redshifts that matched the Ca ii absorber redshifts
(Figures 4.1-4.3 and Table 4.1). The fact that matches required spectroscopic data for both the quasar and the nearby galaxy meant that these could only be found using rare, overlapping, plates, which resulted in the serendipitous identification of only four galaxies. The four absorbers have rest equivalent widths and redshifts lying in the intervals Wλ3934
0 =
[0.48, 1.3] ˚A and zabs = [0.04, 0.24]. The four associated galaxies have impact parameters and luminosities lying in the intervals b = [5, 25] kpc and L = [0.1, 0.9] L∗. The identification and luminosity of one galaxy is unclear, but the three others are clearly star-forming galaxies, exhibiting strong hydrogen Balmer emission lines as well as emission from [O ii], [O iii], [N ii], and [S ii] (Figure4.2). We also investigated the population of galaxies associated with the Ca ii absorbers by stacking and forming composite images. These investigations were necessarily statistical in nature, since we did not utilize any spectroscopic galaxy redshifts, but simply inferred their properties by assuming that any excess galaxy light above the background had z = zabs. The background-subtracted composite images, which covered the redshift intervals zabs = [0.20, 0.40] and [0.40, 0.65], revealed excess light along the Ca ii absorber quasar sightlines (Figures 4.11-4.14). We found that the strong Ca ii absorbers had a more concentrated and steeper light profile than the weak Ca ii absorbers. The first moments of the excess light distributions were used to derive the luminosity-weighted impact parameters, R1, for the weak and strong absorbers. If we average the various r-band and g-band results (Tables 4.3 and 4.4) in the most reliable zabs = [0.20, 0.40] interval, the weak Ca ii absorbers have R1 ≈ 48 kpc, while the strong Ca ii absorbers have R1 ≈ 26 kpc.
Thus, the results from Chapters2-4give rise to a consistent picture. The two-component rest equivalent rest distribution (Chapter 2, Paper 1) was the first clue indicating two ab- sorber populations. The strong Ca ii absorbers were then shown to have metal abundance ra- tios and dust properties consistent with a contribution from disk- and halo-type gas, whereas the weak Ca ii absorbers show properties consistent with only halo-type gas; the strong Ca ii absorbers contain nearly six times more dust than weak Ca ii absorbers, which puts them among the most dusty quasar absorbers (Chapter 3, Paper 2). The metal abundance ra- tio and dust results lead to the expectation that typical strong Ca ii absorbers might have smaller impact parameters than typical weak Ca ii absorbers. And indeed, in this third paper of the series, the strong Ca ii absorbers were found to have associated galaxies with
typical luminosity-weighted impact parameters of ≈ 26 kpc, which is ∼ 54% of the impact- parameter (or∼ 29% of the cross-sectional area) that characterizes the weak Ca ii absorbers. Thus, the various derived observables are qualitatively correlated as might be expected.
But while the combination of results leads to a consistent picture, the results also raise a number of issues which should be investigated into the future. As discussed in these last three Chapters, previous results on Ca ii absorbers have provided evidence that a significant fraction of them contain high column densities of neutral as well as molecular gas and dust conducive to star-formation. Ultimately direct measurements of H i column densities, H2 col- umn densities, and depletions on to dust grains in a significant subsample of Ca ii absorbers are needed to study this and better understand the Ca ii absorber populations. At the same time, our results show that much of this gas lies at large impact parameters, typically ranging between 20− 60 kpc, and there appear to be cases where an associated absorbing galaxy can not be identified to very faint levels within∼ 100 kpc. This provides important clues about inflows, outflows, and chemical enrichment in the extended circumgalactic regions surround- ing galaxies and possibly the intergalactic medium. The existence of such regions far away from the centers of galaxies would also require theoretical explanation.
Therefore, these new results raise a number of issues which should be investigated into the future. As discussed in this series of papers, previous results on Ca ii absorbers have provided evidence that at a significant fraction of them contain high column densities of neutral gas, and also molecular gas conducive to star-formation. Ultimately direct measurements of H i and H2 column densities in a significant subsample of Ca ii absorbers are ideally needed to confirm this. At the same time, our results show that much of this gas lies at large impact parameters, typically ranging between 20− 60 kpc, predominately from galaxies with dwarf-like or near dwarf-like luminosities. This provides important clues about inflows, outflows, and chemical enrichment in the extended circumgalactic regions surrounding this under-luminous galaxy population.