The MgII Absorber–Galaxy Catalog
MAGIICAT

The catalog is a compilation of the MgII absorber–galaxy pairs in the literature with spectroscopic galaxy redshifts. In Paper I, we describe the surveys included, the methods for standardizing absorption and galaxy properties, and tabulate the data. Only isolated galaxies are included in Paper I. See Paper VI below for group environments.

(Left) The relative locations of each galaxy (points) and the associated background quasar (plus sign) for a majority of the survey data included in MAGIICAT.

With the isolated galaxy sample, we investigated the dependence of MgII absorption characteristics on host galaxy properties. This work focused on equivalent widths, covering fractions, and luminosity-scaled absorption radii for various galaxy luminosities, colors, impact parameters, and redshifts. The MgII circumgalactic medium is more extended and has a larger covering fraction around higher luminosity, bluer, and higher redshift galaxies.

(Right) MgII equivalent width is anti-correlated with quasar–galaxy impact parameter at the 7.9σ level and the dependence is fit well by a log-linear relation.

Using halo abundance matching, we obtained galaxy virial masses and radii for MAGIICAT isolated galaxies and studied the mass-dependence of equivalent widths and covering fractions. The circumgalactic medium is self-similar with halo mass. Constant covering fractions with mass indicate that outflows or sub-halos contribute to absorption in more massive galaxies despite the prediction that cold mode accretion is quenched.

(Left) More massive galaxies have larger equivalent widths at a given impact parameter than less massive galaxies. This mass segregation vanishes when normalizing impact parameter by the virial radius.

Following up on the self-similarity of the CGM with mass, we found that MgII primarily resides within 0.3 R_vir regardless of halo mass and for all equivalent width measurements. We also investigated the dependence of MgII on the cooling radius, finding that the cooling radius is a poor indicator of MgII absorption strength. The presence of absorption outside this radius suggests that cool/warm CGM gas does not solely originate from fragmentation and condensation out of the hot CGM. Our halo abundance matching methods are detailed in Appendix A.

(Right) The MgII equivalent width is constant with mass, contrary to previous absorber–galaxy cross-correlation studies.

A subset of about 40 MAGIICAT absorbers (and an additional 30 non-absorbers) have high-resolution spectral coverage with HIRES/Keck and/or UVES/VLT. With this sample, we studied the absorber kinematics and column densities as a function of galaxy redshift and impact parameter.

(Left) The kinematics and column densities for absorbers associated with blue galaxies do not evolve with redshift, indicating an active baryon cycle to replenish the CGM with cool gas. For red galaxies, the velocity dispersions decrease and column densities increase with redshift. The CGM of red galaxies appears to be impacted by star formation quenching.

Galaxy morphological properties were modeled from HST or SDSS images for 88 absorbing and 35 non-absorbing galaxies. We investigated the dependence of MgII covering fractions on the azimuthal angle: the angle between the projected galaxy major axis and the quasar sightline. MgII absorption is more likely found within 20° of the major axis and within 50° of the minor axis.

(Right) The azimuthal angle dependence of MgII is most strongly seen in blue star-forming galaxies, whereas the distribution is flat for red passive galaxies.

Baryon cycle processes are expected to exhibit differing kinematic and column density properties depending on the orientation of the host galaxy. We investigated these differences for a sample of 30 galaxies with MgII in high-resolution quasar spectra and modeled galaxy morphologies (inclinations and azimuthal angles) from HST images.

Gas entrained in biconical outflows along the minor axis of star-forming galaxies appears to be fragmented with large velocity dispersions. Accreting/rotating gas along galaxy major axes has smaller velocity dispersions and may be more coherent. Quiescent galaxies appear to exhibit little-to-no outflows along their minor axes, but may host rotating gas along their major axes.

(Left) Absorbers hosted by blue, face-on galaxies have the largest velocity dispersions, indicative of outflows, whereas red, face-on galaxies have the smallest velocity dispersions, suggesting a lack of outflowing gas. For absorbers hosted by edge-on galaxies, there are no significant differences in their kinematics regardless of the host galaxy star formation activity.

Working with a sample of 29 group environments (2–5 galaxies within 200 kpc of the quasar sightline and within a line-of-sight velocity separation of 500 km s^-1), we examined the MgII intragroup medium. A superposition of multiple galaxy CGM predicts much of the observed equivalent width, but greatly overpredicts the kinematics. MgII is likely deposited into an intragroup medium via outflowing winds and/or from tidal stripping of interacting member galaxies.

(Right, top) Group environments, with connected triangles, have larger median equivalent widths and covering fractions than isolated galaxies, gray points and downward arrows.

(Right, bottom) Comparing average absorption profiles, group absorbers display more optical depth at larger velocities (roughly 100 km s^-1) than isolated absorbers.