We have used Galaxy IMage 2D (GIM2D) to quantify the morphological properties of 40 intermediate‐redshift Mg ii absorption‐selected galaxies [0.03 ≤ Wr(2796) ≤ 2.9 Å], imaged with WFPC‐2/Hubble Space Telescope (HST), and compared them to the halo gas properties measured from HIRES/Keck and UVES/VLT quasar spectra. We find that as the quasar–galaxy separation, D, increases the Mg ii equivalent decreases with large scatter, implying that D is not the only physical parameter affecting the distribution and quantity of halo gas. Our main result shows that inclination correlates with Mg ii absorption properties after normalizing out the relationship (and scatter) between the absorption properties and D. We find a 4.3σ correlation between Wr(2796) and galaxy inclination, normalized by impact parameter, i/D. Other measures of absorption optical depth also correlate with i/D at greater than 3.2σ significance. Overall, this result suggests that Mg ii gas has a co‐planer geometry, not necessarily disc‐like, that is coupled to the galaxy inclination. It is plausible that the absorbing gas arises from tidal streams, satellites, filaments, etc., which tend to have somewhat co‐planer distributions. This result does not support a picture in which Mg ii absorbers with Wr(2796) ≲ 1 Å are predominantly produced by star formation driven winds. We further find that: (1) Mg ii host galaxies have quantitatively similar bulge and disc scalelength distribution to field galaxies at similar redshifts and have a mean disc and bulge scalelength of 3.8 and 2.5 kpc, respectively; (2) Galaxy colour and luminosity do not correlate strongly with absorption properties, implying a lack of a connection between host galaxy star formation rates and absorption strength; and (3) parameters such as scalelengths and bulge‐to‐total ratios do not significantly correlate with the absorption parameters, suggesting that the absorption is independent of galaxy size or mass.

We investigate the dependence of gas kinematics and column densities in the Mg II-absorbing circumgalactic medium on galaxy color, azimuthal angle, and inclination to trace baryon cycle processes. Our sample of 30 foreground isolated galaxies at 0.3 < z gal < 1.0, imaged with the Hubble Space Telescope, are probed by background quasars within a projected distance of 20 < D < 110 kpc. From the high-resolution (∆v 6.6 km s −1 ) quasar spectra, we quantify the extent of the absorber velocity structure with pixel-velocity twopoint correlation functions. Absorbers with the largest velocity dispersions are associated with blue, face-on (i < 57 • ) galaxies probed along the projected minor axis (Φ ≥ 45 • ), while those with the smallest velocity dispersions belong to red, face-on galaxies along the minor axis. The velocity structure is similar for edge-on (i ≥ 57 • ) galaxies regardless of galaxy color or azimuthal angle, for red galaxies with azimuthal angle, and for blue and red galaxies probed along the projected major axis (Φ < 45 • ). The cloud column densities for face-on galaxies and red galaxies are smaller than for edge-on galaxies and blue galaxies, respectively. These results are consistent with biconical outflows along the minor axis for star-forming galaxies and accreting and/or rotating gas, which is most easily observed in edge-on galaxies probed along the major axis. Gas entrained in outflows may be fragmented with large velocity dispersions, while gas accreting onto or rotating around galaxies may be more coherent due to large path lengths and smaller velocity dispersions. Quiescent galaxies may exhibit little-to-no outflows along the minor axis, while accretion/rotation may exist along the major axis.

We compared the quantified morphological properties of 37 intermediate redshift (0.3 ≤ z ≤ 1) Mg II absorption selected galaxies to the properties of the absorbing halo gas [0.03 ≤ W r (2796) ≤ 2.90 Å], observed in the spectra of background quasars. The galaxy morphologies were measured using GIM2D modeling of Hubble Space Telescope WFPC-2 images and the absorbing gas properties were obtained from HIRES/Keck and UVES/VLT quasar spectra. We found a 3.1 σ correlation between galaxy morphological asymmetries normalized by the quasar-galaxy projected separations, A/D, and the Mg II rest-frame equivalent widths. Saturation effects cause increased scatter in the relationship with increasing W r (2796). We defined a subsample for which the fraction of saturated pixels in the absorption profiles is f sat < 0.5. This criterion resulted in a subsample of 28 systems with W r (2796) ≤ 1.39 Å. The correlation strengthened to 3.3 σ. We also find a paucity of small morphological asymmetries for galaxies selected by Mg II absorption as compared to those of the general population of field galaxies, as measured in the Medium Deep Survey. The K-S probability that the two samples are drawn from the same galaxy population is ruled out at a 99.8% confidence level. Based upon four different measures of galaxy asymmetry, it is evident that the morphological perturbations of galaxies selected by Mg II absorption are "minor" and centrally concentrated. The A/D-W r (2796) correlation suggests a connection between the processes that perturb galaxies and the quantity of gas in their halos, normalized by the impact parameter. Since the perturbations are minor, it is clear that dramatic processes or events are not required for a galaxy to have an extended halo; the galaxies appear "normal". We suggest that common, more mild processes that populate halos with gas, such as satellite galaxy merging, accretion of the local cosmic web, and longer-range galaxy-galaxy interactions, consequently also induce the observed minor perturbations in the galaxies.

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The equivalent widths of Mg II absorption in the circumgalactic medium (CGM) trace the global star formation rate up to z<6, are larger for star-forming galaxies than passively evolving galaxies, and decrease with increasing distance from the galaxy. We delve further into the physics involved by investigating gas kinematics and cloud column density distributions as a function of galaxy color, redshift, and projected distance from the galaxy (normalized by galaxy virial radius, D/R vir). For 39 isolated galaxies at 0.3<z gal <1.0, we have detected Mg II absorption in high-resolution (Δv;6.6 km s −1) spectra of background quasars within a projected distance of 7<D<190 kpc. We characterize the absorption velocity spread using pixel-velocity two-point correlation functions. Velocity dispersions and cloud column densities for blue galaxies do not differ with redshift nor with D/R vir. This suggests that outflows continually replenish the CGM of blue galaxies with high velocity dispersion, large column density gas out to large distances. Conversely, absorption hosted by red galaxies evolves with redshift where the velocity dispersions (column densities) are smaller (larger) at z gal <0.656. After taking into account larger possible velocities in more massive galaxies, we find that there is no difference in the velocity dispersions or column densities for absorption hosted by red galaxies with D/R vir. Thus, a lack of outflows in red galaxies causes the CGM to become more quiescent over time, with lower velocity dispersions and larger column densities toward lower z gal. The quenching of star formation appears to affect the CGM out to D/R vir =0.75.