The radial and azimuthal profiles of Mg II absorption around 0.5 < z < 0.9 zCOSMOS galaxies of different colors, masses and environments (1106.0616v2)

Abstract: We map the radial and azimuthal distribution of Mg II gas within 200 kpc (physical) of 4000 galaxies at redshifts 0.5 < z < 0.9 using co-added spectra of more than 5000 background galaxies at z > 1. We investigate the variation of Mg II rest frame equivalent width as a function of the radial impact parameter for different subsets of foreground galaxies selected in terms of their rest-frame colors and masses. Blue galaxies have a significantly higher average Mg II equivalent width at close galactocentric radii as compared to the red galaxies. Amongst the blue galaxies, there is a correlation between Mg II equivalent width and galactic stellar mass of the host galaxy. We also find that the distribution of Mg II absorption around group galaxies is more extended than that for non-group galaxies, and that groups as a whole have more extended radial profiles than individual galaxies. Interestingly, these effects can be satisfactorily modeled by a simple superposition of the absorption profiles of individual member galaxies, assuming that these are the same as those of non-group galaxies, suggesting that the group environment may not significantly enhance or diminish the Mg II absorption of individual galaxies. We show that there is a strong azimuthal dependence of the Mg II absorption within 50 kpc of inclined disk-dominated galaxies, indicating the presence of a strongly bipolar outflow aligned along the disk rotation axis. There is no significant dependence of Mg II absorption on the apparent inclination angle of disk-dominated galaxies.

Overview of "The Radial and Azimuthal Profiles of Mg II Absorption Around 0.5 < z < 0.9 Cosmos Galaxies of Different Colors, Masses and Environments"

The paper by Bordoloi et al. provides an in-depth analysis of Mg II absorption characteristics around galaxies within a redshift range of $0.5 < z < 0.9$. Utilizing a substantial dataset from the zCOSMOS survey, the paper investigates how the properties of galaxies, such as color, mass, and environment, influence the radial and azimuthal distribution of Mg II gas. The methodology involves co-adding spectra from 5000 background galaxies, focusing on the Mg II absorption feature. This research contributes significant insights into the interplay between galaxy characteristics and the circumgalactic medium (CGM).

Key Findings

1. Color and Mg II Absorption:
   • The research identifies a notable discrepancy in Mg II absorption strength based on galaxy color, with blue galaxies exhibiting substantially stronger absorption than red galaxies, especially at smaller impact parameters.
   • For blue galaxies, there is a clear mass dependence of Mg II equivalent width, indicating a relationship between stellar mass and CGM enrichment and dynamics.
2. Radial Profiles and Environmental Influence:
   • Non-group galaxies display a radial dependence of Mg II absorption, modeled effectively with a singular isothermal sphere (SIS). For isolated galaxies, the radius of effective gas absorption is around 115.2 kpc with significant variation between blue and red galaxies.
   • In group environments, the absorption profiles extend beyond 140 kpc and can be explained by a simple superposition model, suggesting group environments do not significantly alter individual galaxies' absorption characteristics.
3. Azimuthal Dependencies:
   • A bipolar distribution of Mg II absorption is observed, with stronger absorption along the disk axis compared to the disk plane in inclined disk galaxies at small impact parameters. This finding aligns with models of bipolar outflows.
   • No significant correlation between absorption strength and disk inclination is detected, reinforcing the dominance of azimuthal angle in dictating Mg II distribution.

Implications

The significance of Mg II absorption as a tracer of the CGM is evidenced by its strong dependence on galaxy characteristics, particularly star-formation activities indicated by color and mass. The paper's findings on the azimuthal profile support models of starburst-driven outflows, which have implications for understanding galaxy evolution and the enrichment of the intergalactic medium. Furthermore, the ability to explain group environments through superposition suggests a promising approach for modeling complex galaxy interactions without necessitating enhanced mechanisms.

Theoretical and Practical Relevance

Theoretically, the work underscores the role of galaxy dynamics, particularly outflows, in shaping the CGM. It provides a structured approach to discriminating between inflow and outflow processes, supported by strong empirical evidence. Practically, these insights offer a pathway to refine models of galactic feedback and gas recycling in semi-analytic simulations.

Future Directions

Future work could deepen the paper of group environments, exploring how variations within these environments could subtly affect Mg II distributions. Additionally, exploring the relation of Mg II absorbers with other metal lines could provide a more comprehensive chemical picture of the CGM. Machine learning methods could also be applied to further stratify Mg II profiles, discovering subtle patterns that traditional stacking methods might overlook.

Overall, Bordoloi et al.'s research delivers a robust understanding of the factors influencing Mg II absorption around galaxies and highlights the nuanced interplay between a galaxy’s intrinsic and environmental properties on its circumgalactic medium.