Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-Line Spectroscopy: II. Methods and Models (1704.00235v1)

Abstract: We present basic data and modeling for a survey of the cool, photo-ionized Circum-Galactic Medium (CGM) of low-redshift galaxies using far-UV QSO absorption line probes. This survey consists of "targeted" and "serendipitous" CGM subsamples, originally described in Stocke et al. (2013, Paper 1). The targeted subsample probes low-luminosity, late-type galaxies at $z<0.02$ with small impact parameters ($\langle\rho\rangle = 71$ kpc), and the serendipitous subsample probes higher luminosity galaxies at $z\lesssim0.2$ with larger impact parameters ($\langle\rho\rangle = 222$ kpc). HST and FUSE UV spectroscopy of the absorbers and basic data for the associated galaxies, derived from ground-based imaging and spectroscopy, are presented. We find broad agreement with the COS-Halos results, but our sample shows no evidence for changing ionization parameter or hydrogen density with distance from the CGM host galaxy, probably because the COS-Halos survey probes the CGM at smaller impact parameters. We find at least two passive galaxies with H I and metal-line absorption, confirming the intriguing COS-Halos result that galaxies sometimes have cool gas halos despite no on-going star formation. Using a new methodology for fitting H I absorption complexes, we confirm the CGM cool gas mass of Paper 1, but this value is significantly smaller than found by the COS-Halos survey. We trace much of this difference to the specific values of the low-$z$ meta-galactic ionization rate assumed. After accounting for this difference, a best-value for the CGM cool gas mass is found by combining the results of both surveys to obtain $\log{(M/M_{\odot})}=10.5\pm0.3$, or ~30% of the total baryon reservoir of an $L \geq L^*$, star-forming galaxy.

Characterizing the Circumgalactic Medium of Nearby Galaxies with COS and STIS Absorption-Line Spectroscopy: Methods and Models

The paper presents an in-depth paper of the Circumgalactic Medium (CGM) of low-redshift galaxies using far-ultraviolet (FUV) spectroscopy data obtained from the Hubble Space Telescope (HST) instruments—COS and STIS. This work expands upon previously published efforts to survey the CGM, focusing specifically on methodological approaches and modeling strategies employed to analyze absorption line data and derive CGM properties.

Data and Methodology

The researchers compiled a dataset comprising two primary subsamples: a targeted subsample and a serendipitous subsample. The targeted subsample consists of low-luminosity, late-type galaxies at redshifts less than 0.02, probed with small impact parameters averaging about 71 kpc. The serendipitous subsample includes higher-luminosity galaxies up to redshifts nearing 0.2, with larger impact parameters averaging 222 kpc. These subsamples were explored through detailed FUV spectroscopy of absorption lines sourced from background quasars (QSOs). The diversity in redshift and impact parameter allows the authors to capture variations across a span of galactic environments at low redshift.

Key to the analysis are the absorption lines from species such as H, C, Si, and O, which serve as indicators of the ionization states and relative abundance of elements, and consequently the physical conditions of the CGM. The authors also compare their results with those from the COS-Halos survey, finding broad agreement in the data but noting differences in specifics such as the absence of an evident ionization parameter gradient with increasing CGM distance here, differing from findings in COS-Halos data.

Results

• Comparative Findings: The CGM observed shows consistency with predictions from the COS-Halos results, although differences surface due to the methodologies of absorption line fitting and model assumptions. Notably, while prior investigations like COS-Halos depict a decreasing ionization parameter with increased distance from the host galaxy, this paper does not find a similar trend, likely due to the larger impact parameters probed here.
• Ionization and Metallicity: Using CLOUDY ionization models, the researchers provide evidence aligning with previous observations, confirming that CGM structures do not reveal statistically significant changes in ionization parameter or hydrogen density as a function of spatial distance from their host galaxies. The paper scrutinizes multiple ionization models to determine metallicities and cloud masses, finding notable variations depending on the underlying assumptions regarding the ionizing UV background radiation.
• Mass Estimation: The total mass of the cool CGM is estimated at a logarithmic value of approximately 10.5 with a margin of uncertainty, which is substantially smaller than the COS-Halos survey results. This discrepancy primarily arises from differing assumptions about the metagalactic ionization rate, reinforcing the need for consistent modeling approaches in CGM studies.

Implications and Future Work

This investigation into nearby galaxies' CGM challenges previous models and adds granularity to our understanding of galactic halos. The results have significant implications for the paper of galaxy formation and the role of CGM in regulating star formation through baryonic processes. They highlight the necessity of accurately characterizing ionizing backgrounds and the importance of understanding CGM cloud line-of-sight thickness and mass estimates.

For future developments, the paper suggests refined modeling techniques and a concerted focus on understanding the ionizing radiation field variations, as these are pivotal in reconciling differences observed across various surveys. Continued detailed analyses coupled with richer datasets will be crucial for models seeking to accurately represent the complicated dynamics and interactions present within the CGM, especially as it relates to larger galactic ecosystems.

In summary, this work details significant strides towards characterizing the CGM and emphasizes methodological robustness, against which future observational efforts can be calibrated to deduce more precise astrophysical inferences.