On the connection between galaxy orientation and halo absorption properties

TL;DR

This study tests whether metal-line absorption in the low-redshift CGM exhibits azimuthal anisotropy tied to galaxy disk orientation. Using a uniformly selected sample of 87 isolated galaxies with quantified morphologies from DECaLS imaging and MgII/CaII absorption constraints from high-quality QSO spectra, the authors compute azimuthal angles and deprojected distances to evaluate absorber strength as a function of geometry. They find no statistically significant azimuthal dependence for MgII or CaII within ~50 kpc, with only a marginal, non-significant hint of stronger MgII along the major axis in a well-constrained subset. The results suggest the inner CGM around these low-z galaxies is consistent with a randomly distributed metal-enriched gas distribution, highlighting the need for larger samples to detect subtle anisotropies and to reconcile previous conflicting findings across redshifts and galaxy populations.

Abstract

We present a systematic investigation of the azimuthal dependence of metal-line absorption in the circumgalactic medium (CGM) using a uniformly selected sample of 87 isolated galaxies at z < 0.4 from the Magellan MagE MgII (M3) halo survey. High-quality archival imaging enables quantitative morphological measurements -- including disk inclination and position angle -- for every galaxy, providing a robust framework for assessing how absorber strength depends on the geometric alignment between galaxies and the QSO sightlines. All galaxies have associated constraints on MgII lambda 2796 absorption, and a subset of 56 galaxies also have measurements of CaII lambda 3934. We compare rest-frame MgII and CaII equivalent widths with both projected distance and deprojected galactocentric distance. Across the full sample, we find no statistically significant correlation between absorption strength and azimuthal angle. Restricting to the 71 galaxies with well-determined disk orientations reveals a mild excess of strong MgII absorbers near the projected major axis, but a Kendall's tau test confirms that this trend is not statistically significant. CaII absorption, which exhibits a low covering fraction of kappa_CaII = 0.18^{+0.06}_{-0.04} within 50 kpc for W_r(3934) > 0.1 Ang, shows no measurable azimuthal dependence. To assess potential biases, we quantify the effects of projection, disk inclination, and variations in imaging quality. After accounting for these systematics, the spatial distribution of low-redshift MgII and CaII absorbers is consistent with arising from a randomly distributed population, with no compelling evidence for azimuthal anisotropy at d <~ 50 kpc. A larger sample with robust constraints on the disk orientation will be required to uncover or rule out subtle anisotropic trends.

Figures (5)

• Figure 1: Summary of galaxy properties for the adopted sample. Panel (a) displays the projected distance ($d$) versus redshift $z$, while panel (b) displays the distribution of H$\alpha$ equivalent width (EW) relative to stellar mass, ${M}_{\rm star}$. Open symbols in panel (a) are galaxies with no Mg$\;$ absorption detected to a sensitive upper limit (see § \ref{['sec:sample']}), while galaxies with no detected H$\alpha$ emission are shown as downward arrows in panel (b) to mark 2-$\sigma$ upper limits on the underlying H$\alpha$ emission flux. As a proxy for characterizing the star formation history, the observed EW(H$\alpha$) is converted to specific star formation rate (sSFR) following Huang2021 and displayed on the right of panel (b).
• Figure 2: Examples of galaxies in the dataset taken from DECaLs Sky Browser feature ordered by impact parameter from $d = 14.4$ kpc to $d = 47.7$ kpc from left to right and top to bottom. The images are 30 arcsecs on a side and are centered on the galaxies with the associated quasar being the adjacent bright blue object. Included in the images are the redshift, Mg$\;$ absorption strength, impact parameter, and the best-fit orientation angle $\phi$ of the galaxy's major axis relative to the QSO sightline. Note that the residuals are constructed by removing nearby bright galaxies from the input image in addition to the quasar light.
• Figure 3: Summary of galaxy alignments relative to the background QSO sightlines. Panel (a) shows the inclination-angle distribution inferred from galaxy axis ratios for the full sample (red) and for the subsample with well-constrained morphological parameters (black; defined as having orientation uncertainties $\Delta\,\phi<\!15^\circ$). Panel (b) shows the corresponding distribution of azimuthal angles, $\phi$. In both panels, the expected distributions for a randomly oriented disk population are shown in blue. The observed inclination and azimuthal-angle distributions are broadly consistent with expectations for a randomly oriented galaxy population. See the main text for details regarding the construction of the theoretical distributions.
• Figure 4: Observed Mg$\;$ (top panels) and Ca$\;$ (bottom panels) absorption strengths versus distance for galaxies in our sample. The sample is divided into three subsamples based on their azimuthal constraints. Galaxies whose long axis are reliably aligned closer to the QSO sightline ($\phi\leq 45^\circ$ and $|\Delta\phi|<15^\circ$) are shown as squares; those with reliably determined minor-axis orientation ($\phi>45^\circ$ and $|\Delta\phi|<15^\circ$) are shown as diamonds; systems with poorly constrained azimuthal angles ($|\Delta\phi|>15^\circ$) are shown as open symbols. The left panels show the rest-frame absorption equivalent widths as a function of projected distance, $d$. The right panels present the corresponding equivalent widths relative to the deprojected galactocentric distance, $r_d$, computed along the extended disk plane based on the best-fit inclination and orientation angles following Equation \ref{['eq:rd']}.
• Figure 5: Observed variations in absorber strength as a function of galaxy geometric alignment relative to the QSO sightlines. Each panel shows the projected position of the QSO sightline relative to the semi-major and semi-minor axes in the rest-frame disk plane of the absorbing galaxy placed at (0,0) of the panel. Galaxies are grouped into four subsamples based on their rest-frame equivalent width. For Mg$\;$ (top row), systems are classified as strong ($W_r > 1$ Å; magenta), intermediate ($0.3 \leq W_r \leq 1$ Å; cyan), weak ($W_r < 0.3$ Å; dark green), or non-detections (crosses). For Ca$\;$ (bottom row), the strong, intermediate, weak, and non-detection categories correspond to $W_r > 0.2$ Å (blue), $0.1 \leq W_r \leq 0.2$ Å (orange), $W_r < 0.1$ Å (green), and non-detections (crosses), respectively. Uncertainties in the azimuthal angle, $\phi$, are indicated by dashed curves associated with each galaxy. The left panels display all galaxies with available absorption constraints, whereas the right panels show only those with well-determined azimuthal angles, defined by orientation uncertainties satisfying $|\Delta\phi| < 15^\circ$. The distribution of absorber strengths relative to the galaxy's major and minor axes illustrates the degree to which gas properties correlate with disk geometry (see the main text for details).