Low Metallicity Gas on the Outskirts of the Local Group: the Circumgalactic Medium of Sextans B

TL;DR

This work probes the circumgalactic medium (CGM) of the Local Group dwarf Sextans B using UV absorption along two near-central sightlines (at $ ho o$ 4.1 and 8.0 kpc). Through HST/COS spectroscopy and 21 cm HI context, the authors detect low-ion and high-ion absorption (Si II, Si III, Si IV, C II) with declining radial profiles, and derive metallicities via Cloudy photoionization models: [Si/H] ≈ -1.7 and [C/H] ≈ -2.1, at log $n_{ m H} oughly<-2.7$. The inferred cool CGM mass within 8 kpc is $M_{ m cool}^{ m CGM} obreak\approx obreak 8 imes10^{8} m\,M_{amily}$, making the CGM the dominant baryon reservoir by about a factor of 20 over the HI and stellar mass. The results extend LG CGM measurements to multiple sightlines, showing a low-metallicity, underabundant CGM consistent with Sextans B’s low halo mass and offering important constraints on CGM structure and chemical evolution in quiescent environments.

Abstract

We present a UV absorption-line analysis of the circumgalactic medium (CGM) of Sextans B, a dwarf irregular galaxy at 1.3 Mpc distance on the outer frontier of the Local Group. Using HST/COS spectroscopy of two AGN sightlines passing through the Sextans B CGM at small impact parameters of 4 kpc and 8 kpc (0.04 and 0.08 r_vir), we detect the CGM in Si II, Si III, Si IV, and C II absorption. All four ions show a column-density profile that declines with radius. The profiles fall below the average CGM profiles of other nearby dwarfs (by 0.3-0.6 dex, depending on ion), likely due to the low halo mass and low metallicity of Sextans B. Using Cloudy photoionization models and interferometric measurements of the H I column density, we find low gas-phase silicon and carbon abundances in the Sextans B CGM, [Si/H]=-1.7+/-0.2 and [C/H]=-2.1+/-0.2, among the lowest gas-phase abundances anywhere in the Local Group. We calculate a cool CGM gas mass within 8 kpc of $\sim8\times10^8 M_\odot$, making the CGM the dominant baryon reservoir of the galaxy, ~20 times larger than both the H I mass and the stellar mass.

Figures (7)

• Figure 1: Location of the two HST/COS AGN sightlines (stars) relative to the H i 21 cm emission in Sextans B (contours) and a DSS image of the galaxy showing the stellar component namumba2018. The H i data is from KAT-7 with an effective spatial resolution (FWHM) of 4.4$\times$3.2. The H i contours are at 5.4, 10.8, 21.6, 43.2, 86.4, 172.8, and 345.6 $\times10^{18}$ cm$^{-2}$.
• Figure 2: Normalized absorption-line profiles of UV metal lines at the Sextans B velocity in the HST/COS spectra of two AGN, J1000+0524 (left, $\rho$=4.1 kpc) and J0959+0503 (right; $\rho$=8.0 kpc). The top panel shows the H i 21 cm emission profile from the EBHIS survey winkel2016 or LAB survey kalberla2005. The COS data are binned by two pixels. Individual Voigt components are shaded in distinct colors with their central velocities marked with short vertical lines. The thick green line shows the overall model, the thick vertical dot-dashed line shows the systemic velocity of Sextans B (295 km s$^{-1}$), and the thin vertical dashed lines show the velocity integration range. The label (N) for O i 1302 refers to night-only reduction, and 'B' indicates a blend. A third sightline (PG1001+054 at $\rho$=27 kpc) gives no Sextans B detections in any of the UV lines zheng2024 and is not shown.
• Figure 3: Normalized apparent column density profiles of UV metal absorption in the Sextans B CGM. The left column shows the J1000+0524 sightline and the right column shows J0959+0503. Different pairs of ions are shown in each panel, allowing us to assess which ions are co-spatial in the Sextans B CGM. Each profile is normalized to its maximum value. Contaminated velocity regions (for C ii 1334 and C iv 1548) are shown in pale color and can be ignored.
• Figure 4: Radial profiles (column density vs. normalized impact parameter) of four ions in the Sextans B CGM compared to the average CGM profiles from zheng2024 from nearby dwarf galaxies (gray). Sextans B detections are shown with blue stars, and non-detections (upper limits on log $N$) are shown with yellow stars. Column densities from saturated lines are shown as lower limits. The data points at $\rho_{\rm proj}/R_{200}$=0.28 (PG1001+054 sightline) are taken from zheng2024.
• Figure 5: Results of Cloudy photoionization analysis showing the column density of C ii, Si ii, and Si iii against gas density $n_{\rm H}$, assuming log $N$(H i)=18.0 and 17.0 for the J1000+0524 (left column) and J0959+0503 (right column) sightlines, respectively. The model predictions are shown with connected symbols, and the observations are shown as colored horizontal bars. Saturated lines give lower limits on log $N$, and non-detections give upper limits. The best-fit values of log $n_{\rm H}<-2.74$ and $<-2.60$ derived from the observed Si iii/Si ii ratios are shown as the vertical gray lines. These models yield the gas-phase silicon and carbon abundances in the Sextans B CGM in each sightline, as annotated in the legend.