Kinematic analysis of an Ultra-Strong MgII absorber at z~1.13 linking to Circumgalactic Gas Structures

TL;DR

This study probes the circumgalactic medium of a star-forming galaxy at $z_{gal} \approx 1.1334$ by analyzing an ultrastrong MgII absorber at $z_{abs} \approx 1.133$ seen in a background quasar spectrum, at a projected distance of $\rho \approx 18$ kpc. By combining high-resolution UVES absorption spectroscopy with VLT/MUSE IFU imaging, the authors decompose the MgII profile into 26 kinematic components and perform component-by-component Bayesian ionization modeling with Cloudy to infer metallicities, densities, and the total $\log N(HI) \approx 22.5$, indicating a sub-DLA/DLA-like system. The MUSE data yield a single, relatively massive, rotating disk–like galaxy with $V_{\rm max} \approx 186$ km s$^{-1}$ and $R_{vir} \approx 152$ kpc, and the absorption kinematics show both gas consistent with disk rotation and substantial extraplanar or random-velocity material. Overall, the ultrastrong MgII absorption arises from a complex, multi-phase CGM, comprising corotating gas and additional halo structures, illustrating how integrated absorption and IFU observations map the baryon cycle and gas accretion/recycling around galaxies.

Abstract

We present a spectroscopic and imaging analysis of the $z_{gal} \approx 1.1334$ ultra-strong MgII absorption system identified in the $VLT$/UVES spectrum of a background quasar located at $ρ\approx 18$ kpc from a star-forming galaxy. Low ionization metal lines like MgI, FeII, and CaII are also detected for this absorber. The HI lines are outside of the wavelength coverage. The MgII has a rest-frame equivalent width of $W_r(2796) =3.185 +/- 0.032 A^{\circ} $, with the absorption spread across $Δv \approx 460$ km~s$^{-1}$ in several components. A component-by-component ionization modeling shows several of these components having solar and higher metallicities. The models also predict a total HI column density of $log[N(HI)/cm^{-2}] \approx 22.5$, consistent with ultra-strong MgII absorbers being sub-Damped Lyman Alpha and Damped Lyman Alpha systems. The absorber is well within the virial radius of the nearest galaxy which has a stellar mass $M_* = 4.7 \times 10^{10}$~M$_\odot$, and a star formation rate of $\approx 8.3$~M$_\odot$~yr$^{-1}$. The absorption is along the projected major axis of the galaxy with a velocity spread that is wider than the galaxy's disk rotation. From the kinematic analysis of the absorber and the galaxy, the origin of the absorption can be attributed to a combination of circumgalactic gas structures, some corotating with the disk and the rest at line-of-sight velocities outside of the disk rotation.

Figures (6)

• Figure 1: Top-Left : White light image from the $VLT$/MUSE with dimensions of $1^{\prime} \times 1^{\prime}$ centered on the quasar Q $1621-0042$. The quasar light is removed by subtracting the PSF as indicated in § \ref{['sec:ifudata']}. Top-Right: A narrow band image of the same field centered around the [O ii] $\lambda\lambda3726, 3728$ Å doublet at $z_{gal} = 1.1334$. The [O ii] doublet lines are unresolved at the resolution of MUSE. The line of sight to the background quasar Q $1621-0042$ is indicated with the cross-wire symbol. The circles with dotted lines represent $\rho = 50$ kpc and $\rho =100$ kpc projected separations around the galaxy at $z_{gal}$ respectively. The foreground field of the quasar does not show any other extended source at the redshift of $z_{gal}\approx$1.1334. Bottom: The galaxy spectrum obtained from MUSE is shown in grey. The blue curve is the binned spectrum. The redshifted [O ii] emission lines are used to establish the redshift of the galaxy coinciding with the absorber, and determine star-formation rate of the galaxy.
• Figure 2: The system plot centered on the emission redshift of z = 1.1334 of the nearest galaxy, showing the lines detected at $>\,3$ sigma and their respective Voigt profile fits (red) imposed over the QSO UVES spectra (grey). The colored vertical lines show the different components of each line, with their respective number labeled, used to ascertain the component structure for photoionization modeling. The Mg ii profile has 26 components out of which, component structure in the central saturated region is ascertained from the unsaturated Ca ii doublet lines, the Fe ii multiplets, and the Mg i line. The Fe ii$\lambda2586$ Å and Fe ii$\lambda2374$ Å lines fall in the Ly$\alpha$ forest region.
• Figure 3: This figure shows the component profiles of metal lines predicted by photoionization models for the 26 components seen in Mg ii. The modeling results shown here are summarised in Table \ref{['tab:cloudproperties']}.
• Figure 4: Left: The inclination-angle-corrected rotation curve obtained from GalPaK$^{3D}$. Right: The detected absorption profiles with normalized flux (+ offset) along the X-axis. We exclude all FeII features except the $\lambda2586$ Å line, as the others are too saturated or have low S/N.This line is also truncated on both sides in the plot due to contamination from the forest, making it visually cleaner for comparison. The dotted lines mark the range of velocities covered by the rotation curve. The central dotted line at 0 km s$^{-1}$ is for the redshift of the galaxy.
• Figure 5: Left: MUSE pseudo-narrow band image spanning the wavelength range $7940 - 7960$ Å centered on the redshifted position of the [O ii] $\lambda\lambda3727, 3729$ Å emission. The faint residuals after removing the quasar light are seen at the top-left corner of the image.A Gaussian filter of 2 sigma has also been applied to smoothen the image. Right: The galaxy velocity field map derived using Galpak$^{3D}$ along with the quasar line of sight (white filled circle). The color grid indicates the corresponding velocities in the map. The yellow dotted line represents the major axis of the galaxy at the position angle. The concentric ellipsoids define the iso-flux contours. The azimuthal angle between the quasar line of sight and the galaxy's major axis is $\alpha \approx 21.4^{\circ}$. The quasar line of sight probes a region close to the projected major axis where the emitting gas has a velocity of $\approx -150$ km s$^{-1}$, tracing the flat part of the rotation curve.