Disturbed cold gas in galaxy and structure formation

TL;DR

The paper investigates how cold gas in the CGM influences galaxy formation at high redshift by targeting ultra-strong MgII absorbers with extensive kinematic widths. Using a multi-wavelength approach (VLT/MUSE, JWST/NIRCam, and ALMA) they detect two Lyα emitters around a z~4.86 USMgII pair and a dusty star-forming galaxy at z=2.566 near another USMgII system, revealing complex gas environments. CLOUDY ionization modeling and dynamical analyses indicate metallicity and ionization differences between the two absorbers and suggest possible rotating disk kinematics in the z=2.566 galaxy, while simulations show that high-column-density HI gas can reside along multiple axes in disk-like halos. The findings imply that cold CGM gas can drive disk formation and mediate metal/dust exchange in overdense regions at z>2, highlighting the need for deeper IFU surveys and JWST follow-up to map CGM–galaxy kinematics in three dimensions.

Abstract

Cold and cool gas (T $\leq 10^4$ K) in the circumgalactic medium (CGM) and its interaction with galaxies remain poorly understood. Simulations predict that cold gas flows into galaxies through cosmic filaments, determining the disk formation and galaxy evolution. The cold gas accretion modes in the CGM and their dependence on dark matter halo mass and redshift remain puzzling. Resolving the kiloparsec-scale kinematics and dynamics of cold gas interacting with the disk, dust, and metals in different environments is particularly lacking at z > 2. Here we report two disturbed cold gas structures traced by ultra-strong MgII absorbers (rest-frame equivalent width Wr > 2 Å) exhibiting high kinematic velocities (> 500 km/s) and their environments at z ~ 4.9 and z ~ 2.6. Observations were conducted with VLT/MUSE, JWST/NIRCam, and ALMA to detect Lya and nebular emission lines, as well as dust continuum emission in the vicinity of these two absorbing gas structures. We identify two Lya emitters associated with a strong MgII absorber pair separated by ~1000 km/s at z~ 4.87. The pair exhibits relative differences in metallicity, dust content, and ionization states, suggesting internal metal and dust exchange within the ultra-large cold gas structure. For the strong MgII absorber at z = 2.5652, we detect a dusty star-forming galaxy at a projected distance of $D = 38$ kpc. This galaxy exhibits prominent HeI, [SIII], and Paschen$γ$ lines, along with significant dust continuum. It has a star formation rate of ~ 121 +/- 33 $M_{\odot}$/yr and likely harbors a rotating disk. These findings tentatively suggest that cold gas at high redshifts plays a critical role in driving disk formation and actively participates in the transfer of metals and dust within the overdense regions of the CGM.

Figures (12)

• Figure 1: $Left.$ Spatial distribution of the absorbing gas along J1306+0356 (located at the center of the VLT/MUSE cube) and the two LAEs, which are offset by $\sim 200$ kpc from the gas. The background image is the variance-weighted white-light image generated by MUSELET (see details in Section \ref{['sec:emission_line']}). Narrow-band images of the two detected LAEs are presented. The absorbing gas structure is composed of two VSMGII systems, with a total velocity width $\Delta v$ greater than 1000 km s$^{-1}$. $Right.$ Spatial distribution of the absorbing gas along the J0305--3150 sightline (upper right corner) and the host galaxy, as detected with JWST/NIRCam data. The red dashed line is the fitted galaxy major axis.
• Figure 2: Emission profile of the Ly$\alpha$ lines detected with VLT/MUSE and the absorption profile of the Mg ii absorber observed by Magellan/FIRE along the J1306+0356 sightline at $z = 4.8651$, shown in velocity space. The upper panel shows the spectra of the two LAEs, which are at distances of 201 kpc (red) and 197 kpc (blue) from the absorbing gas, respectively. LAE1 is centered at $z = 4.867$ at the peak of the strongest component. The grey dashed lines indicate the potential subcomponents of LAE 1. The shaded regions indicate the wavelength interval used to compute the equivalent width (EW) of the Ly$\alpha$ lines. For LAE1, the blue bump is tentative, therefore, we do not include it in the EW measurement. The lower panels show the Mg ii$\lambda2796$ and Mg ii$\lambda2803$ lines, respectively, with the blue dashed line representing the zero point at $z = 4.8651$, where the first major subcomponents are located. Complete Voigt profile fitting of the two systems is provided in Table \ref{['table:absorption']} and Appendix Figure \ref{['fig:J1306_abs_complete']}.
• Figure 3: The absorption profiles of the VSMGII at $z$ = 2.5662, along with the 1D spectra of the detected galaxy which is 4.65$\hbox{$^{\prime\prime}$}$ (38 kpc) away from the absorber. The galaxy shows He i (with Gaussian fit), [S iii] and Paschen emission lines at $z$ = 2.5643 in the JWST NIRCam F356W band. The zero point is centered on the strongest Mg ii subcomponent (the same in each panel).
• Figure 4: GALFIT fitting of galaxy J0305M31-He1-2708 in F200W (upper) and F356W (lower). We use GALFIT to measure the effective radius ($R_e$), Sérsic index, axis ratio, and position angle. The cyan dashed line marks the major axis. Each row shows (left to right) the JWST image, GALFIT model, and residuals (data minus model).
• Figure 5: $Upper$: Comparison of VSMGII-Ly$\alpha$ line flux with other DLA-Ly$\alpha$ hosts. The DLA-Ly$\alpha$ data points are color-coded by the velocity width $v_{90}$ of the Si ii($\lambda$1808) line. The Mg ii system ($W_r = 2.033 \pm 0.187$ Å and $v_{90}(\lambda$2796) = 355 km s$^{-1}$) at $z = 3.188$ in the MAGG survey marta24 is plotted as a red triangle. The column density of H i for the two Mg ii systems, including our $z = 4.8651$ VSMGII system, is extrapolated from the $W_r$(Mg ii)-$N$(H i) relation in lan18. $Lower:$ Equivalent widths of LAEs detected in the MUSE Hubble Ultra Deep Field and the JADES survey at $2 < z < 8$.