Chemodynamic evidence of pristine gas accretion in the void galaxy VGS 12

TL;DR

We investigate metal-poor gas accretion in the void galaxy VGS 12 using VLA HI maps, Fabry-Perot H_alpha kinematics, and long-slit spectroscopy to measure gas-phase abundances. The oxygen abundance is $12+\log(\mathrm{O/H}) = 7.67 \pm 0.07$, about $0.7$ dex below the value expected from the galaxy’s luminosity, while $\log(\mathrm{N/O}) = -1.29 \pm 0.04$, consistent with accretion diluting O/H but preserving N/O; this supports pristine gas inflow. The HI polar disk shows clumpy morphology and northern-southern asymmetry with kinematic distortions, and the ionized gas exhibits prolate rotation that tracks the HI disk, suggesting the HI gas is accreted and ionized in the central regions. Collectively, the results provide strong, multiwavelength evidence for ongoing cold gas accretion from the cosmic web in a nearby void galaxy, illustrating that such accretion can contribute to galaxy growth at low redshift.

Abstract

Accretion of metal-poor gas is expected to be an important channel of gas replenishment in galaxy evolution studies. However, observational evidence of this process is still relatively scarce. The unusual polar disk galaxy VGS 12 was found in the Void Galaxy Survey. It appears to be isolated and resides in the cosmological wall between two large voids. The suggested formation scenario for this peculiar system is accretion of metal-poor gas from the void interior. We present the data on the gas-phase chemical abundance of VGS 12 obtained with the Russian 6m telescope BTA. We complement our analysis with HI data obtained with VLA and the data on the kinematics of the ionized gas. VGS 12 appears to be a strong outlier from the "metallicity - luminosity" relation, with gas oxygen abundance ~0.7dex lower than expected for its luminosity. The nitrogen abundance, on the other hand, is higher than what is typically observed in galaxies with similar metallicity, but is consistent with the metallicity expected given its luminosity. Such behavior is what is expected in the case of metal-poor gas accretion. The HI reveals clear morphological and kinematical asymmetry between the northern and southern parts of the disk, which are likely related to its unsettled state due to the recent accretion event. The kinematics of the ionized gas seen in Halpha reveal prolate rotation and follow closely the rotation of the HI disk, so we suggest this is accreted HI gas ionized by the stars in the central region of the galaxy. Together, our findings provide strong, multiwavelength evidence of ongoing cold gas accretion in a galaxy caught in the act of growing from the cosmic web. This is one of the very few individual galaxies where a convincing case can be made for such a process, and demonstrates the potential for cold accretion to contribute to galaxy growth even in the low-redshift universe.

Figures (11)

• Figure 1: Cosmic structures in the vicinity of the galaxy VGS 12. Three orthogonal cuts across the density field are shown (clockwise): $z$-Dec, RA-Dec, RA-$z$ (RA and Dec centered on VGS 12). The red cross marks the position of VGS 12. In the RA-Dec slice we show galaxies in a $\pm10$ Mpc slice in $z$ as black dots. The density field $\log (1 + \delta)$ is shown in the background. The regions colored in black are located outside the SDSS mask.
• Figure 2: The H i moment maps for VGS 12 at three distinct resolutions: $\sim$ 17$\arcsec$ (top row), $\sim$ 9$\arcsec$ (middle row), and $\sim$ 5$\arcsec$ (bottom row). Left panels show the H i contours overlaid on DECaLS g-band optical images. The plotted H i contours, starting at 2$\sigma$ level, are 3.7$\times$10$^{19}$$\times$ ($\sqrt{3})^{n}$ atoms cm$^{-2}$ , n=0, 1, 2, 3,... at 17$\arcsec$, 7.4$\times$10$^{19}$$\times$ ($\sqrt{3})^{n}$ atoms cm$^{-2}$ , n=0, 1, 2, 3,... at 9$\arcsec$, and 20.3$\times$10$^{19}$$\times$ ($\sqrt{3})^{n}$ atoms cm$^{-2}$ , n=0, 1, 2, 3,... at 5$\arcsec$. Middle panels display velocity fields (isovelocity contours are spaced at intervals of 20 km s$^{-1}$), while right panels present Moment 2 maps. The Gaussian beam is shown on the bottom left of each map.
• Figure 3: Top row: the colored Legacy Survey grz-image (left-hand panel); the map of $H\alpha$ fluxes (middle panel); the map of the ionized-gas velocity dispersions (right-hand panel). Bottom row: the observed ionized-gas velocity field (left-hand panel); the velocity field obtained with a tilted-ring model (middle panel); the map of residuals after subtracting the model velocity field from the observed one (right-hand panel). The H$\alpha$ contours are overlaid on the top-middle, top-right and bottom-right panels.
• Figure 4: Diagnostic BPT diagrams. Black and gray curved lines separating the areas of different mechanisms of excitation are from Kewley2001 and Kauffmann2003, respectively, while the straight line is from Kewley2006. The colors of the symbols correspond to H$\alpha$ brightness.
• Figure 5: The colored Legacy Survey grz-image (the top panel), map of H$\alpha$ velocity dispersion (the middle panel), distribution of the emission-line fluxes ratios (the bottom panel) along the slit PA = 88. The horizontal dashed lines in the top and middle panels show the position of the slit overlaid. The vertical dotted lines denote the positions of the galaxy center and the bright SF clump on the disk periphery.