MUSE study of two giant low surface brightness galaxies with compact satellites

TL;DR

This work uses deep MUSE mosaics to map the spatially resolved ionized gas kinematics and metallicity, and to perform stellar-population analysis in two giant low-surface-brightness galaxies with compact satellites. It finds a counter-rotating gaseous disk in UGC 1382 and a co-rotating disk in AGC 192040, both with flat gas-phase metallicity gradients and evidence of past mergers on Gyr timescales. The authors argue for different merger-driven formation channels: multiple gas-rich, retrograde mergers for UGC 1382, and gas accretion from filaments plus a prograde, intermediate-mass merger for AGC 192040, with the compact satellites possibly contributing to the disks but not fully accounting for their mass. The results imply that gLSBGs can arise from diverse evolutionary histories that preserve large, dynamically cold disks while incorporating substantial external material.

Abstract

Giant low-surface-brightness disk galaxies (gLSBGs) are rare objects with disk radii up-to 160 kpc and dynamical masses of an order of up to 10$^{12}$ $M_{\odot}$. Their very existence challenges currently accepted theories of galaxy formation and evolution, as it is difficult to build such large, dynamically cold disks through mergers without destroying them. We present deep MUSE mosaic observations of two nearby gLSBGs with compact elliptical satellites: UGC 1382, which hosts a globally counter-rotating gaseous disk, and AGC 192040, which does not. We analyze properties of ionized gas and present spatially resolved kinematics and metallicity maps; as well as stellar population analysis for the central regions of the galaxies. The radial gradients of gas-phase metallicities are flat for both galaxies. Our estimates of the effective oxygen yield suggest 'passive' gas in the outskirts of both stellar systems that is not involved in star formation. Our observational data indicate that both galaxies experienced mergers several Gyrs ago. However, the scenarios of formation of giant disks appear to be slightly different for these two systems. For AGC 192040 we propose the gas accretion from the filament followed by the intermediate-mass ratio merger with the companion on a prograde orbit. For UGC 1382 multiple gas-rich mergers with companions on retrograde orbits are preferred by the data.

Figures (10)

• Figure 1: Gas-phase metallicity maps for AGC 192040 (left) ans UGC 1382 (right). The red outline indicates the field of view of the MUSE mosaics. The map for AGC 192040 is based on O3N2 calibration Marino2013AA...559A.114M, while for UGC 1382 the S-calibration Pilyugin2016 is used. Each dot corresponds to a spatial bin; its color represents the oxygen abundance relative to the solar value, assuming $12 + \log{(\mathrm{O}/\mathrm{H})}_\odot = 8.69$Asplund2009ARAA..47..481A. The transparency of the dots reflects the uncertainty in the metallicity measurement, with more transparent points corresponding to higher uncertainties. The red arrows demonstrate the positions of the satellites. For AGC 192040 the cE satellite is the one on the right-hand side.
• Figure 2: Parameter maps and image of UGC 1382. Top left: HSC image hsc2019, with the red outline indicating the field of view of the MUSE mosaics. The magenta hexagon shows the field of view of the MaNGA SDSS spectrum. Light blue and yellow outlines indicate the spatial binning used for fitting emission lines and stellar components, respectively (Sec. \ref{['subsec:nbursts_fit']}). Bottom left: Emission line velocity map. In the top right corner, the same map is shown using MaNGA data for the central region (magenta hexagon), with the same color bar and scale (in kpc). Other maps, from left to right and top to bottom: Maps of stellar velocity, stellar velocity dispersion, and SSP-equivalent luminosity weighted age and metallicity for the central region of the galaxy. In each plot, the top right corner shows the corresponding MaNGA-based map for the central region (same color bar and scale in kpc), and the bottom left corner shows a cutout highlighting the companion (with a velocity shift of $-120$ km s$^{-1}$).
• Figure 3: Parameter maps and image of AGC 19204. Top left: Legacy Survey image Dey2019, with the red outline indicating the field of view of the MUSE mosaics. Light blue and yellow outlines indicate the spatial binning used for fitting emission lines and stellar components, respectively (Sec. \ref{['subsec:nbursts_fit']}). Bottom left: Emission line velocity map. Other maps, from left to right and top to bottom: Maps of stellar velocity, stellar velocity dispersion, and SSP-equivalent luminosity-weighted age and metallicity for the galaxy. The two stellar companions --- located above the center and in the top right corner --- have systemic velocities relative to the galaxy of $+180$ km s$^{-1}$ and $-270$ km s$^{-1}$, respectively (the stellar velocity bins corresponding to these companions were shifted accordingly).
• Figure 4: BPT diagrams for AGC 192040 and UGC 1382. Symbols are color-coded corresponding to their galactocentric distance, the cross in the bottom-left corner represent the median uncertainties of the parameters. The blue dashed line represents the demarcation line introduced by Kauffmann2003, which distinguishes between the star-forming and the so-called composite regions. The additional demarcation lines are taken from Kewley2001. Density plots of emission line measurements from the RCSED database Chilingarian2017ApJS..228...14C are shown in grey.
• Figure 5: Radial metallicity distribution for UGC 1382 and AGC 192040. Solid lines are linear regression of the data points. Horizontal grey dashed line represents solar metallicity.