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Register a new userDiscovery of Counter-Rotating Gas in the Galaxies NGC1596 and NGC3203 and the Incidence of Gas Counter-Rotation in S0 Galaxies
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We have identified two new galaxies with gas counter-rotation (NGC1596 and NGC3203) and have confirmed similar behaviour in another one (NGC128), this using results from separate studies of the ionized-gas and stellar kinematics of a well-defined sample of 30 edge-on disc galaxies. Gas counter-rotators thus represent 10+/-5% of our sample, but the fraction climbs to 21+/-11% when only lenticular (S0) galaxies are considered and to 27+/-13% for S0s with detected ionized-gas only. Those fractions are consistent with but slightly higher than previous studies. A compilation from well-defined studies of S0s in the literature yields fractions of 15+/-4% and 23+/-5%, respectively. Although mainly based on circumstantial evidence, we argue that the counter-rotating gas originates primarily from minor mergers and tidally-induced transfer of material from nearby objects. Assuming isotropic accretion, twice those fractions of objects must have undergone similar processes, underlining the importance of (minor) accretion for galaxy evolution. Applications of gas counter-rotators to barred galaxy dynamics are also discussed.
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We present Australia Telescope Compact Array (ATCA) HI imaging of the edge-on galaxy NGC 1596, which was recently found to have counter-rotating ionized gas in its center (<15). We find a large HI envelope associated with a nearby companion, the dwarf irregular galaxy NGC 1602. The HI covers a region ~11.9x13.4 (62x70 kpc^2) and the total HI mass detected is 2.5+/-0.1x10^9 Msun (assuming an 18 Mpc distance). The HI is centered on NGC 1602 but appears to have two tidal tails, one of which crosses over NGC 1596. The HI located at the position of NGC 1596 has a velocity gradient in the same sense as the ionized gas, i.e. opposite to the stellar rotation. Both the existence of a large gas reservoir and the velocity gradient of the HI and the ionized gas strongly suggest that the ionized gas in NGC 1596 originated from NGC 1602. From the length of the HI tails we conclude that the interaction started at least 1 Gyr ago, but the unsettled, asymmetric distribution of the ionized gas suggests that the accretion occured more recently. NGC 1596 thus provides a good example where the presence of counter-rotating gas can be directly linked to an accretion event. After the accretion has stopped or the merging is complete, NGC 1596 may evolve to a system with more extended counter-rotating gas but no obvious signature of interaction. There is a substantial local HI peak in one of the two tails, where we also find a faint stellar counterpart. The M_HI/L_B ratio in this region is too high for a normal dwarf elliptical or a low surface brightness galaxy, so we conclude that a tidal dwarf is currently forming there.
We present a spectral decomposition technique that separates the contribution of different kinematic components in galaxies from the observed spectrum. This allows to study the kinematics and properties of the stellar populations of the individual components (e.g., bulge, disk, counter-rotating cores, orthogonal structures). Here, we discuss the results of this technique for galaxies that host counter-rotating stellar disks of comparable size. In all the studied cases, the counter-rotating stellar disk is the less massive, the youngest and has different chemical content (metallicity and alpha-elements abundance ratio) than the main galaxy disk. Further applications of the spectral decomposition technique are also discussed.
Very little work has been done on star formation in dwarf lenticular galaxies (S0s). We present 2D-spectroscopic and millimetre observations made by Centro Astronomico Hispano Aleman (CAHA) 3.5 m optical and the IRAM-30 m millimetre telescopes, respectively, for a sample of four dwarf S0 galaxies with multiple star formation regions in the field environment. We find that although most of the sources deviate from the star forming main sequence relation, they all follow the Kennicutt-Schmidt law. After comparing the stellar and Halpha kinematics, we find that the velocity fields of both stars and ionized gas do not show regular motion and the velocity dispersions of stars and ionized gas are low in the regions with high star formation, suggesting these star-forming S0 galaxies still have significant rotation. This view can be supported by the result that most of these dwarf S0 galaxies are classified as fast rotators. The ratio of average atomic gas mass to stellar mass (~ 47%) is much greater than that of molecular gas mass to stellar mass (~ 1%). In addition, the gas-phase metallicities in the star-forming regions are lower than that of the non-star-forming regions. These results indicate that the extended star formation may originate from the combination of abundant atomic hydrogen, long dynamic time scale and low-density environment.
We present a spectral decomposition technique and its applications to a sample of galaxies hosting large-scale counter-rotating stellar disks. Our spectral decomposition technique allows to separate and measure the kinematics and the properties of the stellar populations of both the two counter-rotating disks in the observed galaxies at the same time. Our results provide new insights on the epoch and mechanism of formation of these galaxies.
A key feature of a large population of low-mass, late-type disk galaxies are star-forming disks with exponential light distributions. They are typically also associated with thin and flat morphologies, blue colors, and dynamically cold stars moving along circular orbits within co-planar thin gas disks. However, the latter features do not necessarily always imply the former, in fact, a variety of different kinematic configurations do exist. In this work, we use the cosmological hydrodynamical IllustrisTNG Simulation to study the nature and origin of dynamically hot, sometimes even counter-rotating, star-forming disk galaxies in the lower stellar mass range (between $5times 10^9,mathrm{M_{odot}}$ and $2times 10^{10},mathrm{M_{odot}}$). We find that being dynamically hot arises in most cases as an induced transient state, for example due to galaxy interactions and merger activities, rather than as an age-dependent evolutionary phase of star-forming disk galaxies. The dynamically hot but still actively star-forming disks show a common feature of hosting kinematically misaligned gas and stellar disks, and centrally concentrated on-going star formation. The former is often accompanied by disturbed gas morphologies, while the latter is reflected in low gas and stellar spins in comparison to their dynamically cold, normal disk counterparts. Interestingly, observed galaxies from MaNGA with kinematic misalignment between gas and stars show remarkably similar general properties as the IllustrisTNG galaxies, and therefore are plausible real-world counterparts. In turn, this allows us to make predictions for the stellar orbits and gas properties of these misaligned galaxies.
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