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Register a new userDetectability of large-scale counter-rotating stellar disks in galaxies with integral-field spectroscopy
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In recent years integral-field spectroscopic surveys have revealed that the presence of kinematically decoupled stellar components is not a rare phenomenon in nearby galaxies. However, complete statistics are still lacking because they depend on the detection limit of these objects. We investigate the kinematic signatures of two large-scale counter-rotating stellar disks in mock integral-field spectroscopic data to address their detection limits as a function of the galaxy properties and instrumental setup. We built a set of mock data of two large-scale counter-rotating stellar disks as if they were observed with the Multi-Unit Spectroscopic Explorer (MUSE). We accounted for different photometric, kinematic, and stellar population properties of the two counter-rotating components as a function of galaxy inclination. We extracted the stellar kinematics in the wavelength region of the calcium triplet absorption lines by adopting a Gauss-Hermite (GH) parameterization of the line-of-sight velocity distribution (LOSVD). We confirm that the strongest signature of the presence of two counter-rotating stellar disks is the symmetric double peak in the velocity dispersion map, already known as the $2sigma$ feature. The size, shape, and slope of the 2$sigma$ peak strongly depend on the velocity separation and relative light contribution of the two counter-rotating stellar disks. When the $2sigma$ peak is difficult to detect due to the low signal-to-noise ratio of the data, the large-scale structure in the $h_3$ map can be used as a diagnostic for strong and weak counter-rotation. The counter-rotating kinematic signatures become fainter at lower viewing angles as an effect of the smaller projected velocity separation between the two counter-rotating components. We confirm that the observed frequency of $2sigma$ galaxies represents only a lower limit of the stellar counter-rotation phenomenon.
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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.
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.
By obtaining imaging data in two photometric bands for 60 lenticular galaxies - members of 8 southern clusters - with the Las Cumbres Observatory one-meter telescope network, we have analyzed the structure of their large-scale stellar disks. The parameters of radial surface-brightness profiles have been determined (including also disk thickness), and all the galaxies have been classified into pure exponential (Type I) disk surface-brightness profiles, truncated (Type II) and antitruncated (Type III) piecewise exponential disk surface-brightness profiles. We confirm the previous results of some other authors that the proportion of surface-brightness profile types is very different in environments of different density: in the clusters the Type-II profiles are almost absent while according to the literature data, in the field they constitute about one quarter of all lenticular galaxies. The Type-III profiles are equally presented in the clusters and in the field, while following similar scaling relations; but by undertaking an additional structural analysis including the disk thickness determination we note that some Type-III disks may be a combination of a rather thick exponential pseudobulge and an outer Type-I disk. Marginally we detect a shift of the scaling relation toward higher central surface brightnesses for the outer segments of Type-III disks and smaller thickness of the Type-I disks in the clusters. Both effects may be explained by enhanced radial stellar migration during disk galaxy infall into a cluster that in particular represents an additional channel for Type-I disk shaping in dense environments.
We present the kinematics and stellar population properties of a sample of 53 galaxies (50 are Early-Type galaxies, ETGs) with Counter-Rotating Disks (CRD) extracted from a sample of about 4000 galaxies of all morphological types in the MaNGA survey (DR16). The kinematic maps were used to select galaxies based on evidence of counter-rotation in the velocity maps or two peaks in the velocity dispersion maps. For about 1/3 of the sample, the counter-rotating components can also be separated spectroscopically. We then produced the age and metallicity maps, and compared the stellar population properties to those of the general ETGs population. We found that CRDs have similar trends in age and metallicity, but they are generally less metallic at low masses. The metallicity gradients are similar; instead, age gradients are typically flatter and confined within a smaller range of values. We compared the velocity fields of the ionized gas and the stars, and found that in 25 cases the gas corotates with either the inner (13 cases) or the outer (12 cases) disk, and in 9 cases the gaseous and stellar disks are misaligned. With one exception, all misaligned cases have stellar masses less than $3 times 10^{10}$M$_odot$. We also compared stellar and gaseous disks with age maps and found that in most cases the gas corotates with the younger disk. We looked for evidences of multimodality in the stellar populations, and found it in 25 galaxies, plus 11 cases with evidences of ongoing star formation, and the latter are the youngest and least massive galaxies; 13 galaxies, instead, exhibit unimodality, and are the oldest and most massive CRDs. As a general result, our work supports different formation scenarios for the kinematic class of counter-rotators.
We present a high resolution simulation of an idealized model to explain the origin of the two young, counter-rotating, sub-parsec scale stellar disks around the supermassive black hole SgrA* at the Center of the Milky Way. In our model, the collision of a single molecular cloud with a circum-nuclear gas disk (similar to the one observed presently) leads to multiple streams of gas flowing towards the black hole and creating accretion disks with angular momentum depending on the ratio of cloud and circum-nuclear disk material. The infalling gas creates two inclined, counter-rotating sub-parsec scale accretion disks around the supermassive black hole with the first disk forming roughly 1 Myr earlier, allowing it to fragment into stars and get dispersed before the second, counter-rotating disk forms. Fragmentation of the second disk would lead to the two inclined, counter-rotating stellar disks which are observed at the Galactic Center. A similar event might be happening again right now at the Milky Way Galactic Center. Our model predicts that the collision event generates spiral-like filaments of gas, feeding the Galactic Center prior to disk formation with a geometry and inflow pattern that is in agreement with the structure of the so called mini-spiral that has been detected in the Galactic Center.
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