Counter-rotating Stellar Disks Research Articles

Probing the History of the Galaxy Assembly of the Counterrotating Disk Galaxy PGC 66551

Stellar counterrotation in disk galaxies directly relates to the complex phenomenon of the disk mass assembly believed to be driven by external processes, such as accretion and mergers. The study of such systems makes it possible to reveal the source of external accretion and establish the details of this process. In this paper, we investigate the galaxy PGC 66551 (MaNGA ID 1-179561), which hosts two large-scale counterrotating (CR) stellar disks identified in the Sloan Digital Sky Survey MaNGA data and then confirmed using deep follow-up spectroscopy with the 10 m Southern African Large Telescope. We measured the properties of ionized gas and stellar populations of both CR disks in PGC 66551. We found that the CR disk is compact, contains young stars with subsolar metallicity, and has a stellar mass of 5 × 109 M ⊙, which amounts to ≈20% of the galaxy’s total. Surprisingly, the main 8 Gyr old disk has a significantly lower metallicity of −0.8 dex than other CR galaxies. We developed a simple analytic model of the history of the metal enrichment, which we applied to PGC 66551 and constrained the parameters of the galactic outflow wind, and estimated the metallicity of the infalling gas that formed the CR disk to be −0.9... −0.5 dex. Our interpretation prefers a merger with a gas-rich satellite over cold accretion from a cosmic filament as a source of gas, which then formed the CR disk in PGC 66551.

Uncovering the formation of the counter-rotating stellar discs in SDSS J074834.64+444117.8

ABSTRACT Using the integral field spectroscopic data from Mapping Nearby Galaxies at Apache Point Observatory survey, we study the kinematics and stellar population properties of the two counter-rotating stellar discs in a nearby galaxy SDSS J074834.64+444117.8. We disentangle the two stellar discs by three methods, including Ca ii λ8542 double Gaussian fit, penalized pixel fitting (pPXF) spectral decomposition, and orbit-based dynamical model. These three different methods give consistent stellar kinematics. The pPXF spectral decomposition provides the spectra of two stellar discs, with one being more luminous across the whole galaxy named primary disc, and the other named secondary disc. The primary disc is counter-rotating with ionized gas, while the secondary disc is co-rotating with ionized gas. The secondary disc has younger stellar population and poorer stellar metallicity than the primary disc. We estimate the stellar mass ratio between the primary and secondary discs to be ∼5.2. The DESI g, r, z colour image does not show any merger remnant feature in this galaxy. These findings support a scenario that the counter-rotating stellar discs in SDSS J074834.64+444117.8 formed through gas accretion from the cosmic web or a gas-rich companion.

SDSS-IV MaNGA: integral-field kinematics and stellar population of a sample of galaxies with counter-rotating stellar discs selected from about 4000 galaxies

ABSTRACT We present the integral-field kinematics and stellar population properties of 64 galaxies (61 are Early-Type galaxies; ETGs) with Counter-Rotating stellar Disks (CRD) selected from about 4000 galaxies in the MaNGA survey, based on evidence of counter-rotation or two velocity dispersion peaks in the kinematic maps. For 17 CRDs, the counter-rotating components can also be separated spectroscopically. The frequency of CRDs in MaNGA is <5 per cent for ellipticals, <3 per cent for lenticulars, and <1 per cent for spirals (at 95 per cent confidence level), consistent with previous estimates. We produced 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 to ETGs, but are less metallic at low masses, and show flatter age and steeper metallicity gradients, on average. A comparison of the velocity fields of the ionized gas and the stars reveals that in 33 cases the gas corotates with either the inner (15 cases) or outer (18 cases) stellar disc, and in nine cases it is misaligned. In most cases the gas corotates with the younger disc. Evidence of multimodality in the stellar population is found in 31 galaxies, while the 14 youngest and least massive galaxies show ongoing star formation; 14 galaxies, instead, exhibit unimodality, and are the oldest and most massive. As a general result, our work indicates that CRDs form primarily via gas accretion in retrograde rotation with respect to a pre-existing stellar disc.

Detectability of large-scale counter-rotating stellar disks in galaxies with integral-field spectroscopy

Context. 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. Aims. 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. Methods. 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). Results. 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 2σ feature. The size, shape, and slope of the 2σ peak strongly depend on the velocity separation and relative light contribution of the two counter-rotating stellar disks. When the 2σ peak is difficult to detect due to the low signal-to-noise ratio of the data, the large-scale structure in the h3 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 2σ galaxies represents only a lower limit of the stellar counter-rotation phenomenon. Conclusions. The parameterization with a single GH function does not provide a good description of the LOSVD in the presence of strong counter-rotation. However, using GH parametric solutions is a practical way to reveal the large-scale counter-rotating stellar disks and could be used to detect faint counter-rotating components to improve the statistics of stellar counter-rotation.

Spotting a counter-rotating galaxy

AbstractThe presence of counter-rotating (CR) components in galaxies is not that rare but their origin is still unclear. Important clues to the formation and evolution of CR galaxies are provided by galaxy kinematics, such as the mass distribution and the shape of the gravitational potential. In order to better understand the origin and incidence of CR galaxies, we aim at modeling CR stellar disks, as they would be observed with Integral Field Units (IFU) instruments, and measuring the kinematics of these peculiar astrophysical objects to reveal the CR signatures. In the bi-dimensional maps of analysed models, the double sigma signature is the best diagnostic to spot the presence of a CR disk component.

The properties of the kinematically distinct components in NGC 448 and NGC 4365

Aims. We study the kinematically distinct components in two early-type galaxies NGC 448 and NGC 4365 aided by integral-field observations with the Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope. The former galaxy has previously been shown to host a counter-rotating stellar disc while the latter harbours a central (apparently) decoupled core that has been suggested to not be physically distinct from the main body and instead stems from the different orbital types in the core and main body due to its triaxial nature. We aim to measure the brightness profiles, kinematics, and stellar population properties of the peculiar kinematic structures in these galaxies and shed light on their true nature and formation mechanism.Methods. We use a kinematic decomposition technique to separate the individual contributions to the spectra of the two distinct kinematic components observed at each spatial position in the field of view. Furthermore, by folding back the outcome of a photometric decomposition we reduce the intrinsic degeneracies in recovering the kinematics and the best-fitting stellar spectral templates. Finally, by extracting the Lick line-strength indices for the individual components and fitting them to single stellar population models we derive their ages, metallicities, andα/Fe overabundances.Results. The two kinematically decoupled stellar components in NGC 448 have similar ages, but different chemical compositions. The distinct kinematic feature in NGC 448 has a nearly exponential surface-brightness light profile, dominates in the innermost ∼10″, is smaller in size, and is very likely an embedded counter-rotating disc as also indicated by its kinematics. It has higher metallicity than the main galaxy stellar body and lowerα/Fe overabundance. By contrast, we do not find evidence for true decoupling in the two distinct kinematic components in NGC 4365. This confirms earlier work suggesting that the kinematically distinct core is likely not a separate dynamical structure, but most certainly likely a projection effect stemming from the orbital structure of this galaxy that was previously found to be intrinsically triaxial in shape.Conclusions. Our findings indicate that the kinematically decoupled component in NGC 448 is truly decoupled, has external origin, and was formed through either the acquisition of gas and a subsequent star-formation episode or from the direct accretion of stars from a companion. Conversely, the presence of a kinematically distinct component in NGC 4365 is not associated to a true kinematic decoupling and is instead most likely due to a projection effect stemming from the triaxial nature of this galaxy.

Evidence for the formation of the young counter-rotating stellar disk from gas acquired by IC 719

Aims. The formation scenario of extended counter-rotating stellar disks in galaxies is still debated. In this paper, we study the S0 galaxy IC 719 known to host two large-scale counter-rotating stellar disks in order to investigate their formation mechanism. Methods. We exploit the large field of view and wavelength coverage of the Multi Unit Spectroscopic Explorer (MUSE) spectrograph to derive two-dimensional (2D) maps of the various properties of the counter-rotating stellar disks, such as age, metallicity, kinematics, spatial distribution, the kinematical and chemical properties of the ionized gas, and the dust map. Results. Due to the large wavelength range, and in particular to the presence of the Calcium Triplet λλ8498, 8542, 8662 Å (CaT hereafter), the spectroscopic analysis allows us to separate the two stellar components in great detail. This permits precise measurement of both the velocity and velocity dispersion of the two components as well as their spatial distribution. We derived a 2D map of the age and metallicity of the two stellar components, as well as the star formation rate and gas-phase metallicity from the ionized gas emission maps. Conclusions. The main stellar disk of the galaxy is kinematically hotter, older, thicker and with larger scale-length than the secondary disk. There is no doubt that the latter is strongly linked to the ionized gas component: they have the same kinematics and similar vertical and radial spatial distribution. This result is in favor of a gas accretion scenario over a binary merger scenario to explain the origin of counter-rotation in IC 719. One source of gas that may have contributed to the accretion process is the cloud that surrounds IC 719.

Stellar counter-rotation in lenticular galaxy NGC 448

The counter-rotation phenomenon in disc galaxies directly indicates a complex galaxy assembly history which is crucial for our understanding of galaxy physics. Here we present the complex data analysis for a lenticular galaxy NGC 448, which has been recently suspected to host a counter-rotating stellar component. We collected deep long-slit spectroscopic observations using the Russian 6-m telescope and performed the photometric decomposition of Sloan Digital Sky Survey (SDSS) archival images. We exploited (i) a non-parametric approach in order to recover stellar line-of-sight velocity distributions and (ii) a parametric spectral decomposition technique in order to disentangle stellar population properties of both main and counter-rotating stellar discs. Our spectral decomposition stays in perfect agreement with the photometric analysis. The counter-rotating component contributes $\approx$30 per cent to the total galaxy light. We estimated its stellar mass to be $9.0^{+2.7}_{-1.8}\cdot10^{9}M_\odot$. The radial scale length of counter-rotating disc is $\approx$3 times smaller than that of the main disc. Both discs harbour old stars but the counter-rotating components reveals a detectable negative age gradient that might suggest an extended inside-out formation during $3\dots4$ Gyrs. The counter-rotating disc hosts more metal-rich stars and possesses a shallower metallicity gradient with respect to the main disc. Our findings rule out cosmological filaments as a source of external accretion which is considered as a potential mechanism of the counter-rotating component formation in NGC 448, and favour the satellite merger event with the consequent slow gas accretion.

The difference in age of the two counter-rotating stellar disks of the spiral galaxy NGC 4138

The Sa spiral NGC 4138 is known to host two counter-rotating stellar disks, with the ionized gas co-rotating with one of them. We measured the kinematics and properties of the two counter-rotating stellar populations to constrain their formation scenario. A spectroscopic decomposition of the observed major-axis spectrum was performed to disentangle the relative contribution of the two counter-rotating stellar and one ionized-gas components. The line-strength indices of the two counter-rotating stellar components were measured and modeled with single stellar population models that account for the alpha/Fe overabundance. The counter-rotating stellar population is younger, marginally more metal poor, and more alpha-enhanced than the main stellar component. The younger stellar component is also associated with a star-forming ring. The different properties of the counter-rotating stellar components of NGC~4138 rule out the idea that they formed due to bar dissolution. Our findings support the results of numerical simulations in which the counter-rotating component assembled from gas accreted on retrograde orbits from the environment or from the retrograde merging with a gas-rich dwarf galaxy.

Counter-rotating disks in galaxies: dissecting kinematics and stellar populations with 3D spectroscopy

AbstractWe 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 α-elements abundance ratio) than the main galaxy disk. Further applications of the spectral decomposition technique are also discussed.

Regrowth of stellar discs in mature galaxies: the two-component nature of NGC 7217 revisited with VIRUS-W

Previous studies have reported the existence of two counter-rotating stellar disks in the early-type spiral galaxy NGC7217. We have obtained high-resolution optical spectroscopic data (R ~ 9000) with the new fiber-based Integral Field Unit instrument VIRUS-W at the 2.7m telescope of the McDonald Observatory in Texas. Our analysis confirms the existence of two components. However, we find them to be co-rotating. The first component is the more luminous (~ 77% of the total light), has the higher velocity dispersion (~ 170 km/s) and rotates relatively slowly (projected $v_{max}$ = 50 km/s). The lower luminosity second component, (~ 23% of the total light), has a low velocity dispersion (~ 20 km/s) and rotates quickly (projected $v_{max}$ = 150 km/s). The difference in the kinematics of the two stellar components allows us to perform a kinematic decomposition and to measure the strengths of their Mg and Fe Lick indices separately. The rotational velocities and dispersions of the less luminous and faster component are very similar to those of the interstellar gas as measured from the [OIII] emission. Morphological evidence of active star formation in this component further suggests that NGC7217 may be in the process of (re)growing a disk inside a more massive and higher dispersion stellar halo. The kinematically cold and regular structure of the gas disk in combination with the central almost dust-free morphology allows us to compare the dynamical mass inside of the central 500pc with predictions from a stellar population analysis. We find agreement between the two if a Kroupa stellar initial mass function is assumed.

Spectroscopic evidence of distinct stellar populations in the counter-rotating stellar disks of NGC 3593 and NGC 4550

We present the results of integral-field spectroscopic observations of the two disk galaxies NGC 3593 and NGC 4550 obtained with VIMOS/VLT. Both galaxies are known to host 2 counter-rotating stellar disks, with the ionized gas co-rotating with one of them. We measured in each galaxy the ionized gas kinematics and metallicity, and the surface brightness, kinematics, mass surface density, and the stellar populations of the 2 stellar components to constrain the formation scenario of these peculiar galaxies. We applied a novel spectroscopic decomposition technique to both galaxies, to separate the relative contribution of the 2 counter-rotating stellar and one ionized-gas components to the observed spectrum. We measured the kinematics and the line strengths of the Lick indices of the 2 counter-rotating stellar components. We modeled the data of each stellar component with single stellar population models that account for the alpha/Fe overabundance. In both galaxies we successfully separated the main from the secondary stellar component that is less massive and rotates in the same direction of the ionized-gas component. The 2 stellar components have exponential surface-brightness profiles. In both galaxies, the two counter-rotating stellar components have different stellar populations: the secondary stellar disk is younger, more metal poor, and more alpha-enhanced than the main galaxy stellar disk. Our findings rule out an internal origin of the secondary stellar component and favor a scenario where it formed from gas accreted on retrograde orbits from the environment fueling an in situ outside-in rapid star formation. The event occurred ~ 2 Gyr ago in NGC 3593, and ~ 7 Gyr ago in NGC 4550. The binary galaxy merger scenario cannot be ruled out, and a larger sample is required to statistically determine which is the most efficient mechanism to build counter-rotating stellar disks (abridged).

Disentangling the stellar populations in the counter-rotating disc galaxy NGC 4550

In order to try and understand its origins, we present high-quality long-slit spectral observations of the counter-rotating stellar discs in the strange S0 galaxy NGC 4550. We kinematically decompose the spectra into two counter-rotating stellar components (plus a gaseous component), in order to study both their kinematics and their populations. The derived kinematics largely confirm what was known previously about the stellar discs, but trace them to larger radii with smaller errors; the fitted gaseous component allows us to trace the hydrogen emission lines for the first time, which are found to follow the same rather strange kinematics previously seen in the [OIII] line. Analysis of the populations of the two separate stellar components shows that the secondary disc has a significantly younger mean age than the primary disc, consistent with later star formation from the associated gaseous material. In addition, the secondary disc is somewhat brighter, also consistent with such additional star formation. However, these measurements cannot be self-consistently modelled by a scenario in which extra stars have been added to initially-identical counter-rotating stellar discs, which rules out Evans & Collett's (1994) elegant "separatrix-crossing" model for the formation of such massive counter-rotating discs from a single galaxy, leaving some form of unusual gas accretion history as the most likely formation mechanism.

Counter-rotating stellar discs around a massive black hole: self-consistent, time-dependent dynamics

We formulate the collisionless Boltzmann equation (CBE) for dense star clusters that lie within the radius of influence of a massive black hole in galactic nuclei. Our approach to these nearly Keplerian systems follows that of Sridhar and Touma (1999): Delaunay canonical variables are used to describe stellar orbits and we average over the fast Keplerian orbital phases. The stellar distribution function (DF) evolves on the longer time scale of precessional motions, whose dynamics is governed by a Hamiltonian, given by the orbit-averaged self-gravitational potential of the cluster. We specialize to razor-thin, planar discs and consider two counter-rotating ("$\pm$") populations of stars. To describe discs of small eccentricities, we expand the $\pm$ Hamiltonian to fourth order in the eccentricities, with coefficients that depend self-consistently on the $\pm$ DFs. We construct approximate $\pm$ dynamical invariants and use Jeans' theorem to construct time-dependent $\pm$ DFs, which are completely described by their centroid coordinates and shape matrices. When the centroid eccentricities are larger than the dispersion in eccentricities, the $\pm$ centroids obey a set of 4 autonomous ordinary differential equations. We show that these can be cast as a two-degree of freedom Hamiltonian system which is nonlinear, yet integrable. We study the linear instability of initially circular discs and derive a criterion for the counter-rotating instability. We then explore the rich nonlinear dynamics of counter-rotating discs, with focus on the variety of steadily precessing eccentric configurations that are allowed. The stability and properties of these configurations are studied as functions of parameters such as the disc mass ratios and angular momentum.

Dating the formation of the counter-rotating stellar disc in the spiral galaxy NGC 5719 by disentangling its stellar populations

Abstract We present the results of the VLT/VIMOS integral-field spectroscopic observations of the inner 28 × 28 arcsec2 (3.1 × 3.1 kpc2) of the interacting spiral NGC 5719, which is known to host two cospatial counter-rotating stellar discs. At each position in the field of view, the observed galaxy spectrum is decomposed into the contributions of the spectra of two stellar and one ionized-gas components. We measure the kinematics and the line strengths of the Lick indices of the two stellar counter-rotating components. We model the data of each stellar component with single stellar population models that account for the α/Fe overabundance. We also derive the distribution and kinematics of the ionized-gas disc, that is associated with the younger, less rich in metals, more α-enhanced, and less luminous stellar component. They are both counter rotating with respect the main stellar body of the galaxy. These findings prove the scenario where gas was accreted first by NGC 5719 on to a retrograde orbit from the large reservoir available in its neighbourhoods as a result of the interaction with its companion NGC 5713, and subsequently fuelled the in situ formation of the counter-rotating stellar disc.

Galactic Center Youth: Orbits and Origins of the Young Stars in the Central Parsec

We present new proper motions for the massive, young stars at the Galactic Center, based on 10 years of diffraction limited data from the Keck telescopes. Our proper motion measurements now have uncertainties of only 1-2 km/s and allow us to explore the origin of the young stars that reside within the sphere of inflience of the supermassive black hole whose strong tidal forces make this region inhospitable for star formation. Their presence, however, may be explained either by in situ star formation in an accretion disk or as the remnants of a massive stellar cluster which spiraled in via dynamical friction. Earlier stellar velocity vectors were used to postulate that all the young stars resided in two counter-rotating stellar disks, which is consistent with both of the above formation scenarios. Our precise proper motions allow us, for the frst time, to determine the orbital parameters of each individual star and thereby to test the hypothesis that the massive stars reside in two stellar disks. Of the 26 young stars in this study that were previously proposed to lie on the inner, clockwise disk, we find that nearly all exhibit orbital constraints consistent with such a disk. On the other hand, of the 7 stars in this study previously proposed to lie in the outer, less well-defhed counter-clockwise disk, 6 exhibit inclinations that are inconsistent with such a disk, bringing into question the existence of the outer disk. Furthermore, for stars in the inner disk that have eccentricity constraints, we find several that have lower limits to the eccentricity of more than 0.4, implying highly eccentric orbits. This stands in contrast to simple accretion disk formation scenarios which typically predict predominantly circular orbits.

NGC 5719/13: interacting spirals forming a counter–rotating stellar disc

Context. When a galaxy acquires material from the outside, it is likely that the resulting angular momentum of the accreted material is decoupled from that of the pre-existing galaxy. The presence of stars counter-rotating with respect to other stars and/or gas represents an extreme case of decoupling. Aims. NGC5719, an almost edge-on Sab galaxy with a prominent skewed dust lane, shows a spectacular on-going interaction with its face-on Sbc companion NGC 5713. Observations of such interacting systems provide insight into the processes at work in assembling and reshaping galaxies. Methods. Studies were made of the distribution and kinematics of neutral hydrogen in the NGC 5719/13 galaxy pair and the ionised gas and stellar kinematics along the major axis of NGC 5719. Results. Two HI tidal bridges that loop around NGC 5719 and connect to NGC 5713, and two HI tidal tails departing westward from NGC 5713 were detected. There is a correspondence between the HI condensations and the location of clumps of young stars within and outside the disc of NGC 5719. The low-mass satellite PGC 135857 at the tip of the northern tail was detected in HI, and is likely a by-product of the interaction. The neutral and ionised hydrogen in the disc of NGC 5719 are counter-rotating with respect to the main stellar disc. The counter-rotating stellar disc contains about 20% of the stars in the system, and has the same radial extension as the main stellar disc. This is the first interacting system in which a counter-rotating stellar disc has been detected. Conclusions. The data support a scenario where HI from the large reservoir available in thq galaxy's surroundings was accreted by NGC 5719 onto a retrograde orbit and subsequently fuelled the in situ formation of the counter-rotating stellar disc.