Low Stellar Velocity Dispersion Research Articles

Origin of the correlation between stellar kinematics and globular cluster system richness in ultradiffuse galaxies

ABSTRACT Observational surveys have found that the dynamical masses of ultradiffuse galaxies (UDGs) correlate with the richness of their globular cluster (GC) system. This could be explained if GC-rich galaxies formed in more massive dark matter haloes. We use simulations of galaxies and their GC systems from the E-MOSAICS project to test whether the simulations reproduce such a trend. We find that GC-rich simulated galaxies in galaxy groups have enclosed masses that are consistent with the dynamical masses of observed GC-rich UDGs. However, simulated GC-poor galaxies in galaxy groups have higher enclosed masses than those observed. We argue that GC-poor UDGs with low stellar velocity dispersions are discs observed nearly face on, such that their true mass is underestimated by observations. Using the simulations, we show that galactic star formation conditions resulting in dispersion-supported stellar systems also leads to efficient GC formation. Conversely, conditions leading to rotationally supported discs lead to inefficient GC formation. This result may explain why early-type galaxies typically have richer GC systems than late-type galaxies. This is also supported by comparisons of stellar axis ratios and GC-specific frequencies in observed dwarf galaxy samples, which show GC-rich systems are consistent with being spheroidal, while GC-poor systems are consistent with being discs. Therefore, particularly for GC-poor galaxies, rotation should be included in dynamical mass measurements from stellar dynamics.

The Velocity Dispersion Function for Massive Quiescent and Star-forming Galaxies at 0.6 < z ≤ 1.0

We present the first direct spectroscopic measurement of the stellar velocity dispersion function (VDF) for massive quiescent and star-forming galaxies at 0.6 < z ≤ 1.0. For this analysis we use individual measurements of stellar velocity dispersion from high-signal-to-noise-ratio spectra from the public Large Early Galaxy Astrophysics Census (LEGA-C) survey. We report a remarkable stability of the VDF for both quiescent and star-forming galaxies within this redshift range, though we note the presence of weak evolution in the number densities of star-forming galaxies. We compare both VDFs with previous direct and inferred measurements at local and intermediate redshifts, with the caveat that previous measurements of the VDF for star-forming galaxies are poorly constrained at all epochs. We emphasize that this work is the first to directly push to low stellar velocity dispersion (σ ⋆ > 100 km s−1) and extend to star-forming galaxies. We are largely consistent with the high-sigma tail measured from BOSS, and we find that the VDF remains constant from the median redshift of LEGA-C, z ∼ 0.8, to the present day.

SDSS-IV MaNGA – gas rotation velocity lags in the final sample of MaNGA galaxies

ABSTRACT We consider the largest sample of 561 edge-on galaxies observed with integral field units by the MaNGA survey and find 300 galaxies where the ionized gas shows a negative vertical gradient (lag) in its rotational speed. We introduce the stop altitude as the distance to the galactic mid-plane at which the gas rotation should stop in the linear approximation. We find correlations between the lags, stop altitude and galactic mass, stellar velocity dispersion, and overall Sersic index. We do not find any correlation of the lags or stop altitude with the star formation activity in the galaxies. We conclude that low-mass galaxies (log(M*/M⊙) &amp;lt; 10) with low-Sersic index and with low-stellar velocity dispersion possess a wider ‘zone of influence’ in the extragalactic gas surrounding them with respect to higher mass galaxies that have a significant spherical component. We estimated the trend of the vertical rotational gradient with radius and find it flat for most of the galaxies in our sample. A small subsample of galaxies with negative radial gradients of lag has an enhanced fraction of objects with aged low-surface brightness structures around them (e.g. faint shells), which indicates that noticeable accretion events in the past affected the extraplanar gas kinematics and might have contributed to negative radial lag gradients. We conclude that an isotropic accretion of gas from the circumgalactic medium plays a significant role in the formation of rotation velocity lags.

Diffuse post-starburst galaxies: a smoking gun of formation of ultra-diffuse galaxies via ram pressure stripping

AbstractUltra-diffuse galaxies (UDGs) have sizes comparable to the Milky Way and stellar masses of about 1/1000 of it. They attracted a lot of attention as possible “dark galaxies” heavily dominated by dark matter, however, no reliable dynamical mass estimates were done because of their extremely low surface brightness. We have recently found 13 gas free diffuse young (300–500 Myr) post-starburst galaxies (PSGs) without ongoing star formation in Coma and Abell 2147, which, should they continue to evolve passively, will become UDGs in 5 Gyr. We obtained deep spectroscopic observations for 11 diffuse PSGs and derived their internal kinematics and stellar population properties. All of them possess disk-like kinematics (substantial rotation, low stellar velocity dispersion) and likely experienced starburst episodes prior to the star formation quenching by ram pressure stripping. Our results suggest that at least some UDGs were “normal” intermediate to large-sized disk galaxies in the past, which were later quenched by dense environment.

Outflows in the Seyfert 2 galaxy NGC 5643 traced by the [S iii] emission

AbstractWe use Gemini Multi-Object Spectrograph integral Field Unit observations of the inner 285 × 400 pc2 region of the Seyfert 2 galaxy NGC 5643 to map the [S iii]λ9069 emission line flux distribution and kinematics, as well as the stellar kinematics, derived by fitting the Ca iiλλλ8498,8542,8662 triplet, at a spatial resolution of 45 pc. The stellar velocity field shows regular rotation, with a projected velocity of 100 km s−1 and kinematic major axis along a position angle of –36°. A ring of low stellar velocity dispersion values (∼70 km s−1), attributed to young/intermediate age stellar populations, is seen surrounding the nucleus with a radius of 50 pc. We found that the [S iii] flux distribution shows an elongated structure along the east–west direction and its kinematics is dominated by outflows within a bi-cone at an ionised gas outflow rate of 0.3 M⊙ yr−1. In addition, velocity slices across the [S iii]λ9069 emission line reveal a kinematic component attributed to rotation of gas in the plane of the galaxy.

No evidence for small disk-like bulges in a sample of late-type spirals

About 20% of low-redshift galaxies are late-type spirals with a small or no bulge component. Although they are the simplest disk galaxies in terms of structure and dynamics, the role of the different physical processes driving their formation and evolution is not yet fully understood. We investigated whether small bulges of late-type spirals follow the same scaling relations traced by ellipticals and large bulges and if they are disk-like or classical bulges. We derived the photometric and kinematic properties of 9 nearby late-type spirals. To this aim, we analyzed the surface brightness distribution from the i-band images of the Sloan Digital Sky Survey and obtained the structural parameters of the galaxies from a two-dimensional photometric decomposition. We measured the line-of-sight stellar velocity distribution within the bulge effective radius from the long-slit spectra taken with high spectral resolution at the Telescopio Nazionale Galileo. We used the photometric and kinematic properties of the sample bulges to study their location in the Fundamental Plane, Kormendy, and Faber-Jackson relations defined for ellipticals and large bulges. We found that our sample bulges satisfy some of the photometric and kinematic prescriptions for being considered disk-like bulges such as small sizes and masses with nearly exponential light profiles, small bulge-to-total luminosity ratios, low stellar velocity dispersions, and ongoing star formation. However, each of them follows the same scaling relations of ellipticals, massive bulges, and compact early-type galaxies so they cannot be classified as disk-like systems. We find a single population of galaxy spheroids that follow the same scaling relations, where the mass seems to lead to a smooth transition in the photometric and kinematic properties from less massive bulges to more massive bulges and ellipticals.

On the origin of the stellar halo and multiple stellar populations in the globular cluster NGC 1851

We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence for its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ~ -2 and shows no tidal tails within ~200pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy.

Intermediate-age stars as the origin of low stellar velocity dispersion nuclear rings: the case of Mrk 1157

We have used the Gemini Near-Infrared Integral-Field Spectrograph to map the age distribution of the stellar population in the inner 400 pc of the Seyfert 2 galaxy Mrk 1157 (NGC 591), at a spatial resolution of 35 pc. We have performed wavelet and principal component analysis on the data in order to remove instrumental signatures. An old stellar population component (age ≳ 5 Gyr) is dominant within the inner ≈130 pc which we attribute to the galaxy bulge. Beyond this region, up to the borders of the observation field, young- to intermediate-age components (0.1–0.7 Gyr) dominate. As for Mrk 1066, previously studied by us, we find a spatial correlation between this intermediate-age component and a partial ring of low stellar velocity dispersion (σ*). Low-σ* nuclear rings have been observed in other active galaxies, and our results for Mrk 1157 and Mrk 1066 reveal that they are formed by intermediate-age stars. Such age is consistent with a scenario in which the origin of the low-σ* rings is a past event which triggered an inflow of gas and formed stars which still keep the colder kinematics of the gas from which they have formed. No evidence for the presence of an unresolved featureless continuum and hot dust component – as found in Mrk1066 – is found for Mrk 1157.

INTERMEDIATE-AGE STARS AS ORIGIN OF THE LOW-VELOCITY DISPERSION NUCLEAR RING IN Mrk 1066

We report the first two-dimensional stellar population synthesis in the near-infrared of the nuclear region of an active galaxy, namely Mrk1066. We have used integral field spectroscopy with adaptive optics at the Gemini North Telescope to map the to map the age distribution of the stellar population in the inner 300 pc at a spatial resolution of 35 pc. An old stellar population component (age >5Gyr) is dominant within the inner ~160pc, which we attribute to the galaxy bulge. Beyond this region, up to the borders of the observation field (~300 pc), intermediate age components (0.3-0.7Gyr) dominate. We find a spatial correlation between this intermediate age component and a partial ring of low stellar velocity dispersions (sigma). Low-sigma nuclear rings have been observed in other active galaxies and our result for Mrk1066 suggests that they are formed by intermediate age stars. This age is consistent with an origin for the low-sigma rings in a past event which triggered an inflow of gas and formed stars which still keep the colder kinematics (as compared to that of the bulge) of the gas from which they have formed. At the nucleus proper we detect, in addition, two unresolved components: a compact infrared source, consistent with an origin in hot dust with mass ~1.9x10^{-2} M_Sun, and a blue featureless power-law continuum, which contributes with only ~15% of the flux at 2.12 microns.

Lost and found dark matter in elliptical galaxies

The kinematical properties of elliptical galaxies formed during the mergers of equal mass, stars+gas+dark matter spiral galaxies are compared to the observed low velocity dispersions found for planetary nebulae on the outskirts of ellipticals, which have been interpreted as pointing to a lack of dark matter in ellipticals (which poses a problem for the standard model of galaxy formation). We find that the velocity dispersion profiles of the stars in the simulated ellipticals match well the observed ones. The low outer stellar velocity dispersions are mainly caused by the radial orbits of the outermost stars, which, for a given binding energy must have low angular momentum to reach their large radial distances, usually driven out along tidal tails.

The structure and clustering of Lyman-break galaxies

The number density and clustering properties of Lyman-break galaxies (LBGs) are consistent with them being the central galaxies of the most massive dark haloes present at z ̃ 3. This conclusion holds in all currently popular hierarchical models for structure formation, and is almost independent of the global cosmological parameters. We examine whether the sizes, luminosities, kinematics and star formation rates of LBGs are also consistent with this identification. Simple formation models tuned to give good fits to low-redshift galaxies can predict the distribution of these quantities in the LBG population. The LBGs should be small (with typical half-light radii of 0.6–2 h−1kpc), should inhabit haloes of moderately high circular velocity (180–290 km s−1) but have low stellar velocity dispersions (70–120 km s−1) and should have substantial star formation rates (15–100 M⊙ yr−1). The numbers here refer to the predicted median values in the LBG sample of Adelberger et al.; the first number assumes an ω0=1 universe and the second number a flat universe with ω0=0.3. For either cosmology these predictions are consistent with the current (rather limited) observational data. Following the work of Kennicutt, we assume stars to form more rapidly in gas of higher surface density. This predicts that LBG samples should preferentially contain objects with low angular momentum, and therefore small size, for their mass. In contrast, samples of damped Ly α systems (DLSs) should be biased towards objects with large angular momentum. Bright LBGs and DLSs may therefore form distinct populations, with very different sizes and star formation rates, LBGs being smaller and more metal-rich than DLSs of similar mass and redshift.

NGC 6776 'The Pentagon' - an old merger?

We present photometric and spectroscopic observations of NGC 6776. Photographic and CCD images show that this galaxy has perturbed and chaotic outer isophotes and possesses at least one tidal tail. Spectroscopic observations show that the system is rapidly rotating, with low stellar velocity dispersion, atypical of early-type systems. These characteristics strongly suggest that this galaxy has undergone a merger with a second system. However, the colour distribution shows no evidence for the presence of young stars, while IRAS data indicate that the system is relatively free of dust. We conclude that this galaxy is the remnant of a merger occurring at least 109 yr ago, making it the oldest merger known, and involving at least one dust-poor disc system.

A wave focusing model for starbursts

view Abstract Citations (9) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Wave Focusing Model for Starbursts Lubow, Stephen H. Abstract We consider a model for starbursts in which spiral density waves provide the primary triggering mechamism by achieving largest amplitude within about 1 kpc of a galactic center, a process which we refer to as wave focusing. Radio observations of molecular gas are suggested to test the model. We first consider simple linear, WKB, steady state density waves. Wave focusing occurs when the disk's effective velocity dispersion does not increase rapidly near the galactic center. This condition can be achieved by either having a fairly low stellar velocity dispersion in the central regions (Q ~ 1) or by having a significant (~10% by mass) low-velocity dispersion gaseous component in the central region. To consider nonlinear effects, we adopt a marginally applicable formalism used in the context of planetary rings. The model indicates that wave nonlinearities enhance wave focusing, when it is predicted by linear theory. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/185148 Bibcode: 1988ApJ...328L...3L Keywords: Density Wave Model; Galactic Nuclei; Spiral Galaxies; Star Formation; Stellar Flares; Angular Momentum; Gas Density; Gas Flow; Linear Equations; Molecular Clouds; Astrophysics; GALAXIES: INTERNAL MOTIONS; STARS: FORMATION full text sources ADS |