71 research outputs found
Elliptical galaxies with rapidly decreasing velocity dispersion profiles: NMAGIC models and dark halo parameter estimates for NGC 4494
NGC 4494 is one of several intermediate-luminosity elliptical galaxies
inferred to have an unusually diffuse dark matter halo. We use the
chi^2-made-to-measure particle code NMAGIC to construct axisymmetric models of
NGC 4494 from photometric and various kinematic data. The extended kinematics
include light spectra in multiple slitlets out to 3.5 R_e, and hundreds of
planetary nebulae velocities out to ~7 R_e, thus allowing us to probe the dark
matter content and orbital structure in the halo. We use Monte Carlo
simulations to estimate confidence boundaries for the halo parameters, given
our data and modelling set-up. We find that the true potential of the dark
matter halo is recovered within Delta G (merit function)<26 (Delta chi^2<59) at
70% confidence level (C.L.), and within Delta G<32 (Delta chi^2<70) at 90%
C.L.. These numbers are much larger than the usually assumed Delta chi^2=2.3
(4.6) for 70% (90%) C.L. for two free parameters, perhaps case-dependent, but
calling into question the general validity of the standard assumptions used for
halo and black hole mass determinations. The best-fitting models for NGC 4494
have a dark matter fraction of about 0.6\pm0.1 at 5R_e (70% C.L.), and are
embedded in a dark matter halo with circular velocity ~200 km/s. The total
circular velocity curve (CVC) is approximately flat at v_c=220 km/s outside
~0.5R_e. The orbital anisotropy of the stars is moderately radial. These
results are independent of the assumed inclination of the galaxy, and edge-on
models are preferred. Comparing with the halos of NGC 3379 and NGC 4697, whose
velocity dispersion profiles also decrease rapidly from the center outwards,
the outer CVCs and dark matter halos are quite similar. NGC 4494 shows a
particularly high dark matter fraction inside ~3R_e, and a strong concentration
of baryons in the center.Comment: 21 pages, 23 figures, 1 table. Accepted for publication in MNRA
The extended Planetary Nebula Spectrograph (ePN.S) early type galaxy survey: the kinematic diversity of stellar halos
In this contribution we report on a kinematic study for 33 early type
galaxies (ETGs) into their outer halos (average 6 effective radii, Re). We use
planetary nebulae (PNe) as tracers of the main stellar population at large
radii, where absorption line spectroscopy is no longer feasible. The ePN.S
survey is the largest survey to-date of ETG kinematics with PNe, based on data
from the Planetary Nebula Spectrograph (PN.S), counter-dispersed imaging, and
high-resolution PN spectroscopy. We find that ETGs typically show a kinematic
transition between inner regions and halos. Slow rotators have increased
rotational support at large radii. Most of the ePN.S fast rotators show a
decrease in rotation, due to the fading of the stellar disk in the outer, more
slowly rotating spheroid. 30% of these fast rotators are dominated by rotation
also at large radii, 40% show kinematic twists or misalignments, indicating a
transition from oblate to triaxial in the halo. Despite this variety of
kinematic behaviors, the ePN.S ETG halos have similar angular momentum content,
independently of fast/slow rotation of the central regions. Estimated kinematic
transition radii in units of Re are ~1-3 Re and anti-correlate with stellar
mass. These results are consistent with cosmological simulations and support a
two-phase formation scenario for ETGs.Comment: 5 pages, 1 figure, proceeding of the 6th Focus Meeting (FM6) at the
XXXth IAU GA 201
Re-growth of stellar disks 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 = 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 = 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.Comment: accepted for publication by MNRA
Understanding the Unique Assembly History of Central Group Galaxies
Central Galaxies (CGs) in massive halos live in unique environments with
formation histories closely linked to that of the host halo. In local clusters
they have larger sizes () and lower velocity dispersions (sigma) at fixed
stellar mass M_star, and much larger R_e at a fixed than field and
satellite galaxies (non-CGs). Using spectroscopic observations of group
galaxies selected from the COSMOS survey, we compare the dynamical scaling
relations of early-type CGs and non-CGs at z~0.6, to distinguish possible
mechanisms that produce the required evolution. CGs are systematically offset
towards larger R_e at fixed compared to non-CGs with similar M_star.
The CG R_e-M_star relation also shows differences, primarily driven by a
sub-population (~15%) of galaxies with large , while the M_star-sigma
relations are indistinguishable. These results are accentuated when double
Sersic profiles, which better fit light in the outer regions of galaxies, are
adopted. They suggest that even group-scale CGs can develop extended components
by these redshifts that can increase total and M_star estimates by
factors of ~2. To probe the evolutionary link between our sample and cluster
CGs, we also analyze two cluster samples at z~0.6 and z~0. We find similar
results for the more massive halos at comparable z, but much more distinct CG
scaling relations at low-z. Thus, the rapid, late-time accretion of outer
components, perhaps via the stripping and accretion of satellites, would appear
to be a key feature that distinguishes the evolutionary history of CGs.Comment: 18 pages, 14 Figures, ApJ in pres
The properties and the formation mechanism of the stellar counter-rotating components in NGC 4191
We disentangle two counter-rotating stellar components in NGC 4191 and
characterize their physical properties (kinematics, morphology, age,
metallicity, and abundance ratio). We performed a spectroscopic decomposition
on integral field data to separate the contribution of two stellar components
to the observed galaxy spectrum across the field of view. We also performed a
photometric decomposition, modelling the galaxy with a S\'ersic bulge and two
exponential disks of different scale length, with the aim of associating these
structural components with the kinematic components. We measured the equivalent
width of the absorption line indices on the best fit that represent the
kinematic components and compared our measurements to the predictions of
stellar population models. We have evidence that the line-of-sight velocity
distributions (LOSVDs) are consistent with the presence of two distinct
kinematic components. The combined information of the intensity of the LOSVDs
and photometry allows us to associate the S\'ersic bulge and the outer disk
with the main kinematic component, and the inner disk with the secondary
kinematic component. The two kinematic stellar components counter-rotate with
respect to each other. The main component is the most luminous and massive, and
it rotates slower than the secondary component, which rotates along the same
direction as the ionized gas. We also found that the two kinematic components
have the same solar metallicity and sub-solar abundance ratio, without the
presence of significant radial gradients. On the other hand, their ages show
strong negative gradients and the possible indication that the secondary
component is the youngest. We interpret our results in light of recent
cosmological simulations and suggest gas accretion along two filaments as the
formation mechanism of the stellar counter-rotating components in NGC 4191
(Abridged).Comment: 10 pages, 10 figure. Accepted for publication in Astronomy and
Astrophysic
Spectroscopic decomposition of NGC 3521: unveiling the properties of the bulge and disc
We study the kinematics and the stellar populations of the bulge and disc of
the spiral galaxy NGC 3521. At each position in the field of view, we separate
the contributions of the bulge and the disc from the total observed spectrum
and study their kinematics, age, and metallicities independently. Their
properties are clearly distinct: the bulge rotates more slowly, has a higher
velocity dispersion, and is less luminous than the disc. We identify three main
populations of stars in NGC 3521: old ( Gyr), intermediate ( 3
Gyr), and young (1 Gyr). The mass and light of NGC 3521 are dominated by
the intermediate stellar population. The youngest population contributes mostly
to the disc component and its contribution increases with radius. We also study
the luminosity-weighed properties of the stars in NGC 3521. Along the
photometric major axis, we find: i) no age gradient for the stars in the bulge,
and a negative age gradient for the stars in the disc; ii) negative metallicity
gradients and sub-solar -enhancement for both the bulge and the disc.
We propose the following picture for the formation of NGC 3521: initial
formation a long time ago ( Gyr), followed by a second burst of star
formation or a merger ( 3 Gyrs ago), which contributed predominantly
to the mass-build up of the bulge. Recently ( Gyr), the disc of NGC
3521 experienced an additional episode of star formation that started in the
innermost regions.Comment: 13 pages, 11 figures, accepted for publication in MNRA
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