94 research outputs found
Evidence for non-axisymmetry in M31 from wide-field kinematics of stars and gas
As the nearest large spiral galaxy, M31 provides a unique opportunity to
learn about the structure and evolutionary history of this galaxy type in great
detail. Among the many observing programs aimed at M31 are microlensing
studies, which require good three-dimensional models of the stellar mass
distribution. Possible non-axisymmetric structures like a bar need to be taken
into account. Due to M31's high inclination, the bar is difficult to detect in
photometry alone. Therefore, detailed kinematic measurements are needed to
constrain the possible existence and position of a bar in M31. We obtained
220 separate fields with the optical IFU spectrograph VIRUS-W,
covering the whole bulge region of M31 and parts of the disk. We derive stellar
line-of-sight velocity distributions from the stellar absorption lines, as well
as velocity distributions and line fluxes of the emission lines H,
[OIII] and [NI]. Our data supersede any previous study in terms of spacial
coverage and spectral resolution. We find several features that are indicative
of a bar in the kinematics of the stars, we see intermediate plateaus in the
velocity and the velocity dispersion, and correlation between the higher moment
and the velocity. The gas kinematics is highly irregular, but is
consistent with non-triaxial streaming motions caused by a bar. The morphology
of the gas shows a spiral pattern, with seemingly lower inclination than the
stellar disk. We also look at the ionization mechanisms of the gas, which
happens mostly through shocks and not through starbursts.Comment: 23 pages, 39 figures; accepted for publication in A&
Unravelling stellar populations in the Andromeda Galaxy
To understand the history and formation mechanisms of galaxies it is crucial
to determine their current multidimensional structure. Here we focus on stellar
population properties, such as metallicity and [/Fe] enhancement. We
devise a new technique to recover the distribution of these parameters using
spatially resolved, line-of-sight averaged data. Our chemodynamical method is
based on the made-to-measure (M2M) framework and results in an -body model
for the abundance distribution. We test our method on a mock data set and find
that the radial and azimuthal profiles are well-recovered, however only the
overall shape of the vertical profile matches the true profile. We apply our
procedure to spatially resolved maps of mean [Z/H] and [/Fe] for the
Andromeda Galaxy, using an earlier barred dynamical model of M31. We find that
the metallicity is enhanced along the bar, with possible maxima at the ansae.
In the edge-on view the [Z/H] distribution has an X shape due to the
boxy/peanut bulge; the average vertical metallicity gradient is equal to
dex/kpc. We identify a metallicity-enhanced ring around the
bar, which also has relatively lower [/Fe]. The highest [/Fe]
is found in the centre, due to the classical bulge. Away from the centre, the
-overabundance in the bar region increases with height, which could be
an indication of a thick disc. We argue that the galaxy assembly resulted in a
sharp peak of metallicity in the central few hundred parsecs and a more gentle
negative gradient in the remaining disc, but no [/Fe] gradient. The
formation of the bar lead to the re-arrangement of the [Z/H] distribution,
causing a flat gradient along the bar. Subsequent star formation close to the
bar ends may have produced the metallicity enhancements at the ansae and the
[Z/H] enhanced lower- ring.Comment: 20 pages, 17 figures, 1.9MB, accepted to A&
The Supermassive Black Hole and Double Nucleus of the Core Elliptical NGC 5419
We obtained adaptive-optics assisted SINFONI observations of the central regions of the giant elliptical galaxy NGC5419 with a spatial resolution of 0.2 arcsec (≈55 pc). NGC5419 has a large depleted stellar core with a radius of 1.58 arcsec (430 pc). HST and SINFONI images show a point source located at the galaxy\u27s photocentre, which is likely associated with the low-luminosity AGN previously detected in NGC5419. Both the HST and SINFONI images also show a second nucleus, off-centred by 0.25 arcsec (≈70 pc). Outside of the central double nucleus, we measure an almost constant velocity dispersion of σ∼350 km/s. In the region where the double nucleus is located, the dispersion rises steeply to a peak value of ∼420 km/s. In addition to the SINFONI data, we also obtained stellar kinematics at larger radii from the South African Large Telescope. While NGC5419 shows low rotation (v\u3c50 km/s), the central regions (inside ∼4rb) clearly rotate in the opposite direction to the galaxy\u27s outer parts. We use orbit-based dynamical models to measure the black hole mass of NGC5419 from the kinematical data outside of the double nuclear structure. The models imply MBH=7.2+2.7−1.9×109 M⊙. The enhanced velocity dispersion in the region of the double nucleus suggests that NGC5419 possibly hosts two supermassive black holes at its centre, separated by only ≈70 pc. Yet our measured MBH is consistent with the black hole mass expected from the size of the galaxy\u27s depleted stellar core. This suggests, that systematic uncertainties in MBH related to the secondary nucleus are small
Dark matter content and internal dynamics of NGC 4697: nmagic particle models from slit data and planetary nebula velocities
We present a dynamical study of NGC 4697, an almost edge-on, intermediate-luminosity, E4 elliptical galaxy, combining new surface brightness photometry, new as well as published long-slit absorption-line kinematic data, and published planetary nebula (PN) velocity data. The combined kinematic data set extends out to ≃5 arcmin (≃4.5Re) and allows us to probe the galaxy's outer halo. For the first time, we model such a data set with the new and flexible χ2-made-to-measure particle code nmagic. We extend nmagic to include seeing effects, introduce an efficient scheme to estimate the mass-to-light ratio, and incorporate a maximum-likelihood technique to account for discrete velocity measurements. For modelling the PN kinematics, we use line-of-sight velocities and velocity dispersions computed on two different spatial grids, and we also use the individual velocity measurements with the likelihood method, in order to make sure that our results are not biased by the way we treat the PN measurements. We generate axisymmetric self-consistent models as well models including various dark matter haloes. These models fit all the mean velocity and velocity dispersion data with χ2/N < 1, both in the case with only luminous matter and in potentials including quite massive haloes. The likelihood analysis together with the velocity histograms suggest that models with low-density haloes such that the circular velocity vc≲ 200 km s−1 at 5Re are not consistent with the data. A range of massive haloes with vc≃ 250 km s−1 at 5Re fit the PN data best. To derive stronger results would require PN velocities at even larger radii. The best-fitting models are slightly radially anisotropic; the anisotropy parameter β≃ 0.3 at the centre, increasing to β≃ 0.5 at radii ≳2R
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
- …