66 research outputs found
NIR spectroscopy of the most massive open cluster in the Galaxy: Westerlund 1
Using ISAAC/VLT, we have obtained individual spectra of all NIR-bright stars
in the central 2'x2' of the cluster Westerlund 1 (Wd 1) with a resolution of
R~9000 at a central wavelength of 2.30 micron. This allowed us to determine
radial velocities of ten post-main-sequence stars, and from these values a
velocity dispersion. Assuming virial equilibrium, the dispersion of sigma=8.4
km/s leads to a total dynamical cluster mass of 1.25x10^5 solar masses,
comparable to the photometric mass of the cluster. There is no extra-virial
motion which would have to be interpreted as a signature of cluster expansion
or dissolution.Comment: To appear in the proceedings of IAU 246: "Dynamical Evolution of
Dense Stellar Systems" (E. Vesperini, M. Giersz, A. Sills, eds.
ALFA & 3D: integral field spectroscopy with adaptive optics
One of the most important techniques for astrophysics with adaptive optics is
the ability to do spectroscopy at diffraction limited scales. The extreme
difficulty of positioning a faint target accurately on a very narrow slit can
be avoided by using an integral field unit, which provides the added benefit of
full spatial coverage. During 1998, working with ALFA and the 3D integral field
spectrometer, we demonstrated the validity of this technique by extracting and
distinguishing spectra from binary stars separated by only 0.26". The
combination of ALFA & 3D is also ideally suited to imaging distant galaxies or
the nuclei of nearby ones, as its field of view can be changed between
1.2"x1.2" and 4"x4", depending on the pixel scale chosen. In this contribution
we present new results both on galactic targets, namely young stellar objects,
as well as extra-galactic objects including a Seyfert and a starburst nucleus.Comment: SPIE meeting 4007 on Adaptive Optical Systems Technology, March 200
Stellar Dynamics and the implications on the merger evolution in NGC6240
We report near-infrared integral field spectroscopy of the luminous merging
galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by
1.6arcsec are the central parts of inclined, rotating disk galaxies with equal
mass bulges. The dynamical masses of the nuclei are much larger than the
stellar mass derived from the K-band light, implying that the progenitor
galaxies were galaxies with massive bulges. The K-band light is dominated by
red supergiants formed in the two nuclei in starbursts, triggered ~2x10^7 years
ago, possibly by the most recent perigalactic approach. Strong feedback effects
of a superwind and supernovae are responsible for a short duration burst
(~5x10^6 years) which is already decaying. The two galaxies form a
prograde-retrograde rotating system and from the stellar velocity field it
seems that one of the two interacting galaxies is subject to a prograde
encounter. Between the stellar nuclei is a prominent peak of molecular gas
(H_2, CO). The stellar velocity dispersion peaks there indicating that the gas
has formed a local, self-gravitating concentration decoupled from the stellar
gravitational potential. NGC 6240 has previously been reported to fit the
paradigm of an elliptical galaxy formed through the merger of two galaxies.
This was based on the near-infrared light distribution which follows a
r^1/4-law. Our data cast strong doubt on this conclusion: the system is by far
not relaxed, rotation plays an important role, as does self-gravitating gas,
and the near-infrared light is dominated by young stars.Comment: 34 pages, 11 figures, using AASTEX 5.0rc3.1, paper submitted to the
Astrophysical Journal, revised versio
Confirmation of two extended objects along the line of sight to PKS1830-211 with ESO-VLT adaptive optics imaging
We report on new high-resolution near-infrared images of the gravitationally
lensed radio source PKS1830-211, a quasar at z=2.507. These adaptive optics
observations, taken with the Very Large Telescope (VLT), are further improved
through image deconvolution. They confirm the presence of a second object along
the line of sight to the quasar, in addition to the previously known spiral
galaxy. This additional object is clearly extended in our images. However, its
faint luminosity does not allow to infer any photometric redshift. If this
galaxy is located in the foreground of PKS1830-211, it complicates the modeling
of this system and decreases the interest in using PKS1830-211 as a means to
determine H0 via the time delay between the two lensed images of the quasar.Comment: Accepted in A&A Letter
K-band Spectroscopy of Clusters in NGC 4038/4039
Integral field spectroscopy in the K-band (1.9-2.4um) was performed on four
IR-bright star clusters and the two nuclei in NGC 4038/4039 (``The Antennae'').
Two of the clusters are located in the overlap region of the two galaxies, and
together comprise ~25% of the total 15um and ~10% of the total 4.8 GHz emission
from this pair of merging galaxies. The other two clusters, each of them
spatially resolved into two components, are located in the northern galaxy, one
in the western and one in the eastern loop of blue clusters. Comparing our
analysis of Brgamma, CO band-heads, He I (2.058um), Halpha (from archival HST
data), and V-K colors with stellar population synthesis models indicates that
the clusters are extincted (A_V ~ 0.7 - 4.3 mags) and young, displaying a
significant age spread (4-13 Myrs). The starbursts in the nuclei are much older
(65 Myrs), with the nucleus of NGC 4038 displaying a region of recent star
formation northward of its K-band peak. Using our derived age estimates and
assuming the parameters of the IMF (Salpeter slope, upper mass cut-off of 100
M_sun, Miller-Scalo between 1 M_sun and 0.1 M_sun), we find that the clusters
have masses between 0.5 and 5 * 10^6M_sun.Comment: 10 pages, 3 figures, ApJ accepte
Medium resolution 2.3 micron spectroscopy of the massive Galactic open cluster Westerlund 1
The Galactic open cluster Westerlund 1 was found only a few years ago to be
much more massive than previously thought, with evidence suggesting its mass to
be in excess of ~10^5 Msun, in the range spanned by young extragalactic star
clusters. Unlike those clusters its proximity makes spatially resolved studies
of its stellar population feasible. It is therefore the ultimate template for a
young, massive star cluster, permitting direct comparison of its properties
with measurements of velocity dispersion and dynamical mass for spatially
unresolved extragalactic clusters.
To this end, we used the long slit near-infrared spectrograph VLT/ISAAC to
observe the CO bandhead region near 2.29 micron scanning the slit across the
cluster centre during the integration. Spatially collapsing the spectra along
the slit results in a single co-added spectrum of the cluster, comparable to
what one would obtain in the extragalactic cluster context.
This spectrum was analysed the same way as the spectra of almost point-like
extragalactic clusters, using red superiant cluster members as velocity
templates.
We detected four red supergiants which are included in the integrated
spectrum, and our measured velocity dispersion is 5.8 km/s. Together with the
cluster size of 0.86 pc, derived from archival near-infrared SOFI-NTT images,
this yields a dynamical mass of 6.3x10^4 Msun. While this value is not to be
considered the final word, there is at least so far no sign for rapid expansion
or collapse.Comment: 6 pages, 5 figures, A&A accepte
Molecular Hydrogen Excitation in Ultraluminous Infrared Galaxies
We report medium resolution VLT ISAAC K-band spectroscopy of the nuclei of
seven ultraluminous infrared galaxies. After accounting for stellar absorption
features, we have detected several molecular hydrogen (H_2) v=1-0, 2-1, and 3-2
vibrational emission lines, as well as the HI Br\gamma and HeI 2^1P-2^1S
recombination lines. The relative H_2 line intensities show little variation
between the objects, suggesting that the H_2 excitation mechanisms in the
nuclei are similar in all the objects. The 1-0 emissions appear thermalised at
temperatures T\sim1000K. However, the 2-1 and 3-2 emissions show evidence of
being radiatively excited by far-ultraviolet (FUV) photons, suggesting that the
H_2 excitation in the ULIRGs may arise in dense photon dominated regions
(PDRs). We show that the line ratios in the nuclei are consistent with PDRs
with cloud densities between 10^4 to 10^5cm^{-3}, exposed to far ultraviolet
(FUV) radiation fields at least 10^3 times more intense than the ambient FUV
intensity in the local interstellar medium. We have constructed starburst
models for the ULIRGs based on their H_2 properties, as well as on the
intensities of the recombination lines. Our models provide a consistent picture
of young 1-5Myr star clusters surrounded by relatively dense PDRs which are
irradiated by intense FUV fluxes. Comparison to the inner few hundred parsecs
of the Milky Way indicates that the star formation efficiency in ULIRGs is
10--100 times higher than in the Galactic Center.Comment: accepted by ApJ (32 pages including figures
Discovery of a nuclear gas bar feeding the active nucleus in Circinus
We report the discovery of gas inflow motions towards the active nucleus of
the Circinus galaxy caused by the non-axisymmetric potential of a nuclear gas
bar. Evidence for dust associated with the bar comes from the HST/NICMOS H-K
color map, whereas the streaming motions along the gas bar are seen in the
velocity field of the H2 S(1)(1-0) emission line. The gas bar is about 100 pc
long with a visual extinction in excess of 10 mag. Indication for the gaseous
nature of this bar comes from the lack of a stellar counterpart even in the K
band where the extinction is greatly reduced.
We also use the NICMOS emission line images (Pa-alpha, [SiVI], and [FeII]) to
study the innermost region of the ionization cones and the nuclear star forming
activity. We discuss the possible relationship of these components with the
gaseous bar.Comment: 14 pages, 7 figures (3 color plates), accepted for publication in Ap
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