59 research outputs found
Nuclear Star Clusters
The centers of most galaxies in the local universe are occupied by compact,
barely resolved sources. Based on their structural properties, position in the
fundamental plane, and integrated spectra, these sources clearly have a stellar
origin. They are therefore called "nuclear star clusters" (NCs) or "stellar
nuclei". NCs are found in galaxies of all Hubble types, suggesting that their
formation is intricately linked to galaxy evolution. Here, I review some recent
studies of NCs, describe ideas for their formation and subsequent growth, and
touch on their possible evolutionary connection with both supermassive black
holes and globular clusters.Comment: invited talk at IAU Symp. 266 "Star Clusters: Galactic Building
Blocks through Space and Time
A SINFONI view of circum-nuclear star-forming rings in spiral galaxies
We present near-infrared (H- and K-band) SINFONI integral-field observations
of the circumnuclear star formation rings in five nearby spiral galaxies. We
made use of the relative intensities of different emission lines (i.e. [FeII],
HeI, Brg) to age date the stellar clusters present along the rings. This
qualitative, yet robust, method allows us to discriminate between two distinct
scenarios that describe how star formation progresses along the rings. Our
findings favour a model where star formation is triggered predominantly at the
intersection between the bar major axis and the inner Lindblad resonance and
then passively evolves as the clusters rotate around the ring ('Pearls on a
string' scenario), although models of stochastically distributed star formation
('Popcorn' model) cannot be completely ruled out.Comment: 4 pages, 3 figures, contribution to the Proceedings of the IAU
Symposium 245, "Formation and Evolution of Galaxy Bulges", held at Oxford,
U.K., July 2007, Eds. M. Bureau, E. Athanassoula, B. Barbu
An Accreting Black Hole in the Nuclear Star Cluster of the Bulgeless Galaxy NGC 1042
We present spectroscopic evidence for a low-luminosity, low-excitation active
galactic nucleus (AGN) in NGC 1042, powered by an intermediate-mass black hole.
These findings are significant in that the AGN is coincident with a compact
star cluster known to reside in the nucleus, thus providing an example where
the two types of central mass concentration coexist. The existence of a central
black hole is additionally remarkable in that NGC 1042 lacks a stellar bulge.
Objects such as NGC 1042 may have an important role in testing theories for the
genesis of massive black holes in galaxy nuclei, and the extent to which they
are in symbiosis with the larger stellar host.Comment: 15 pages, 6 figures, accepted for publication in Ap
Stellar populations and star formation histories of the nuclear star clusters in six nearby galaxies
The majority of spiral and elliptical galaxies in the Universe host very
dense and compact stellar systems at their centres known as nuclear star
clusters (NSCs). In this work we study the stellar populations and star
formation histories (SFH) of the NSCs of six nearby galaxies with stellar
masses ranging between and (four late-type
spirals and two early-types) with high resolution spectroscopy. Our
observations are taken with the X-Shooter spectrograph at the VLT. We make use
of an empirical simple stellar population (SSP) model grid to fit composite
stellar populations to the data and recover the SFHs of the nuclei. We find
that the nuclei of all late-type galaxies experienced a prolonged SFH, while
the NSCs of the two early-types are consistent with SSPs. The NSCs in the
late-type galaxies sample appear to have formed a significant fraction of their
stellar mass already more than Gyr ago, while the NSCs in the two
early-type galaxies are surprisingly younger. Stars younger than Myr are
present in at least two nuclei: NGC 247 and NGC 7793, with some evidence for
young star formation in NGC 300's NSC. The NSCs of the spirals NGC 247 and NGC
300 are consistent with prolonged \in situ star formation with a gradual
metallicity enrichment from dex more than Gyr ago, reaching
super-Solar values few hundred Myr ago. NGC 3621 appears to be very metal rich
already in the early Universe and NGC 7793 presents us with a very complex SFH,
likely dominated by merging of various massive star clusters coming from
different environments.Comment: Accepted for publication in MNRA
The Toomre Sequence Revisited with HST NICMOS: Nuclear Brightness Profiles and Colors of Interacting and Merging Galaxies
We discuss the near-infrared (NIR) properties of the nuclei in the 11 merging galaxies of the Toomre sequence, based on high spatial resolution J, H, and K imaging data using NICMOS on board the Hubble Space Telescope (HST). The observations are less affected by dust extinction than our previous HST WFPC2 observations and offer higher spatial resolution than existing ground-based NIR data. Nuclear positions are generally found to be consistent with those reported from data in other wave bands. In NGC 7764A we detect for the first time two nuclei with a separation of about 260 pc, consistent with its placement in the middle of the merging sequence. We see a marginal trend for the nuclei to become bluer with advancing merger stage, which we attribute to a dispersal of dust at late times in the merging process. Our data also indicate a statistically significant trend for the nuclei in the sequence to become more luminous, within an aperture of fixed physical size and after correcting for dust extinction, with advancing merger stage. We derive K-band surface brightness profiles for those nuclei for which the morphology allows a meaningful isophotal analysis, and fit the profiles with a "Nuker" law for comparison with other samples of galaxies observed with HST. The majority of the nuclei have steep profiles that can be characterized as power-law type. In general, the Toomre-sequence galaxies tend to have steeper profiles and higher central luminosity surface densities than E/S0 galaxies. Our findings can be explained if the Toomre-sequence galaxies have newly formed stars that are concentrated toward their centers. We derive V - K color profiles for the nuclei to further address this possibility, but find that the large amounts of dust extinction complicate their interpretation. Overall, our results are consistent with the generic predictions of N-body simulations of spiral galaxy mergers. If left to evolve and fade for several gigayears, it is possible that the properties of the Toomre-sequence nuclei would resemble the properties of the nuclei of normal E/S0 galaxies. Our results therefore support the view that mergers of spiral galaxies can lead to the formation of early-type galaxies
CO-bandhead spectroscopy of IC 342: mass and age of the nuclear star cluster
We have used the NASA Infra-Red Telescope Facility (IRTF) to observe the
nuclear stellar cluster in the nearby, face-on, giant Scd spiral IC 342. From
high resolution (R = 21500) spectra at the 12CO (2-0) bandhead at 2.3 micron we
derive a line-of-sight stellar velocity dispersion sigma = (33 +- 3) km/s.
To interpret this observation we construct dynamical models based on the
Jeans equation for a spherical system. The light distribution of the cluster is
modeled using an isophotal analysis of an HST V-band image from the HST Data
Archive, combined with new ground-based K-band imaging. Under the assumption of
an isotropic velocity distribution, the observed kinematics imply a K-band
mass-to-light ratio M/L_K = 0.05, and a cluster mass M ~ 6 times 10^6 Msun. We
model the mass-to-light ratio with the `starburst99' stellar population
synthesis models of Leitherer and collaborators, and infer a best-fitting
cluster age in the range 63-630 Myears. Although this result depends somewhat
on a number of uncertainties in the modeling (e.g., the assumed extinction
along the line-of-sight towards the nucleus, the IMF of the stellar population
model, and the velocity dispersion anisotropy of the cluster), none of these
can be plausibly modified to yield a significantly larger age. We discuss the
implications of this result on possible scenarios for the frequency of nuclear
starbursts and their impact on secular evolution of spiral galaxy nuclei.
As a byproduct of our analysis, we infer that IC 342 cannot have any central
black hole more massive than 0.5 million solar masses. This is ~ 6 times less
massive than the black hole inferred to exist in our Galaxy, consistent with
the accumulating evidence that galaxies with less massive bulges harbor less
massive black holes.Comment: 27 pages, incl. 9 figures, submitted to The Astronomical Journa
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