Torsten Böker

MASSES OF STAR CLUSTERS IN THE NUCLEI OF BULGELESS SPIRAL GALAXIES

In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10 6 –10 8 L� )butcompact( re � 5 pc) nuclear star clusters in very late type spirals with the Ultraviolet and Visual Echelle Spectrograph at the VLT. We find that the velocity dispersions of the nine clusters in our sample range from 13 to 34 km s � 1 . Using photometric data from the Hubble Space Telescope WFPC2 and spherically symmetric dynamical models, we determine masses between 8 ; 10 5 and 6 ; 10 7 M� . The mass-to-light ratios range from 0.2 to 1.5 in the I band. This indicates a young mean age for most clusters, in agreement with previous studies. Given their high masses and small sizes, we find that nuclear clusters

Stellar populations in the nuclei of late-type spiral galaxies

As part of an ongoing effort to study the stellar nuclei of very late type, bulge-less spirals, we present results from a high-resolution spectroscopic survey of nine such nuclear star clusters, undertaken with the VLT UVES. We fit the spectra with population synthesis models and measure Lick-type indices to determine mean luminosity-weighted ages, which range from 4.1 × 107 to 1.1 × 1010 yr and are insensitive to assumed metallicity or internal extinction. The average metallicity of nuclear clusters in late-type spirals is slightly subsolar (Z = 0.015) but shows significant scatter. Most of the clusters have moderate extinctions of 0.1-0.3 mag in the I band. The nuclear cluster spectra are best described by age-composite stellar populations, as only such models yield mass-to-light ratios that match those obtained from dynamical measurements. For our nine sample clusters, the last star formation episode was on average 34 Myr ago, while all clusters experienced some star formation in the last 100 Myr. We thus conclude that the nuclear clusters undergo repeated episodes of star formation. Our results are robust to possible contamination from the underlying galaxy disk, as demonstrated by comparison to a similar analysis using smaller aperture spectra obtained with the HST STIS. Also considering the 2005 results from Walcher et al., we have thus shown that the stellar nuclei of bulge-less galaxies are massive and dense star clusters that form stars recurrently until the present day. This set of properties is unique among the various classes of star clusters. To elucidate whether and how these unique properties are related to the nuclear location of the cluster in its host galaxy remains a challenging question.

A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. II. Cluster Sizes and Structural Parameter Correlations

We investigate the structural properties of nuclear star clusters in late-type spiral galaxies. More specifically, we fit analytical models to Hubble Space Telescope images of 39 nuclear clusters in order to determine their effective radii after correction for the instrumental point-spread function. We use the results of this analysis to compare the luminosities and sizes of nuclear star clusters to those of other ellipsoidal stellar systems, in particular the Milky Way globular clusters. Our nuclear clusters have a median effective radius of e = 3.5 pc, with 50% of the sample falling in the range 2.4 pc ≤ re ≤ 5.0 pc. This narrow size distribution is statistically indistinguishable from that of Galactic globular clusters, even though the nuclear clusters are, on average, 4 mag brighter than the old globular clusters. We discuss some possible interpretations of this result. From a comparison of nuclear cluster luminosities with various properties of their host galaxies, we confirm that more luminous galaxies harbor more luminous nuclear clusters. It remains unclear whether this correlation mainly reflects the influence of galaxy size, mass, and/or star formation rate. Since the brighter galaxies in our sample typically have stellar disks with a higher central surface brightness, nuclear cluster luminosity also correlates with this property of their hosts. On the other hand, we find no evidence for a correlation between the presence of a nuclear star cluster and the presence of a large-scale stellar bar.

Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type*

We study the nuclear star clusters (NCs) in spiral galaxies of various Hubble types using spectra obtained with the STIS on board the Hubble Space Telescope (HST). We observed the nuclear clusters in 40 galaxies, selected from two previous HST WFPC2 imaging surveys. At a spatial resolution of ~02 the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have a sufficiently high signal-to-noise ratio for detailed stellar population analysis. For the other half we only measure the continuum slope, as quantified by the B - V color. To infer the star formation history, metallicity, and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise ratio spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. Approximately half of the sample clusters contain a population younger than 1 Gyr. The luminosity-weighted ages range from 10 Myr to 10 Gyr. The stellar populations of NCs are generally best fit as a mixture of populations of different ages. This indicates that NCs did not form in a single event, but that instead they had additional star formation long after the oldest stars formed. On average, the sample clusters in late-type spirals have a younger luminosity-weighted mean age than those in early-type spirals ( = 8.37 ± 0.25 vs. 9.23 ± 0.21). The average mass-weighted ages are older by ~0.7 dex, indicating that there often is an underlying older population that does not contribute much light but does contain most of the mass. The average cluster masses are smaller in late-type spirals than in early-type spirals ( = 6.25 ± 0.21 vs. 7.63 ± 0.24) and exceed the masses typical of globular clusters. The cluster mass correlates loosely with total galaxy luminosity. It correlates more strongly with both the Hubble type of the host galaxy and the luminosity of its bulge. The latter correlation has the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity. The properties of both nuclear clusters and black holes in the centers of spiral galaxies are therefore intimately connected to the properties of the host galaxy, and in particular its bulge component. Plausible formation scenarios have to account for this. We discuss various possible selection biases in our results, but conclude that none of them can explain the differences seen between clusters in early- and late-type spirals. The inability to infer spectroscopically the populations of faint clusters does introduce a bias toward younger ages, but not necessarily toward higher masses.

N-Band Imaging of Seyfert Nuclei and the Mid-Infrared-X-Ray Correlation

We present new mid-infrared (N-band) images of a sample of eight nearby Seyfert galaxies. In all of our targets, we detect a central unresolved source, which in some cases has been identified for the first time. In particular, we have detected the mid-infrared emission from the active nucleus of NGC 4945, which previously remained undetected at any wavelength but hard X-rays. We also detect circumnuclear extended emission in the Circinus galaxy along its major axis and find marginal evidence for extended circumnuclear emission in NGC 3281. The high spatial resolution (17) of our data allows us to separate the flux of the nuclear point sources from the extended circumnuclear starburst (if present). We complement our sample with literature data for a number of nonactive starburst galaxies and relate the nuclear N-band flux to published hard (2-10 keV) X-ray fluxes. We find tight and well-separated correlations between the nuclear N-band flux and X-ray flux for both Seyfert and starburst nuclei that span over 3 orders of magnitude in luminosity. We demonstrate that these correlations can be used as a powerful classification tool for galactic nuclei. For example, we find strong evidence against NGC 1808 currently harboring an active Seyfert nucleus based on its position in the mid-infrared-X-ray diagram. On the other hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.

THE INCIDENCE OF ACTIVE GALACTIC NUCLEI IN PURE DISK GALAXIES: THE SPITZER VIEW

Using the Spitzer telescope, we have conducted a high-resolution spectroscopic study of 18 bulgeless (Hubble type of Sd or Sdm) galaxies that show no definitive signatures of nuclear activity in their optical spectra. This is the first systematic mid-infrared search for weak or hidden active galactic nuclei (AGNs) in a statistically significant sample of bulgeless (Sd/Sdm) disk galaxies. Based on the detection of the high-ionization [NeV] 14.3 µm line, we report the discovery of an AGN in one out of the 18 galaxies in the sample. This galaxy, NGC 4178, is a nearby edge-on Sd galaxy, which likely hosts a prominent nuclear star cluster (NSC). The bolometric luminosity of the AGN inferred from the [NeV] line luminosity is � 8×10 41 ergs s −1 . This is almost two orders of magnitude greater than the luminosity of the AGN in NGC 4395, the best studied AGN in a bulgeless disk galaxy. Assuming that the AGN in NGC 4178 is radiating below the Eddington limit, the lower mass limit for the black hole is � 6×10 3 M⊙. The fact that none of the other galaxies in the sample shows any evidence for an AGN demonstrates that while the AGN detection rate based on mid-infrared diagnostics is high (30-40%) in optically quiescent galaxies with pseudobulges or weak classical bulges (Hubble type Sbc and Sc), it drops drastically in Sd/Sdm galaxies. Our observations therefore confirm that AGNs in completely bulgeless disk galaxies are not hidden in the optical but truly are rare. Of the three Sd galaxies with AGNs known so far, all have prominent NSCs, suggesting that in the absence of a well-defined bulge, the galaxy must possess a NSC in order to host an AGN. On the other hand, while the presence of a NSC appears to be a requirement for hosting an AGN in bulgeless galaxies, neither the properties of the NSC nor those of the host galaxy appear exceptional in late-type AGN host galaxies. The recipe for forming and growing a central black hole in a bulgeless galaxy therefore remains unknown. Subject headings: Galaxies: Active— Galaxies: black hole physics – dark matter – galaxies: spiral: Galaxies — Infrared: Galaxies

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.

Massive star formation in the central regions of spiral galaxies

Received ; accepted Abstract. Context The morphology of massive star formation in the central regions of galaxies is an important tracer of the dynamical processes that govern the evolution of disk, bulge, and nuclear activity. Aims We present optical imaging of the central regions of a sample of 73 spiral galaxies in the Hline and in optical broad bands, and derive information on the morphology of massive star formation. Methods We obtained images with the William Herschel Telescope, mostly at a spatial resolution of below one second of arc. For most galaxies, no Himaging is available in the literature. We outline the obser ving and data reduction procedures, list basic properties, and present the I-band and continuum-subtracted Himages. We classify the morphology of the nuclear and circumnuclear Hemission and explore trends with host galaxy parameters. Results We confirm that late-type galaxies have a patchy circumnucle ar appearance in H�, and that nuclear rings occur pri- marily in spiral types Sa-Sbc. We identify a number of previously unknown nuclear rings, and confirm that nuclear rings ar e predominantly hosted by barred galaxies. Conclusions Other than in stimulating nuclear rings, bars do not influenc e the relative strength of the nuclear Hpeak, nor the circumnuclear Hmorphology. Even though our selection criteria led to an over-abundance of galaxies with close massive companions, we do not find any significant influence of the presence or absence of a close com panion on the relative strength of the nuclear Hpeak, nor on the Hmorphology around the nucleus.

A hubble space telescope WFPC2 investigation of the nuclear morphology in the Toomre sequence of merging galaxies

We report on the properties of nuclear regions in the Toomre sequence of merging galaxies, based on imaging data gathered with the Hubble Space Telescope WFPC2 camera. We have imaged the 11 systems in the proposed evolutionary merger sequence in the F555W and F814W broadband filters, and in Hα+[N II] narrowband filters. The broadband morphology of the nuclear regions varies from nonnucleated starburst clumps through dust-covered nuclei to a nucleated morphology. There is no unambiguous trend in the morphology with merger stage. The emission-line morphology is extended beyond the nucleus in most cases, but centrally concentrated (within 1 kpc) emission-line gas can be seen in the four latest-stage merger systems. We have quantified the intrinsic luminosity densities and colors within the inner 100 pc and 1 kpc of each identified nucleus. We find little evidence for a clear trend in nuclear properties along the merger sequence other than a suggestive rise in the nuclear luminosity density in the most evolved members of the sequence. The lack of clear trends in nuclear properties is likely due both to the effects of obscuration and geometry, as well as the physical variety of galaxies included in the Toomre sequence.

Herschel/SPIRE observations of the dusty disk of NGC 4244

We present Herschel/SPIRE images at 250, 350, and 500 mu m of NGC 4244, a typical low-mass, disk-only and edge-on spiral galaxy. The dust disk is clumpy and shows signs of truncation at the break radius of the stellar disk. This disk coincides with the densest part of the Hi disk. We compare the spectral energy distribution (SED), including the new SPIRE fluxes, to 3D radiative transfer models; a smooth model disk and a clumpy model with embedded heating. Each model requires a very high value for the dust scale-length (h(d) = 2-5 h(*)), higher dust masses than previous models of NGC 4244 (M-d = 0.47-1.39 x 10(7) M-circle dot) and a face-on optical depth of tau(f.o.)(V) = 0.4-1.12, in agreement with previous disk opacity studies. The vertical scales of stars and dust are similar. The clumpy model much better mimics the general morphology in the sub-mm images and the general SED. The inferred gas-to-dust mass ratio is compatible with those of similar low-mass disks. The relatively large radial scale-length of the dust disk points to radial mixing of the dusty ISM within the stellar disk. The large vertical dust scale and the clumpy dust distribution of our SED model are both consistent with a scenario in which the vertical structure of the ISM is dictated by the balance of turbulence and self-gravity.