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T. Boeker

Publications and source records attributed to T. Boeker.

30 records · Page 2Linked to original sources
arXiv2003

Elusive Active Galactic Nuclei

A fraction of active galactic nuclei do not show the classical Seyfert-type signatures in their optical spectra, i.e. they are optically "elusive". X-ray observations are an optimal tool to identify this class of objects. We combine new Chandra observations with archival X-ray data in order to obtain a first estimate of the fraction of elusive AGN in local galaxies and to constrain their nature. Our results suggest that elusive AGN have a local density comparable to or even higher than optically classified Seyfert nuclei. Most elusive AGN are heavily absorbed in the X-rays, with gas column densities exceeding 10^24 cm^-2, suggesting that their peculiar nature is associated with obscuration. It is likely that in elusive AGN, the nuclear UV source is completely embedded and the ionizing photons cannot escape, which prevents the formation of a classical Narrow Line Region. Elusive AGN may contribute significantly to the 30 keV bump of the X-ray background.

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arXiv2003

Molecular Gas and the Nuclear Star Cluster in IC342: Sufficient Inflow for Recurring Star Formation Events?

We present high spatial resolution (1.2") mm-interferometric observations of the CO(2-1) line emission in the central 300pc of the late-type spiral galaxy IC342. The data, obtained with the Owens Valley Radio Observatory, allow first-time detection of a molecular gas disk that coincides with the luminous young stellar cluster in the nucleus of IC342. The nuclear CO disk has a diameter of ~30pc and a molecular gas mass of M(H2) ~ 2x10^5 M_sun. It connects via two faint CO bridges to the well-known, 100pc diameter circumnuclear gas ring. Analysis of the gas kinematics shows that the line-of-nodes in the inner 15pc is offset by about 45degree from the major kinematic axis, indicating non-circular motion of the gas within a few parsec of the dynamical center of IC342. Both the morphology and the kinematics of the CO gas indicate ongoing inflow of molecular gas into the central few parsec of IC342. We infer a gas inflow rate between 0.003 and 0.14 M_sun/yr, based on the observed surface density of the nuclear gas disk and estimates of the radial velocities of the surrounding gas. Inflow rates of this order can support repetitive star formation events in the nucleus of IC342 on timescales much smaller than a Hubble time.

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arXiv2003

Molecular gas in the central regions of the latest-type spiral galaxies

Using the IRAM 30m telescope, we have surveyed an unbiased sample of 47 nearby spiral galaxies of very late (Scd-Sm) Hubble-type for emission in the CO(1-0) and CO(2-1) lines. The sensitivity of our data (a few mK) allows detection of about 60% of our sample in at least one of the CO lines. The median detected H2 mass is $1.4\times 10^7 \msun$ within the central few kpc, assuming a standard conversion factor. We use the measured line intensities to complement existing studies of the molecular gas content of spiral galaxies as a function of Hubble-type and to significantly improve the statistical significance of such studies at the late end of the spiral sequence. We find that the latest-type spirals closely follow the correlation between molecular gas content and galaxy luminosity established for earlier Hubble types. The molecular gas in late-type galaxies seems to be less centrally concentrated than in earlier types. We use Hubble Space Telescope optical images to correlate the molecular gas mass to the properties of the central galaxy disk and the compact star cluster that occupies the nucleus of most late-type spirals. There is no clear correlation between the luminosity of the nuclear star cluster and the molecular gas mass, although the CO detection rate is highest for the brightest clusters. It appears that the central surface brightness of the stellar disk is an important parameter for the amount of molecular gas at the galaxy center. Whether stellar bars play a critical role for the gas dynamics remains unclear, in part because of uncertainties in the morphological classifications of our sample.

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arXiv2001

HST Observations of the Nuclear Regions of the Toomre Sequence of Merging Galaxies

We present first results from an HST WFPC2 imaging and STIS spectroscopy program to investigate the structural and star forming properties in the nuclear regions of the Toomre Sequence of merging galaxies. Here we discuss V-band, I-band and H-alpha images of the nuclei. We comment briefly on the connection between the nuclear morphology of the ionized gas and the merger stage.

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arXiv2001

Properties of PACE-I HgCdTe Detectors in Space - the NICMOS Warm-up Monitoring Program

We summarize the results of a monitoring program which was executed following the cryogen exhaustion of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) onboard the Hubble Space Telescope. During the subsequent warm-up, detector parameters such as detective quantum efficiency, dark current, bias offsets, and saturation levels have been measured over the temperature range 62K to about 100K. The measurements provide a unique database of the characteristics of PACE-I HgCdTe detector arrays in the space environment. A surprising result of the analysis is the fact that all three NICMOS detectors showed an enhanced dark current in the temperature range between 77K and 85K. However, a subsequent laboratory experiment designed to replicate the on-orbit warm-up did not reproduce the anomaly, despite the fact that it employed a flight-spare detector of the same pedigree. The mechanism behind the on-orbit dark current anomaly is therefore believed to be unique to the space environment. We discuss possible explanations for these unexpected observational results, as well as their implications for future NICMOS operations.

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arXiv2000

A Young Stellar Cluster in the Nucleus of NGC 4449

We have obtained 1-2 A resolution optical Echellette spectra of the nuclear star cluster in the nearby starburst galaxy NGC 4449. The light is clearly dominated by a very young (6-10 Myr) population of stars. For our age dating, we have used recent population synthesis models to interpret the observed equivalent width of stellar absorption features such as the HI Balmer series and the CaII triplet around 8500 A. We also compare the observed spectrum of the nuclear cluster to synthesized spectra of simple stellar populations of varying ages. All these approaches yield a consistent cluster age. Metallicity estimates based on the relative intensities of various ionization lines yield no evidence for significant enrichment in the center of this low mass galaxy: the metallicity of the nuclear cluster is about one fourth of the solar value, in agreement with independent estimates for the disk material of NGC 4449.

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arXiv1999

Star Formation in the Nuclei of Spiral Galaxies

Recent observations with the Hubble Space Telescope (HST) have revealed that a large fraction of late-type (Sc and later) spiral galaxies harbor a bright, compact stellar cluster in their dynamical centers. Statistics of the mass, age, and star formation history of these clusters as a function of their host galaxy's Hubble type can be used to constrain models of secular galaxy evolution. Since late-type spirals by definition do not possess a prominent bulge, their nuclear clusters are more easily separated from the underlying disk population. Their spectroscopic properties can thus be studied from ground-based observations. Here, I will discuss plans for, and first results of, a program to study a sample of known nuclear clusters in late-type spirals. For one galaxy (IC 342), we have used high-resolution near infrared spectroscopy to determine the cluster mass directly via its stellar velocity dispersion. The analysis conclusively shows a very low mass-to-light ratio for the nuclear cluster in IC 342, indicative of a young cluster age (about 50 Myrs). From probability arguments, this result favors the scenario that such bursts are a recurrent phenomenon in late-type spiral nuclei.

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arXiv1999

The NICMOS Snapshot Survey of nearby Galaxies

We present ``snapshot'' observations with the NearInfrared Camera and MultiObject Spectrometer (NICMOS) on board the Hubble Space Telescope (HST) of 94 nearby galaxies from the Revised Shapley Ames Catalog. Images with 0.2 as resolution were obtained in two filters, a broad-band continuum filter (F160W, roughly equivalent to the H-band) and a narrow band filter centered on the Paschen alpha line (F187N or F190N, depending on the galaxy redshift) with the 51x51 as field of view of the NICMOS camera 3. A first-order continuum subtraction is performed, and the resulting line maps and integrated Paschen alpha line fluxes are presented. A statistical analysis indicates that the average Paschen alpha surface brightness {\bf in the central regions} is highest in early-type (Sa-Sb) spirals.

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arXiv1999

Imaging and Nulling with the Space Interferometry Mission

We present numerical simulations for a possible synthesis imaging mode of the Space Interferometer Mission (SIM). We summarize the general techniques that SIM offers to perform imaging of high surface brightness sources, and discuss their strengths and weaknesses. We describe an interactive software package that is used to provide realistic, photometrically correct estimates of SIM performance for various classes of astronomical objects. In particular, we simulate the cases of gaseous disks around black holes in the nuclei of galaxies, and zodiacal dust disks around young stellar objects. Regarding the first, we show that a Keplerian velocity gradient of the line-emitting gaseous disk -- and thus the mass of the putative black hole -- can be determined with SIM to unprecedented accuracy in about 5 hours of integration time for objects with H_alpha surface brigthness comparable to the prototype M 87. Detections and observations of exo-zodiacal dust disks depend critically on the disk properties and the nulling capabilities of SIM. Systems with similar disk size and at least one tenth of the dust content of beta Pic can be detected by SIM at distances between 100 pc and a few kpc, if a nulling efficiency of 1/10000 is achieved. Possible inner clear regions indicative of the presence of massive planets can also be detected and imaged. On the other hand, exo-zodiacal disks with properties more similar to the solar system will not be found in reasonable integration times with SIM.

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arXiv1999

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.

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arXiv1998

Mid-infrared [NeII] line emission from the nucleus of NGC 253

We report on mid-infrared (MIR) continuum and line emission mapping of the nucleus of NGC 253. The data, with a resolution of 1\as.4, reveal a double-peaked arc-like [NeII] emission region. Comparison with published data shows that the [NeII] arc is centered on the nucleus of the galaxy. The brightest [NeII] source coincides with the infrared continuum peak. The interpretation of these results is complicated by the edge-on orientation of NGC 253, but a self-consistent explanation is starformation triggered by dynamical resonances in a barred potential.

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