arXiv subjects

Reinhard Genzel

Publications and source records attributed to Reinhard Genzel.

133 records · Page 8Linked to original sources

arXiv2000

The Nuclear Star Cluster of the Milky Way: Star Formation, Dynamics and Central Black Hole

High spatial resolution, near-infrared imaging and spectroscopy of the nuclear star cluster have given key new insights about the dynamics, evolution and mass distribution in the Milky Way Center. The central parsec is powered by a cluster of hot, massive stars which must have formed a few million years ago. Either star formation was triggered in the central parsec by the infall of a very dense cloud, or a dense, young star cluster formed outside of the central parsec sank rapidly into the nuclear region through dynamical friction. The presence of luminous asymptotic giant branch (AGB) stars suggests that there were earlier such star formation episodes. Measurements of radial and proper motions for more than 200 stars delineate the stellar dynamics to a scale of a few light days from the dynamic center which is coincident with the compact radio source (SgrA*) within ~0.1" (800 AU). The stellar velocities increase toward SgrA* with a Kepler law (to >1000 km/s for the innermost stars), implying the presence of a three million solar mass central (dark) mass. The observations make a compelling case that this mass concentration is a black hole which is currently accreting at a low rate or radiating at low efficiency. With the exception of the young, massive stars the velocity field of the central stellar cluster is close to isotropic. The young stars are characterized by a turbulent rotation pattern that still carries the imprint of the angular momentum distribution in the original cloud or star cluster.

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arXiv2000

Extragalactic Results from the Infrared Space Observatory

More than a decade ago the IRAS satellite opened the realm of external galaxies for studies in the 10 to 100 micron band and discovered emission from tens of thousands of normal and active galaxies. With the 1995-1998 mission of the Infrared Space Observatory the next major steps in extragalactic infrared astronomy became possible: detailed imaging, spectroscopy and spectro-photometry of many galaxies detected by IRAS, as well as deep surveys in the mid- and far- IR. The spectroscopic data reveal a wealth of detail about the nature of the energy source(s) and about the physical conditions in galaxies. ISO's surveys for the first time explore the infrared emission of distant, high-redshift galaxies. ISO's main theme in extragalactic astronomy is the role of star formation in the activity and evolution of galaxies.

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arXiv2000

Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum

The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a compilation of UV to IR narrow emission line data to determine the spectral energy distribution (SED) of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We search a large grid of gas cloud models and SEDs for the combination that best reproduces the observed line fluxes and NLR geometry. Our best fit model reproduces the observed line fluxes to better than a factor of 2 on average and is in general agreement with the observed NLR geometry. It has two gas components that are consistent with a clumpy distribution of dense outflowing gas in the center and a more extended distribution of less dense and more clumpy gas farther out that has no net outflow. The best fit SED has a deep trough at ~4 Ryd, which is consistent with an intrinsic Big Blue Bump that is partially absorbed by ~6x10^19 cm^-2 of neutral hydrogen interior to the NLR.

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arXiv2000

Near infrared imaging spectroscopy of NGC1275

We present H and K band imaging spectroscopy of the core regions of the cD/AGN galaxy NGC1275. The spectra, including lines from H2, H, 12CO bandheads, [FeII], and [FeIII], are exploited to constrain the star formation and excitation mechanisms in the galaxy's nucleus. The near-infrared properties can largely be accounted for by ionized gas in the NLR, dense molecular gas, and hot dust concentrated near the active nucleus of NGC1275. The strong and compact H2 emission is mostly from circumnuclear gas excited by the AGN and not from the cooling flow. The extended emission of latetype stars is diluted in the center by the thermal emission of hot dust.

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arXiv1999

The Sgr A* Stellar Cluster: New NIR Imaging and Spectroscopy

We report preliminary results based on new near-infrared observations of the central stellar cluster of our Galaxy conducted with the infrared spectrometer ISAAC at the ESO VLT UT1 and the MPE speckle camera SHARP at the ESO NTT. We obtained lambda / dlambda >= 3000 K-band spectroscopy of the 2.058 um He I, 2.165 um Br gamma emission lines, and 2.29 um CO bandhead absorption line. These data demonstrate clearly that there is no strong CO bandhead absorption originating in the northern part (S1/S2 area) of the central stellar cluster at the position of Sgr A*. This makes it likely that these K~14.5 stars are O9 - B0.5 stars with masses of 15 to 20 M(sun). Weaker CO bandhead absorption in the southern part of the cluster (S10/S11 area) could be due to contributions from neighbouring stars. We also report the detection of Br gamma line emission at the position of the central stellar cluster which could be associated with the 'mini-spiral' rather than with the Sgr A* cluster itself. At the NTT we obtained another epoch epoch of proper motion measurments. The changes of source positions are consistent with the proper motion velocities derived from earlier epochs.

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arXiv1998

Infrared spectroscopy of NGC4151: Probing the obscured ionizing AGN continuum

The ISO-SWS infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC4151, which are described in a companion paper, are used together with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We present a new method to determine the best fitting photoionizing continuum and emission line cloud model from a heterogeneous set of emission line data. For NGC4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and which is consistent with the observed geometry of the optical Narrow Line Region (NLR). Our model consists of a clumpy, optically thick (ionization bounded) gas distribution, with a hydrogen gas density of ~1000 cm^-3 and a volume filling factor of 6.5*10^-4. Our best fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again towards 100 eV. In particular, it does not display a `Big Blue Bump' signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100--500 pc scale, does not have enough UV photons to produce the observed BLR recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a Big Blue Bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of \~5*10^19 cm^-2 of neutral hydrogen. Such an absorber was independently detected by UV absorption lines (Kriss et al. 1992, 1995).

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arXiv1997

Colliding Winds in the Stellar Core at the Galactic Center: Some Implications

We point out that a high number density of stars in the core of a dense star cluster such as the central stellar cluster at the Galactic center, where many stars possess strong stellar winds, should result in collisions of those winds. The wind collisions in the dense stellar core at the Galactic center would result in production of strong X-ray flares with the rate of $\sim 10^{-4}~ (N_w/10^3)^2 / yr$ and duration of $\sim 1$ week, where $N_w$ is the number of the wind producing stars in the core. Presence of a massive black hole would enhance the stellar density around it and would make collisions of the winds in the core substantially more frequent. Collisions of the stellar winds in the cluster have also a number of interesting observable implications, including generation of $γ$-rays by particles accelerated by the shocks from the colliding winds. These processes are also expected to be relevant to compact regions of intense star formation elsewhere.

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