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R. Genzel

Publications and source records attributed to R. Genzel.

At least 379 records · Page 21Linked to original sources
arXiv2003

A Geometric Determination of the Distance to the Galactic Center

We report new astrometric and spectroscopic observations of the star S2 orbiting the massive black hole in the Galactic Center, which were taken at the ESO VLT with the adaptive optics assisted, near-IR camera NAOS/CONICA and the near-IR integral field spectrometer SPIFFI. We use these data to determine all orbital parameters of the star with high precision, including the Sun-Galactic Center distance, which is a key parameter for calibrating stellar standard candles and an important rung in the extragalactic distance ladder. Our deduced value of Ro = 8.0 +/- 0.4 kpc is the most accurate primary distance measurement to the center of the Milky Way and has minimal systematic uncertainties of astrophysical origin. It is in excellent agreement with other recent determinations of Ro.

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arXiv2003

SINFONI - Integral Field Spectroscopy at 50 milli-arcsecond resolution with the ESO VLT

SINFONI is an adaptive optics assisted near-infrared integral field spectrometer for the ESO VLT. The Adaptive Optics Module (built by the ESO Adaptive Optics Group) is a 60-elements curvature-sensor based system, designed for operations with natural or sodium laser guide stars. The near-infrared integral field spectrometer SPIFFI (built by the Infrared Group of MPE) provides simultaneous spectroscopy of 32 x 32 spatial pixels, and a spectral resolving power of up to 3300. The adaptive optics module is in the phase of integration; the spectrometer is presently tested in the laboratory. We provide an overview of the project, with particular emphasis on the problems encountered in designing and building an adaptive optics assisted spectrometer.

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arXiv2003

The [CII] 158 um Line Deficit in Ultraluminous Infrared Galaxies Revisited

We present a study of the [CII] 157.74 um fine-structure line in a sample of 15 ultraluminous infrared (IR) galaxies (L_IR>10^12 Lsun; ULIRGs) using the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We confirm the observed order of magnitude deficit (compared to normal and starburst galaxies) in the strength of the [CII] line relative to the far-IR dust continuum emission found in our initial report (Luhman et al. 1998), but here with a sample that is twice as large. This result suggests that the deficit is a general phenomenon affecting 4/5 ULIRGs. We present an analysis using observations of generally acknowledged photodissociation region (PDR) tracers ([CII], [OI] 63 and 145 um, and FIR continuum emission), which suggests that a high UV flux G_o incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including CO (1-0), [CI] (1-0), and 6.2 um polycyclic aromatic hydrocarbon (PAH) emission, suggest that high G_o/n PDRs alone cannot produce a self-consistent solution that is compatible with all of the observations. We propose that non-PDR contributions to the FIR continuum can explain the apparent [CII] deficiency. Here, unusually high G_o and/or n physical conditions in ULIRGs as compared to those in normal and starburst galaxies are not required to explain the [CII] deficit. Dust-bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [CII] emission, offer one possible physical origin for this additional non-PDR component. Such environments may also contribute to the observed suppression of FIR fine-structure emission from ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The implications for observations at higher redshifts are also revisited.

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arXiv2003

The Stellar Cusp Around the Supermassive Black Hole in the Galactic Center

We analyze deep near-IR adaptive optics imaging as well as new proper motion data of the nuclear star cluster of the Milky Way. The surface density distribution of faint stars peaks within 0.2" of the black hole candidate SgrA*. The radial density distribution of this stellar 'cusp' follows a power law of exponent 1.3-1.4. The K-band luminosity function of the overall nuclear stellar cluster (within 9" of SgrA*) resembles that of the large scale, Galactic bulge, but shows an excess of stars at K<14. We find that most of the massive early type stars at distances 1-10" from SgrA* are located in two rotating and geometrically thin disks. These disks are inclined at large angles and counter-rotate with respect to each other. Their stellar content is essentially the same, indicating that they formed at the same time. The star closest to SgrA* in 2002, S2, exhibits a 3.8 micron excess. We propose that the mid-IR emission either comes from the accretion flow around the black hole itself, or from dust in the accretion flow that is heated by the ultra-violet emission of S2.

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arXiv2003

The Galactic Center stellar cluster: The central arcsecond

With 10 years of high-resolution imaging data now available on the stellar cluster in the Galactic Center, we analyze the dynamics of the stars at projected distances $\leq1.2''$ from the central black hole candidate Sagittarius A* (Sgr A*). We find evidence for radial anisotropy of the cluster of stars surrounding Sgr A*. We confirm/find accelerated motion for 6 stars, with 4 stars having passed the pericenter of their orbits during the observed time span. We calculated/constrained the orbital parameters of these stars. All orbits have moderate to high eccentricities. The center of acceleration coincides with the radio position of Sgr A*. From the orbit of the star S2, the currently most tightly constrained one, we determine the mass of Sgr A* to $3.3\pm0.7\times10^{6}$M$_{\odot}$ and its position to $2.0\pm2.4$ mas East and $2.7\pm4.5$ mas South of the nominal radio position. The data provide compelling evidence that Sgr A* is a single supermassive black hole.

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arXiv2003

The Position, Motion, and Mass of Sgr A*

We report progress on measuring the position of Sgr A* on infrared images, placing limits on the motion of the central star cluster relative to Sgr A*, and measuring the proper motionof Sgr A* itself. The position of Sgr A* has been determined to within 10 mas on infrared images. To this accuracy, the gravitational source (sensed by stellar orbits) and the radiative source (Sgr A*) are coincident. Proper motions of four stars measured both in the infrared and radio indicate that the central star cluster moves with Sgr A* to within 70 km/s. Finally, combining stellar orbital information with an upper limit of 8 km/s for the intrinsic proper motion of Sgr A* (perpendicular to the Galactic plane), we place a lower limit on the mass of Sgr A* of 4 x 10^5 Msun.

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arXiv2003

Infrared spectroscopy of faint 15 micron sources in the Hubble Deep Field South: first hints at the properties of the sources of the IR background

We present a spectroscopic analysis of 21 galaxies with z=0.2-1.5 drawn from a 25 sq. arcmin ultra-deep 15 micron ISOCAM survey centered in the HDFS. VLT/ISAAC spectra are reported for 18 sources, aimed at detecting the redshifted Ha. Optical data come from VLT/FORS2 and NTT/EMMI. Ha line emission is detected in all the observed objects. Our analysis of emission lines, morphology, and SEDs shows evidence for AGN in only two of these sources. The Ha luminosities indicate star formation rates (SFR) between 0.5 and 20 Msun, without extinction corrections. We find good correlations between the mid-IR, the radio and Ha luminosities, confirming the mid-IR light as a good tracer of star formation (while SFR based on Ha flux show large scatter and offset, still to be understood). We have estimated the baryonic masses in stars by fitting the optical-IR SED, and found that the host galaxies of ISO sources are massive members of groups with SFR=10 to 300 Msun/yr. We have finally compared this ongoing SF activity with the already formed stellar masses to estimate the timescales t(SF) for the stellar build-up, which turn-out to be widely spread between 0.1 Gyrs to more than 10 Gyr. The faint ISOCAM galaxies appear to form a composite population, including moderately active but very massive spiral-like galaxies, and very luminous ongoing starbursts, in a continuous sequence. From the observed t(SF), assuming typical starburst timescales, we infer that, with few exceptions, only a fraction of the galactic stars can be formed in any single starburst, while several of such episodes during a protracted SF history are required for the whole galactic build-up.

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arXiv2003

Ha Rotation Curves of z~1 Galaxies: Unraveling the Evolution of the Tully-Fisher Relation

We describe the first results of a programme to obtain rotation curves of z~1 disc galaxies in the near-infrared using the Ha emission line in order to study the Tully-Fisher relation. To put any observed evolution into perspective and to investigate any possible selection biases, we constructed a control sample of low redshift galaxies that had rotation velocities and images available for measuring their dynamical, photometric, and morphological properties. Compared to local objects with isophotal sizes similar to the high redshift targets, we find that our sample of galaxies with spatially resolved rotation curves, the most distant sample so far ( ~0.9), clearly reveals a brightening of ~1.1 mag in the rest-frame B-band. The observed offset can be explained by a combination of increasing surface brightness, decreasing rotation speeds, and slightly smaller disc scale lengths of the high redshift galaxies.

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arXiv2002

II. Accurate Positions and Proper Motions of Stellar SiO Masers near the Galactic Center

Menten, Reid, Eckart and Genzel (1997) accurately determined the position of Sgr A* on an infrared image, by aligning the infrared image with positions measured for SiO masers, associated with infrared-bright evolved stars, at radio wavelengths. We now report greatly improved radio positions and, for the first time, proper motions of many stellar SiO masers at the Galactic Center. These positions and motions, coupled with better infrared imaging, allow a much improved location of Sgr A* on infrared images. With current data, infrared stars can be placed in a reference frame tied to Sgr A*, to an accuracy of approximately 10 mas in position and 1 mas/yr in motion. The position of Sgr A* is, within uncertainties, consistent with stellar accelerations and the measured orbital focus of the star S2. The star S2 was observed within 16 mas (130 AU in projection) of Sgr A* on 2002 May 02. Finally, we find that the central stellar cluster moves with Sgr A* to within approximatly 70 km/s.

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arXiv2002

Closest Star Seen Orbiting the Supermassive Black Hole at the Centre of the Milky Way

Measurements of stellar velocities and variable X-ray emission near the centre of the Milky Way have provided the strongest evidence so far that the dark mass concentrations seen in many galactic nuclei are likely supermassive black holes, but have not yet excluded several alternative configurations. Here we report ten years of high resolution astrometric imaging that allow us to trace two thirds of the orbit of the star currently closest to the compact radio source and massive black hole candidate SgrA*. In particular, we have observed both peri- and apocentre passages. Our observations show that the star is on a bound, highly elliptical Keplerian orbit around SgrA*, with an orbital period of 15.2 years and a peri-centre distance of only 17 light hours. The orbital elements require an enclosed point mass of 3.7+-1.5x10^6 solar masses. The data exclude with high confidence that the central dark mass consists of a cluster of astrophysical objects or massive, degenerate fermions, and strongly constrain the central density structure.

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arXiv2002

Infrared spectroscopy around 4micron of Seyfert 2 galaxies: Obscured broad line regions and coronal lines

The state of the matter that is obscuring a small circumnuclear region in AGN can be probed by observations of its broad emission lines. Infrared lines are particularly useful since they penetrate significant columns of obscuring matter, the properties of which can be constrained by comparing infrared and X-ray obscuration. We report on new 4um spectroscopy of a sample of 12 Seyfert 2 galaxies, probing for broad components to the Brackett alpha 4.05um line. Broad components are observed in 3 to 4 objects. All objects with a broad component exhibit relatively low X-ray obscuring columns, and our results are consistent with a Galactic ratio of 4um obscuration to the BLR and X-ray column. In combination with observations of a NON-Galactic ratio of VISUAL obscuration of BLRs and X-ray obscuring column in Seyferts, and interpreted in a unified AGN scheme, this result can be reconciled with two interpretations. Either the properties of dust near the AGN are modified towards larger grains, for example through coagulation, in a way that significantly flattens the optical/IR extinction curve, or the ratio of dust obscuration to X-ray column varies for different viewing angles with respect to the axis of symmetry of the putative torus. Our spectra also provide a survey of emission in the [SiIX] 3.94um coronal line, finding variation by an order of magnitude in its ratio to Br alpha. The first extragalactic detection of the [CaVII] 4.09um and [CaV] 4.16um coronal lines is reported in the spectrum of the Circinus galaxy.

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arXiv2002

Kinematics of Galaxies in the Hubble Deep Field South: Discovery of a Very Massive Spiral at z=0.6

We report the first results from a study of the internal kinematics, based on spatially resolved H_alpha velocity profiles, of three galaxies at redshift z~0.6 and one at redshift z~0.8, detected by ISOCAM in the Hubble Deep Field South. The kinematics are derived from high resolution near-infrared VLT spectroscopy. One of the galaxies is a massive spiral which possesses a very large rotational velocity of 460 km/s and contains a mass of 10^12 M_solar (within 20 kpc), significantly higher than the dynamical masses measured in most other local and high redshift spirals. Two of the galaxies comprise a counter-rotating interacting system, while the fourth is also a large spiral. The observed galaxies are representative examples of the morphologies encountered among ISOCAM galaxies. The mass-to-light (M /L_bol) ratios of ISOCAM galaxies lie between those of local luminous IR galaxies and massive spirals. We measure an offset of 1.6+/-0.3 mag in the rest frame B-band and of 0.7+/-0.3 mag in the rest frame I-band when we compare the four ISOCAM galaxies to the local Tully-Fisher B and I-band relations. We conclude that the large IR luminosity of the ISOCAM population results from a combination of large mass and efficient triggering of star formation. Since ISOCAM galaxies contribute significantly to the Cosmic Infrared Background our results imply that a relatively small number of very massive and IR luminous objects contribute significantly to the IR background and star formation activity near z~0.7.

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arXiv2002

Mid-Infrared line diagnostics of Active Galaxies -- A spectroscopic AGN survey with ISO-SWS

We present medium resolution (R approx. 1500) ISO-SWS 2.4--45 micron spectra of a sample of 29 galaxies with active nuclei. This data set is rich in fine structure emission lines tracing the narrow line regions and (circum-)nuclear star formation regions, and it provides a coherent spectroscopic reference for future extragalactic studies in the mid-infrared. We use the data set to briefly discuss the physical conditions in the narrow line regions (density, temperature, excitation, line profiles) and to test for possible differences between AGN sub-types. Our main focus is on new tools for determining the propertibes of dusty galaxies and on the AGN-starburst connection. We present mid-IR line ratio diagrams which can be used to identify composite (starburst + AGN) sources and to distinguish between emission excited by active nuclei and emission from (circum-nuclear) star forming regions. For instance, line ratios of high to low excitation lines like [O IV]25.9um/[Ne II]12.8um, that have been used to probe for AGNs in dusty objects, can be examined in more detail and with better statistics now. In addition, we present two-dimensional diagnostic diagrams that are fully analogous to classical optical diagnostic diagrams, but better suited for objects with high extinction. Finally, we discuss correlations of mid-infrared line fluxes to the mid- and far-infrared continuum. We compare these relations to similar relations in starburst galaxies in order to examine the contribution of AGNs to the bolometric luminosities of their host galaxies. The spectra are available in electronic form from the authors.

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arXiv2002

Ultr-Luminous Infrared Galaxies: QSOs in Formation?

We present new near-infrared Keck and VLT spectroscopic data on the stellar dynamics in late stage, ultra-luminous infrared galaxy (ULIRG) mergers . We now have information on the structural and kinematic properties of 18 ULIRGs, 8 of which contain QSO-like active galactic nuclei. The host properties (velocity dispersion, effective radius, effective surface brightness, M_K) of AGN-dominated and star formation dominated ULIRGs are similar. ULIRGs fall remarkably close to the fundamental plane of early type galaxies. They populate a wide range of the plane, are on average similar to L*-rotating ellipticals, but are well offset from giant ellipticals and optically/UV bright, low-z QSOs/radio galaxies. ULIRGs and local QSOs/radio galaxies are very similar in their distributions of bolometric and extinction corrected near-IR luminosities, but ULIRGs have smaller effective radii and velocity dispersions than the local QSO/radio galaxy population. Hence, their host masses and inferred black hole masses are correspondingly smaller. The latter are more akin to those of local Seyfert galaxies. ULIRGs thus resemble local QSOs in their near-IR and bolometric luminosities because they are (much more) efficiently forming stars and/or feeding their black holes, and not because they have QSO-like, very massive black holes. We conclude that ULIRGs as a class cannot evolve into optically bright QSOs. They will more likely become quiescent, moderate mass field ellipticals or, when active, might resemble the X-ray bright, early type galaxies that have recently been found by the Chandra Observatory.

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arXiv2002

An ISO--SWS survey of molecular hydrogen in Starburst and Seyfert Galaxies

We present results from a survey of molecular hydrogen emission from a sample of Starburst and Seyfert galaxies carried out with the Infrared Space Observatory (ISO). Pure rotational H_2 emission has been detected in a number of extragalactic objects and a variety of environments. A number of transitions from S(7) to S(0) are detected in both Starbursts and Seyferts. Using excitation diagrams we derive temperatures and masses of the ``warm'' molecular hydrogen. We find that the temperature of the ``warm'' gas is similar in Starbursts and Seyferts (those Seyferts for which we have firm detections of the S(0) line) with a value of around T~150 K. This ``warm'' gas accounts for as much as 10% of the total galactic mass (as probed by CO molecular observations) in Starbursts. The fraction of ``warm'' gas is overall higher in Seyferts, ranging between 2--35%. We then investigate the origin of the warm H_2 emission. Comparison with published theoretical models and Galactic templates implies that although emission from photodissociation regions (PDR) alone could explain the emission from Starbursts and Seyferts, most likely a combination of PDR, shock emission and gas heated by X-rays (mostly for the Seyferts) is responsible for H_2 excitation in extragalactic environments. Finally, we find that although PAH and H_2 line emission correlate well in Starbursts and the large scale emission in AGN, H_2 emission is much stronger compared to PAH emission in cases where a ``pure'' AGN dominates the energy output.

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arXiv2002

Properties of mm galaxies: Constraints from K-band blank fields

We have used the IRAM Plateau de Bure mm interferometer to locate with subarcsecond accuracy the dust emission of three of the brightest 1.2mm sources in the NTT Deep Field (NDF) selected from our 1.2mm MAMBO survey at the IRAM 30m telescope. We combine these results with deep B to K imaging and VLA interferometry. Strikingly, none of the three accurately located mm galaxies MMJ120546-0741.5, MMJ120539-0745.4, and MMJ120517-0743.1 has a K-band counterpart down to the faint limit of K>21.9. This implies that these three galaxies are either extremely obscured and/or are at very high redshifts (z>~4). We combine our results with literature data for 11 more (sub)mm galaxies that are identified with similar reliability. In terms of their K-band properties, the sample divides into three roughly equal groups: (i) undetected to K~22, (ii) detected in the near-infrared but not the optical and (iii) detected in the optical with the possibility of optical follow-up spectroscopy. We find a trend in this sample between near-infrared to submm and submm to radio spectral indices, which in comparison to spectral energy distributions (SEDs) of low redshift infrared luminous galaxies suggests that the most plausible primary factor causing the extreme near-infrared faintness of our objects is their high redshift. We show that the near-infrared to radio SEDs of the sample are inconsistent with SEDs that resemble local far-infrared cool galaxies with moderate luminosities, which were proposed in some models of the submm sky. We briefly discuss the implications of the results for our understanding of galaxy formation.

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arXiv2002

Stellar Orbits Near Sagittarius A*

We present new SHARP/NTT stellar proper motion and accelaration data covering an interval from 1992 to 2000: 1) We combine the high precision but shorter time scale NIRC/Keck data with the lower precision but longer time scale SHARP/NTT data set; 2) We statistically correct the observed accelerations for geometrical projection effects; 3) We exclude star S8 from the analysis of the amount and position of the central mass. We show that the stars S2, and most likely S1 and S8 as well, are on bound, fairly inclined ($60^o<i<80^o$), and eccentric ($0.4<e<0.95$) orbits around a central dark mass. The combination of both data sets results in a position of this central mass of 48$^{+54}_{-24}$ $mas$ E and 18$^{+42}_{-61}$ $mas$ S of the nominal radio position of Sgr A*. The mean statistically corrected enclosed mass derived from accelerations is M_acc=(5 +/- 3) x 10^6 solar masses with current radial separations of S1 and S2 from SgrA* of about $8-10 mpc$. This enclosed mass estimate is derived from individual stellar orbits as close to the massive black hole at the center of the Milky Way as currently possible. Although the uncertainties are large this estimate is fully consistent with the enclosed mass range of (2.6-3.3)x10^6 solar masses derived by Genzel et al. (2000) from radial and/or proper motion velocities of a homogenized sample of sources.

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