arXiv subjects

F. Eisenhauer

Publications and source records attributed to F. Eisenhauer.

At least 217 records · Page 12Linked to original sources

arXiv2006

Variations in the spectral slope of Sgr A* during a NIR flare

We have observed a bright flare of Sgr A* in the near infrared with the adaptive optics assisted integral field spectrometer SINFON. Within the uncertainties, the observed spectrum is featureless and can be described by a power law. Our data suggest that the spectral index is correlated with the instantaneous flux and that both quantities experience significant changes within less than one hour. We argue that the near infrared flares from Sgr A* are due to synchrotron emission of transiently heated electrons, the emission being affected by orbital dynamics and synchrotron cooling, both acting on timescales of ~20 minutes.

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arXiv2006

The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation

We report the definite spectroscopic identification of 41 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ~80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick (<|h|/R>~0.14) rotating disks. These stellar disks have fairly sharp inner edges (R~1") and surface density profiles that scale as R^{-2}. We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the `counter-clockwise' disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (core density > 3x10^8 sunmass/pc^3), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ~6+/-2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ~1.5x10^4 sunmass. Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.

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arXiv2006

Atmospheric Turbulence Compensation with Laser Phase Shifting Interferometry

Laser guide stars with adaptive optics allow astronomical image correction in the absence of a natural guide star. Single guide star systems with a star created in the earth's sodium layer can be used to correct the wavefront in the near infrared spectral regime for 8-m class telescopes. For possible future telescopes of larger sizes, or for correction at shorter wavelengths, the use of a single guide star is ultimately limited by focal anisoplanatism that arises from the finite height of the guide star. To overcome this limitation we propose to overlap coherently pulsed laser beams that are expanded over the full aperture of the telescope, traveling upwards along the same path which light from the astronomical object travels downwards. Imaging the scattered light from the resultant interference pattern with a camera gated to a certain height above the telescope, and using phase shifting interferometry we have found a method to retrieve the local wavefront gradients. By sensing the backscattered light from two different heights, one can fully remove the cone effect, which can otherwise be a serious handicap to the use of laser guide stars at shorter wavelengths or on larger telescopes. Using two laser beams multiconjugate correction is possible, resulting in larger corrected fields. With a proper choice of laser, wavefront correction could be expanded to the visible regime and, due to the lack of a cone effect, the method is applicable to any size of telescope. Finally the position of the laser spot could be imaged from the side of the main telescope against a bright background star to retrieve tip-tilt information, which would greatly improve the sky coverage of the system.

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arXiv2005

GCIRS34W: An irregular variable in the Galactic Centre

We report the results of time-resolved photometric and spectroscopic near-infrared observations of the Ofpe/WN9 star and LBV candidate GCIRS34W in the Galactic Centre star cluster. Diffraction limited resolution photometric observations obtained in H and K bands show a strong, non-periodic variability on time scales from months to years in both bands accompanied by variations of the stellar colour. Three K band spectra obtained in 1996, 2003 and 2004 with integral field spectrometers are identical within their accuracies and exclude significant spectroscopic variability. The most probable explanation of the stellar photometric variability is obscuration by circumstellar material ejected by the star. The approximated position of GCIRS34W in a HR diagram is located between O supergiants and LBVs, suggesting that this star is a transitional object between these two phases of stellar evolution.

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arXiv2005

GRAVITY: The AO-Assisted, Two-Object Beam-Combiner Instrument

We present the proposal for the infrared adaptive optics (AO) assisted, two-object, high-throughput, multiple-beam-combiner GRAVITY for the VLTI. This instrument will be optimized for phase-referenced interferometric imaging and narrow-angle astrometry of faint, red objects. Following the scientific drivers, we analyze the VLTI infrastructure, and subsequently derive the requirements and concept for the optimum instrument. The analysis can be summarized with the need for highest sensitivity, phase referenced imaging and astrometry of two objects in the VLTI beam, and infrared wavefront-sensing. Consequently our proposed instrument allows the observations of faint, red objects with its internal infrared wavefront sensor, pushes the optical throughput by restricting observations to K-band at low and medium spectral resolution, and is fully enclosed in a cryostat for optimum background suppression and stability. Our instrument will thus increase the sensitivity of the VLTI significantly beyond the present capabilities. With its two fibers per telescope beam, GRAVITY will not only allow the simultaneous observations of two objects, but will also push the astrometric accuracy for UTs to 10 micro-arcsec, and provide simultaneous astrometry for up to six baselines.

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arXiv2005

Near IR diffraction-limited integral-field SINFONI spectroscopy of the Circinus galaxy

Using the adaptive optics assisted near infrared integral field spectrometer SINFONI on the VLT, we have obtained observations of the Circinus galaxy on parsec scales. The morphologies of the H_2(1-0)S(1) 2.12um and Br_gamma 2.17um emission lines are only slightly different, but their velocity maps are similar and show a gradient along the major axis of the galaxy, consistent with rotation.Since V_rot/sigma is approximately 1 suggests that random motions are also important, it is likely that the lines arise in a rotating spheroid or thickened disk around the AGN. Comparing the Br_gamma flux to the stellar continuum indicates that the star formation in this region began almost 10^8 yr ago. We also detect the [SiVI] 1.96um,[AlIX] 2.04um and [CaVIII] 2.32um coronal lines. In all cases we observe a broad blue wing, indicating the presence of two or more components in the coronal line region. A correlation between the ionisation potential and the asymmetry of the profiles was found for these high excitation species.

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arXiv2005

SINFONI in the Galactic Center: young stars and IR flares in the central light month

We report 75 milli-arcsec resolution, near-IR imaging spectroscopy within the central 30 light days of the Galactic Center [...]. To a limiting magnitude of K~16, 9 of 10 stars in the central 0.4 arcsec, and 13 of 17 stars out to 0.7 arcsec from the central black hole have spectral properties of B0-B9, main sequence stars. [...] all brighter early type stars have normal rotation velocities, similar to solar neighborhood stars. We [...] derive improved 3d stellar orbits for six of these S-stars in the central 0.5 arcsec. Their orientations in space appear random. Their orbital planes are not co-aligned with those of the two disks of massive young stars 1-10 arcsec from SgrA*. We can thus exclude [...] that the S-stars as a group inhabit the inner regions of these disks. They also cannot have been located/formed in these disks [...]. [...] we conclude that the S-stars were most likely brought into the central light month by strong individual scattering events. The updated estimate of distance to the Galactic center from the S2 orbit fit is Ro = 7.62 +/- 0.32 kpc, resulting in a central mass value of 3.61 +/- 0.32 x 10^6 Msun. We happened to catch two smaller flaring events from SgrA* [...]. The 1.7-2.45 mum spectral energy distributions of these flares are fit by a featureless, red power law [...]. The observed spectral slope is in good agreement with synchrotron models in which the infrared emission comes from [...] radiative inefficient accretion flow in the central R~10 Rs region.

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arXiv2004

Census of the Galactic Centre early-type stars using spectro-imagery

The few central parsecs of the Galaxy are known to contain a surprising population of early-type stars, including at least 30 Wolf-Rayet stars and luminous blue variables (LBV), identified thanks to their strong emission lines. Despite the presence of emission from ionised interstellar material in the same lines, the latest advances in spectro-imaging have made it possible to use the absorption lines of the OB stars to characterise them as well. This stellar population is particularly intriguing in the deep potential well of the 4 million solar mass black hole Sgr A*. We will review the properties of these early-type stars known from spectro-imagery, and discuss possible formation scenarios.

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arXiv2004

SPIFFI observations of the starburst SMMJ14011+0252: Already old, fat, and rich by z = 2.565

Using the SPectrometer for Infrared Faint Field Imaging (SPIFFI) on the ESO VLT, we have obtained J, H, and K band integral field spectroscopy of the z = 2.565 luminous submillimeter galaxy SMMJ14011+0252. A global spectrum reveals the brighter of this spatially resolved system's two components as an intense starburst that is remarkably old, massive, and metal-rich for the early epoch at which it is observed. We see a strong Balmer break implying a > 100 Myr timescale for continuous star formation, as well as nebular emission line ratios implying a supersolar oxygen abundance on large spatial scales. Overall, the system is rapidly converting a large baryonic mass into stars over the course of only a few hundred Myr. Our study thus adds new arguments to the growing evidence that submillimeter galaxies are more massive than Lyman break galaxies, and more numerous at high redshift than predicted by current semi-analytic models of galaxy evolution.

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arXiv2004

Submillimeter Galaxies as Tracers of Mass Assembly at Large M

Luminous submillimeter galaxies (SMGs) have been known for some time to produce a substantial fraction of the far-IR/submillimeter background. New observations show that these systems are not only very luminous, but also very massive. Here we present results from (a) a survey of SMGs' molecular line emission, which has confirmed Keck optical redshifts and revealed their high gas and dynamical masses; and (b) a detailed case study of the rest-frame optical emission from one SMG, which has revealed its high metallicity and old stellar population. We infer a high z~2.7 comoving number density of galaxies with baryonic masses greater than 10^11 solar masses; this is not accounted for in current semi-analytic models of galaxy evolution.

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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 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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arXiv2001

Scientific Potential of Enhancing the Integral-Field Spectrometer SPIFFI with a Large Detector and High Spectral Resolution

SPIFFI is the near-infrared integral-field spectrometer for the VLT. Assisted by the SINFONI adaptive optics module, the instrument will be offered to the astronomical community in 2004. We outline the scientific rationale for infrared integral-field spectroscopy at the VLT, and specifically for the enhancement of SPIFFI with a larger detector and higher spectral resolution gratings. We give examples of a broad variety of astronomical research which will gain specifically from the high angular and spectral resolution provided by SPIFFI, including studies of high red-shift galaxies, merging galaxies, starburst galaxies, superstar clusters, galactic nuclei, extra-solar planets, and circum-stellar discs.

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arXiv2001

CROMOS: A cryogenic near-infrared, multi-object spectrometer for the VLT

We discuss a cryogenic, multi-object near-infrared spectrometer as a second generation instrument for the VLT. The spectrometer combines 20 to 40 independent integral eld units (IFUs), which can be positioned by a cryogenic robot over the entire unvignetted eld of the VLT (~7'). Each IFU consists of a contiguous cluster of 20 to 30 pixels (0.15 to 0.25" per pixel). The individual IFUs have cold fore-optics and couple into the spectrograph with integrated bers-microlenses. The spectrometer has lambda/d-lambda~4000 and simultaneously covers the J-, H-, and K-bands with three HAWAII 2 detectors. The system is designed for operation both in seeing limited and MCAO modes. Its speed is approximately 3500 times greater than that of ISAAC and 60 times greater than NIRMOS (in H-band). The proposed instrument aims at a wide range of science, ranging from studies of galaxies/clusters in the high-z Universe (dynamics and star formation in z>1 galaxies, evolution of ellipticals, properties of distant, obscured far-IR and X-ray sources), to investigations of nearby starbursts, star clusters and properties of young low mass stars and brown dwarfs.

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arXiv2001

Multi-Conjugate Adaptive Optics with Laser Guide Stars

The Laser Guide Star Facility for the VLT will be commissioned during 2003. This, of course, can only be the first step towards enhancing the scientific output of the VLT through adaptive optics. So in this contribution we propose the development of a laser guide star multi-conjugate adaptive optics system. We consider geometries for the adaptive optics system, and discuss the technology needed to project up to 5 laser beacons. The rationale for such a project is provided by an outline of two primary science drivers.

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arXiv2001

NGC 3603 - a Local Template for Massive Young Clusters

We present a study of the star cluster associated with the massive Galactic HII region NGC3603 based on near-IR broad-- and narrowband observations taken with ISAAC/VLT under excellent seeing conditions (<0.4''). We discuss color-color diagrams and address the impact of the high UV flux on the disk evolution of the low-mass stars.

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arXiv2001

Adaptive Optics Integral Field Spectroscopy of the Young Stellar Objects in LkH_alpha 225

Progress in understanding the embedded stars in LkHa225 has been hampered by their variability, making it hard to compare data taken at different times, and by the limited resolution of the available data, which cannot probe the small scales between the two stars. In an attempt to overcome these difficulties, we present new near-infrared data on this object taken using the ALFA adaptive optics system with the MPE 3D integral field spectrometer and the near-infrared camera Omega-Cass. The stars themselves have K-band spectra which are dominated by warm dust emission, analagous to class I-II for low mass YSOs, suggesting that the stars are in a phase where they are still accreting matter. On the other hand, the ridge of continuum emission between them is rather bluer, suggestive of extincted and/or scattered stellar light rather than direct dust emission. The compactness of the CO emission seen toward each star argues for accretion disks (which can also account for much of the K-band veiling) rather than a neutral wind. In contrast to other YSOs with CO emission, LkHa225 has no detectable Br_gamma emission. Additionally there is no H_2 detected on the northern star, although we do confirm that the strongest H_2 emission is on the southern star, where we find it is excited primarily by thermal mechanisms. A second knot of H_2 is observed to its northeast, with a velocity shift of -75kms and a higher fraction of non-thermal emission. This is discussed with reference to the H2O maser, the molecular outflow, and [S II] emission observed between the stars.

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