We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxiesWe combine the survey results with a detailed case study, based on arc-(L IR º 1012 L _ ). second resolution, near-IR, and millimeter imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near-and mid-IR characteristics of these ultraluminous galaxies to ISO and literature data of 30 starburst and active galactic nuclei (AGN) template galaxies. We Ðnd the following :

We analyze deep near-IR adaptive optics imaging (taken with NAOS/CONICA on the VLT) 1 as well as new proper motion data of the nuclear star cluster of the Milky Way. The surface density distribution of faint (H≤ 20, K s ≤ 19) 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 α ∼ 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 s ≤ 14. It fits population synthesis models of an old, metal rich stellar population with a contribution from young, early and late-type stars at the bright end. In contrast, the cusp within ≤ 1.5 ′′ of SgrA ⋆ appears to have a featureless luminosity function, suggesting that old, low mass horizontal branch/red clump stars are lacking. Likewise there appear to be fewer late type giants. The innermost cusp also contains a group of moderately bright, early type stars that are tightly bound to the black hole. We interpret these results as evidence that the stellar properties change significantly from the outer cluster (≥ a few arcsecs) to the dense innermost region around the black hole.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. We conclude that of the possible formation scenarios for these massive stars the most probable one is that 5-8 million years ago two clouds fell into the center, collided, were shock compressed and then formed two rotating (accretion) disks orbiting the central black hole. For the OB-stars in the central arcsecond, on the other hand, a stellar merger model is the most appealing explanation. These stars may thus be 'super-blue-stragglers', formed and 'rejuvenated' through mergers of lower mass stars in the very dense (≥ 10 8 M ⊙ pc −3 ) environment of the cusp. The 'collider model' also accounts for the lack of giants within the central few arcseconds.The star closest to SgrA ⋆ in 2002, S2, exhibits a 3.8 µm 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 S2.1 Based on observations obtained at the European Southern Observatory, Chile

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We report D1A resolution K-band (2 km) imaging spectroscopy of the central parsec of our Galaxy. The derived radial velocities for 223 early-and late-type stars probe the nuclear mass distribution to spatial scales of 0.1 pc. We Ðnd a statistically very signiÐcant increase of projected stellar velocity dispersion from about 55 km s~1 at p D 5 pc to 180 km s~1 at p D 0.1 pc. The stars are also rotating about the dynamic center. The late-type stars follow general Galactic rotation, while the early-type stars show counter-rotation. Fitting simultaneously the observed projected surface densities and velocity dispersions, we derive the intrinsic volume densities and radial velocity dispersions as a function of distance from the dynamic center for both types of stars. We then derive the mass distribution between 0.1 and 5 pc from the Jeans equation assuming an isotropic velocity Ðeld. Our analysis requires a compact central dark mass of 2.5È3.2 ] 106 at 6È8 p signiÐcance. The dark mass has a density of 109 pc~3 or M _ , M _ greater and a mass to 2 km luminosity of º 100. The increase in mass-to-luminosity ratio can be reduced but not eliminated even if extreme anisotropic velocity destributions are considered. The dark mass cannot be a cluster of solar mass remnants (such as neutron stars). It is either a compact cluster of 10È20 black holes or a single massive black hole. M _

We report near-infrared integral field spectroscopy of the luminous merging galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by 1. ′′ 6 are the central parts of inclined, rotating disk galaxies with equal mass bulges. The dynamical masses of the nuclei are much larger than the stellar mass derived from the K-band light, implying that the progenitor galaxies were galaxies with massive bulges. The K-band light is dominated by red supergiants formed in the two nuclei in starbursts, triggered ≈ 2× 10 7 years ago, possibly by the most recent perigalactic approach. Strong feedback effects of a superwind and supernovae are responsible for a short duration burst (≈ 5 × 10 6 years) which is already decaying. The two galaxies form a prograderetrograde rotating system and from the stellar velocity field it seems that one of the two interacting galaxies is subject to a prograde encounter. Between the stellar nuclei is a prominent peak of molecular gas (H 2 , CO). The stellar velocity dispersion peaks there indicating that the gas has formed a local, self-gravitating concentration decoupled from the stellar gravitational potential. NGC 6240 has previously been reported to fit the paradigm of an elliptical galaxy formed through the merger of two galaxies. This was based on the near-infrared light distribution which follows a r 1/4 -law. Our data cast strong doubt on this conclusion: the system is by far not relaxed, rotation plays an important role, as does self-gravitating gas, and the near-infrared light is dominated by young stars.NGC 6240 was observed with the MPE near-infrared imaging spectrograph 3D (Weitzel et al. 1996) in conjunction with the tip-tilt correction adaptive optics system ROGUE (Thatte et al. 1995) in two observing runs. 3D is an integral field spectrograph that simultaneously obtains spectra for each of 256 spatial pixels covering a square field of view with over 95% fill factor. In both observing runs the spectral resolving power (R ≡ λ/∆λ) was 2000 and Nyquist sampled using two settings of a piezo-driven flat mirror.The first observing run took place in April 1996 at the ESO 2.2 m telescope on La Silla, Chile. The pixel scale was 0. ′′ 3 per pixel, and a wavelength range from 2.18µm to 2.45µm was covered. The total on-source integration time was 4600 s with individual frame integration times of 300 s or 400 s. The same amount of time was spent off-source 1 ′ E and W of the nuclear region of NGC 6240 for sky background subtraction. The seeing during the observations was better than 0. ′′ 8.

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