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 present the Ðrst results of a spectroscopic survey of Lyman break galaxies (LBGs) in the nearinfrared aimed at detecting the emission lines of [O II], [O III], and Hb from the H II regions of normal star-forming galaxies at z^3. From observations of 15 objects with the Keck telescope and the Very Large Telescope augmented with data from the literature for an additional four objects, we reach the following main conclusions. The rest-frame optical properties of LBGs at the bright end of the luminosity function are remarkably uniform, their spectra are dominated by emission lines, [O III] is always stronger than Hb and [O II], and projected velocity dispersions are between 50 and 115 km s~1. Contrary to expectations, the star formation rates deduced from the Hb luminosity are on average no larger than those implied by the stellar continuum at 1500 presumably any di †erential extinction between A ; rest-frame optical and UV wavelengths is small compared to the relative uncertainties in the calibrations of these two star formation tracers. For the galaxies in our sample, the abundance of oxygen can only be determined to within 1 order of magnitude without recourse to other emission lines ([N II] and Ha), which are generally not available. Even so, it seems well established that LBGs are the most metalenriched structures at z^3, apart from quasi-stellar objects, with abundances greater than about 1/10 solar and generally higher than those of damped Lya systems at the same epoch. They are also signiÐ-cantly overluminous for their metallicities ; this is probably an indication that their mass-to-light ratios are low compared to present-day galaxies. At face value, the measured velocity dispersions imply virial masses of about 1010 within half-light radii of 2.5 kpc. The corresponding mass-to-light ratios, M _ M/L B 0.15 in solar units, are indicative of stellar populations with ages between 108 and 109 yr, consistent with the UV-optical spectral energy distributions. However, we are unable to establish conclusively whether or not the widths of the emission lines reÑect the motions of the H II regions within the gravitational potential of the galaxies, even though in two cases we see hints of rotation curves. All 19 LBGs observed show evidence for galactic-scale superwinds ; such outÑows have important consequences for regulating star formation, distributing metals over large volumes, and allowing Lyman continuum photons to escape and ionize the intergalactic medium.

Many galaxies are thought to have supermassive black holes at their centres-more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 +/- 1.5) x 10(6) solar masses (M(*)). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.

We present the evolution of the luminosity-size and stellar massYsize relations of luminous (L V k 3:4 ; 10 10 h À2 70 L) and massive (M Ã k 3 ; 10 10 h À2 70 M) galaxies in the last $11 Gyr. We use very deep near-infrared images of the Hubble Deep FieldYSouth and the MS 1054-03 field in the J s , H, and K s bands from FIRES to retrieve the sizes in the optical rest frame for galaxies with z > 1. We combine our results with those from GEMS at 0:2 < z < 1 and SDSS at z $ 0:1 to achieve a comprehensive picture of the optical rest-frame size evolution from z ¼ 0 to 3. Galaxies are differentiated according to their light concentration using the Sérsic index n. For less concentrated objects, the galaxies at a given luminosity were typically $3 AE 0:5 (AE2) times smaller at z $ 2:5 than those we see today. The stellar massYsize relation has evolved less: the mean size at a given stellar mass was $2 AE 0:5 times smaller at z $ 2:5, evolving proportionally to (1 þ z) À0:40AE0:06. Simple scaling relations between dark matter halos and baryons in a hierarchical cosmogony predict a stronger (although consistent within the error bars) than observed evolution of the stellar massYsize relation. The observed luminosity-size evolution out to z $ 2:5 matches well recent infall model predictions for Milky WayYtype objects. For low-n galaxies, the evolution of the stellar massYsize relation would follow naturally if the individual galaxies grow inside out. For highly concentrated objects, the situation is as follows: at a given luminosity, these galaxies were $2:7 AE 1:1 times smaller at z $ 2:5 (or, put differently, were typically $2:2 AE 0:7 mag brighter at a given size than they are today), and at a given stellar mass the size has evolved proportionally to (1 þ z) À0:45AE0:10 .

We use very deep near-infrared photometry of the Hubble Deep Field-South taken with ISAAC on the Very Large Telescope to identify a population of high-redshift galaxies with rest-frame optical colors similar to those of nearby galaxies. The galaxies are chosen by their infrared colors , aimed at selecting galaxies with J ϪK 1 2.3 s s redshifts above 2. When applied to our data set, we find 14 galaxies with , corresponding to a surface K ! 22.5 s density of arcmin. The photometric redshifts all lie above 1.9, with a median of 2.6 and an rms of Ϫ2 3 ‫ע‬ 0.8 0.7. The spectral energy distributions of these galaxies show a wide range. One is very blue in the rest-frame UV and satisfies the normal Lyman break criteria for high-redshift, star-forming galaxies. Others are quite red throughout the observed spectral range and are extremely faint in the optical, with a median. Hence, V p 26.6 these galaxies would not be included in photometric samples based on optical ground-based data, and spectroscopic follow-up is difficult. The spectral energy distributions often show a prominent break, identified as the Balmer break or the 4000 Å break. The median age is 1 Gyr when fitted with a constant star formation model with dust or 0.7 Gyr when fitted with a single burst model. Although significantly younger ages cannot be excluded when a larger range of models is allowed, the results indicate that these galaxies are among the oldest at these redshifts. The volume density to is half that of Lyman break galaxies at. Since the mass-to-light ratios K p 22.5 z ≈ 3 s of the red galaxies are likely to be higher, the stellar mass density is inferred to be comparable to that of Lyman break galaxies. These red galaxies may be the descendants of galaxies that started to form stars at very high redshifts, and they may evolve into the most massive galaxies at low redshift.

We present a low-resolution mid-infrared spectroscopic survey of an unbiased sample of 62 ultraluminous infrared galaxies (ULIRGs) z ¹ 0.3) using ISOPHOT-S on board the Infrared (L IR [ 1012L _ , Space Observatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 km polycyclic aromatic hydrocarbon (PAH) emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We Ðnd that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, for starburst-dominated ULIRGs is very L PAH /L IR , similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable e †ect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S ULIRGs in order to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems, and all show distorted morphologies. Of the 23 double-nuclei systems, 17 appear at linear separations between 4 and 14 kpc, with a mean separation of 6.5 kpc. Using the separations measured from our new near-infrared imaging as well as from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity. In fact, we Ðnd that systems in the early stages of merging may well put out maximum luminosity. We also Ðnd that the total mass of interstellar gas, as estimated from the CO (1 ] 0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we Ðnd that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

Abstract. We use mid-infrared spectral decomposition to separate the 6 µm mid-infrared AGN continuum from the host emission in the ISO low resolution spectra of 71 active galaxies and compare the results to observed and intrinsic 2−10 keV hard X-ray fluxes from the literature. We find a correlation between mid-infrared luminosity and absorption corrected hard X-ray luminosity, but the scatter is about an order of magnitude, significantly larger than previously found with smaller statistics. Main contributors to this scatter are likely variations in the geometry of absorbing dust, and AGN variability in combination with non-simultaneous observations. There is no significant difference between type 1 and type 2 objects in the average ratio of midinfrared and hard X-ray emission, a result which is not consistent with the most simple version of a unified scheme in which an optically and geometrically thick torus dominates the mid-infrared AGN continuum. Most probably, significant non-torus contributions to the AGN mid-IR continuum are masking the expected difference between the two types of AGN.

We present deep 3.6-8 mm imaging of the Hubble Deep Field-South with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We study distant red galaxies (DRGs) at z 1 2 selected by J s ϪK s 1 2.3 and compare them with a sample of Lyman break galaxies (LBGs) at z p 2-3. The observed UV-to-8 mm spectral energy distributions are fitted with stellar population models to constrain star formation histories and derive stellar masses. We find that 70% of the DRGs are best described by dust-reddened star-forming models and 30% are very well fitted with old and "dead" models. Using only the IϪK s and K s Ϫ4.5 mm colors, we can effectively separate the two groups. The dead systems are among the most massive at z ∼ 2.5 (mean stellar mass AM * S p 0.8 #10 11 M , ) and likely formed most of their stellar mass at z 1 5. To a limit of 0.5 #10 11 M , , their number density is ∼10 times lower than that of local early-type galaxies. Furthermore, we use the IRAC photometry to derive rest-frame near-infrared J, H, and K fluxes. The DRGs and LBGs together show a large variation (a factor of 6) in the rest-frame K-band mass-to-light ratios (M/L K ), implying that even a Spitzer 8 mm-selected sample would be very different from a mass-selected sample. The average M/L K of the DRGs is about 3 times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/L K values and ages of the two samples appear to correlate with derived stellar mass, with the most massive galaxies being the oldest and having the highest mass-to-light ratios, similar to what is found in the low-redshift universe.