Jorge Melnick

The massive star initial mass function of the Arches cluster

The massive Arches cluster near the Galactic center should be an ideal laboratory for investigating massive star formation under extreme conditions. But it comes at a high price: the cluster is hidden behind several tens of magnitudes of visual extinction. Severe crowding requires space or AO-assisted instruments to resolve the stellar populations, and even with the best instruments interpreting the data is far from direct. Several investigations using NICMOS and the most advanced AO imagers on the ground revealed an overall top-heavy IMF for the cluster, with a very flat IMF near the center. There are several effects, however, that could potentially bias these results, in particular the strong differential extinction and the problem of transforming the observations into a standard photometric system in the presence of strong reddening. We present new observations obtained with the NAOS-Conica (NACO) AO-imager on the VLT. The problem of photometric transformation is avoided by working in the natural photometric system of NACO, and we use a Bayesian approach to determine masses and reddenings from the broad-band IR colors. A global value of Γ= −1.1 ± 0. 2f or the high-mass end (M > 10 M� ) of the IMF is obtained, and we conclude that a power law of Salpeter slope cannot be discarded for the Arches cluster. The flattening of the IMF towards the center is confirmed, but is less severe than previously thought. We find Γ= −0.88 ± 0.20, which is incompatible with previous determinations. Within 0. 4p c we derive at otal mass of∼2.0(±0.6) × 10 4 M�

The morphology of H II galaxies

We present CCD images of a sample of 39 HII galaxies taken at the Danish 1.54m telescope on La Silla. The images are used to analyse the morphology of these emission line dwarfs, and the structural properties of the knots of star formation and of the underlying galaxy. The sizes of the starbursts are measured. We propose a morphological classification based on the presence or absence of signs of tails, extensions, or distorted outer isophotes. This criterion segregates the objects into two broad morphological types with different physical properties: the more disturbed and extended (type I) HII galaxies having larger luminosities and velocity dispersions than the more compact and regular (type II) objects. The relative position of HII galaxies and of a sample of dwarf elliptical galaxies in the [R -- sigma] diagram support the hypothesis of a possible evolutionary link between the two types of galaxy.

Seyfert 1 Mutation of the Classical Seyfert 2 Nucleus NGC 7582

We report the transition toward a type 1 Seyfert experienced by the classical type 2 Seyfert nucleus in NGC 7582. The transition, found at most 20 days from its maximum peak, presents a unique opportunity to study these rare events in detail. At maximum, the Ha line width is about 12,000 km s 21 . We examine three scenarios that could potentially explain the transition: capture of a star by a supermassive black hole, a reddening change in the surrounding torus, and the radiative onset of a Type IIn supernova exploding in a compact nuclear/ circumnuclear starburst. Subject headings: galaxies: active — galaxies: individual (NGC 7582) — galaxies: Seyfert — galaxies: starburst — supernova remnants

Integral field near-infrared spectroscopy of II Zw 40

We present integral field spectroscopy in the near-infrared of the nearby starburst galaxy II Zw 40. Our new observations provide an unprecedented detailed view of the interstellar medium and star formation of this galaxy. The radiation emitted by the galaxy is dominated by a giant HII region, which extends over an area of more than 400 pc in size. A few clusters are present in this area, however, one, in particular, appears to be the main source of ionizing photons. We derive the properties of this object and compare them with those of the 30 Doradus cluster in the Large magellanic cloud (LMC). We study the spatial distribution and velocity field of different components of the inetrstellar medium (ISM), mostly through the Bracket series lines, the molecular hydrogen spectrum, and [Fell]. We find that [Fell] and H 2 are mostly photon excited, but while the region emitting [Fell] is almost coincident with the giant HII region observed in the lines of atomic H and He, the H 2 has a quite different distribution in space and velocity. The age of the stellar population in the main cluster is such that no supernova (SN) should be present yet so that the gas kinematics must be dominated by the young stars. We do not see, in the starbursting region, any geometrical or dynamical structure that can be related to the large scale morphology of the galaxy.

The ionising cluster of 30 Doradus ? IV. Stellar kinematics

On the basis of multislit spectroscopy of 180 stars in the ionising cluster of 30 Doradus we present reliable radial velocities for 55 stars. We calculate a radial velocity dispersion of ~35 km/s for the cluster and we analyse the possible influence of spectroscopic binaries in this rather large velocity dispersion. We use numerical simulations to show that the observations are consistent with the hypothesis that all the stars in the cluster are binaries, and the total mass of the cluster is ~5E+5 solar masses. A simple test shows only marginal evidence for dynamical mass segregation which if present is most likely not due to dynamical relaxation.

Population synthesis of H ii galaxies

We study the stellar population of galaxies with active star formation, determining ages of the stellar components by means of spectral population synthesis of their absorption spectra. The data consist of optical spectra of 185 nearby (

APEX: the Atacama Pathfinder EXperiment

z \leq 0.075

Stellar populations in Seyfert 2 galaxies I. Atlas of near-UV spectra ?;??

) emission line galaxies. They are mostly HII galaxies, but we also include some Starbursts and Seyfert 2s, for comparison purposes. They were grouped into 19 high signal-to-noise ratio template spectra, according to their continuum distribution, absorption and emission line characteristics. The templates were then synthesized with a star cluster spectral base. The synthesis results indicate that HII galaxies are typically age-composite stellar systems, presenting important contribution from generations up to as old as 500 Myr. We detect a significant contribution of populations with ages older than 1 Gyr in two groups of HII galaxies. The age distributions of stellar populations among Starbursts can vary considerably despite similarities in the emission line spectra. In the case of Seyfert 2 groups we obtain important contributions of old population, consistent with a bulge. From the diversity of star formation histories, we conclude that typical HII galaxies in the local universe are not systems presently forming their first stellar generation.

Very Large Telescope and Hubble Space Telescope Observations of the Host Galaxy of GRB 990705

APEX, the Atacama Pathfinder Experiment, has been successfully commissioned and is in operation now. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the worlds outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of 17-18 μm makes APEX suitable for observations up to 200 μm, through all atmospheric submm windows accessible from the ground.

The Scale-Free Character of the Cluster Mass Function and the Universality of the Stellar Initial Mass Function

We have carried out a uniform spectroscopic survey of Seyfert 2 galaxies to study the stellar populations of the host galaxies. New spectra have been obtained for 79 Southern galaxies classied as Seyfert 2 galaxies, 7 normal galaxies, and 73 stars at a resolution of 2.2 A over the wavelength region 3500{5300 A. Cross-correlation between the stellar spectra is performed to group the individual observations into 44 synthesis standard spectra. The standard groups include a solar abundance sequence of spectral types from O5 to M3 for dwarfs, giants, and supergiants. Metal-rich and metal-weak F-K giants and dwarfs are also included. A comparison of the stellar data with previously published spectra is performed both with the individual spectra and the standard groups. For each galaxy, two distinct spatial regions are considered: the nucleus and the external bulge. Spectroscopic variations from one galaxy to another and from the central to the external region are briefly discussed. It is found that the central region of a Seyfert 2 galaxy, after subtracting the bulge stellar population, always shows a near-UV spectrum similar to one of three representative categories: a) many strong emission lines and only two visible absorption lines (Ca ii Ka ndG band) (Sey2e); b) few emission lines, many absorption lines, and a redder continuum than the previous category (Sey2a); c) an almost flat continuum and high-order Balmer lines seen in absorption (Sey2b). The proportion of Seyfert 2 galaxies belonging to each class is found to be 22%, 28%, and 50% respectively. We nd no signicative dierences between morphology distributions of Seyfert 2 galaxies with Balmer lines detected in absorption and the rest of the sample. This quick look through the atlas indicates that half of Seyfert 2 galaxies harbour a young stellar population (about or less than 100 Myr) in their central region, clearly unveiled by the high order Balmer series seen in absorption.