arXiv subjects

Jorge Melnick

Publications and source records attributed to Jorge Melnick.

24 records · Page 2Linked to original sources

arXiv2009

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 $Γ=-1.1 \pm 0.2$ for the high-mass end ($M>10M_{\sun}$) 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 $Γ=-0.88\pm0.20$, which is incompatible with previous determinations. Within 0.4 pc we derive a total mass of $\sim2.0(\pm0.6)\times10^{4}M_{\sun}$ for the cluster and a central mass density $ρ= 2(\pm0.4)\times10^{5} M_{\sun}pc^{-3}$ that confirms Arches as the densest known young massive cluster in the Milky Way.

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arXiv2008

The scale-free character of the cluster mass function and the universality of the stellar IMF

Our recent determination of a Salpeter slope for the IMF in the field of 30 Doradus (Selman and Melnick 2005) appears to be in conflict with simple probabilistic counting arguments advanced in the past to support observational claims of a steeper IMF in the LMC field. In this paper we re-examine these arguments and show by explicit construction that, contrary to these claims, the field IMF is expected to be exactly the same as the stellar IMF of the clusters out of which the field was presumably formed. We show that the current data on the mass distribution of clusters themselves is in excellent agreement with our model, and is consistent with a single spectrum {\it by number of stars} of the type $n^β$ with beta between -1.8 and -2.2 down to the smallest clusters without any preferred mass scale for cluster formation. We also use the random sampling model to estimate the statistics of the maximal mass star in clusters, and confirm the discrepancy with observations found by Weidner and Kroupa (2006). We argue that rather than signaling the violation of the random sampling model these observations reflect the gravitationally unstable nature of systems with one very large mass star. We stress the importance of the random sampling model as a \emph{null hypothesis} whose violation would signal the presence of interesting physics.

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arXiv2006

The K-band properties of Seyfert 2 galaxies

It is well known that the [O {\sc iii}]$λ$5007 emission line and hard X-ray(2-10keV) luminosities are good indicators of AGN activities and that the near and mid-infrared emission of AGN originates from re-radiation of dusty clouds heated by the UV/optical radiation from the accretion disk. In this paper we present a study of the near-infrared K-band (2.2$μ$m) properties for a sample of 65 Seyfert 2 galaxies. By using the AGN/Bulge/Disk decomposition technique, we analyzed the 2MASS K$_{\rm S}$-band images for Seyfert 2 galaxies in order to derive the K$_{\rm S}$- band magnitudes for the central engine, bulge, and disk components. We find that the K$_{\rm S}$-band magnitudes of the central AGN component in Seyfert 2 galaxies are tightly correlated with the [O {\sc iii}]$λ$5007 and the hard X-ray luminosities, which suggests that the AGN K-band emission is also an excellent indicator of the nuclear activities at least for Seyfert 2 galaxies. We also confirm the good relation between the central black hole masses and bulge's K-band magnitudes for Seyfert 2s.

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arXiv2005

Emission Line Properties of Seyfert 2 Nuclei

We present a study of the global properties of Joguet's sample of 65 Seyfert 2 galaxies. We use the population synthesis models of our Paper II to derive `pure' emission-line spectra, and thus explore the statistical properties of the nuclear nebular components and their relation to the stellar populations. We find that the emission line clouds suffer substantially more extinction than the starlight, and confirm the correlations between stellar and nebular velocity dispersions and between emission line luminosity and velocity dispersions. Nuclear luminosities correlate with stellar velocity dispersions. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation, $L \propto σ_\star^4$, consistent with the Faber-Jackson law. Emission line ratios indicative of the gas excitation such as [OIII]/H$β$ and [OIII]/[OII] are statistically smaller for Seyferts with significant star-formation, implying that ionization by massive stars is responsible for a substantial, and sometimes even a dominant, fraction of the H$β$ and [OII] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden AGN. We correlate this fraction with classical indicators of AGN photo-ionization: X-ray luminosity and nebular excitation, but find no significant correlations. Thus, while there is a strong contribution of starbursts to the excitation of the nuclear nebular emission in low-luminosity Seyferts, the contribution of the hidden AGN remains elusive even in hard X-rays. (abridged)

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arXiv1999

HII Galaxies as deep cosmological probes

We re-investigated the use of the Hubble diagram to measure the cosmological constant ($Λ$) and the mass density of the Universe ($Ω_M$). We find an important focusing effect in $Λ$ for redshifts about 3. This effect implies that the apparent magnitude of a standard candle at redshifts z=2-3 has almost no dependence on $Λ$ for $Ω_M>0.2$. This means that $Ω_M$ can be measured independently of $Ω_Λ$ by targeting the redshift range according to an estimate of the value of $Ω_M$. We explore the evidence in support of the suggestion that extreme starburst galaxies also known as HII galaxies can be used as distance estimators over a wide range of redshifts and reaching very high values. We have compiled literature data of HII galaxies up to $z\sim 3$ and found a good correlation between their luminosity and velocity dispersion measured from their strong emission lines, thus confirming the correlation already known to exist for HII galaxies in the nearby Universe. Several systematic effects such as age, extinction, kinematics, and metallicity are discussed as well as the effects of different cosmologies.

astro-ph↗

arXiv1997

The Morphology of HII 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.

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