Browsing by Author "Melnick, J."

Now showing 1 - 6 of 6
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    Diffuse intracluster light at intermediate redshifts: intracluster light observations in an X-ray cluster at z=0.29
    (2011) Toledo, I.; Melnick, J.; Selman, F.; Quintana, H.; Giraud, E.; Zelaya, P.
    The diffuse intracluster light (ICL) contains a significant fraction of the total stellar mass in clusters of galaxies, and contributes in roughly equal proportion as the hot intracluster medium to the total baryon content of clusters. Because of the potential importance of understanding the origin of the ICL in the context of the formation and evolution of structure in the Universe, the field has recently undergone a revival both in the quality and quantity of observational and theoretical investigations. Because of cosmological dimming, the observational work has mostly concentrated on low-redshift clusters, but clearly observations at higher redshifts can provide interesting clues about the evolution of the diffuse component. In this paper we present the first results of a programme to characterize the ICL of intermediate-redshift clusters. We find that at z similar to 0.3, the X-ray cluster RXJ0054.0-2823 already has a significant ICL and that the fraction of the total light in the ICL and the brightest cluster galaxy (BCG) is comparable to that of similar clusters at lower redshift. We also find that the kinematics of the ICL is consistent with it being the remnant of tidally destroyed galaxies streaming in the central regions of the cluster, which has three central giant elliptical galaxies acting as an efficient 'galaxy grinding machine'. Our cluster has a bi-modal radial velocity distribution and thus two possible values for the velocity dispersion. We find that the cluster fits well in the correlation between BCG+ICL fraction and cluster mass for a range of velocity dispersions, leading us to question the validity of a relevant correlation between these two quantities.
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    Galaxy Evolution in a Pilot Survey up to z=1 and CDM Halos
    (2011) Giraud, E.; Melnick, J.; Gu, Q. -S.; Quintana, H.; Selman, F.; Toledo, I.; Zelaya, P.
    We study spectral evolution of galaxies in a magnitude limited sample of 550 galaxies from z = 1 down to z = 0.3 on a pencil beam of approximate to 10' x 10'. We concentrate on the large apparent cosmological structures along the line of sight, and we bin our individual galaxy spectra to obtain representative high S/N spectra based upon these structures. We divide the resulting average spectra in three groups to facilitate the analysis: galaxies with pure absorption line spectra, galaxies with emission lines and blue continua, and galaxies with emission lines and red continua. We revisit the question of downsizing in emission-line galaxies between z = 0.9 and z = 0.45 in our pencil-beam and find the following results: strong star formation in emission line galaxies, aging in emission line galaxies, and aging in absorption systems, are shifting from bright to faint systems as cosmological time increases. Each redshift bin is repopulated in new starbursts. Therefore at redshifts z <= 1 galaxy formation is downsizing both in luminosity and number density. Our observations indicate that at z <= 1 star formation and hierarchical structure formation of CDM halos are not in phase.
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    Integral field near-infrared spectroscopy of II Zw 40
    (EDP SCIENCES S A, 2008) Vanzi, L.; Cresci, G.; Telles, E.; Melnick, J.
    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 [FeII]. We find that [FeII] and H-2 are mostly photon excited, but while the region emitting [FeII] 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.
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    Intergalactic stellar populations in intermediate redshift clusters
    (2012) Melnick, J.; Toledo, Ignacio; Quintana Godoy, Hernán
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    The massive star initial mass function of the Arches cluster
    (2009) Espinoza, P.; Selman, F. J.; Melnick, J.
    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 Gamma = -1.1 +/- 0.2 for the high-mass end (M > 10 M-circle dot) 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 Gamma = -0.88 +/- 0.20, which is incompatible with previous determinations. Within 0.4 pc we derive a total mass of similar to 2.0(+/- 0.6) x 10(4) M-circle dot for the cluster and a central mass density rho = 2(+/- 0.4) x 10(5) M-circle dot pc(-3) that confirms Arches as the densest known young massive cluster in the Milky Way.