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Petropoulou V.,Institute Astrofisica Of Andalucia Csic | Vilchez J.,Institute Astrofisica Of Andalucia Csic | Iglesias-Pramo J.,Institute Astrofisica Of Andalucia Csic | Iglesias-Pramo J.,Centro Astronomico Hispano Alemn | And 4 more authors.
Astrophysical Journal | Year: 2011

Spatially resolved spectroscopy has been obtained for a sample of 27 star-forming (SF) galaxies selected from our deep Hα survey of the Hercules cluster. We have applied spectral synthesis models to all emission-line spectra of this sample using the population synthesis code STARLIGHT and have obtained fundamental parameters of stellar components such as mean metallicity and age. The emission-line spectra were corrected for underlying stellar absorption using these spectral synthesis models. Line fluxes were measured and O/H and N/O gas chemical abundances were obtained using the latest empirical calibrations. We have derived the masses and total luminosities of the galaxies using available Sloan Digital Sky Survey broadband photometry. The effects of cluster environment on the chemical evolution of galaxies and on their mass-metallicity (MZ) and luminosity-metallicity (LZ) relations were studied by combining the derived gas metallicities, the mean stellar metallicities and ages, the masses and luminosities of the galaxies, and their existing H I data. Our Hercules SF galaxies are divided into three main subgroups: (1) chemically evolved spirals with truncated ionized-gas disks and nearly flat oxygen gradients, demonstrating the effect of ram-pressure stripping; (2) chemically evolved dwarfs/irregulars populating the highest local densities, possible products of tidal interactions in preprocessing events; and (3) less metallic dwarf galaxies that appear to be "newcomers" to the cluster and are experiencing pressure-triggered star formation. Most Hercules SF galaxies follow well-defined MZ and LZ sequences (for both O/H and N/O), though the dwarf/irregular galaxies located at the densest regions appear to be outliers to these global relations, suggesting a physical reason for the dispersion in these fundamental relations. The Hercules cluster appears to be currently assembling via the merger of smaller substructures, providing an ideal laboratory where the local environment has been found to be a key parameter in understanding the chemical history of galaxies. © 2011. The American Astronomical Society. All rights reserved.

Villegas T.A.,Institute Astrofisica Of Andalucia Csic | Alfaro E.J.,Institute Astrofisica Of Andalucia Csic | Cabrera-Cao J.,Institute Astrofisica Of Andalucia Csic | Cabrera-Cao J.,University of Seville | And 21 more authors.
Astronomical Journal | Year: 2010

This paper presents the characterization of the optical range of the ALHAMBRA photometric system, a 20 contiguous, equal-width, medium-band CCD system with wavelength coverage from 3500 Å to 9700 Å. The photometric description of the system is done by presenting the full response curve as a product of the filters, CCD, and atmospheric transmission curves, and using some first- and second-order moments of this response function. We also introduce the set of standard stars that defines the system, formed by 31 classic spectrophotometric standard stars which have been used in the calibration of other known photometric systems, and 288 stars, flux calibrated homogeneously, from the Next Generation Spectral Library (NGSL). Based on the NGSL, we determine the transformation equations between Sloan Digital Sky Survey ugriz photometry and the ALHAMBRA photometric system, in order to establish some relations between both systems. Finally, we develop and discuss a strategy to calculate the photometric zero points of the different pointings in the ALHAMBRA project. © 2010. The American Astronomical Society.

Wang J.,Pennsylvania State University | Wang J.,Harvard - Smithsonian Center for Astrophysics | Feigelson E.D.,Pennsylvania State University | Townsley L.K.,Pennsylvania State University | And 4 more authors.
Astrophysical Journal | Year: 2010

We present high spatial resolution Chandra X-ray images of the NGC2237 young stellar cluster on the periphery of the Rosette Nebula. We detect 168 X-ray sources, 80% of which have stellar counterparts in USNO, Two Micron All Sky Survey, and deep FLAMINGOS images. These constitute the first census of the cluster members with 0.2 ≲ M ≲ 2 MȮ. Star locations in near-infrared color-magnitude diagrams indicate a cluster age around 2Myr with a visual extinction of 1 ≲ AV ≲ 3 at 1.4kpc, the distance of the Rosette Nebula's main cluster NGC2244. We derive the K-band luminosity function and the X-ray luminosity function of the cluster, which indicate a population 400-600 stars. The X-ray-selected sample shows a K-excess disk frequency of 13%. The young ClassII counterparts are aligned in an arc 3 pc long suggestive of a triggered formation process induced by the O stars in NGC2244. The diskless Class III sources are more dispersed. Several X-ray emitting stars are located inside the molecular cloud and around gaseous pillars projecting from the cloud. These stars, together with a previously unreported optical outflow originating inside the cloud, indicate that star formation is continuing at a low level and the cluster is still growing. This X-ray view of young stars on the western side of the Rosette Nebula complements our earlier studies of the central cluster NGC2244 and the embedded clusters on the eastern side of the Nebula. The large-scale distribution of the clusters and molecular material is consistent with a scenario in which the rich central NGC2244 cluster formed first, and its expanding H II region triggered the formation of the now-unobscured satellite clusters Rosette Molecular Cloud (RMC) XA and NGC2237. A large swept-up shell material around the H II region is now in a second phase of collect-and-collapse fragmentation, leading to the recent formation of subclusters. Other clusters deeper in the molecular cloud appear unaffected by the Rosette Nebula expansion. © 2010. The American Astronomical Society. All rights reserved.

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