1,623 research outputs found
Low redshift star-forming galaxies: What can they teach us about primeval galaxies?
The analysis of the UV plus optical spectra of three star-forming galaxies, Mrk 496, Mrk 357, TOL1924-416, obtained by matching the size of the optical aperture with that of IUE, has given unexpected results. These can be summarized as follows: (1) the dereddened Ly(alpha)/H(beta) ratios are consistent with the prediction of case B recombination for nebular emission, within the uncertainties; (2) the decrease of the Ly(alpha)/H(beta) ratio with increasing metallicities is not confirmed in our three objects, although the sample is too small to consider this result definitive. The first result is surprising, mainly because at least the two Markarian galaxies have a large enough H1 content to markedly increase the optical depth for the Ly(alpha) photons and to trigger their absorption by dust. This finding can probably be explained as an effect of the inhomogeneous distribution of gas and dust within the galaxies. On the basis of these results, we conclude that the detection of the Ly(alpha) emission line in searching for primeval galaxies (PG's) can be still considered a valid technique
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Nearby Galaxies with Spitzer
We review the main advances brought by the Spitzer Space Telescope in the field of nearby galaxies studies, concentrating on a few subject areas, including: (1) the physics of the Polycyclic Aromatic Hydrocarbons that generate the mid–infrared features between 3.5 ìm and 20 ìm; (2) the use of the mid– and far–infrared emission from galaxies as star formation rate indicators; and (3) the improvement of mid–infrared diagnostics to discriminate between thermal (star–formation)and non–thermal (AGN) emission in galaxies and galaxy centers
Characterizing Dust Attenuation in Local Star-Forming Galaxies: Near-Infrared Reddening and Normalization
We characterize the near-infrared (NIR) dust attenuation for a sample of
~5500 local (z<0.1) star-forming galaxies and obtain an estimate of their
average total-to-selective attenuation . We utilize data from the
United Kingdom Infrared Telescope (UKIRT) and the Two Micron All-Sky Survey
(2MASS), which is combined with previously measured UV-optical data for these
galaxies. The average attenuation curve is slightly lower in the far-UV than
local starburst galaxies, by roughly 15%, but appears similar at longer
wavelengths with a total-to-selective normalization at V-band of
. Under the assumption of energy balance,
the total attenuated energy inferred from this curve is found to be broadly
consistent with the observed infrared dust emission () in a small
sample of local galaxies for which far-IR measurements are available. However,
the significant scatter in this quantity among the sample may reflect large
variations in the attenuation properties of individual galaxies. We also derive
the attenuation curve for sub-populations of the main sample, separated
according to mean stellar population age (via ), specific star
formation rate, stellar mass, and metallicity, and find that they show only
tentative trends with low significance, at least over the range which is probed
by our sample. These results indicate that a single curve is reasonable for
applications seeking to broadly characterize large samples of galaxies in the
local Universe, while applications to individual galaxies would yield large
uncertainties and is not recommended.Comment: 14 pages, 10 figures, 1 table. Accepted for publication in Ap
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A METHOD FOR MEASURING VARIATIONS IN THE STELLAR INITIAL MASS FUNCTION
We present a method for investigating variations in the upper end of the stellar initial mass function (IMF) by probing the production rate of ionizing photons in unresolved, compact star clusters with ages ≤107 yr and with different masses. We test this method by performing a pilot study on the young cluster population in the nearby galaxy NGC5194 (M51a), for which multi-wavelength observations from the Hubble Space Telescope are available. Our results indicate that the proposed method can probe the upper end of the IMF in galaxies located out to at least ~10 Mpc, i.e., a factor 200 further away than what is possible by counting individual stars in young compact clusters. Our results for NGC5194 show no obvious dependence of the upper mass end of the IMF on the mass of the star cluster down to 103 M , although more extensive analyses involving lower mass clusters and other galaxies are needed to confirm this conclusion
The Very Massive Star Content of the Nuclear Star Clusters in NGC 5253
The blue compact dwarf galaxy NGC 5253 hosts a very young starburst
containing twin nuclear star clusters, separated by a projected distance of 5
pc. One cluster (#5) coincides with the peak of the H-alpha emission and the
other (#11) with a massive ultracompact H II region. A recent analysis of these
clusters shows that they have a photometric age of 1+/-1 Myr, in apparent
contradiction with the age of 3-5 Myr inferred from the presence of Wolf-Rayet
features in the cluster #5 spectrum. We examine Hubble Space Telescope
ultraviolet and Very Large Telescope optical spectroscopy of #5 and show that
the stellar features arise from very massive stars (VMS), with masses greater
than 100 Msun, at an age of 1-2 Myr. We further show that the very high
ionizing flux from the nuclear clusters can only be explained if VMS are
present. We investigate the origin of the observed nitrogen enrichment in the
circum-cluster ionized gas and find that the excess N can be produced by
massive rotating stars within the first 1 Myr. We find similarities between the
NGC 5253 cluster spectrum and those of metal poor, high redshift galaxies. We
discuss the presence of VMS in young, star-forming galaxies at high redshift;
these should be detected in rest frame UV spectra to be obtained with the James
Webb Space Telescope. We emphasize that population synthesis models with upper
mass cut-offs greater than 100 Msun are crucial for future studies of young
massive star clusters at all redshifts.Comment: 11 pages, 7 figures, accepted for publication in Astrophysical
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