1,615 research outputs found

    Low redshift star-forming galaxies: What can they teach us about primeval galaxies?

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    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

    Characterizing Dust Attenuation in Local Star-Forming Galaxies: Near-Infrared Reddening and Normalization

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    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 k(λ)k(\lambda). 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 RV=3.67+0.44−0.35R_V=3.67\substack{+0.44 \\ -0.35}. 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 (LTIRL_{\rm{TIR}}) 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 Dn4000D_n4000), 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

    The Very Massive Star Content of the Nuclear Star Clusters in NGC 5253

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    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 Journa
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