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The nature of luminous IRAS galaxies
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|Title:||The nature of luminous IRAS galaxies|
|Authors:||DePoy, Darren Lee|
|Keywords:||Galaxies -- Spectra|
Stars -- Formation
|Abstract:||A flux limited sample of 23 optically identified galaxies has been selected from Infrared Astronomical Satellite (IRAS) data that have far-infrared luminosities greater than ~5 x 10^10 L⊙ , substantially larger than those of normal spiral galaxies. If star formation is the cause of the large luminosities, then strong hydrogen recombination line emission should be present, although substantial extinction is probable. Accordingly, the sample has been observed for Bra (n = 5 → 4) hydrogen recombination line emission; 15 of the galaxies have had Brγ (n = 7 → 4) line measurements. The observations were made with multi-channel grating spectrometers, allowing high sensitivity, accurate photometric precision, and moderate velocity resolution. Sixteen of the sample galaxies were detected in one or both of the lines. Photometry through N (10 µm), Q (20 µm), and/or 25 µm broad-band filters has been obtained for 16 of the sample galaxies, while 13 have been measured through a 12.5 µm narrow-band filter. Further narrow-band filter observations have measured the 9.7 µm silicate absorption optical depth for 7 galaxies. The photometry, line ratios, and silicate optical depths have been used to correct the observed line strengths for the effects of extinction and spatially-extended emission. For comparison, the bright IRAS source 17138-1017 has also been studied. The galaxy is close to the Galactic plane and is identified with an optically uncataloged spiral galaxy at a redshift of 0.018 and an implied bolometric luminosity of -2.5 x 10^11 L⊙. The source has among the highest 60 um flux densities of any previously uncataloged galaxy. Strong [NII] and Hα, Brγ, and Brα hydrogen recombination lines are observed from the nuclear region with line profiles < 600 km s^-1 wide. The [NII]/Hα ratio is similar to that found in Galactic HII regions. Strong radio emission at 6 cm and 20 cm is observed that has structure which resembles that observed in B and I CCD images. Photometry of the nucleus from 1.25-25 µm has also been obtained; off-nuclear measurements show that the peak of the infrared emission is coincident with the radio and optical peaks and that the galaxy is extended at 10 µm. Many of the observed properties of the galaxy are due to extinction within the galaxy, which is heaviest around the center and which drops to the Galactic value well outside of the nucleus. Most of the galaxies, including IRAS 17138-1017, have line strengths that can be adequately explained by recombination of hydrogen ionized by the continuum radiation from high mass stars. In particular, the ratio of the galaxies' luminosity to the estimated number of ionizing continuum photons is what would be expected from a burst of star formation. The lines are also spectrally unresolved, as expected for a collection of low internal dispersion HII regions taking part in only galactic rotation. Therefore, most of the galaxies in the luminosity range observed appear to derive most of their luminosities from the formation of high mass stars. The high-mass star formation rate is so large that the initial mass function describing the star formation must have comparatively few low-mass stars. Most of the activity occurs with ~1 kpc of the nucleus, even in the most luminous examples. Molecular hydrogen emission was not detected from IRAS 17138-1017. Combined with results on other galaxies, this implies that extragalactic H2 emission is probably not associated with star formation. There are several galaxies that have weak lines and very few ionizing continuum photons for their luminosities, however. The best example is IC 4553 (= Arp 220). The Bra emission in IC 4553 is ~10 times smaller than expected from a starburst and has a very broad profile, with a FWHM of ~1200 km s^-1 and possibly even broader wings. NGC 6240 and the known Seyfert galaxies Mkn 231 and NGC 7469 also have relatively weak lines and broad profiles, with ~900 km s^-1 and ~1000 km s^-1 FWHM respectively. Several other galaxies have weak lines, but not conclusively broadened profiles (e.g., NGC 4418 and IC 694). These characteristics are contrary to what is expected from a starburst, but are similar to the observed properties of other Seyfert galaxies. Therefore, these objects may have their luminosities generated by an active nucleus similar to those found in Seyfert galaxies. Within the luminosity range observed, if the percentage of Seyfert activity in all infrared-luminous galaxies is the same as in the sample, then the total space density of galaxies with some Seyfert activity may be ~50% larger than hitherto measured. Even the galaxies that show some evidence for the presence of an AGN have an appreciable amount of their luminosities contributed by star formation. In particular, star formation and Seyfert-like nuclear activity contribute roughly equally to the total far-infrared luminosity observed from the sample galaxies.|
xi, 175 leaves, bound ill. 29 cm
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|Appears in Collections:||
Ph.D. - Astronomy|
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