Research

The Role of Interactions in the Evolution of Early-type Spirals

We present a search for the signatures of galaxy-galaxy interactions in the neutral gas of early-type spirals. New neutral hydrogen observations for four highly star-forming early-type spirals are presented here, along with HI data for three additional galaxies from other sources. HI maps of six of seven galaxies reveal unambiguous signs of a recent encounter, via tidal tails and HI bridges. Most of these galaxies appear undisturbed in the optical, and these interactions probably would have gone unnoticed without HI mapping. Such high rates of interaction suggest that galactic encounters may play an important role in the evolution of early-type spiral galaxies.

Keywords: galaxies evolution, galaxies nteractions, ISM, spiral galaxies

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The Discovery of a Giant Ionized Filament in NGC 7213

The nearby Seyfert galaxy NGC 7213 has been imaged in Ha and H? with the Cerro Tololo Inter-American Observatory 1.5 m telescope and with the Australia Telescope Compact Array, respectively. Optically, NGC 7213 looks undisturbed and relatively featureless, but the continuum-subtracted Ha image shows a 19 kpc long filament located approximately 18.6 kpc from the nucleus. The H? filament could be neutral gas photoionized by the active nucleus, as has been suggested for the Seyfert galaxy NGC 5252 or shock-ionized by a jet interacting with the surrounding H?, as has been suggested for the radio galaxy PKS 2240241. The H? map reveals NGC 7213 to be a highly disturbed system, suggesting a past merging event.

Keywords: galaxies, NGC 7213, peculiar, Seyfert, spiral, PKS 2240241

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H-alpha Imaging of Early-type (Sa-Sab) Spiral Galaxies I.

H? and continuum images are presented for 27 nearby early-type (SaS–ab) spiral galaxies. Contrary to popular perception, the images reveal copious massive star formation in some of these galaxies. A determination of the H? morphology and a measure of the H? luminosity suggest that early-type spirals can be classified into two broad categories based on theluminosity of the largest H II region in the disk. The first category includes galaxies for which the individual H II regions have LH? < 1039 ergs s-1. Most of the category 1 galaxies appear to be morphologically undisturbed but show a wide diversity in nuclear H? properties. The second category includes galaxies that have at least one H II region in the disk with LH?   1039 ergs s-1. All category 2 galaxies show either prominent dust lanes or other morphological peculiarities such as tidal tails, which suggests that the anomalously luminous H IIregions in category 2 galaxies may have formed as a result of a recent interaction. The observations, which are part of an ongoing H? survey, reveal early-type spirals to be a heterogeneous class of galaxies that are evolving in the current epoch. We have also identified some systematic differences between the classifications of spiral galaxies in the Second General Catalog and the Revised Shapley-Ames Catalog that may be traced to subtle variations in the application of the criteria used for classifying spiral galaxies. An examination of earlier studies suggests that perceptions concerning the Hubble-type dependence of star formation rates among spiral galaxies depends on the choice of catalog.

Keywords: galaxies interactions, galaxies, spiral, HII regions, stars, stars formation

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Massive Star Formation Along the Hubble Sequence

Far-infrared fluxes have been extracted from the IRAS database for galaxies in the Nearby Galaxies Catalog. The IRAS data are used to determine the Hubble type dependance of the far-infrared luminosity, the L(FIR)/L(Blue) luminosity ratio and, for a subset of the galaxies, the L(FIR)/M(H2) ratio. Adopting the far-infrared luminosity as a measure of a massive star formation rate, one finds that the IRAS data reveal a previously unexpected populatin of early-type (Sa-Sub) spirals with massive star formation rates that rival the most prodigiously star-forming Sc galaxies. Thus, the IRAS data do not support the distinction between the global star-forming capabilities of early- and late-type spirals that have been reported previously in the published literature.

Keywords: IRAS database, star-forming galaxies,far-infrared luminosity

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