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The Interstellar Medium of Star-forming Irregular Galaxies: The View with ISO
We present mid-infrared imaging and far-infrared (FIR) spectroscopy offive IBm galaxies observed by ISO as part of our larger study of theinterstellar medium of galaxies. Most of the irregulars in our sampleare very actively forming stars, and one is a starburst system. Thus,most are not typical Im galaxies. The mid-infrared imaging was in a bandcentered at 6.75 μm that is dominated by polycyclic aromatichydrocarbons (PAHs) and in a band centered at 15 μm that is dominatedby small dust grains. The spectroscopy of three of the galaxies includes[C II] λ158 μm and [O I] λ63 μm, important coolantsof photodissociation regions (PDRs), and [O III] λ88 μm and [NII] λ122 μm, which come from ionized gas. [O I] λ145μm and [O III] λ52 μm were measured in one galaxy as well.These data are combined with PDR and H II region models to deduceproperties of the interstellar medium of these galaxies. We find adecrease in PAH emission in our irregulars relative to small grain, FIR,and Hα emissions for increasing FIR color temperature, which weinterpret as an increase in the radiation field due to star formationresulting in a decrease in PAH emission. Thef15/fHα ratio is constant for ourirregulars, and we suggest that the 15 μm emission in theseirregulars is being generated by the transient heating of small dustgrains by single-photon events, possibly Lyα photons trapped in HII regions. The low f15/fHα ratio, as wellas the high f[CII]/f15 ratio, in our irregularscompared to spirals may be due to the lower overall dust content,resulting in fewer dust grains being, on average, near heating sources.We find that, as in spirals, a large fraction of the [C II] emissioncomes from PDRs. This is partly a consequence of the high averagestellar effective temperatures in these irregulars. However, ourirregulars have high [C II] emission relative to FIR, PAH, and smallgrain emission compared to spirals. If the PAHs that produce the 6.75μm emission and the PAHs that heat the PDR are the same, then themuch higher f[CII]/f6.75 ratio in irregulars wouldrequire that the PAHs in irregulars produce several times more heat thanthe PAHs in spirals. Alternatively, the carrier of the 6.75 μmfeature tracks, but contributes only a part of, the PDR heating, that isdue mostly to small grains or other PAHs. In that case, our irregularswould have a higher proportion of the PAHs that heat the PDRs comparedto the PAHs that produce the 6.75 μm feature. The highf[OIII]/f[CII] ratio may indicate a smaller solidangle of optically thick PDRs outside the H II regions compared tospirals. The very high L[CII]/LCO ratios among oursample of irregulars could be accounted for by a very thick [C II] shellaround a tiny CO core in irregulars, and PDR models for one galaxy areconsistent with this. The average densities of the PDRs andfar-ultraviolet stellar radiation fields hitting the PDRs are muchhigher in two of our irregulars than in most normal spirals; the thirdirregular has properties like those in typical spirals. We deduce thepresence of several molecular clouds in each galaxy with masses muchlarger than typical GMCs. Based on observations made with ISO, an ESAproject with instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom),and with the participation of ISAS and NASA.
| The supergalactic plane redshift survey
Redshift measurements, about 1000 of which are new, are presented for1314 galaxies in a survey toward the apex of the large-scale streamingflow for ellipticals. The velocity histogram shows that the excess ingalaxy number counts in this area is due to a substantial concentrationof galaxies with discrete peaks at V about 3000 km/s and V about 4500km/s. After correction for the sampling function, the centroid of thedensity distribution is found to be near V about 4500 km/s.Normalization to the more extensive SSRS survey, which was selected bythe same criteria, shows that the region studied contains a considerableoverdensity of galaxies from 2000 to 6000 km/s. This result is in goodagreement with the 'great attractor' model suggested by Lynden-Bell etal. (1988) which attributes the peculiar motions of elliptical galaxiesover a large region of space to an extensive mass overdensity whichincludes the Hydra-Centaurus and Pavo-Indus superclusters. The centroidof the density enhancement is also consistent with new data by Dresslerand Faber (1990) of peculiar motions of elliptical and spiral galaxies,both of which show a zero crossing of the Hubble line at approximately4500-5000 km/s.
| Southern Galaxy Catalogue.
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