Structure and Evolution of Irregular Galaxies

Abstract

INTRODUCTIONIrregular galaxies (Irrs) usually are smaller, less massive, and optically dimmer than commonly studied giant spirals, SOs, and classical ellipticals. At first giance they also appear to be rare objects that make up only a few percent of the major bright galaxy catalogs (87, 300). Is there then much reward in pursuing such faint and elusive galaxies? The answer to this question turns out to be a surprisingly strong "yes." An examination of more nearly complete samples of galaxies (213,341,366) reveals that Irrs account for it substantial (1/3-1/2) fraction of all galaxies, and that they certainly are dominant by number density among actively star-forming galaxies. A diversity of nearby examples therefore abound [including, of course, the Large and Small Magellanic Clouds (LMC and SMC)], making Irrs prime targets for detailed investigations of galactic stellar content and star fo rmation processes. Recognition of their structural simplicity has provided an additional stimulus for recent interest in Irrs as tests for theories of galaxy structure and evolution. These galaxies also are comparatively unevolved; and thus they may yield further rewards by allowing conditions to be defined in the poorly understood realm of low density extragalactic systems, which retain considerable information about galaxy formation (101, 327).In the broadest sense, the irregular galaxy class is loosely defined (e.g. see illustrations in 9, 23, 378,379). Hubble (175, 176) originally bu�lt on earlier nebular classification schemes (e.g. 153) and considered galaxies to be "irregular" if they showed chaotic, non symmetrical blue-light distributions, in contrast with the axial symmetry of normal "regular" systems. Later classification systems subdivide irregular galaxies into tWb major groups : Magellanic systems (Irr I, 1m), which resemble the Magellanic Clouds; and peculiar, often amorphous galaxies, which are classified as Irr II or 10 systems (72,174,290,292).Unfortunately, irregular structures may arise from a vari�ty of physical causes, and as a result a wide range of physical types of irregular galaxies are known to exist : (a) There may be substantial chaos in the projected stellar mass distributions in galaxies, although such nonequilibrium structures are unlikely to survive for more than a fe w rotation periods ("" 109 yr). Most currently kilOwn galaxies in disturbed states seem to be involved in galaxy galaxy intetactions (3 11, 358, 359, 384, 392, 393), although newly fo rmed galaxies could also find themselves in this situation (45). Some galaxies classified as Irr II or IO also belong in the category of interacting galaxies. (b) Similarly, unusual distributions of dense interstellar gas across the face of a galaxy may produce an optical image that is mottled by dark lanes. The dusty Irr II galaxies discussed by Krienke & Hodge (214) belong in this group. As these authors note, extended interstellar matter also can lead to emission filaments and reflection nebulae on galactic scales that give rise to abnormal optica...

The dwarf LSB galaxy population of the Virgo cluster -- I. The faint-end slope of the luminosity function

“…Deep H i pointed observations of 59 Virgo cluster dEs from the VCC by Conselice et al (2003) yielded only 7 clear detections (just 2 of which are new) for objects having a mean blue absolute magnitude MB of -16 (range: -14.2 to -17.0) and with an average MHI =2.5±3.6 10 8 M⊙, (MHI/LB )⊙ = 0.57±0.52 and FWHM line width W50=125±93 km s −1 . Even if the expected detection rate was low, we carried out H i follow ups of a sub-sample of our galaxies, because dwarf galaxies in clusters do include star-forming, and generally H i-rich, dwarf irregulars (e.g., Gallagher & Hunter 1984) as well as quiescent, gas-poor dwarf elliptical/spheroidals (e.g., Ferguson & Binggeli 1994;Gallagher & Wyse 1994). H i observations are required for a clearer understanding of their nature, in order to confirm their cluster membership, place lower limits on the H i mass of the non-detected objects, obtain MHI /LB ratios or upper limits and look for environmental effects on their gas content.…”

Radial variations of the SFHs of dwarf irregular galaxies

“…Gallagher & Hunter 1984). Compared to the larger spiral galaxies, gas-rich dIrrs are smaller, bluer, metal-poor, and less luminous.…”

Starbursts in Isolated Galaxies?

“…Variations in the star formation rate (SFR) in dwarf galaxies have been deduced from several observations like direct measurements of individual star formation histories (Dohm-Palmer et al 2002), a large variation in the specific SFR, or short gas depletion times (Gallagher & Hunter 1984). Especially interesting is the group of isolated starburst galaxies, because no external triggers can be invoked for their high activity.…”

“…enhanced gas densities. Already Gallagher & Hunter (1984) stressed that the observation of starburst dIrrs implies a mechanism to organise large-scale star formation which cannot operate all the time due to fast consumption of fuel. On the other hand, the instability leading to an episodic SFR should only work in a small parameter range, because most of the isolated dIrrs seem to evolve in a highly self-regulated manner.…”