Galaxies irregular

NGC 6822 is an isolated irregular galaxy of the Local Group located at 495 kpc from our Galaxy and at 880 kpc from M31, there are no small galaxies in its neighbourhood, and it does not show tidal effects. 

Dwarf galaxies are common objects in the nearby universe. They present a well pronounced dichotomy between the two main classes of dwarf spheroidals (Matteucci, these proceedings) and dwarf irregulars (DIGs). 

The three broad classifications (elliptical, spiral and irregular) of star clusters that also cluster together to form the Local Group that contains the Milky Way and the Andromeda Galaxy, along with the Small and Large Magellanic Clouds... 

Fig. 3.25. Trends of nebular line strengths in H n regions with oxygen abundance. This figure shows oxygen abundance in H n regions of the Milky Way and spiral and irregular galaxies (determined using measured electron temperatures) vs. log R23, after Pilyugin (2003) the p parameter is the line ratio [O iii]/([0 11] + [O hi]).

Fig. 8.1. Ne/O ratio vs. O/H from (mainly optical) observations of H n regions in spiral and irregular galaxies and the Sun. Filled and open symbols represent results from different authors. After Garnett (2004).

Fig. 8.11. N/O ratio vs. O/H in H 11 regions of irregular (circles) and spiral galaxies (plus signs), adapted from Pilyugin, Thuan and Vilchez (2003). Solar values are indicated by the axes.

Fig. 8.12. Relation between oxygen abundance of H II regions in irregular (open squares) and spiral galaxies (filled circles, taking abundances at 0.4 of the de Vaucouleurs isophotal radius R25) plotted against the gas fraction, after Pilyugin, Vilchez and Contini (2004). The heavy curve shows expectation from the Simple model with an oxygen yield of 0.0027 (or about 0.5 Z ) and the broken curves show the same with the yield 1.5 x higher or lower, whereas the dotted curve shows a yield 4 x lower. The effective yield, defined as Zo/(— ln/z), increases systematically with luminosity, and the gas fraction decreases.

Fig. 8.13. H ii region oxygen abundance against rotational velocity for irregular and spiral galaxies (at a radius of 0.4/ s), after Pilyugin, Vflchez and Contini (2004).

Fig. 11.2. Mean stellar iron abundances as a function of luminosity in dwarf spheroidals (filled circles), dwarf ellipticals (open circles), dSph/dlrr transition galaxies (filled diamonds) and dwarf irregulars (open diamonds). Baryonic luminosity in the right panel includes the additional luminosity that irregulars would have if their gas were converted into stars. After Grebel, Gallagher and Harbeck (2003).

Assuming that the initial mass function is invariable, we may calculate the average production of the various star generations, born with the same metaUicity, and estimate their contribution to the evolution of the galaxy (see Appendix 4). The abundances produced by a whole population are not as discontinuous and irregular as those shown in the table of individual yields (Table A4.1). This is because the latter are averaged over the mass distribution. 

The consequences of interstellar dust may be seen with the unaided eye. Under good seeing conditions dark patches can be observed in the Milky Way. These dark areas, we now know, do not result from the irregular distribution of stars in our galaxy but rather from the very effective obscuration of starlight by irregular clouds of small particles, the interstellar dust. 

In these lectures I present a highly opinionated review of the observed patterns of metallicity and element abundance ratios in nearby spiral, irregular, and dwarf elliptical galaxies, with connection to a number of astrophysical issues associated with chemical evolution. I also discuss some of the observational and theoretical issues associated with measuring abundances in H II regions and gas and stellar surface densities in disk galaxies. Finally, I will outline a few open questions that deserve attention in future investigations. 

Here I present an overview of the patterns of metallicity and element abundance ratios observed in spiral and irregular galaxies. I will discuss the results for both types of galaxies rather than separately many aspects can be discussed for the combined groups, although there are a number of differences that could constitute the topic of an entire... 

Neutral gas is the largest component of gas in most spirals and irregulars, as determined from H I 21-cm hyperfine line measurements. The 21-cm line has been well-mapped in many nearby galaxies. With regard to determining gas fractions and surface densities, a few points should be kept in mind ... 

Figure 10. Effective yields yeff for nearby spiral and irregular galaxies versus rotation speed Vrot (Garnett 2002). Filled squares represent the data for spirals while the crosses show the data for irregulars.

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