Study of dust and ionized gas in early-type galaxies

Abstract We present results of optical broad-band and narrow-band H α observations of a sample of forty nearby early-type galaxies. The majority of sample galaxies are known to have dust in various forms viz. dust lanes, nuclear dust and patchy/filamentary dust. A detailed study of dust was performed for 12 galaxies with prominent dust features. The extinction curves for these galaxies run parallel to the Galactic extinction curve, implying that the properties of dust in these galaxies are similar to those of the Milky-Way. The ratio of total to selective extinction ( R V ) varies between 2.1 and 3.8, with an average of 2.9 ± 0.2, fairly close to its canonical value of 3.1 for our Galaxy. The average relative grain size a > a Gal of dust particles in these galaxies turns out to be 1.01 ± 0.2, while dust mass estimated using optical extinction lies in the range ∼ 10 2 to 10 4 M ⊙ . The H α emission was detected in 23 out of 29 galaxies imaged through narrow-band filters with the H α luminosities in the range 10 38 –10 41 erg s −1 . The mass of the ionized gas is in the range ∼10 3 –10 5 M ⊙ . The morphology and extent of ionized gas is found similar to those of dust, indicating possible coexistence of dust and ionized gas in these galaxies. The absence of any apparent correlation between blue luminosity and normalized IRAS dust mass is suggestive of merger related origin of dust and gas in these galaxies.