Astron. Astrophys. 333, 803-808 (1998)

1. Introduction

Dust is now known to be frequently present in early-type galaxies, detected through optical (Hawarden et al. 1981, Ebneter & Balick 1985, Ebneter et al. 1988) as well as FIR emissions (Jura et al. 1987, Knapp et al. 1989). These studies have reported the dust detection rate in E's as 36% and in SO's as 50%, suggesting that dust is more frequent in SO's than in E's. The study of Goudfrooij et al. (1994a) revealed an optical detection rate of dust in E's as 41%. However, taking into consideration the misclassification of E's and SO's (selection effects) could very well raise the estimate to a value as high as 80% in case of E's. The HST survey of these galaxies has shown presence of dust in the nuclei of almost every galaxy (Jaffe et al. 1994).

The other phases of the interstellar matter (ISM) i.e. hot, warm and cool gas are also observed in these galaxies. Hot gas ( 10⁷ K) is observed through X-ray emission (Forman et al. 1985, Fabbiano et al. 1992) of coronal gases having typical mass of the order of 10⁹ - 10¹⁰ , which is consistent with the expected amount from the accumulated stellar mass loss, heated by collision and gravity (Fabian et al. 1987, Sarazin 1986 and references therein). Warm gas ( 10⁴ K) observed through line emission contributes only 10³ - 10⁵ to the total ISM content in these galaxies (Phillips et al. 1986, Caldwell 1984). Around 55% of these galaxies are known to contain this less dominant but physically interesting component (Sadler 1984, Kim 1989, Goudfrooij et al. 1994a). Substantial amount of cool gas ( 10² K) is also detected in these galaxies, through HI mapping and far infrared emission from them (Knapp et al. 1985, 1989). These galaxies are also found to contain the molecular clouds, as evident from CO emission detected in many of them (Lees et al. 1991, Wang et al. 1992, Wiklind et al. 1995).

Dusty ellipticals have received systematic attention after Bertola and Galleta (1978) realized their importance in determining three dimensional structure of these galaxies (van Albada et al. 1982), and in studying the origin and subsequent evolution of the interstellar matter and the underlying galaxy (Faber and Gallagher 1976, Schweizer 1987). Further, the broad dust lane in these galaxies were also used to investigate properties of dust in extragalactic environments (Rodgers 1978, Brosch et al. 1985, 1990, 1991, Goudfrooij et al. 1994b).

Orientation of dust lanes can be used as an additional information to infer the intrinsic shapes of ellipticals. The cold gas and dust is expected to be in simple closed orbits in the galaxy potential, which occur in a small number of preferred planes (Gunn 1979, Habe & Ikeuchi 1985, 1988). Thus the kinematics and morphology of these disks/lanes can be used to constrain intrinsic shapes of the underlying galaxy.

Early-type galaxies are suitable targets to study the dust grain properties in the extragalactic environments. The data can be used to model the size distribution of grains (Rowan-Robinson 1992), to predict the different mechanisms operating (de Jong et al. 1990, Drain & Saltpeter 1979, Barlow 1978), to make the existence of dust possible in a wide variety of environments. As pointed out by Goudfrooij et al. (1994b) the physical properties of dust are a function of time and can be used as indicator for the time elapsed since the dust was last substantially replenshied. The basic tool to study the dust properties is to examine the behaviour of dust extinction in different wavebands i.e. the extinction curve. There are two main methods to determine the wavelength dependence of dust extinction in galaxies:
(i) Direct method - This method is used to study the dust properties of our own Galaxy. The extinction curve is plotted with the help of stellar photometry of individual stars, whose intrinsic luminosity is known by means of some other methods. It is found that the Galactic extinction curve is a function of , the ratio of total selective extinction in V band to the selective extinction between B and V bands.
(ii) Indirect method - In extragalactic objects photometry of individual stars is not possible and hence indirect methods for calculating are used. Comparison between the original galaxy (extinguished) and a smooth model (unextinguished) gives an estimate of extinction by dust; this exercise at a number of wavelength gives the extinction law. Several workers have used these indirect methods and determined values of for spiral galaxies (Knapen et al. 1991, Lequeux 1988, Walterbos & Kennicutt 1988) quite similar to that in our Galaxy. On the other hand the value of for early-type galaxies is smaller than the Galactic value (Goudfrooij et al. 1994b).

In this paper we report detailed study of the dusty galaxy NGC 2076. This galaxy has been classified as lenticular in the Third Reference Catalogue (RC3) of Bright Galaxies (de Vaucouleurs et al. 1991). As the dust lies parallel to the major axis, Ebneter & Balick (1985) have categorised the dust configuration as oblate. Mollenhoff et al. (1992) carried out 20cm. radio observation for a sample of dusty ellipticals to examine the connection between optical morphology of radio galaxies and the orientation of the radio jets. All dust lane ellipticals with radio jets show a strong tendency for radio jets to be orthogonal to the dust lane. NGC 2076 posses a strong dust lane along its apparent major axis. The low resolution radio map shows an extended radio source of size x centered on the optical source. However, its high resolution radio maps does not reveal any radio structure. NGC 2076 is also enlisted in the 1.425 GHz atlas of IRAS Bright Galaxy Sample studied by Condon et al. (1996). Its optical, infra-red and radio properties collected from RC3 (1991), Knapp et al. (1989), and Condon et al. (1996), respectively, are given in Table 1.

Table 1. Observational parameters of NGC 2076

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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