Research conducted during the last decade gave a new view of nearby
elliptical galaxies previously considered as old, uniform systems with
little gas or dust. Images from the Hubble Space Telescope (HST) have
shown that many early-type galaxies have a large amount of dust
(
), either in the form of a nuclear disk or in the
more diverse form of filaments. Among different studies there is a
large variation in the detection rates which may be due to the
different methods, resolutions, and sensitivities of the observations
(Sadler & Gerhard 1985, 40%; Goudfrooij et al. 1994, 41%; van
Dokkum & Franx 1995, 48%; Ferrari et al. 1999, 75%; Tomita et al.
2000, 56%; Rest et al. 2001, 43%; Tran et al. 2001 (IRAS bright
sample), 78%), but the general conclusion is that dust is common in
nearby ellipticals.
Establishing the presence of dust in nearby early-type galaxies is only the first step towards determining the role of dust in these systems. It is already a well-known fact that radio-loud ellipticals often have large amounts of dust but there are some open questions, especially for the radio-weak sources. Verdoes Kleijn et al. (1999) found that the incidence of dust in radio-loud early type galaxies is 89% while Tran et al. (2001) has a value of 43% for the occurrence of dust in their snapshot sample of relatively radio-quiet nearby early-type galaxies (for a description of the sample see Sect. 2). In the same sample, 66% of dusty galaxies have NRAO VLA Sky Survey (NVSS) 1.4 GHz flux detections (Condon et al. 1998), while only 8% of galaxies without dust are listed as radio sources.
These results raise a question: how important is the presence of dust for radio emission in the nuclei of ellipticals? Plausibly, dust indicates the presence of gas, and gas is necessary to fuel the activity of a central massive black hole (BH). However, this line of reasoning is highly incomplete. Gas may be present without dust. Dust may be present but not visually detectable (Goudfrooij & de Jong 1995). Dust and gas that have fed a BH in the past may not be observable at the time when the nuclear activity is observed. These arguments justify a careful study of the relation between dust and nuclear radio emission to determine the relevance of radio luminosity, dust morphology and other effects. There are two approaches to the study of extragalactic radio sources. The first one is based on catalogs of discrete radio sources followed by an analysis of the optical counterparts. The second involves searching for radio emission from optically chosen objects. The first approach (e.g. de Koff et al. 2000) is relatively efficient in finding radio galaxies, but emphasizes powerful radio sources and may not provide a good counter-sample of radio-quiet galaxies. The second approach conversely emphasizes weak radio sources (e.g. Sadler et al. 1989; Wrobel 1991; Wrobel & Heeschen 1991; Sadler et al. 2002).
Both types of radio surveys are important. Here we have chosen the second method primarily so that the optical selection of the sample, including Hubble type and especially dust content, is not biased by a priori selection for radio emission or other "interesting" properties of the galaxies. The survey objects are selected on their optical/IR properties only and then observed with the VLA with the purpose of establishing the presence of nuclear AGNs. We compare our dusty and non-dusty parts of the sample to investigate the importance of dust (as a fuel reservoir) for the existence of nuclear activity.
In Sect. 2 we present the sample and discuss the observations and the data reduction. In Sect. 3 we present the results of our study. They are followed with a discussion in Sect. 4. Sect. 5 brings a discussion on correlation of dust with radio emission. The conclusions are given in Sect. 6.
Copyright ESO 2002