Astron. Astrophys. 358, 65-71 (2000)

1. Introduction

The prototypical starburst galaxy M 82 is the prime example of a galaxy in which the violent star formation activity gives rise to the formation of a bipolar outflow and an associated extended halo, which is visible in various regimes of the electro-magnetic spectrum. Outflowing material was first evident in the H light (Lynds & Sandage 1963; McCarthy et al. 1987; Bland & Tully 1988). The kinematics of the outflowing material and the geometry of the cone have been thoroughly worked out by McKeith et al. (1995). The recent detection of H emission 11 kpc away from the plane of M 82 (Devine & Bally 1999) determines the cap of this outflow. The whole scenario is further corroborated by the existence of vertical magnetic field lines (Reuter et al. 1994), along which relativistic particles partake in the outflow, thus forming a radio halo (Seaquist & Odegard 1991). This halo exhibits a filamentary structure away from the plane (Reuter et al. 1992). Hot gas has been also found in some kind of halo which extends for several arcminutes along the minor axis (Fabbiano 1988; Schaaf et al. 1989; Bregman et al. 1995). This X-ray emission correlates well with the H if observed with sufficiently high angular resolution (Watson et al. 1984).

In this context, the search for neutral gas and dust away from the plane of M 82 has seen a number of attempts. Owing to the close interaction between M 81 and M 82, neutral hydrogen is seen enveloping M 82, but whether this gas stems from M 81 or has been expelled from M 82 is still a matter of debate (Yun et al. 1994). Molecular gas associated with the dusty filaments outside the plane of M 82 has been reported by Stark & Carlson (1984) and has been observed by Sofue et al. (1992) out to a projected distance of 2 kpc. Clear evidence for the existence of scattering dust away from the plane of M 82 comes from measurements of optical polarization (Bingham et al. 1976; Notni et al. 1981; Neininger et al. 1990).

Owing to its high luminosity in all spectral bands, M 82 has been a prime target for first-light experiments in the mm and submm regime (Elias et al. 1978; Jura et al. 1978). With the improvement of bolometric measurements detailed studies of the distribution of the cold dust in this galaxy have become feasible; thus the recent past has seen an increasing number of such investigations (2 mm: Kuno & Matsuo 1997; 1 mm: Thronson et al. 1989; Hughes et al. 1990; Krügel et al. 1990a; Alton et al. 1999; 0.5 mm: Jaffe et al. 1984; Smith et al. 1990; Alton et al. 1999). Carlstrom & Kronberg (1991) have shown that at 3 mm thermal free-free emission still dominates the overall spectrum.

Apart from these dedicated bolometric measurements, continuum maps at millimeter wavelengths have been produced as by-products in various observations of spectral lines (e.g. Neininger et al. 1998). As a result of the discoveries of a galactic wind and halo in M 82, the more recent bolometric measurements aimed at detecting a dust halo at mm wavelengths. CO emission away from the plane was reported by Stark & Carlson (1984), Nakai et al. (1986) and by Sofue et al. (1992). An outflow of molecular gas was claimed by Nakai et al. (1987). A dust continuum halo was first mentioned by Hughes et al. (1990). Observations with higher resolution (Kuno & Matsuo 1997) suggest emission at 2 mm out to 400 pc from the plane. Recently Alton et al. (1999) reported a dust outflow from the central region of M 82, based on their submm images.

Here we report observations of M 82 in the mm continuum performed with the MPIfR 19-channel bolometer at the IRAM 30-m telescope. In Sect. 2 we describe the observations and data analysis, with consideration of the possible influence of the error beam. In Sect. 3 the distribution of the cold dust in M 82 will be presented, with a discussion conducted in Sect. 4, along with a comparison with other published measurements. In Sect. 5 we give a short summary of our results.

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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