SCUBA and the Galactic Plane

	A sub-millimetre survey of the Galactic Plane

Introduction
Impact
Target selection
Images:
Technique
Preliminary results
The next stage
Partners

Introduction

We are conducting a survey of part of the Galactic Plane using the bolometer camera SCUBA at the JCMT. A survey of an area 2 square degrees in extent takes about 64 hours of telescope time. The beamwidth of the JCMT at 850µm is about 13.5 arcsec, and the images are essentially fully-sampled at 850µm wavelength. The survey noise level would be about 0.001 solar masses per beam area for cool dust. We have also surveyed the same region at 1200µm with the bolometer camera SIMBA on the European Southern Observatory's SEST.

A preliminary account of this work, presented as a poster at the 2004 CASCA meeting, is available here as a pdf file and as a PowerPoint file; you may need to adjust the zoom factor on your acrobat reader to fit the pdf poster on your screen.

Impact

The SCUBA and SIMBA images will provide a census of the dust mass along each line of sight. The SCUBA and SIMBA data alone will not distinguish between clouds at different distances, however, because they see only the continuum. We will therefore combine the SCUBA/SIMBA data in the first instance with ¹³CO 1-0 data from the BU-FCRAO Galactic Ring Survey (GRS) and multi-wavelength mid-infrared images from the MSX satellite to obtain accurate dust properties of individual clouds for each line of sight.

In a subsequent phase of this project we will obtain images of the same region of the Galaxy in CO isotopomers using the forthcoming heterodyne array receiver HARP in combination with a new spectrometer (ACSIS), both of which are expected to become available within the next year or so. These data will eventually provide images with an angular resolution very similar to those obtained by SCUBA and SIMBA. We will also compare the SCUBA and SIMBA data with data from the Spitzer legacy survey GLIMPSE, the aims of which are to study the inner Galaxy as traced by stars, the statistics and physics of star formation, and the physics and structure of the interstellar medium.

In due course the data will be placed in the public domain on these web pages.

Target selection

We have chosen a region of the Galactic Plane in which interesting CO and mid-IR features are known. The images below show the observed SCUBA/SIMBA survey region (the tilted rectangle) overlaid on the 8µm MSX image (top) and three versions of the same field from the GRS, respectively integrated over all survey velocities, and over velocity ranges corresponding to relatively 'near' (1-2 kpc) and 'far' (8-12 kpc) material. The field covers an area 4 by 0.5 degrees in length and height respectively, and is slightly tilted from the Galactic plane to allow the observation of the cold quiescent filamentary structures at 25 km/sec (at longitude 45.5° in the third panel), through W49, which appears at longitude 43.2° at intermediate velocities in the second panel and shines brightly at 8µm, to the multitude of smaller features which appear at greater distances (bottom panel). Note for example the filament of relatively distant small 'blobs' which run parallel to, and along the length of, the survey rectangle in the 'integrated' and 'far' ¹³CO images. The region around 45.5° is officially known as G45.5 but is also referred to as the 'Klingon Warbird' (see the GRS 'far' map).

BU-FCRAO image: 13CO 1-0, all velocities

BU-FCRAO image: 13CO 1-0, near distances

Technique

The observations with SCUBA are executed in "scan-mapping" mode, in which the arrays are scanned across the target at angles fixed in Nasmyth coordinates while the secondary mirror is chopping. The scan rate is 48 arcsec/sec. A complete map is obtained from six combinations of chop distance (3) and chop direction (2). Each individual "segment" in the survey is about 0.5 × 0.25 degrees in size and takes about 2 hours elapsed time. A total of 16 segments is required to complete the survey area. Data reduction involves "basket-weaving in Fourier space", in which the dual-beam signature of chopping is restored in the Fourier plane before being removed in the map plane. With SEST we exploited the 'Fastscanning Mapping' capability; the scan speed is 160 arcsec/sec. In this case 20 segments were required, with each segment taking about 45 minutes' telescope time.

Given the sensitivity of SCUBA/JCMT and SIMBA/SEST the survey detects (at ~1kpc distance) a dust mass of about 2 - 5 × 10^-3 M

of cool dust per beam area. Furthermore, since we will have data at both 850µm and 1200µm, we will be able to map out the temperature of the dust, together with its beta index (which is determined by the physical properties of the dust).

Preliminary results

The following image is a preliminary view of the survey region at 850µm, taken from our CASCA poster. Comparison with the MSX and CO fields above shows strong 850µm emission associated with the G45.5 region at longitude 45.5°, and with the W49 star-forming region at longitude 43.2°. Also, several point-like sources are discernible at both 850µm and 1200µm.

The next stage

We are planning to extend the SCUBA survey to include the region around Galactic longitude 28°. The Galaxy in this region differs considerably from that in our first survey area. It lies in the 'Galactic Ring' region, and includes several IR-bright, active star-forming regions covering a wide range of distances. It contains two of the darkest infrared dark clouds in the Milky Way; these are potential candidates for the earliest stages of star/cluster formation. It also contains the 'Lockman hole', which may be a Giant Molecular Cloud in the making. The new region is shown below in the GRS ¹³CO 1-0 survey; this shows the integrated emission images of the covering 'near' and 'far' velocity ranges. The blank regions have not yet been surveyed by the GRS.

Updated: 16 October 2004

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