We have surveyed a ~0.9-square-degree area of the W3 giant molecular cloud and star-forming region in the 850-micron continuum, using the SCUBA bolometer array on the James Clerk Maxwell Telescope. A complete sample of 316 dense clumps was detected with a mass range from around 13 to 2500 Msun. Part of the W3 GMC is subject to an interaction with the HII region and fast stellar winds generated by the nearby W4 OB association. We find that the fraction of total gas mass in dense, 850-micron traced structures is significantly altered by this interaction, being around 5% to 13% in the undisturbed cloud but ~25 - 37% in the feedback-affected region. The mass distribution in the detected clump sample depends somewhat on assumptions of dust temperature and is not a simple, single power law but contains significant structure at intermediate masses. This structure is likely to be due to crowding of sources near or below the spatial resolution of the observations. There is little evidence of any difference between the index of the high-mass end of the clump mass function in the compressed region and in the unaffected cloud. The consequences of these results are discussed in terms of current models of triggered star formation.Comment: 13 pages, 8 figures, 1 table (full source table available on request). Accepted for publication in Monthly Notices of the Royal Astronomical Society (Main Journal

Context. The Red MSX Source (RMS) survey is an ongoing effort to return a large, well-selected sample of massive young stellar objects (MYSOs) within our Galaxy. 2000 candidates have been colour-selected from the Mid-course Space Experiment (MSX) point source catalogue (PSC). A series of ground-based follow-up observations are being undertaken in order to remove contaminant objects (ultra-compact HII (UCHII) regions, planetary nebulae (PN), evolved stars), and to begin characterising these MYSOs. Aims. As a part of these follow-up observations, high resolution (∼1 ) mid-IR imaging aids the identification of contaminant objects which are resolved (UCHII regions, PN) as opposed to those which are unresolved (YSOs, evolved stars) as well as identifying YSOs near UCHII regions and other multiple sources. Methods. We present 10.4 µm imaging observations for 346 candidate MYSOs in the RMS survey in the Southern Hemisphere, primarily outside the region covered by the GLIMPSE Spitzer Legacy Survey. These were obtained using TIMMI2 on the ESO 3.6 m telescope in La Silla, Chile. Our photometric accuracy is of order 0.05 Jy, and our astrometric accuracy is 0.8 , which is an improvement over the nominal 2 accuracy of the MSX PSC. Results. Point sources are detected in 64% of our observations, which are expected to be either YSOs or evolved stars. 24% contain only sources of extended emission, which are likely to be either UCHII regions or, in a few cases, PN. This is confirmed by comparison with radio continuum observations. We find that, as expected for a dusty HII region, the strength of 10.4 µm and radio continuum emission is related. The remaining targets (12%) result in non-detections. While for 63% of our targets we detect only one mid-infrared source, 25% show multiple sources. In these cases, our observations will allow the apportioning of the flux from larger beam measurements between the different sources. Within these multiple source targets, we find some point sources on or near UCHII regions. Our improved astrometric information will allow more accurate targeting of spectroscopy, which will be used to identify unresolved sources in cases where it is not clear whether they are YSOs or evolved stars.

We present new results for the molecular gas, dust emission, and the ionized gas in J1023+1952, an H iYrich intergalactic star-forming tidal dwarf galaxy candidate. It is located at the projected intersection of two faint stellar tidal streams wrapped around the interacting pair of galaxies NGC 3227/6 (Arp 94). Using the IRAM 30 m telescope, emission from 12 CO(1Y0) and 12 CO(2Y1) was detected across the entire extent of the neutral hydrogen cloud associated with J1023+1952, a region of the size of 8:9 ; 5:9 kpc. The molecular gas is found to be abundant over the entire H i cloud, with H 2 -to-H i gas mass ratios between 0.5 and 1.7. New Spitzer mid-infrared observations at 3.6, 4.5, 5.8, 8.0, 15, and 24 m show that young SF is restricted to the southern part of the cloud. Despite the relatively uniform H 2 and H i column density across the cloud, young SF occurs only in those regions where the velocity dispersion in the CO and H i is a factor of $2 lower (FWHM of 30Y70 km s À1 ) than elsewhere in the cloud (FWHM of 80Y120 km s À1). Thus, the kinematics of the gas, in addition to its column density, seems to be a crucial factor in triggering SF. Optical /infrared spectral energy distributions (SEDs) and H photometry confirm that all the knots are young. Optical spectroscopy of the brightest SF region allowed us to determine the metallicity [12 þ log (O/H) ¼ 8:6 AE 0:2] and the extinction (A B ¼ 2:4). This shows that J1023+1952 is made from metal-enriched gas which is inconsistent with the hypothesis that it represents a preexisting dwarf galaxy. Instead, it must be formed from recycled, metal-enriched gas, expelled from NGC 3227 or NGC 3226 in a previous phase of the interaction.

We present 12CO, 13CO and C18O J= 3 → 2 maps of the W3 giant molecular cloud (GMC) made at the James Clerk Maxwell Telescope. We combine these observations with Five College Radio Astronomy Observatory CO J= 1→0 data to produce the first map of molecular‐gas temperatures across a GMC and the most accurate determination of the mass distribution in W3 yet obtained. We measure excitation temperatures in the part of the cloud dominated by triggered star formation (the high‐density layer, HDL) of 15–30 K, while in the rest of the cloud, which is relatively unaffected by triggering (low‐density layer), the excitation temperature is generally less than 12 K. We identify a temperature gradient in the HDL which we associate with an age sequence in the embedded massive star‐forming regions. We measure the mass of the cloud to be 4.4 ± 0.4 × 105 M⊙, in agreement with previous estimates. Existing submillimetre continuum data are used to derive the fraction of gas mass in dense clumps as a function of position in the cloud. This fraction, which we interpret as a clump formation efficiency (CFE), is significantly enhanced across the HDL, probably due to the triggering. Finally, we measure the 3D rms Mach number, , as a function of position and find a correlation between and the CFE within the HDL only. This correlation is interpreted as due to feedback from the newly formed stars, and a change in its slope between the three main star‐forming regions is construed as another evolutionary effect. We conclude that triggering has affected the star formation process in the W3 GMC primarily by creating additional dense structures that can collapse into stars. Any traces of changes in CFE due to additional turbulence have since been overruled by the feedback effects of the star‐forming process itself.

We have selected red MSX sources (RMS) that have the colours of massive young stellar objects (MYSOs). Our aim is to generate a large, systematically selected sample to address questions such as their luminosity function, lifetimes, clustering and triggering. Other objects such as UCHIIs, PN, PPN and AGB stars have similar IR colours and a large programme of ground-based follow-up observations is underway to identify and eliminate these from the sample of the red MSX sources. These include radio continuum observations, kinematic distances, groundbased mid-IR imaging, near-IR imaging and spectroscopy to distinguish. We report the progress of these campaigns on the 3000 candidates, with initial indications showing that a substantial fraction are indeed massive YSOs.