The Astrophysical Journal, 543:L157–L161, 2000 November 10 q 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
SUBMILLIMETER OBSERVATIONS OF MIDCOURSE SPACE EXPERIMENT GALACTIC INFRARED-DARK CLOUDS Sean J. Carey,1 P. A. Feldman,2 R. O. Redman,2 M. P. Egan,3 J. M. MacLeod,2 and S. D. Price3 Received 2000 May 11; accepted 2000 August 4; published 2000 October 30
ABSTRACT We present 850 and 450 mm continuum images of infrared-dark clouds (IRDCs) taken with the Submillimeter Common-User Bolometer Array (SCUBA) submillimeter camera at the James Clerk Maxwell Telescope. The IRDCs are large (1–10 pc diameter) molecular cores with gas densities ∼106 cm23 and temperatures ≈15 K. We detected strong submillimeter sources with peak flux densities of ≈1 Jy beam21 at 850 mm in all eight clouds that were observed. The submillimeter emission generally lies within the envelope of the mid-infrared extinction where dense gas has been detected using H2CO as a tracer. The dust temperatures in the bright, compact sources are calculated to lie in the range 10–25 K. The masses of these sources are estimated to be in the range of several tens up to about a thousand solar masses. The corresponding gas column densities range over an order of magnitude, up to about 1023 cm22. Several of the sources are detected in emission at both 850 and 8 mm. Two of the sources have HCO1 line profiles characteristic of molecular infall. It is likely that the bright, compact sources seen in the SCUBA images are in various early stages of star formation, from preprotostellar cores to class I objects. Subject headings: infrared: ISM: continuum — ISM: clouds — stars: formation — submillimeter non-Gaussian, raising the possibility that molecular outflows may be present. The IRDCs are preferentially located at the latitudes of tangent points of spiral arms and the 4 kpc molecular ring (Egan et al. 1999). For the IRDCs observed by C98, the LSR velocities of the IRDCs are similar to the LSR velocities of recombination lines of H ii regions within 309 of the IRDCs (Kuchar & Clark 1997). Continuum maps at 450 and 850 mm of the Galactic center region by Lis & Carlstrom (1994) show that several of the IRDCs contain bright, submillimeter objects. Clearly, IRDCs are a population of very large molecular cores with physical properties similar to molecular cores in massive star-forming regions such as Orion, except that the IRDCs contain few star formation tracers such as far-infrared point sources. Recently, submillimeter continuum mapping of massive starforming regions such as Orion (Johnstone & Bally 1999) and ultracompact H ii regions in the Galactic plane (Hatchell et al. 2000) have elucidated their structure. Analogous studies of IRDCs should probe an earlier evolutionary stage of these starforming regions. To better characterize the dust properties of the IRDCs, and to search for embedded protostellar objects, we have started a submillimeter continuum mapping campaign of the MSX IRDCs. This Letter describes our initial results with particular emphasis on the embedded compact sources.
1. INTRODUCTION
Mid-infrared imaging surveys of the Galactic plane with the Infrared Space Observatory and Midcourse Space Experiment (MSX; Price 1995) have identified objects with significant extinctions. These objects (hereafter called infrared-dark clouds, or IRDCs) are seen in silhouette against the bright, diffuse, mid-infrared emission of the Galactic plane. Pe´rault et al. (1996) reported ISOCAM observations of clouds with extinction at 15 mm and suggested that the objects have visual extinctions greater than 25 mag. From the Galactic plane survey using the SPIRIT III infrared telescope on MSX, Egan et al. (1998) have identified a population of several thousand objects with extinctions of 1–2 mag from 8 to 25 mm. Egan et al. note that the IRDCs have no excess far-infrared emission above the local backgrounds. From the lack of emission between 8 and 100 mm and the high mid-infrared opacities, Egan et al. conclude that IRDCs contain large columns of cold (T ! 13 K) dust and suggest that they are dense molecular cores. Observations of millimeter transitions of H2CO toward 10 IRDCs by Carey et al. (1998, hereafter C98) confirm that these objects contain dense molecular gas. There is good agreement between the millimeter and mid-infrared morphologies of the seven clouds that were partially mapped in the 212–111 line. Large velocity gradient (LVG) modeling of several lines of H2CO indicate that IRDCs have gas densities ∼106 cm23 and kinetic temperatures of 10–20 K. The available data strongly suggest that the gas and dust are in thermal equilibrium. Kinematic distances of 1.0–8.5 kpc were derived for several of the IRDCs, with corresponding linear diameters ranging from 0.4 to 15 pc. Because many of the clouds are filamentary in shape, the diameters correspond to spherical model clouds subtending equivalent solid angles. Most of the observed line profiles are
2. OBSERVATIONS
Observations were obtained at 850 and 450 mm with the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) located on Mauna Kea in Hawaii. The resolution for SCUBA is 140 at 850 mm and 80 at 450 mm (Holland et al. 1999). Two different mapping modes, “jiggle-mapping” and “scanmapping,” were employed to produce Nyquist sampled images. Jiggle-mapping produces small (29. 3), roughly hexagonal images which are useful for mapping compact sources or surveying large numbers of objects. We jiggle-mapped our sample of eight IRDCs and made 8 0 # 8 0 scan maps of the two most interesting IRDCs, G11.1120.12 and G28.3410.06. In both cases, sky subtraction is provided by chopping to a nearby location. We chose offsets of 900–1200 and chop po-
1 Institute for Scientific Research, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467;
[email protected]. 2 National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada; paul.feldman@ nrc.ca,
[email protected],
[email protected]. 3 Department of the Air Force, Air Force Research Laboratory, VSBC, 29 Randolph Road, Hanscom AFB, MA 01731-3010; michael.egan2@ hanscom.af.mil,
[email protected].
L157
L158
GALACTIC INFRARED-DARK CLOUDS
Vol. 543
TABLE 1 Submillimeter Properties of Bright Compact Sources Source G353.8510.23 P1a . . . . . . G353.5120.33 P1b . . . . . . G357.5110.33 P1c . . . . . . G10.7420.13 P1 . . . . . . . . G11.1120.12 P1d . . . . . . . G19.3010.07 P1 . . . . . . . . G19.3010.07 P2e . . . . . . . G28.3410.06 P1 . . . . . . . . G28.3410.06 P2 . . . . . . . . G79.310.3 P1 . . . . . . . . . . . G79.310.3 P3 . . . . . . . . . . . Fig. 1.—SCUBA jiggle maps of the 850 mm emission from the regions of the IRDC G79.310.3 around the sources P1 and P3. (Source P2 has not been labeled because it plays no role in the discussion.) The dark (negative) features are artifacts of the chopping process. The pixel spacing is 30, and the intensity range is from 20.7 to 1.6 Jy beam21.
sition angles based on the mid-infrared morphology of the IRDCs. For jiggle maps, the telescope also “nods” so that the target is alternately in the “left” and “right” beam positions. Even when a jiggle map has been fully reduced, emission sources will be flanked by two half-intensity negative features. Sources within, or even outside, the field may induce negative features in the image. (This effect can be seen to the northeast of G79.310.3 P1 in Fig. 1.) The data reduction process for scan maps attempts to correct for the chopping, so that sources appear only once, as positive features. However, structures larger than the chop throw may not be reproduced correctly. For both mapping modes, the zero level is uncertain unless the maps are large enough to include some blank sky in the image. Descriptions of SCUBA and its observing modes may be found in Holland et al. (1999) and Jenness et al. (2000). The observations were taken under fair weather conditions (t850 p 0.24–0.30) during 4 hr of actual observing time on
a (J2000) 17 17 17 18 18 18 18 18 18 20 20
29 30 40 09 10 25 25 42 42 32 31
16.5 26.0 49.9 45.9 28.4 58.5 52.5 50.9 52.4 21.8 57.5
d (J2000) 234 234 231 219 219 212 212 204 203 140 140
00 41 14 42 22 03 04 03 59 20 18
06 48 50 04 29 59 48 14 54 08 30
F850 (Jy)
F450 (Jy)
0.7 17 0.9 1.3 1.8 1.3 1.1 1.5 5.5 2.0 1.4
… … … 8.8 18 9.3 7.2 12 48 12 7.2
Note.—F850 and F450 are the source flux densities at 850 and 450 mm. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Background uncertain. b Brightest part of source outside map. c Source extended and at edge of map. d Background very uncertain at 450 mm. e 450 mm peak very close to edge.
1998 August 12 UT. In general, the fields were centered on the locations of the highest obscuration in each cloud (C98). The data were processed using the standard routines provided by the SURF package (Jenness & Lightfoot 1998) for flatfielding, extinction correction, and sky subtraction. Jiggle maps of Uranus were used to calibrate both the jiggle and scan maps of the IRDCs. The flux densities of Uranus at 450 and 850 mm were calculated using the FLUXES program, provided by the Joint Astronomy Centre, which makes use of the measurements reported by Griffin & Orton (1993). Our measured flux densities are good to 515%. The accuracy of the calibration was limited by the uncertainty of sky subtraction in regions with extended emission and by the lack of complete calibration information caused by the shortage of time. Pointing was performed on Uranus and G34.3 and should be better than 20.
Fig. 2.—Left: MSX 8 mm image showing the mid-infrared extinction of the filamentary cloud G11.1120.12. Right: SCUBA scan map of 850 mm emission from the same region. The images are in J(2000) coordinates, using a tangent projection. The pixel spacing is 10. The intensity levels in the 8 mm image range from 7 to 70 MJy sr21. The 850 mm intensity range is from 20.4 to 1.7 Jy beam21.
No. 2, 2000
CAREY ET AL.
L159
Fig. 3.—Left: MSX 8 mm image showing the mid-infrared extinction of G28.3410.06. Right: SCUBA scan map of 850 mm emission from the same region. The color scale of the SCUBA image has been adjusted to emphasize the structure of the clouds. Contours are overlaid on the burned-out core of P2 at the 3.0 and 4.5 Jy beam21 levels to show the location of this very bright pointlike source more clearly. The coordinate system, projection, and pixel spacing are the same as in Fig. 1. The intensity levels in the 8 mm image range from 10 to 60 MJy sr21. The 850 mm intensity range is from 20.4 to 1.6 Jy beam21.
In addition, brief observations were made of the HCO1 (3–2) line toward the locations of two compact, bright sources identified in these fields (called G79.310.3 P1 and P3 in Table 1 and indicated with arrows in Fig. 1). These spectra were obtained under good weather conditions in 1998 December, during commissioning of the RxA3 receiver (Claude et al. 1999). The MSX images presented in Figures 2 and 3 are extracted from the MSX Galactic plane survey images.4 The MSX Galactic plane survey consists of 1680 17. 5 # 17. 5 images covering the Galactic plane within FbF ! 57 in four mid-infrared passbands (centered on 8.3, 12.1, 14.7, and 21.3 mm). The resolution of the images is 200, and the rms noise of the 8.3 mm images is 1.3 MJy sr21. The rms pointing error in the images is 20. The 8.3 mm images presented in this Letter have been resampled to the astrometry and pixel size of the SCUBA images. 3. RESULTS
Bright emission was detected from all eight IRDCs at 850 mm. Emission was also detected at 450 mm, especially from the strongest point sources (see Table 1). The scan maps of G11.1120.12 and G28.3410.06 show that there is generally a good agreement between the morphologies of the 850 mm emission and the 8 mm extinction (see Figs. 2 and 3). Also, 4
Publicly available at http://irsa.ipac.caltech.edu.
the 850 mm emission follows the H2CO emission morphology. However, the 850 mm emission shows considerably more structure than either the extinction or the H2CO emission. The compact sources revealed at 850 mm are unresolved by the larger (450) beam used for the H2CO mapping and are hidden by the opacity of the surrounding dust cloud in the 8 mm images. The compact submillimeter sources identified at 850 mm sometimes appear to be associated with pointlike counterparts in emission at 8 mm. For example, G11.1120.12 P1, and possibly G28.3410.06 P3 and G79.310.3 P1, show such counterparts. However, G28.3410.06 P1 and G79.310.3 P3 do not and are good candidates for class 0 protostars. G28.3410.06 P2 is more complex. The bright source visible just south of P2 in the MSX 8 mm image (see Fig. 3) is the point source IRAS 1840220403. With an angular separation of 230 these are distinctly different, but possibly related, sources. Assuming that the IRAS source is at the kinematic distance (4.8 kpc) of the IRDC (C98), its bolometric luminosity is ≈103 L,, typical of a pre–main-sequence OB star. The proximity between this luminous IRAS source and P2, the strongest submillimeter source in the map (5.5 Jy at 850 mm), suggests that there may be a causal relationship between them. For each compact source, integrated flux densities at 850 and 450 mm were calculated using a 150 diameter circular aperture centered on the source. This aperture closely approxi-
TABLE 2 Estimated Dust Temperatures, Total Masses, and Column Densities Temperature (K)
Total Mass (M,)
Column Density (#1022 cm22)
Source
Da (kpc)
b p 1.5
b p 1.75
b p 2.0
b p 1.5
b p 1.75
b p 2.0
b p 1.5
b p 1.75
b p 2.0
G11.1120.12 P1 . . . . . . G19.3010.07 P1 . . . . . . G19.3010.07 P2 . . . . . . G28.3410.06 P1 . . . . . . G28.3410.06 P2 . . . . . . G79.310.3 P1 . . . . . . . . . G79.310.3 P3 . . . . . . . . .
3.6 2.2 2.2 4.8 4.8 1 1
… 25 21 34 50 18 14
43 18 16 22 27 14 12
25 14 13 17 19 12 10
… 33 36 120 270 17 17
67 58 60 240 660 27 25
150 93 94 400 1200 41 37
… 2.2 2.4 1.7 3.9 5.4 5.5
1.7 3.9 4.1 3.3 9.3 8.3 8.8
3.8 6.3 6.3 5.7 17 12 13
a
Distances from C98.
L160
GALACTIC INFRARED-DARK CLOUDS
Fig. 4.—Spectrum of HCO1 (3–2) from G79.310.3 P1, showing the bluered asymmetry characteristic of infalling gas.
mates the beam size at 850 mm and also reduces the noise of the 450 mm data. Background levels were estimated from cuts through the source extending into regions that appeared to be emission-free and were not obviously contaminated by chopping into the source. The data were calibrated using a jiggle map of Uranus by integrating over an aperture of the same size centered on the planet. The observed flux densities for the bright, compact sources are given in Table 1. We have estimated dust temperatures, total masses, and column densities for the bright, compact sources that had kinematic distances from C98. Color temperatures for the dust were calculated from the flux density ratios F450 /F850, assuming the dust emissivity Qn varies as n b. Table 2 gives these 450/850 color temperatures as functions of b for each source over the range of b from 1.5 to 2. Higher values of b yield temperatures in better agreement with those previously measured for the gas component of these sources by C98. For the IRDC GCM 0.2510.11, Lis & Menten (1998) fit the far-infrared and submillimeter emission with b p 2.8 . This value is even higher than the range of b that we considered and would imply lower temperatures, higher masses, and higher column densities than we estimated in Table 2. Large values of b may suggest the presence of a population of grains with icy mantles; however, other possible explanations include temperature dependence of b for silicate grains (Agladze et al. 1996) and compositional dependencies in grain emissivity (Pollack et al. 1994). The presence of such a grain population is expected in the cold, dense interiors of the IRDCs and is supported by the observed depletion of gas-phase H2CO (C98). Mass estimates of the bright, compact sources were obtained following Hildebrand (1983); namely, Mtot p D 2Fn /k n Bn (T ), where Mtot is the total mass (dust and gas), D is the distance, kn is the dust opacity per unit (dust and gas) mass column density, and Bn(T) is the Planck function for a dust temperature T. We scaled the values of kn, quoted at 1.3 mm by Motte, Andre´, & Neri (1998) and references therein, by nb to obtain k850. We adopted a value k 850 ≈ 0.143 cm 2 g21 at b p 1.5, which is intermediate between the k850 values for prestellar cores and class 0/I sources. Table 2 gives the derived masses as functions of b for each source using the color temperatures derived above. Mean total column densities were estimated from the masses given in Table 2 by assuming the mass was uniformly distributed over the 150 photometric aperture. In the calculation, the mean molecular weight for the gas was taken to be 2.33 (Motte et al.
1998). In several cases, the 450 mm images showed that most of the emission comes from a region much smaller than 150, so the estimates given in Table 2 are lower limits to the true peak column densities. Nevertheless, the values we obtain are in the range (0.2–2) # 10 23 cm22. These results are broadly consistent with the estimates obtained from mid-infrared extinction (Egan et al. 1998) but are somewhat lower (factor of 2–4) than the values obtained from LVG modeling of ortho-H2CO emission lines (C98). Our use of a single-temperature model for the dust emission is likely to account for much of the difference. Multipletemperature models, such as a warm envelope of dust surrounding a cold core or a warm protostar embedded in a cold cloud, would result in larger total columns of dust. Accurate measurements of the spectral energy distributions over a wider range of frequencies will be needed to derive reliable estimates of the temperatures and column densities of the dust. As well, the differences in the total column density estimates may be due, in part, to slight changes in the abundance of gas-phase H2CO. 4. SUMMARY
Observations of eight IRDCs resulted in the discovery of surprisingly strong submillimeter sources in all of the target fields. In spite of the large distances to many of the clouds, peak flux densities were typically *1 Jy beam21 at 850 mm. The submillimeter emission generally lies within the envelope of the mid-infrared extinction. The temperatures of the dust in the bright, compact sources are calculated to be in the range 10–25 K, in agreement with the gas temperatures obtained by C98. The total masses of the compact sources are estimated to be in the range of several tens up to about a thousand M,. The corresponding lower limits to the gas column densities range up to about 1023 cm22. The bright, compact sources clearly show potential as sites for star formation. The 8 mm emission associated with several of the sources suggests the presence of class I protostars. However, many of the submillimeter sources do not show 8 mm emission and therefore may contain class 0 objects. Another indication that the compact sources may be in the earliest stages of star formation is the strong blue-red asymmetry of the HCO1 J p 3–2 line in G79.310.3 P1 (see Fig. 4). This is generally a good indicator of infall (Zhou et al. 1993). The relatively high masses that we deduce for the bright, compact sources suggest that high-mass star formation may be occurring in IRDCs. The proximity of the bright infrared source IRAS 1840220403 to G28.3410.06 P2 suggests that an OB star has already formed in this cloud and may have triggered the formation of G28.3410.06 P2, an even more massive object. These results suggest that mid-infrared extinction is an excellent tracer of the early stages of star formation. IRDCs may therefore be the best places to find large numbers of molecular cores in the earliest stages of star formation. A preliminary catalog of IRDCs lists several thousand of these objects. The JCMT is operated by the JAC on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom, the Netherlands Organisation for Scientific Research, and the National Research Council of Canada.
REFERENCES Agladze, N. I., Sievers, A. J., Jones, S. A., Burlitch, J., & Beckwith, S. V. W.
Vol. 543
1996, ApJ, 462, 1026
No. 2, 2000
CAREY ET AL.
Carey, S. J., Clark, F. O., Egan, M. P., Price, S. D., Shipman, R. F., & Kuchar, T. A. 1998, ApJ, 508, 721 (C98) Claude, S. M. X., et al. 1999, Abstracts of the 26th General Assembly of URSI (Ghent: URSI), 582 Egan, M. P., Carey, S. J., Price, S. D., Shipman, R. F., Feldman, P., & Redman, R. 1999, in The Universe as Seen by ISO, ed. P. Cox & M. F. Kessler (ESA SP-427; Noordwijk: ESA/ESTEC), 671 Egan, M. P., Shipman, R. F., Price, S. D., Carey, S. J., Clark, F. O., & Cohen, M. 1998, ApJ, 494, L199 Griffin, W., & Orton, G. 1993, Icarus, 105, 537 Hatchell, J., Fuller, G. A., Millar, T. J., Thompson, M. A., & Macdonald, G. H. 2000, A&A, 357, 637 Hildebrand, R. H. 1983, QJRAS, 24, 267 Holland, W. S., et al. 1999, MNRAS, 303, 659 Jenness, T., & Lightfoot, J. F. 1998, Starlink User Note 216
L161
Jenness, T., Lightfoot, J. F., Holland, W. S., Greaves, J. S., & Economou, F. 2000, in Imaging at Radio through Submillimeter Wavelengths, ed. J. Mangum & S. Radford (San Francisco: ASP), 205 Johnstone, D., & Bally, J. 1999, ApJ, 510, L49 Kuchar, T. A., & Clark, F. O. 1997, ApJ, 488, 224 Lis, D. C., & Carlstrom, J. E. 1994, ApJ, 424, 189 Lis, D. C., & Menten, K. M. 1998, ApJ, 507, 794 Motte, F., Andre´, P., & Neri, R. 1998, A&A, 336, 150 Pe´rault, M., et al. 1996, A&A, 315, L165 Pollack, J. B., Hollenbach, D., Beckwith, S., Simonelli, D. P., Roush, T., & Fong, W. 1994, ApJ, 421, 615 Price, S. D. 1995, Space Sci. Rev., 74, 81 Zhou, S., Evans, N. J., II, Koempe, C., & Walmsley, C. M. 1993, ApJ, 404, 232
