Anomalous formaldehyde 6-cm CMB absorption in the L1204/S140 region Mónica I. Rodríguez 1, Laurent Loinard 1, Ron J. Allen 2 and Tommy Wiklind 1: Centro de Radiostronomía y Astrofísica, Universidad Nacional Autónoma de México, Apartado Postal (Xangari), Morelia, Michoacán, México m.rodriguez, 2: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD US rjallen, Abstract : The region surrounding the Sharpless 140 HII region and the associated dust/molecular cloud Lynds 1204 has been observed with the Onsala 25-m telescope in the H 2 CO( ) transition. This spectral line is seen here in absorption against the microwave background, and is a tracer for the presence of relatively cold and moderately dense molecular material. We have detected absorption in 16 pointing positions, and confirmed that the strongest absorption occurs deep in the cloud rather than on its surface where the CO(J = 1-0) emission is brightest. We argue that the CO emission is strong on the edge of the cloud because of local heating by a nearby B0 star, but that the H 2 CO 6-cm line may be a better tracer of the bulk of molecular gas, located behind the PDR. Introduction : Since H 2 lacks a permanent dipole moment, it is difficult to detect directly. Thus, the structure and properties of cool molecular clouds are usually studied using emission lines of simple non-symmetric polar molecules, the first rotational transition (J = 1-0) of carbon monoxide (CO), at GHz being the most popular. However, this transition is well-known to be nearly always optically thick, so its intensity is expected to increase monotonically with the kinetic temperature of the emitting gas. Clearly, this could have adverse effects on the distribution of molecular gas deduced from CO observations. On the other hand, absorption lines can be detected even in very cold gas, but sufficiently bright background continuum sources need be present. The scarcity of such sources at the wavelenghts of the common molecular tracers has limited the usefulness of absorption measurements. The 6-cm ( ) transition of ortho-formaldehyde H 2 CO offers an interesting alternative, because it can be observed in absorption against the cosmic microwave background (Palmer et al. 1969). Although the absorption line is weak and its strength depends on physical conditions, the line is an indicator of the presence of relatively low temperature (8 K < T < 30 K) and intermediate density (100 cm -3 < n < 10 5 cm -3 ) gas. The L1204/S140 Region : The dark dust cloud Lynds 1204, is centered at l = 107 o.47, b = +4 o.82 and covers a 2.5 square degree area according to Lynds (1962). Located at the southwest edge there is a prominent compact HII region (S140 − Sharpless 1959). When seen with sufficient angular resolution, S140 appears as an Hα emission arc. The ionization is driven by the nearby star HD The distance to the region is 910 pc (Crampton & Fisher 1974). S140 has been the subject of many observational studies, which have focused on the PDR on the edge of L1204, and on the embedded infrared sources located right behind it (e.g. Preibisch et al. 2001; Hayashi et al. 1987; Preibisch & Smith 2002; Bally et al. 2002). While the dust cloud is seen as an extended dark feature covering more than two square degrees, the CO emission peaks on the infrared sources regions (Helfer & Blitz 1997, Heyer et al. 1996, Evans et al. 1987, Blair et al. 1978). Remarkably, only relatively dim emission extends further into the cloud (Fig. 1). The 6-cm line of H 2 CO was first detected in L1204 near S140 by Blair (1978), and unexpectedly by Evans et al. (1987) in north west of S140. Interestingly, both authors found that the peak of H 2 CO absorption was not strongest where the CO is brightest, but deeper into the dust cloud. 10’, and our pointing precision is better than 20”. Frequency- switching was used. At the observed frequency of MHz, our setup provided a total bandwidth of 96 km s -1 for a velocity resolution 0.12 km s -1. The spectrometer was centered at the systemic velocity of S140, Vlsr = − 8.0 km s -1. The system temperature during our observations varied from 33 to 36 K. In order to map the entire region behind S140, we observed 36 positions centered at l = 107 o, b = +5.3 o. The total integration time for each of these positions was 10 hours, yielding a typical final noise level of 3 mK (T A *) per 0.12 km s -1 velocity channel. The data processing was done with CLASS. 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Dom. Astrophys. Obs., 14, 283 Fig. 2.- Square grid of H 2 CO spectra at 36 positions toward S140. The (0,0) position correspond to l = 107 o and b = +5.3 o and the offsets are in l and b. Fig. 1.-The first panel shows the comparison between the CO and the optical DSS-red image of S140. The second panel shows the comparison between the H 2 CO and the optical DSS-red image of S140. Note the decrease of the CO brightness moving away from the Hα arc, while the H 2 CO absorption covers a big region which extends further, and resembles more the dust absorption feature. Observations : The data were obtained during two observing sessions (January, September-October 2004, respectively) with the 25.6 m telescope of the Onsala Space Observatory (OSO) in Sweden. At 6 cm, the angular resolution of the 25 m is about ___ Results : Formaldehyde absorption was clearly detected in 16 of our 36 observed positions (Fig. 2). The maximum absorption is located near the center of our grid, 10' from the CO peak (Fig. 1). Thus, we confirm that the strongest H 2 CO absorption does not occur near the Hα arc where the CO emission is brightest, but deeper into the dust cloud. Also, the H 2 CO absorption feature is more than half a degree across, significantly more extended than the CO peak (Fig. 1). In summary, although both are acknowledged tracers of molecular material, the CO emission and the H 2 CO absorption exhibit quite different morphologies. How can we explain these differences? Discussion : In the molecular ISM, the CO J = 1-0 line is nearly always optically thick, and is therefore expected to be a better tracer of temperature than of column density. Thus, CO(1-0) observations of the molecular ISM will tend to be biased toward warmer regions (Tilanus & Allen 1993). From the SWAS detection of the CO(5-4) transition toward the bright CO(1-0) peak just behind the S140 arc, it is known that the molecular gas there is at a kinetic temperature above 60 K (Ashby et al. 2000, Ashby et al. 2000). The molecular gas located deeper into the cloud, however, is expected to be much cooler (typically 10 K). But at Tkin = 60 K, the brightness of the CO(1-0) transition is expected to be about 60 K, whereas at Tkin = 10 K, it is expected to be about 7 K, ten times lower. Thus, we naturally expect the CO(1-0) emission to be much brighter just behind the Hα arc than in the rest of the dust cloud, independently of where the column density is highest. For the H 2 CO absorption to be detectable, on the other hand, the molecular gas must be cool and moderately dense, but the exact conditions will not greatly affect the optical depth of the line : the 6 cm line of formaldehyde is a poor diagnostic of physical conditions, but a good tracer of the mere presence of molecular gas. Consequently, we argue that the H 2 CO gives a fair picture of the overall distribution of molecular gas in the L1204/S140 region, whereas the CO(1-0) distribution provides a picture of the temperature distribution in the region. Conclusions : We have obtained a large scale map of the L1204/S140 region in the 6-cm line of formaldehyde. The morphology of the formaldehyde absorption feature is quite different from that of the associated CO(1-0) emission, and we argue that the H 2 CO provides a better description of the total extent of the molecular gas distribution, whereas the CO is largely biased toward warmer gas.