Anomalous H 2 CO Absorption in the Galactic Anticenter A blind Search for Dense Molecular Clouds Mónica I. Rodríguez 1,2 Ron J. Allen 2, Vladimir Escalante 1, Laurent Loinard 1 and Tommy Wiklind 2 1: Centro de Radiostronomía y Astrofísica, Universidad Nacional Autónoma de México, Apartado Postal (Xangari), Morelia, Michoacán, México m.rodriguez, l.loinard, 2: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD US monica, rjallen, Abstract : We have carried out a blind search in the general direction of the Galactic Anticenter for absorption of the Cosmic Microwave Background (CMB) radiation near 4.83 GHz by molecular clouds containing gaseous ortho-formaldehyde (H 2 CO). The observations covers strips in Galactic latitude 1 o ≤ b ≤ +1 o at severals longitudes in the region 170 o ≤ l ≤ 190 o. Successfully we have dectected H 2 CO CMB absorption at ≈ 10% of the survey. We have mapped these regions in more detail and compared the H 2 CO absorption to existing maps of CO(1-0) emission. There appears to be a rough correlation between the velocity- integrated line strength of the CO(1-0) emission and that of the H 2 CO absorption, but the larger scatter in our data suggest that there are important differences of detail wich lead to variations of these tracers in different locations and the effects of different optical dephts for each tracers. The distances toward these regions seems to be between 1.5 and 3.0 Kpc. Bibliography : Avedisova, V.S., 2002, Astron. Zh., 79, 216 Blair, G.N., Evans, N.J., Vanden Bout, P.A., & Peters W.L., 1978, ApJ, 219, Cohen, R.J., Matthews, N., Few, R.W., & Booth, R.S. 1983, MNRAS, 203, 1123 Crampton, D., & Fisher, W.A., 1974, Pub. Dom. Astrophys. Obs., 14, 283 Evans, N..J., II, Kutner, M.L., & Mundy, L.G., 1987, ApJ, 323, 145 Gordon, M.A., & Roberts, M.S. 1971, Apj, 170, 277 Gordon, M.A., & Höglund, B. 1973, Apj, 182, 41 Helfer, T.T., & Blitz, L., 1997, ApJ, 478, 233 Heyer, M..H., Carpenter, J..M., & Ladd, E.F., 1996, ApJ, 463, 630 Lynds, B.T., 1962, ApJS, 7, 1 Sharpless, S. 1959, ApJS, 4, 257 Observations : January, September-October m telescope (OSO) Angular resolution 10’ Frequency : MHz Bandwidth of 199 km s -1 Velocity resolution 0.24 km s positions from 1 o ≤ b ≤ +1 o at every 2 o in the range 170 o ≤ l ≤ 190 o 66 positions centered at l = 182 o, b = 0 o 121 positions centered at l = 190 o, b = 0 o The system temperature 33 to 36 K Typical final noise level of 3.5 mK (T A *) Mapping the Galaxy and nearby galaxies June, 2006 Ishigaki island, Japan Discussion : In order to study the relation between H 2 CO absorption and CO(1-0) emission in more detail, we have computed the moments profiles of Figures 4 and 5, and their values were plotted in the correlation diagram of Figure 8. The rough correspondence between H 2 CO and 12 CO(1-0) was also noted by Cohen et al in their extensive mapping study of H 2 CO and OH in the Orion region. Those authors also found a broad general agreement in that well-known star-forming region, but concluded that the detailed agreement was poor. They found a better correspondence with 13 CO(1-0), and concluded that the reason for this was that the optically-thick 12 CO(1-0) line was primarily tracing gas temperature. In general we agree with this suggestion, although we also suggests that the dependence of the H 2 CO absorption on kinetic temperature is also likely to be playing a role in producing the general correlation. In order to have some idea of the distances to our detections, we have searched the Catalog of Star-Forming Regions in the Galaxy Avedisova 2002 for objects at known distances which may be associated with them. The distances toward these regions seems to be between 1.5 and 3.0 Kpc, and were obtained from both optical (of the HII regions) and radio spectroscopy (CO(1-0) in molecular clouds), and using a model rotation curve of the outer Galaxy. Conclusions : We have detected anomalous CMB absorption by H 2 CO in ~ 10% of our blind survey position. Our success is due to the improvement in sensitivity. We found a rough correspondence between the integrated opacities in H 2 CO absorption and 12 CO line emission in detailed maps of two regions in our survey area. We have argued that both lines generally trace warm and dense gas in the interstellar medium. UNAM Results : For our blind search, integrations were made every 10’ in 11 strips perpendicular to the Galactic plane from -1 o to +1 o at intervals of 2 o in Galactic longitude from 170 o to 190 o The espectrometer was centered at a different systemic velocity for each strip (Table 1). The final (avererage) spectra are shown in Figure 1. We have succefully detected H 2 CO absorption at roughly 10% of the total of 143 position. The presence of two regions of relatively strong absoption can be identified in Figure 1, the first at l = 182 o, and the second at l = 190 o. (Figure 2 and Figure 3) Fig. 1.- Mosaic of H 2 CO spectra at 143 positions towards the Galactic anticenter. Introduction : Following on the discovery of the anomalous absorption of the (CMB) by H 2 CO molecules, several surveys were carried out in hopes of establishing the general Galactic distribution of distant molecular clouds. The first attempts were by Gordon & Roberts (1971) using NRAO 140’ and Gordon & Höglund (1973) using the 25-m radio telescope OSO, but no emission or absorption was found. Our approach is to carry out long integrations at a set of positions near the Galactic anticenter. For these observations we have again used the 25-m OSO, the same telescope used by Gordon & Höglund (1973). Our present success is due entirely to the availability of more sensitive receivers and to generous allocations of observing time on this telescope. STScI Galactic Latitude (degree) Galactic Longitude (degree) Fig. 4.- a. (letf panel) Mosaic of H 2 CO spectra observed in the direction l = 182 o, b = 0 o. b.(right panel) Observations of CO(1-0) at corresponding positions. Fig. 2.- Velocity-integrated contour map H 2 CO absorption in the direction l = 182 o, b = 0 o. The blue triangles show the positions of IRAS sources possibly associated. Fig. 3.- Velocity-integrated contour map H 2 CO absorption in the direction l = 190 o, b = 0 o. The blue triangles show the positions of IRAS sources possibly associated. Galactic Latitude (degree) Galactic Longitude (degree) Fig. 5.- a. (letf panel) Mosaic of H 2 CO spectra observed in the direction l = 190 o, b = 0 o. b.(right panel) Observations of CO(1-0) at corresponding positions. Fig. 7.- Brightnees temperature minus backgroung continuum for different kinetic temperatures T of a 1-pc thick slab. Fig. 6.- Brightnees temperature minus backgroung continuum for different kinetic temperatures T. Fig. 8.- The correation between the H 2 CO absorption line intensity and the CO(1-0) emission line intensity at corresponding points in the two fields surveyed in detail. CO(1-0) H 2 CO H 2 CO absorption and CO(1-0) Comparison : We have mapped these regions in more detail in order to examine the distribution of H 2 CO absorption and pertmit a point-by-point comparison with existing CO(1-0) emission surveys in this region of the Galaxy. Figure 4 and Figure 5 shows that there is an overall correlation between the two tracers, in particular, at every position where we have detected H 2 CO, CO(1-0) emission is also detected. The converse is not always true.