Anomalous H2CO Absorption in the Galactic Anticenter

A blind Search for Dense Molecular CloudsMónica I. Rodríguez1,2

Ron J. Allen2, Vladimir Escalante1, Laurent Loinard1 and Tommy Wiklind2

1: Centro de Radiostronomía y Astrofísica, Universidad Nacional Autónoma de México, Apartado Postal 72--3 (Xangari), 58089 Morelia, Michoacán, México

m.rodriguez, l.loinard, v.escalante@astrosmo.unam.mx

2: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 US

monica, rjallen, wiklind@stsci.edu

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 (H2CO). The observations covers strips in Galactic latitude 1o ≤ b ≤ +1o at severals longitudes in the region 170o ≤ l ≤ 190o. Successfullywe have dectected H2CO CMB absorption at ≈ 10% of the survey.

We have mapped these regions in more detail and compared the H2CO 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 H2CO 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 :

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Observations :

January, September-October 200425.6 m telescope (OSO) Angular resolution 10’Frequency : 4829.660 MHzBandwidth of 199 km s-1

Velocity resolution 0.24 km s-1

143 positions from 1o ≤ b ≤ +1o at every 2o in the range 170o ≤ l ≤ 190o

66 positions centered at l = 182o, b = 0o

121 positions centered at l = 190o, b = 0o

The system temperature 33 to 36 KTypical final noise level of 3.5 mK (TA*)

Mapping the Galaxy and nearby galaxies26 - 30 June, 2006

Ishigaki island, Japan

Discussion :

In order to study the relation between H2CO 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 H2CO and 12CO(1-0) was also noted by Cohen et al. 1983 in their extensive mapping study of H2CO 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 13CO(1-0), and concluded that the reason for this was that the optically-thick 12CO(1-0) line was primarily tracing gas temperature. In general we agree with this suggestion, although we also suggests that the dependence of the H2CO 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 H2CO 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 H2CO absorption and 12CO 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 -1o to +1o at intervals of 2o in Galactic longitude from 170o to 190o 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 H2CO 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 = 182o, and the second at l = 190o. (Figure 2 and Figure 3)

Fig. 1.- Mosaic of H2CO spectra at 143 positions towards the Galactic anticenter.

Introduction :

Following on the discovery of the anomalous absorption of the (CMB) by H2CO 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

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Fig. 4.- a. (letf panel) Mosaic of H2CO spectra observed in the direction l = 182o, b = 0o. b.(right panel) Observations of CO(1-0) at corresponding positions.

Fig. 2.- Velocity-integrated contour map H2CO absorption in the direction l = 182o, b = 0o. The blue triangles show the positions of IRAS sources possibly associated.

Fig. 3.- Velocity-integrated contour map H2CO absorption in the direction l = 190o, b = 0o. The blue triangles show the positions of IRAS sources possibly associated.

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Fig. 5.- a. (letf panel) Mosaic of H2CO spectra observed in the direction l = 190o, b = 0o. 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 H2CO absorption line intensity and the CO(1-0) emission line intensity at corresponding points in the two fields surveyed in detail.

CO(1-0)

H2CO

H2CO absorption and CO(1-0) Comparison :

We have mapped these regions in more detail in order to examine the distribution of H2CO 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 H2CO, CO(1-0) emission is also detected. The converse is not always true.