T E S I S - CRyA, UNAM

Abstract
This thesis presents a study on the Interstellar Medium (ISM) in our Galaxy. The fundamentals
of the ISM are briefly exposed in Chapter 1, while Chapter 2 describes the techniques
of radioastronomy, which are the kind of observations used in this thesis.
The main research project was divided in two parts. In the first part, a study of the
Galactic molecular ISM through observations of formaldehyde is presented. Molecular gas
is commonly studied by the CO (1-0) emission line. But this line has long been known to be
nearly always optically thick, so its intensity is expected to increase monotonically with the
kinetic temperature of the emitting gas. In the other hand, owing to collisions with neutral
particles that selectively overpopulate the lower energy level, the excitation temperature of
the 1 10 → 1 11 transition of formaldehyde lies below 2.7 K (Townes & Cheung 1969), allowing
the transition to be observed in absorption against the cosmic microwave background
(CMB; Snyder et al. 1969). This makes it a potentially powerful tracer of molecular gas in
any direction of the sky. To explore this potential, long-integration-time observations were
carried out using the Onsala 25-m radio telescope toward the Galactic anticenter and the
Galactic dust cloud L1204. The paper containing the results of the H 2 CO survey in the
direction of the Galactic anticenter is presented in Chapter 4. This survey covered strips in
Galactic latitude −1 ◦ ≤ b ≤ +1 ◦ at several longitudes in the region 170 ◦ ≤ l ≤ 190 ◦ , and
H 2 CO CMB absorption was detected at ≈ 10 % of the survey pointings. The observations of
the dust cloud L1204, are shown in the paper presented in Chapter 5. The observed region
includes the S140 Hα arc. When comparing the H 2 CO absorption with existing maps of
CO(1-0) emission in the same regions, a rough correlation between the CO(1-0) emission
and the H 2 CO absorption was found. However, the scatter in this correlation was significantly
larger than the measurement errors, indicating differences of detail at and below
the linear resolution of the observations. Presumably, these differences result from differing
abundances, excitation requirements, and photodissociation energies.
The second part of the project focused on the behavior of the far-infrared – radio continuum
correlation, on scales corresponding to the size of molecular clouds. The existence of
the correlation between the integrated radio continuum (RC) and far-infrared (FIR) emissions
of galaxies was first hinted at more than 30 years ago (van der Kruit 1971, 1973).
It was the IRAS all-sky survey that demonstrated how tight and universal this correlation
really is. It applies to galaxies of many different Hubble types whose luminosities covers
over 4 orders of magnitude with a dispersion of only ∼ 0.2 dex (de Jong et al. 1985, Helou,
Soifer, & Rowan-Rowison 1985). Since both components of the correlation, FIR and RC
emission, seem to be directly associated with star formation, the standard interpretation of
the observed global RC-FIR correlation is that the infrared and radio emission both trace
high-mass star-forming regions. In that picture, the infrared emission is due to dust heated