Bibcode
DOI
Knapen, Johan H.
Referencia bibliográfica
Publications of the Astronomical Society of the Pacific, v.105, p.323
Advertised on:
3
1993
Número de citas
3
Número de citas referidas
3
Descripción
An observational study of large-scale star formation processes in spiral
galaxies with well-defined arm systems is presented. Combining new data
on the current massive star formation (from H-alpha imaging) and on the
total gas distribution (both atomic and molecular hydrogen, from H I and
CO observations), the star formation efficiencies in- and outside the
spiral arms are compared. For the grand-design spiral galaxy NGC~4321,
new 21cm H I (VLA, New Mexico), J>CO (45-m telescope, Nobeyama,
Japan) and H-alpha observations (4.2-m William Herschel Telescope, La
Palma, Spain) were obtained, whereas for M51 data from the literature
were used. The spatial resolution of all these data is high enough
(typically 15") to distinguish arm from inter-arm emission. From an
analysis of the massive star formation efficiency (MSFE) along the main
arms of NGC~4321 and M51, and comparing with the neighboring inter-arm
regions, it is found that the MSFE is higher in the arm regions than
between the arms by an average factor of 2 to 3 in both galaxies. This
enhancement is shown to be due only to enhanced star formation in the
arms, and cannot be caused by e.g. dust extinction in the H-alpha, or by
metallicity effects in the disks of the galaxies that may cause a change
in the CO to H_2 conversion factor. This result is direct evidence for
triggering of the star formation in the arms. In the case of M51, the
arm/inter-arm ratio of the MSFE (called epsilon) shows a well-defined
two-fold symmetric pattern between the two main arms, indicating that a
global mechanism, probably a density wave system, organizes the star
formation in the disc of that galaxy. In NGC 4321 such a pattern is
absent, and epsilon is constant along the arms. The absence of a
symmetric pattern in epsilon may be related to the presence of a bar in
NGC 4321. Evidence for the existence of a bar in this galaxy is
presented: a distortion in the H I velocity field, as well as the
morphology of the galaxy in H-alpha and in the near-infrared. Such an
anti-correlation between the presence of a bar and the presence of a
symmetric pattern in the distribution of the star forming regions along
the arms is seen among a small sample of other grand-design galaxies
studied in H-alpha, although a statistical study is needed to generalize
this finding. From along-the-arm profiles in H I, CO, and H-alpha,
evidence is presented that in both M51 and NGC 4321 most of the atomic
hydrogen in the star forming disk of the galaxy is formed by
photo-dissociation of molecular hydrogen by young massive stars. From
the 21 cm line study of NGC 4321, it is found that the H I disk of NGC
4321, in general limited to the extent of the optical disc, has a large
though faint extension toward the SW, which may well have been caused by
a close passage of the companion galaxy NGC 4322. A large-scale
distortion of the H I velocity field is seen in the same direction.
Other distortions in the velocity field due to the presence of a
non-axisymmetric potential and of density wave streaming motions are
also detected in H I. (SECTION: Dissertation Abstracts)