Bibcode
DOI
Muñoz-Tuñón, Casiana; Caon, N.; Aguerri, J. Alfonso L.
Bibliographical reference
The Astronomical Journal, Volume 127, Issue 1, pp. 58-74.
Advertised on:
1
2004
Citations
32
Refereed citations
29
Description
We revise the explanation for the ``expanding H II regions ring'' of NGC
4736. From the analysis of long-slit spectra, we identify structures
within the emission region previously reported in the literature as a
single ring (former parameters: R~47", FWHM=21"). The H II ring is
located at 35" (1.12 kpc) galactocentric distance, and its thickness is
10". The line width at the ring location is typical of H II regions,
whereas a higher velocity dispersion (chaotic motions) is measured just
outside. The location of the H II ring and of the outer stellar (300")
ring are defined by the resonances obtained from the stellar rotation
curve. After modeling the surface brightness profile into the different
structural components, we conclude that the massive triaxial bulge
(re=10", (1-b/a)re~0.25) produces the
galactic resonance pattern. We also find evidence for a fossil starburst
nucleus of about 150 pc size. The bulge and the inner bar drive disk gas
motion, causing inward movements outside the H II ring and outward just
inside, thereby accumulating material to trigger star formation on the
ring. In the central part the bar drives the gas toward the center,
which explains the substantial amount of gas in the nucleus in spite of
the presence of a fossil starburst. The peculiar motions reported in the
literature in reference to the ionized gas of the H II ring can be
understood as infalling gas encountering the shock waves generated by
the starburst knots on the H II ring and being raised above the galaxy
disk. The scenario of star formation propagating from the nucleus
outward used to explain the apparent expanding motion of the Hα
ring is not fully supported, in light of a comparison of the location of
the Hα ring with that of the FUV ring. The FUV ring peaks at about
45"-48", which might point to an inward-propagating star formation
scenario.