Detalles de publicación
PP 010087
Properties of the HII regions of M51 and NGC 4449 from Halpha images with ACS on HST.
1.Universidad Nacional Autónoma de México, Instituto de Astronomía, Ensenada, B.C.México. 2. Instituto de Astrofísica de Canarias. 3.Consejo Superior de Investigaciones Científicas, España. 4. Facultad de Física, Universidad de La Laguna 5. Universidad Nacional Autónoma de México, Centro de Radioastronomía y Astrofísica, Morelia, México.
We have used the images from the ACS on HST in H , and in the neighboring
continuum, to produce flux calibrated images of the large spiral galaxy M51, and
the dwarf irregular NGC 4449. From these images we have derived the absolute
luminosities in H , the areas, and the positions with respect to the galactic
centers as reference points, of over 2600 Hii regions in M51 and over 270 Hii
regions in NGC 4449. Using this database we have derived luminosity (L)–
volume (V) relations for the regions in the two galaxies, showing that within
the error limits these obey the equation L ∼ V 2/3, which differs from the linear
relation expected for regions of constant uniform electron density. We discuss
briefly possible models which would give rise to this behavior, notably models
with strong density inhomogeneities within the regions. Plotting the luminosity
functions for the two galaxies we find a break in the slope for M51 at log(L) = 38.5
dex (units in erg s−1) for M51 in good agreement with the previous ground-based
study by Rand, and above this luminosity NGC 4449 also shows a sharp decline
in its luminosity function, although the number of regions is too small to plot
the function well at higher luminosities. The cumulative diameter distribution
for the Hii regions of M51 shows dual behaviour, with a break at a radius close
to 100 pc, the radius of regions with the break luminosity. Here too we indicate
the possible physical implications.
continuum, to produce flux calibrated images of the large spiral galaxy M51, and
the dwarf irregular NGC 4449. From these images we have derived the absolute
luminosities in H , the areas, and the positions with respect to the galactic
centers as reference points, of over 2600 Hii regions in M51 and over 270 Hii
regions in NGC 4449. Using this database we have derived luminosity (L)–
volume (V) relations for the regions in the two galaxies, showing that within
the error limits these obey the equation L ∼ V 2/3, which differs from the linear
relation expected for regions of constant uniform electron density. We discuss
briefly possible models which would give rise to this behavior, notably models
with strong density inhomogeneities within the regions. Plotting the luminosity
functions for the two galaxies we find a break in the slope for M51 at log(L) = 38.5
dex (units in erg s−1) for M51 in good agreement with the previous ground-based
study by Rand, and above this luminosity NGC 4449 also shows a sharp decline
in its luminosity function, although the number of regions is too small to plot
the function well at higher luminosities. The cumulative diameter distribution
for the Hii regions of M51 shows dual behaviour, with a break at a radius close
to 100 pc, the radius of regions with the break luminosity. Here too we indicate
the possible physical implications.