Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982a%26a...110...79v&link_type=abstract
Astronomy and Astrophysics, vol. 110, no. 1, June 1982, p. 79-94.
Astronomy and Astrophysics
Astrophysics
129
Astronomical Photometry, Disk Galaxies, Galactic Structure, Mass Distribution, Spiral Galaxies, Star Distribution, Astronomical Models, Brightness Distribution, Chemical Evolution, Galactic Evolution, Isophotes, Mass To Light Ratios, Spheroids, Surface Properties
Scientific paper
We present three-colour surface photometry of the edge-on spiral galaxy NGC 7814. The light distribution of this galaxy is dominated by its spheroid. We fit the light distribution of the spheroid with high accuracy (0.1-0.2 mag arc s-2) using a model in which the radial dependence is an R1/4--law and the isophotes at all radii and in all three colours have an axis ratio of about 0.57. The colours of the spheroid become progressively bluer with increasing radius and the isochromes are closely similar in shape to the isophotes. The colour gradient results, presumably, from a decrease in the mean heavy element abundance with increasing radius.
Subtraction of the light predicted by the spheroid model from the observed total light distributions reveals the residual disk. This has colours similar to those of an old disk population and it has been fitted with the disk model proposed in Paper I of this series. The disk involves only a few percent of the total mass of NGC 7814, 50 that the spheroid dominates the gravitational field. This is presumably the reason that the spheroid of NGC 7814 is not flattened in its outer parts (as is the case in NGC 4565, 891, and 4013 described in Papers 1-111) and why the dust lane and the old stellar disk are much thicker in the z-direction than is the case in the later type spirals.
The sum of our model disk and spheroid reproduces the observed surface brightness distributions everywhere - except near the dust lane - to 0.2 mag arc s-2 or better. There are no appreciable populations with flattenings intermediate between those of the disk and spheroid. This fact, together with the - observed similarity of isochromes and isophotes and the uniformity of isophotal axis ratios in the spheroid present difficulties - for models in which galactic halos form by a gaseous condensation process.
We report Westerbork H I synthesis observations of NGC 7814. These observations show that the H I is located in a flat, thin disk out to 4'.5-5', a radius coinciding with the galaxy's optical extent. There may be a warp on the SE side beyond this radius. The rotation curve is flat from 0;75 galactocentric radius out to at least 4'.5-5' at 225 ± 15 km s-1. The total mass within this radius (22 kpc) is 2 1011 Msun km s-1 assuming a distance of 15 Mpc. Of this mass ∼0.5% is in the form of H I and ∼2% in the form of disk stars.
We estimate that the mass-to-light ratio in the spheroid increases by an order of magnitude from 14 to 160 Msun/Lsun over the range 3-22 kpc in galactocentric distance. We also show that this mass and luminosity distribution in the spheroid combined with the stellar velocity field (Kormendy and Illingworth, 1981) predicts that the velocity dispersion is roughly isotropic in the centre and at greater distances from the centre becomes increasingly anisotropic, the radial velocity dispersion increasing with galactocentric distance. We infer from the abundance structure of the spheroid that metal-rich halo stars formed in the protogalaxy in locations such that they had larger binding energy than metal-poor ones. If the dark matter consists of faint stars or black dwarfs that formed together with luminous halo stars, the observed abundance gradient and the M/L gradient may be causally related.
Searle Leonard
van der Kruit Pieter Corijnus
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