E. Laurikainen et H. Salo, BVRI imaging of M51-type interacting galaxy pairs - III. Analysis of the photometric parameters, M NOT R AST, 324(3), 2001, pp. 685-698
Photometric bulge and disc properties of a sample of 21 M51-type pairs obse
rved in BVRI bands are analysed, and the derived parameters obtained by the
R (1/4)-law and exponential bulge models are compared with the sample of n
ormal galaxies reported by de Jong & van der Kruit in 1994. Some characteri
stic properties of the intensity profiles are then interpreted by 3D N-body
simulations, in which both components of the pair are described by an expo
nential star + gas disc embedded in a live bulge and halo.
The disc central surface brightness mu (0) was found to be similar for the
M51-type and for normal galaxies, being 21.5 +/- 0.8B mag arcsec(-2), when
the exponential bulge model was used. The main galaxies had generally almos
t normal photometric properties, whereas for the companions secular evoluti
on must have played an important role. Even though the mean scalelengths fo
r the M51-type galaxies were smaller than for the comparison galaxies, thei
r bulges were larger: especially the companions had extremely large bulge s
izes relative to their disc scalelengths having [r(e)(bulge)/h] (B) = 0.37
+/- 0.10 in comparison with 0.15 +/- 0.09 for normal galaxies. Consequently
, the bulge-to-disc luminosity ratios for the companions were also generall
y larger than known for any of the Hubble types of normal galaxy.
N-body simulations indicated that the inner disc can be steepened during th
e interaction, and that this steepened disc can be erroneously interpreted
as belonging to the bulge. Simultaneously material in the outer disc is red
istributed, producing shallow outer profiles similar to those observed for
typical M51-type galaxies. These processes were demonstrated by applying ph
otometric bulge-to-disc decompositions for some of the simulated intensity
profiles. The decompositions also showed that one must be cautious when app
lying the R (1/4)-law bulge model, especially for interacting galaxies. A n
early parabolic experiment showed that the redistribution of matter in the
disc during the close passage can be long-lived, suggesting that galaxy int
eractions even with small companions may play an important role in secular
evolution for galaxies in clusters.