Astron. Astrophys. Suppl. Ser. 134, 317-326
Send offprint request: U. Hopp
Universitätssternwarte München, Scheiner Str. 1, D-81679
München, Germany,
e-mail: hopp.at.usm.uni-muenchen.de
Received June 16; accepted August 27, 1998
B and R CCD images and J NICMOS3 frames taken with the Calar Alto 3.5 m
telescope of the dwarf irregular galaxy UGC 685 are presented. The
brightest part of the stellar population is resolved in B and R, very few
also in J. The stellar color-magnitude diagram is discussed. An
estimate of the distance to UGC 685 of 5.5 Mpc is derived
based on the brightest blue supergiant stars. Most of the resolved
bright stars show colors in the range .
The continuum light of the unresolved stars is distributed rather
regular in B, R, and J, showing only minor irregularities. This
surface brightness distribution follows an exponential law in all
three colors with a scale length kpc. The central surface
brightnesses are
,
, and 20.11
0.11 mag
, in B, R, and J,
respectively. The surface
brightness can be traced out to 5
in B and R. Thus, UGC 685
belongs to the class of dwarfs where the HI distribution is much more
extended (here 2.6 times) than the optical (stellar) light
distribution, but contrary to many objects of this type, it does not
belong to the class of low-surface brightness objects. No color
gradient was detected in UGC 685 except that the very center is
slightly bluer. The overall colors are B-R = + 0.97, B-J = +1.55
and the magnitude is B = 14.55 (MB0 = -14.5).
The classification as an irregular dwarf from survey plates results from
the few HII regions of UGC 685 which are all concentrated to the
South-East of the center of the galaxy. On an H Calar Alto
2.2 m telescope CCD image, I identified only 5 HII regions, one of them
being rather bright. The total H
flux corresponds to a mildly
on-going star formation with a rate of 0.003
yr-1, a
low rate even in comparison to other dwarfs.
The available (and limited) data do not indicate any major deviation
from this rather low star formation rate within the last 109 yr.
Key words: galaxies -- dwarfs; galaxies -- structure; galaxies -- distances
Copyright The European Southern Observatory (ESO)