Structure and stellar content of dwarf galaxies - III. B and R photometry of dwarf galaxies in the M 101 group and the nearby field
Astron. Astrophys. Suppl. Ser.
Structure and stellar content of dwarf galaxies?
T. Bremnes 1 2
B. Binggeli 1 2
P. Prugniel 0 1
0 Observatoire de Lyon , F-64561 St. Genis-Laval Cedex , France
1 Send o print requests to: T. Bremnes
2 Astronomical Institute, University of Basel , Venusstrasse 7, CH-4102 Binningen , Switzerland
We have carried out CCD photometry in the Cousins B and R bands of 21 dwarf galaxies in and around the M 101 group. Eleven are members and suspected members of the group and ten are eld galaxies in the projected vicinity of the group. We have derived total magnitudes, e ective radii, e ective surface brightnesses, as well as galaxy diameters at various isophotal levels in both colours. Best- tting exponential parameters and colour gradients are also given for these galaxies. Some of the galaxies show a pronounced luminosity excess above the best- tting exponential at large radii, or surface brightnesses fainter than 26 mag=ut00 in R. This feature, while non-signi cant for a single case and technically di cult to interpret, might be an indication of the existence of an extended old stellar halo in dwarf irregulars. The photometric parameters of the galaxies presented here will be combined with previously published data for nearby dwarf galaxies and statistically analysed in a forthcoming paper.
galaxies; general | galaxies; fundamental parameters | galaxies; photometry | galaxies; irregular | galaxies; structure | galaxies; luminosity function
1. Introduction
A good knowledge of the local galaxian neighbourhood is
prerequisite for an understanding of the distant (early)
universe. Deep images, such as the spectacular Hubble
Deep Field, can only be interpreted properly if the
dwarf galaxy content of the local universe is very well
known. However, studies of nearby dwarf galaxies have
Most galaxies within 10 Mpc distance are organized
into a small number of well-known groups of galaxies;
essentially these are the IC 342, M 81, M 101, CVn I,
Cen A, and Scl groups
(Schmidt & Boller 1992b)
.
Following previous work on the M 81 group dwarfs
(Bremnes et al. 1998; Lesa re et al. 1999)
, herefater
Papers I & II, we here present CCD photometric data in
the B and R photometric bands for the 11 known M 101
group dwarf members, as well as 10 eld dwarfs in the
vicinity of M 101. A short description of the M 101 group
is given in the following section. The photometric data
presented here (Sect. 4) will be combined with those of
Notes: Columns 5 and 6: 2000.0 epoch coordinates taken from the NED.
Column 7: Dwarf type reckoned by B.B. on the system of
Sandage & Binggeli (1984)
.
Columns 8 and 9: Diameter at = 25 mag=ut00 and total apparent blue magnitude from the present photometry, from
Schmidt & Boller (1992a) (1), or from other sources compiled by one of us (B. B.) (2).
Column 10: heliocentric velocity from NED.
previous data papers. An interpretation and scienti c but found only one additional possible member (the BCD
discussion of this material is planned to follow in a future MGC 9-23-21). A recent blind H I survey of the M 101
paper of this series. area has also not resulted in a single new member of the
group (Kraan-Korteweg et al. 1999, in preparation). It will
take surveys of highly increased sensitivity to uncover the
2. Sample and imaging sought-for exponential rise of the luminosity function of
the M 101 group.
The M 101 group, with D 6:5 Mpc (Karachentsev 1996),
is the most distant one in the 10 Mpc volume. It is also
the poorest group of all, including the LG. It is completely In Table 1 we give a complete list of the 11 presently
dominated by M 101 itself: the second-ranked group mem- known members (M) and possible members (PM) of the
ber, NGC 5585, is already 3 magnitudes fainter than M 101 group as well as 10 eld (F) dwarfs that were
M 101. Only 13 members and possible members of the imaged during the same run. This list was prepared by
group are known to date, half of which are lying very close B. Binggeli based on the catalogue of Schmidt & Boller
to M 101 and can therefore be regarded as M 101 satel- (1992a). A map showing the distribution of these objects
lites. With one exception (the dwarf elliptical UGC 8882) on the sky is shown in Fig. 1, where the galaxies are coded
they are all late-type dwarfs (Sd, Sm, Im). One peculiar according to their type and group membership. A gallery
feature of the group is its luminosity function: the popu- of images is given in Fig. 2. It should be noted that the
oblation of very faint and di use dawarfs (elliptical or irreg- jects listed in Table 1 and the images displayed in Fig. 2
inular), which is so frequent elsewhere, is apparently simply clude all M 101 group members known to date with the
exmissing here (the faintest member known is as bright as ception of the two giant members M 101 and NGC 5585 for
MB −14). We have therefore made an attempt to nd which data and images are given in Sandage & Tammann
new candidate members (...truncated)