Structure and stellar content of dwarf galaxies - III. B and R photometry of dwarf galaxies in the M 101 group and the nearby field

Astronomy and Astrophysics Supplement Series, Jun 1999

We have carried out CCD photometry in the Cousins B and R bands of 21 dwarf galaxies in and around the M101 group. Eleven are members and suspected members of the group and ten are field galaxies in the projected vicinity of the group. We have derived total magnitudes, effective radii, effective surface brightnesses, as well as galaxy diameters at various isophotal levels in both colours. Best-fitting exponential parameters and colour gradients are also given for these galaxies. Some of the galaxies show a pronounced luminosity excess above the best-fitting exponential at large radii, or surface brightnesses fainter than in R. This feature, while non-significant for a single case and technically difficult 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.

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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)


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T. Bremnes, B. Binggeli, P. Prugniel. Structure and stellar content of dwarf galaxies - III. B and R photometry of dwarf galaxies in the M 101 group and the nearby field, Astronomy and Astrophysics Supplement Series, 1999, pp. 337-350, Volume 137, Issue 2, DOI: 10.1051/aas:1999486