An H i line search for optically identified dwarf galaxy candidates in the M 81 group
Astron. Astrophys. Suppl. Ser.
An H i line search for optically identi ed dwarf galaxy candidates in the M 81 group
W. van Driel 2
R.C. Kraan-Korteweg 1
B. Binggeli 0
W.K. Huchtmeier 3
0 Astronomical Institute, University of Basel , Venusstrasse 7, CH-4102 Binningen , Switzerland
1 DAEC, Observatoire de Paris , 5 Place Jules Janssen, F-92195 Meudon Cedex , France
2 Nancay Radio Observatory, USN, Observatoire de Paris , 5 Place Jules Janssen, F-92195 Meudon Cedex , France
3 Max-Planck-Institut fu ̈r Radioastronomie , Auf dem Hu ̈gel 69, D-53121 Bonn , Germany
Sensitive 21 cm H i line observations were per- 1. Introduction formed for 23 dwarf members and possible members of the nearby M 81 group of galaxies, including ve objects of a clustering of extremely low-surface brightness objects of unknown nature. With the Nancay decimetric radio telescope the radial velocity range of −529 to 1826 km s−1 was searched to an rms noise of 2 − 4 mJy. Only three objects were detected. However, their high radial velocities (between 600 and 1150 km s−1) show them to lie behind the M 81 group. These three objects, classi ed as dS0: (UGC 4998) and Im (Kar 1N and UGC 5658), have H i masses of 0.4, 1.6 and 2:0 108M , for the assumed distance of 4 Mpc, and H i mass-to-blue light ratios of 0.04, 0.73 and 0.18 M /L ;B, respectively. Considering that half of the observed objects are classi ed as irregular dwarfs, hence expected to be relatively gas-rich, the resulting detection rate of about 1=3 is quite low. However, the mean redshift and velocity dispersion of the M 81 group (< V >= 101 km s−1, = 114 km s−1) suggest that the H i emission of low velocity H i -rich members of the M 81 group may still remain hidden within the strong Galactic H i emission (typically −150 < V < 115 km s−1) or, for the 6 dwarf candidates in the immediate vicinity of M 81, overshadowed by the very extended H i envelope encompassing M 81, M 82, NGC 3077, and NGC 2976 (−280 < V < 355 km s−1).
galaxies; distances and redshifts | galaxies; irregular | galaxies; ISM | radio lines; galaxies | galaxies; clusters of; M 81 group
An extensive survey of M 81 group dwarfs has been
carried out by
B¨orngen et al. (1982)
, resulting in a list
of dwarf members to a limiting absolute magnitude of
−11, for the adopted distance of 4 Mpc to the group.
They furthermore report the clustering of a number of
unusual, very low-surface brightness about 7 southeast of
M 81. Although classi ed as dwarf members of the M 81
group (B¨orngen et al. 1984), the nature of these objects
is uncertain. Are these remarkable objects dwarfs at the
extreme faint end of the luminosity function? And if true,
do they contain any gas and might thus be the lowest
H i -mass objects yet observed?
The goal of the survey presented here is to determine
the H i properties of the dwarfs of the M 81 group. Dwarf
galaxies in groups and in the eld generally are
irregulars
(Binggeli et al. 1990)
{ hence gas-rich. We therefore
searched at Nancay for 21 cm H i line emission in 23
canDriel, e-mail: didate dwarf galaxy members of the M 81 group with
considerably lower rms noise than previously obtained.
W. van
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2. The M 81 group Tautenberg Schmidt survey by Bo¨rngen & Karachentseva
(1982), the photometric work by
B¨orngen et al. (1982)
,
The M 81 group of galaxies is the most nearby rich con- and the photographic atlas of Karachentseva et al. (1985a)
centration of dwarf galaxies beyond the Local Group. For obtained with the 6 m SAO telescope. Note that we
reour study of possible dwarf members of the M 81 group of jected object No. 5 (Kar 54 = UGC 5954, at = 09h17m:6,
galaxies, we used the optically selected catalog of 41 mem- = 75 570) due to its high redshift of 659 km s−1, which
bers and possible members (dwarfs or otherwise) compiled was unknown in 1993. We also recti ed the identi cation
by Binggeli (1993). These data represent a compilation of Ho ix, erroneously named Ho iv in
Binggeli (1993)
.
of various publications, databases and private communi- The basic optical data of these objects are listed in
cations. Binggeli's catalog is strongly based on the 2 m Table 1. Binggeli's numbering was retained throughout
this paper. The coordinates were taken from NED and
are for the epoch 1950.0. All velocities are heliocentric
and calculated according to the conventional optical
definition (v = c = 0). Heliocentric velocities are from
de
Vaucouleurs et al. (1990
, RC3), except for the following
objects: No. 1 Holmberg ii
(Strauss et al. 1992)
, No. 12
Kar 3N
(Tikhonov & Karachentsev 1993)
, No. 19 Garland
(Karachentseva et al. 1985b)
, No. 22 UGC 5423 (Schneider
et al. 1992) and No. 36 Kar 73
(Tikhonov & Karachentsev
1993)
; note that the heliocentric velocity of 180 km/s
listed for No. 27, (...truncated)