We have observed 23 dwarf galaxy candidate members of the M 81 group of galaxies, including 5 peculiar objects of unknown nature. Of the 18 with given morphology, half are early types (dE or dS0: and one dE/Im:), and half are late types (Im or Sm). The latter 9 objects should in principle be gas-rich, hence detectable at Nançay with the sensitivity obtained here.
However, in the present survey only 3 objects were detected in the HI line. The strong signal in the spectrum of No. 11 (Kar 59) at 1323 originates from a nearby bright spiral. None of the 3 detected dwarfs seem to be a member of the M 81 group of galaxies, however, as their redshifts of 632, 570 and 1159 clearly show them to be behind the group. If we calculate distances directly from their recession velocites corrected to the LSR following the precepts given in Sandage & Tammann (1987), and a Hubble constant of , the galaxies UGC 4998, Kar 1N and UGC 5658 are at distances of 15.9, 14.7 and 26.6 Mpc and have HI masses of 0.4, 1.6 and 2.0. The resulting HI mass-to-blue light ratios are 0.04, 0.73 and 0.18 , respectively.
The profile of Kar 1N is narrow ( = 61 ) and clearly Gaussian shaped, that of UGC 4998 is a bit broader (89 ) and more flat-topped, while that of UGC 5658 ( = 127 ) least resembles a typical dwarf line profile.
The two detected objects classified as magellanic irregulars (Kar 1N and UGC 5658) have quite discrepant ratios (0.73 and 0.18 ) for their similar morphological type. In particular, the ratio of 0.18 for UGC 5658 seems very low for an irregular dwarf, whereas the extremely low ratio of 0.04 for the dS0: galaxy UGC 4998 seems quite consistent with the upper limit of about 0.15 found for morphologically pure lenticulars (Knapp et al. 1989).
For comparison, the global optical and HI line properties of all 11 previously detected M 81-group dwarf galaxies are given Table 3 (click here). The values are taken from the Huchtmeier & Richter (1989) and Schmidt & Boller (1992) compilations, with the exception of the integrated fluxes for the systems M 81, M 82, and NGC 3077 interconnected in HI, as well as HoIX, which are adopted from the VLA synthesis analysis by Yun et al. (1994).
|(h m s)||( )||(mag)||(km/s)||(Jy km/s)||(km/s)||()|
|1.||HoII||08 13 53.5||70 52 13||Im||7.9||10.2||158||359.7||79||0.59|
|2.||Kar 52||08 18 43.0||71 11 25||Im||1.3||14.2||114||3.7||38||0.25|
|3.||DDO 53||08 29 33.3||66 21 08||Im||1.5||13.6||19||23.7||46:||0.89|
|4.||UGC 4483||08 32 07.0||69 57 16||Im||1.1||13.9||157||3.1||70||0.16|
|7.||HoI||09 36 00.9||71 24 55||IABm||3.6||12.2||136||49.0||45||0.50|
|9.||NGC 2976||09 43 11.5||68 08 45||SAcp||5.9||10.9||3||63.6||159||0.20|
|13.||M 81||09 51 27.3||69 18 08||Sb||26.9||7.9||-34||859.9||464||0.19|
|14.||M 82||09 51 43.6||69 55 00||Amorph||11.2||9.3||203||245.2||290||0.20|
|17.||HoIX||09 53 28.0||69 16 53||Im||2.5||13.5||46||94.8||120:||3.63|
|18.||NGC 3077||09 59 21.9||68 58 33||Am||5.4||10.6||14||212.5||93||0.57|
|22.||UGC 5423||10 01 25.3||70 36 27||BCD||0.9||13.8||343||2.3||80||0.11|
|26.||IC 2574||10 24 41.3||68 40 18||Sm||13.2||11.0||47||442.5||126||1.52|
|31.||DDO 87||10 46 17.0||65 47 40||Im:||2.4||14.9||338||18.9||80||2.35|
The average 3 upper limit of 10.5 mJy and an assumed line width of 50 imply an upper limit of 0.52 Jy to the HI line flux for the objects not detected in our survey. This implies an upper mass limit of 2.4 106 (and even less for the "anonymous'' low-surface brightness objects of unknown nature) at the adopted distance of 4 Mpc to the M 81 group, and upper limits to the ratios of 0.08 and 0.5 for a 15, respectively a 17 magnitude galaxy.
However, the non-detections are unlikely due to a lack of gas in these dwarfs. Inherent to HI line searches for nearby objects is always the problem of confusion by or with strong Galactic HI lines, i.e. emission lines of external galaxies being lost among strong Galactic lines, or being interpreted as part of the Galactic emission (see, e.g., the case of the discovery of Dwingeloo 1 by Kraan-Korteweg et al. 1994). We have noted the velocity ranges dominated by Galactic confusion for each object in Table 2 (click here), i.e., the range in which we estimate that the profile of a typical dwarf with a peak intensity of 10 mJy would not be recognized as such. The average range is about -150 to 115 \ with about 50 for the 2 narrowest profiles. As the mean velocity of the 18 members of the M 81 group with known redshifts listed in Table 1 (click here) is <V> = 101 , with a dispersion of = 114 , this means that, statistically speaking, more than half of them would be lost among strong Galactic HI lines if they were all gas-rich. In fact, 5 of the galaxies observed at Nançay have known optical redshifts (see Table 2 (click here)), which all fall within the velocity range obscured by Galactic HI.
In the central part of this group, i.e. in an area of about 1.5 around the galaxy M 81, the confusion problem is even worse. Here, M 81, M 82, NGC 3077, and - according to Appleton et al. (1981, their Fig. 2 (click here)) - also NGC 2976 are embedded in a common, very extended HI cloud with HI bridges connecting the major galaxies, and distinct HI clumps, some of which coincide with optically identified dwarfs.
Seven dwarfs (Kar 2N, Kar 3N, A9562+68, Kar 61, Ho IX, Garland, and Kar 5N) reside in this area where the strong HI emission (within about -280 to +355 ) from the larger interacting galaxies makes the detection of gas-rich dwarfs extremely difficult (local estimates for the confusion range are given for each dwarf candidate in the last column of Table 2 (click here)). So far, only the dwarf Ho IX has been unambigously associated with HI. None of the 6 dwarfs observed at Nançay were detected. If we regard Fig. 1 of Yun et al. (1994), however, it seems clear that the irregular galaxy A952+69 must be associated with the second HI concentration visible in that image (the first being Ho IX). Appleton et al. (1981) even suggest that the HI concentration in the southern tip of the spiral arm of M 81 visible in HI is due to Kar 61 (No. 16) and that the south eastern extension in the HI distribution of NGC 3077 might be due to the starforming galaxy Garland.
Even with the above indications for gas in three of the central dwarf galaxies, single dish observations cannot resolve these signals from the stronger emission of their dominant companions - or the Galaxy. In case of the two objects with the broadest confusion range in the Nançay beam (No. 15 = A952+69, -270 to +355 , and No. 16 = Kar 61, -260 to +190 ), we have obtained shorter observations pointed to the galaxy's centre and one HPBW (4') due East and one due West, in order to verify whether part of the signal could be assigned to the dwarf candidate in question. A comparison of these on and off-source profiles (Fig. 3 (click here)) confirms the signal to originate from our Galaxy and the HI complex around M 81, as they have about the same velocity extent at and near each galaxy, whose optical diameter is considerably smaller than the beam width.
Not even the detailed HI maps from synthesis observations can resolve these problems. The assignment of individual HI concentrations (or filaments) to optically identified dwarf galaxies remains ambiguous within this highly active region where the gas might have been swept away from the currently visible starforming dwarfs.
Here spectroscopy, in particular the determination of the recession velocities from emission lines in starforming galaxies, might be the only approach to learn more about the kinematics in the M 81 group - not only in the central part of the group, but also within the larger group boundary.
Three new detections of dwarf candidates in the M 81 group have disproven these objects to be members of the M 81 group. Due to confusion problems the non-detections do not a priori imply that the other observed dwarf candidates are gas-poor, although the high-sensitivity observations clearly demonstrate that they cannot be gas-rich dwarfs just beyond the M 81 group. In order to study the dynamics of the M 81 group in further detail, in particular the behaviour of the dwarfs within the gravitational potential well of the group as such and the massive galaxies within it, spectroscopic observations of the remaining dwarf candidates without velocity information are required.
We would like to thank Drs. L. Bottinelli, L. Gouguenheim and G. Theureau for making their flux density calibration data available to us, and V. Etieve for her help with the illustrations. The Nançay Radio Observatory is the Unité Scientifique Nançay\ of the Observatoire de Paris, associated as Unité de Service et de Recherche (USR) No. B704 to the French Centre National de Recherche Scientifique (CNRS). The Observatory also gratefully acknowledges the financial support of the Région Centre in France. The research by RCKK is being supported with an EC grant. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.