An H  i line search for optically identified dwarf galaxy candidates in the M 81 group

Astronomy and Astrophysics Supplement Series, Feb 1998

Sensitive 21 cm H I  line observations were performed for 23 dwarf members and possible members of the nearby M 81 group of galaxies, including five objects of a clustering of extremely low-surface brightness objects of unknown nature. With the Nançay decimetric radio telescope the radial velocity range of -529 to 1826  was searched to an rms noise of . Only three objects were detected. However, their high radial velocities (between 600 and 1150 ) show them to lie behind the M 81 group. These three objects, classified as dS0: (UGC 4998) and Im (Kar 1N and UGC 5658), have H I   masses of 0.4, 1.6 and , for the assumed distance of 4 Mpc, and H I   mass-to-blue light ratios of 0.04, 0.73 and 0.18 , respectively.
Considering that half of the observed objects are classified as irregular dwarfs, hence expected to be relatively gas-rich, the resulting detection rate of about is quite low. However, the mean redshift and velocity dispersion of the M 81 group ( , ) 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 115 ) 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 ( ).

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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 * * * * * * * * * * * * P P P P P P P P P P P P P P 1. 2. 3. 4. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 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)


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W. van Driel, R. C. Kraan-Korteweg, B. Binggeli, W. K. Huchtmeier. An H  i line search for optically identified dwarf galaxy candidates in the M 81 group, Astronomy and Astrophysics Supplement Series, 1998, pp. 397-408, Volume 127, Issue 3, DOI: 10.1051/aas:1998108