GK Workshop - W.K. Huchtmeier et al.

Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Neutral Hydrogen in a sample of selected Blue Compact Dwarf Galaxies

W.K. Huchtmeier¹, Gopal Krishna², and A. Petrosian³

¹Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²NCRA-TIFR, Pune University Campus, Pune - 411 007, India;
³Astronomical Observatory of Byurakan, Armenia

Received 17th February 1998

Abstract. We present H I-observations of 56 selected nearby blue compact dwarf galaxies with the Effelsberg 100 m radiotelescope of which 45 have been detected. The relative content of H I (M_H I/L_B) of these BCD's is on the average comparable to that of the sample of nearby galaxies for the same range in luminosity.

1. Introduction

A small but remarkable minority of nearby galaxies, called Blue Compact Dwarf Galaxies (BCDG), are characterized by not just a low intrinsic luminosity (M_B ≥ -17) and a barely resolved appearance on the Palomar-Sky-Survey plates, but also a blue color, low metallicity (∼ 10% solar) and sharp narrow emission lines superposed on a blue continuum (Sandage & Binggeli 1984; Thuan 1991). On high-resolution images BCDGs appear as a small group of emission knots (star forming 30 Doradus type) often distributed around the center of an underlying faint fuzz resembling an irregular galaxy. Due to the low metallicity and the considerable gas consumption involved in the violent star formation, it is mostly believed that unlike spirals, the star formation in BCDGs takes place in transient sporadic bursts within either an already evolved galaxy or, possibly, in some cases like I Zw 18, in a relatively young galaxy forming out of unprocessed primeval material (Kunth et al. 1994). Such gas-rich, metal-pour dwarfs offer a rare opportunity to view the galaxy formation from close proximity.

2. Observations

Observations were performed using the 100 m radiotelescope at Effelsberg which has a half power beam width of 9.3' at the wavelength of 21 cm. The 1024 autorrelator was split into four filter banks (256 channels each) using a bandwidth of 6.25 MHz which yielded a velocity resolution of 6 km s^-1 or 10 km s^-1 after Hanning smoothing. A typical observing time of 60 min per source resulted in an r.m.s. noise of 4 mJy (the system noise was 30 K).

3. Data

From our searchlist of 56 we detected 45 galaxies. This high detection rate of 80% is due to the relativly small distance of our sample and to the relativly high H I-content of these 'late-type' systems. The H I profiles are shown in Fig. 1. The narrow profiles correspond to dwarfish galaxies (i.e. M_B,T^0,i ≥ -15). In these profiles we cut the local H I emission (in fact the difference of the local emission between the source and the comparison field) as it was much stronger than the faint extragalactic emission.

[Click here to see Fig. 1!]

References

Kunth D., Lequeux J., Sargent W.L.W., Viallefond F., 1994, A&A 282, 709
Sandage A., Binggeli B., 1984, AJ 89, 919
Thuan T.X., 1991, ApJ 370, 25

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First version:	09th	March,	1998
Last update:	29th	September,	1998