Few systematic searches for WR populations outside the Local Group (or "WR galaxies") have been undertaken. In this Section we briefly summarise the studies explicitly devoted to the detection of WR signatures. A list of candidate WR galaxies resulting from some of these searches or found loosely in the literature is provided in Sect. 6. The WR galaxies issued from the searches described below are included in our list and represent the vast majority of detections. Let us now briefly summarise the properties of the spectroscopic and narrow-band imaging searches.
The first search for WR features in giant HII regions of nearby galaxies was carried out by D'Odorico et al. (1983). The latest update from their study is summarised by Rosa & D'Odorico (1986). Data from their work was included in the quantitative analysis by Arnault et al. (1989).
The most detailed search was undertaken by
Kunth & Joubert
(1985)
from a sample of 45 "lazy" galaxies (blue emission-line galaxies forming
stars by intermittent short bursts) from various sources.
In their statistical approach they measure the excess emission above the
continuum between 4600-4711 Å (rest wavelength) after subtracting
a typical nebular contamination taken as a function of the excitation
level and abundance. Their search yielded 19 regions (15 different objects)
with excess emission above 0.8
.
A systematic search for a broad WR bump in all the HII
galaxies included in the catalogue of
Terlevich et al. (1991)
was presented by
Masegosa et al. (1991).
Earlier publications using a subset of the same observational data had
also reported some WR detections and nebular HeII
4686
(Campbell & Smith
1986,
Campbell et al. 1986).
Positive detections were considered by Masegosa et al. when the "blue bump"
was at least 1 over the
continuum level and clearly discernible from the nebular HeII
4686 line. Their
search yielded 37 detections (~ 10% of the sample); 14 of these objects
have spectra with a spectral resolution of
5 Å FWHM,
which the authors estimate to be "good enough" to reliably detect WR stars.
Only these objects (their Table 2) were included in our list as confirmed
WR galaxies. The remaining objects are classified here as "candidates"
(Sect. 6).
Recently
Pindao (1998) and
Pindao et al. (1999)
have reanalysed the spectra from the Terlevich et al. catalogue and
100 additional
emission line galaxies for their WR content.
Objects with a clear detection of broad HeII
4686 are retained as
WR galaxies here
(See Pindao 1998.
The detection level corresponds to
0.8
, i.e. category 4 of
Pindao et al. 1999).
Category 3 objects (WR bump detection at ~ 0.5
) from
Pindao et al. (1999)
are classified here as "WR candidates".
Robledo-Rella & Conti (1993) presented a search for WR features in a selected sample of northern HII galaxies; candidates are given in Sect. 6.
First results from a new search for WR signatures in young starbursts have been presented by Contini (1996) and Kovo & Contini (1998).
An ongoing systematic search for WR galaxies has been mentioned by Huang et al. (1998).
According to Izotov (1998, private communication) the observational data
gathered primarily for accurate determinations of the helium abundance
since 1993 (see
Izotov et al., 1994,
1996,
1997a;
Thuan et al. 1995,
Izotov & Thuan 1998)
are being systematically re-analysed for their WR content
(Guseva et al. 1998,
Izotov et al. 1998).
Adding 10 newly observed objects, their sample mostly including blue
compact galaxies consists of ~ 70 spectra.
While the initial sample contained essentially very metal-poor
objects, metallicities up to ~ solar are now also included.
The majority of the WR detections have been mentioned in the above papers;
in total
Guseva et al. (1998) and
Izotov et al. (1998)
find 41 WR galaxies, defined
by broad emission between ~ 4620 - 4700 Å. Often several broad
features are pointed out in the blue bump (HeII
4686,
NIII
4640, but also other
lines they identify as NIII
4510,
NII 4565,
NV
4605,4620,
CIV4658).
According to their study 28 spectra also show broad CIV
5808.
Finally, few detections of broad HeII
5412,
CIII5696,
and also
CII4267
are signaled.
We retain all except one WR galaxy (Mrk 1026=NGC 848 showing no broad
HeII 4686)
from their study.
2.2. Narrow-band imaging and others
Drissen et al. (1993)
have conducted a search for HeII
4686
emission via narrow-band imagery in four low mass galaxies (GR8, NGC 2366,
IC 2574, NGC 1569). Two of them are now confirmed WR galaxies
(see above), IC 2574 remains to be studied spectroscopically, and
GR8 yielded negative results (no HeII).
Schmidt-Kaler & Feitzinger (1984) initiated a search for 30 Dor and NGC 604 like objects based on POSS, ESO-Blue, and SRC film. To the best of our knowledge results from this survey have not been published.