Optical studies of Blue Compact Dwarf Galaxies with the 1.23m telescope

P. Papaderos, K.G. Noeske, K.J. Fricke
Universitäts Sternwarte,  Geismarlandstraße 11, 37083 Göttingen, Germany
 
 




Blue Compact Dwarf Galaxies (BCDs) are low-luminosity (MB>-18 mag), gas-rich extragalactic systems

undergoing intense star formation (SF) activity (cf. e.g. Thuan & Martin 1981, hereafter TM81). A first deep
optical survey of such systems (Loose & Thuan 1985) revealed in the vast majority of them an evolved (few Gyr)
stellar low-surface-brightness (LSB) component underlying the regions of active star formation.
This finding went a long way in deciding the fundamental problem whether BCDs are young systems in their formation
process or whether these objects form a class of gas-rich old dwarf galaxies undergoing episodes of strongly enhanced
SF activity (starbursts). On statistical grounds these are separated by longer quiescent phases (Thuan 1991).
The Loose & Thuan imaging survey has furthermore demonstrated the morphological heterogeneity of BCDs and
resulted in the development of the first empirical classification scheme for these systems (cf. Fig. 1).

 
 
 
Mkn 996
Mkn 86
II Zw 40
SBS 0335-052 E

Fig. 1: The four main morphological subdivisions of BCDs in the classification scheme by Loose & Thuan (1985).

  Systems belonging to the nE and iE type comprise roughly 90% of the local BCD population; they exhibit a nuclear
  or irregular high-surface-brightness SF component on top a smooth elliptical/circular LSB host. In BCDs classified
  iI both the starburst and the LSB component are of irregular morphology. In BCDs belonging to the rare i0 type a
  regular stellar LSB host is virtually absent. Extremely metal deficient members of this class are thought to be promising
  candidates of young galaxies.

 

A central question to recent BCD studies concerns the origin of their recurrent starburst activity. Most BCDs

are known to be isolated with a fraction of them being arranged in loose groups close to void boundaries
(e.g. Campos-Aguilar & Moles 1991, Pustil'nik et al. 1995, Lindner et al. 1996, Popescu et al. 1999).
Dynamical interactions or merging with luminous companions are therefore not likely to be the generic
triggering agent for their starburst activity. Interaction with or accretion of a low-mass gaseous companion
offers a viable hypothesis which has motivated a number of recent interferometric studies of BCDs
(e.g. Taylor et al. 1993,1995). Alternatively, the transient starburst phenomenon in BCDs may be accounted
for by intrinsic processes, such as stochastic self-propagating SF (Gerola et al.1980), cloud-cloud collisions
or a cyclic process of gas infall from and expulsion into the halo.
Particularly the last-mentioned scenario calls for an investigation of the role of the older LSB stellar component
on the galaxy-wide SF process. While the LSB host contributes on average only one half of the B band luminosity
of a BCD within its 25 B mag arcsec-2 isophotal radius (Papaderos et al. 1996, Salzer & Norton 1998), it provides
due to its high M/L ratio the bulk of the stellar mass in the galaxy. If, therefore, dark matter does not dominate entirely
the mass of a BCD within its optical size, the stellar LSB host may strongly determine the gravitational potential within
which starburst activity takes place. Indeed, a recent study by Papaderos et al. (1996) provides circumstantial evidence
for a connection between the SF activity in BCDs and the physical properties of their LSB hosts and requires a thorough
investigation of this issue with larger BCD samples.
Surface photometry, i.e. measuring the photometric structure and colors of LSB component, is an essential tool here
and became an important piece of evidence in recent studies of such systems (Marlowe et al. 1997, Doublier et al. 1997;1999,
Telles et al. 1997, Smoker et al. 1999, Makarova et al. 1999, Vennik et al. 2000, Cairos et al. 2000).
Despite the significant progress made in the field in the past few years, a continuation of surface photometry studies
appears necessary for drawing firm conclusions on the build-up and evolutionary status of BCDs over their wide
morphological spectrum and exploring possible evolutionary links between them and other species of the dwarf galaxy
population (e.g. dwarf irregulars or dwarf ellipticals). In continuation of previous studies of BCDs (Papaderos et al. 1996,
Noeske et al. 2000) we recently started a broad-band imaging survey of such systems with the upgraded 1.23m telescope
at Calar Alto (see Calar Alto Newsletter N.1 January 2000). Our aim is to derive surface brightness and colour profiles of
roughly 40 nearby BCDs out to their Holmberg radius and infer therefrom the structural properties and age of their stellar LSB
hosts. In an initial stage of this campaign we observed in February 2000 a subset of 20 sample galaxies compiled from the
TM81 list. The telescope was equipped with a 2k SITe CCD detector yielding an instrumental scale of 0.5 arcsec pixel-1
and a usable field of view of ~11 arcmin in diameter. The typical sky background during the observations was
~22.2 B mag arcsec-2. Exposures were taken in B and R and have been splitted into at least two sub-exposures with
integration times ranging typically between ~15 and 30 min.
Here we shall briefly comment on the performance of the telescope after its recent upgrade regarding the capabilities
it offers for surface photometry studies. For this purpose we discuss next the surface brightness distribution of one of
the sample galaxies, Mkn 178, as derived from a 28 min sub-exposure in the B band. For a detailed review of the
spectrophotometric properties of this Wolf-Rayet galaxy the reader is referred to e.g. Zamorano & Rego (1986),
Gonzalez-Riestra et al. (1988), Conti 1991 and Guseva et al. 2000. An elaborated study of the stellar content of the
starburst component of Mkn 178 based on HST/NICMOS colour-magnitude diagrams has been recently accomplished by
Schulte-Ladbeck et al. (2000).

 

Mkn 178, Calar Alto 1.23m, B band 28 min
 
 

Fig. 2: B band exposure of the iE BCD Mkn 178 (D=4.2 Mpc; cf. Schulte-Ladbeck et al. 2000) taken with the Calar Alto 1.23m telescope.
  The integration time was 28 min and the seeing 1.05 arcsec (FWHM). Note the complex morphology of the high-surface-brightness region
  northeast of the main starburst region comprising an assembly of faint compact sources with a typical absolute B magnitude of -7.5 mag.
  The dominant starburst source a (MB~ -12 mag) contributes ~40% of B band emission in excess of the LSB host (cf. Fig. 3).
 
 
 

Mkn 178, B band, surface brightness profile
 

Fig. 3: Decomposition of the surface brightness profile of Mkn 178 in the B band in an exponential component

      approximating the intensity distribution of the LSB host (thick line) and the starburst emission in excess of the
      former (open circles). The profile is corrected for Galactic extinction (AB=0.08 mag). The effective radius reff
      as well as the isophotal radii of the starburst P25 and of the LSB host E25 at 25 B mag arcsec-2are indicated.

 

The surface brightness profile of Mkn 178 (Fig. 3) is typical of an iE BCD comprising three marked intensity regimes:

a compact intensity excess at small radii , an extended plateau-like feature at intermediate radii and an exponential
intensity decrease in the outskirts of the BCD dominating the light for a surface brightness level fainter than ~25
mag arcsec-2. The first two components are mainly due to the luminosity output of the young stellar content whereas
the last one originates from the old underlying LSB host. A simple decomposition of the SBP shows that the starburst
contributes within the 25 mag arcsec-2 isophotal radius (E25) ~60% of the B band emission. Its line-of-sight contribution
decreases monotonously with increasing equivalent radius R* from ~90% at the center to ~60% at its effective radius and
getting negligible beyond E25. The decreasing contribution of the starburst component in Mkn 178 shows up also in the
gradual increase of its B-R colour index from ~0.6 mag at the effective radius reff to ~1.1 mag for R*>E25.
From Fig. 3 is evident that an integration of ~30 min allows to trace the intensity distribution of the stellar LSB host down
to a surface brightness level fainter than 26.5 B mag arcsec-2. This shows that the upgraded 1.23m telescope offers an
interesting option for pursuing an extensive photometric study of nearby starburst galaxies at a reasonable expense of
observing time.

 From the deep surface photometry for a large number of BCDs we are currently deriving the structural properties

(central surface brightness, exponential scale length, colour gradient) of their LSB hosts and shall discuss them
together with the photometric structure of other dwarf galaxy types (e.g. dwarf irregulars, dwarf ellipticals) in order to
test scenarios for evolutionary connections and to detect clues to the origin of the BCD phenomenon.
Such an investigation will refine earlier results obtained by us (Papaderos et al. 1996).

 
 
 
 

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