Received 11th March 1998
Abstract.
We present first results of a multi-color photometric study of a sample
of Blue Compact Dwarf Galaxies (BCDs) observed with CAFOS at the
Calar Alto 2.2 m telescope.
For the BCDs Mkn 5 and II Zw 33 a and b, surface
brightness- and color profiles were derived from exposures in the Johnson
B-,
R- and
I- bands.
Color profiles and color maps may be used to disentangle the old and young
stellar populations that form the underlying low-surface brightness (LSB)
and starburst components.
Surface brightness profiles provide information on the structural properties
and mass distribution of the LSB component, i.e. parameters necessary for the
understanding of the physical origin of starbursts in BCDs.
1. Introduction
Since common properties of BCDs, such as their high gas content
(Mgas/Mtot ≅0.3,
Thuan & Martin 1981), low metallicity
(Zsun/50 < Z < Zsun/3)
and enhanced star formation are thought to have prevailed during the galaxy
formation epoch, BCDs may serve as nearby laboratories to study processes
which otherwise can be monitored only in high-z environments.
Star formation in BCDs is known to proceed in intermittent bursts
lasting less than 107 yr and being separated by quiescent
phases longer than 1 Gyr (Thuan 1991).
Since most BCDs seem to be isolated (e.g. Lindner et al. 1996),
this phenomenon is likely to result from internal processes only.
To understand such processes, it is crucial to obtain information on e.g.
the stellar and gas mass distributions,
i.e. the baryonic part of the gravitational potential within which gas
collapses in BCDs.
For the star formation histories, color profiles and color maps provide
integral as well as spatially resolved information.
The stellar mass distributions can then be obtained from a decomposition
of the surface-brightness profiles into a starburst component and an older
underlying stellar LSB-component which contains most of the system's baryonic
mass within its optical extent (cf. Papaderos et al. 1996).
2. Observations
The instrumentation on the 2.2 m telescope of the German-Spanish
Astronomical Center at Calar Alto
consisted of the CAFOS focal reducer and a 2010 × 2010 SITe CCD
chip with an instrumental scale of 0.53 arcsec pixel-1;
exposure times for both objects ranged from 5 to 15 minutes.
The seeing was 1.8" for Mkn 5 and 1.2" for II Zw 33.
3. Surface Photometry
Surface brightness profiles (SBPs) and color profiles were derived using
methods described in Papaderos et al. (1996).
A preliminary quantitative analysis of each SBP was carried out in terms of
a simple decomposition into an exponential fitting law for the LSB component
and a residual light distribution for the starburst component
(Fig. 1).
A 3-component decomposition scheme, well suited for describing the great
variety of profiles of BCDs (Papaderos et al. 1996) will be applied to the
data at a later time.
Table 1 lists the derived structural properties of the LSB and starburst
components.
Table 1.
Structural properties of Mkn 5 and II Zw 33
| Name(1) | Band(2)
| µE,0(3) | α(4)
| P25(5)
| mP25(6)
| E25(7)
| mE25(8)
| m25(9)
| CI(10)
|
|---|
| | | mag arcsec-2 | kpc | kpc | mag
| kpc | mag | mag |
|
| Mkn 5 | B | 21.05 | 0.32 | 0.55
| 17.22 | 1.21 | 15.54 | 15.32 | 0.79
|
|---|
| R | 20.24 | 0.34 | 0.55 | 17.29 | 1.47 | 14.74
| 14.63 | --
|
| I | 19.84 | 0.37 | 0.58 | 19.51 | 1.77 | 14.09
| 14.08 | --
|
| II Zw 33 | B | 20.06 | 0.92 | 1.59
| 15.58 | 4.2 | 14.6 | 14.37 | 0.86
|
|---|
| R | 19.30 | 0.85 | 1.70 | 15.09 | 4.43 | 14.0
| 13.66 | --
|
| I | 19.35 | 1.02 | 1.82 | 14.56 | 5.28 | 13.65
| 13.29 | --
|
(3): Central surface brightness of the LSB component, from lin. fit to SBP for
R* > 10", weighted by the photometric
uncertainty of each point; corrected for galactic extinction.
(4): Linear scale length of the LSB component.
(5),(7): Radius P25 of the plateau and E25
of the LSB component as derived at a surface brightness level of
25 mag arcsec-2.
(6),(8): Apparent luminosities of the starburst and LSB components within
P25 and E25, resp.
(9): Apparent luminosity, determined by integration of the profiles.
(10): Concentration Index in B, cf. Papaderos et al. (1996).
[Click here to see Fig. 1!]
4. Results
The LSB components of Mkn 5 and II Zw 33 can be described by an
exponential fitting law; the SBP of Mkn 5 exhibits a plateau feature at
intermediate radii, while for II Zw 33 the luminosity decrease of
the starburst component is nearly exponential.
Color and Halpha emission maps show that star formation is
currently confined to the south of Mkn 5.
For radii >10", the B-R and R-I colors of Mkn 5
become nearly constant at ≅0.8 and 0.6 mag, respectively, consistent
with an age of 2-4 Gyr.
The color distributions of II Zw 33 imply stellar ages older than
≅2 Gyr.
The LSB-companion of II Zw 33, II Zw 33 b,
exhibits very blue integrated B-R and R-I colors suggestive of
a relatively young (≤0.5 Gyr) unevolved system.
For Mkn 5, we obtained B fluxes of (15.36±0.07) and
(15.21±0.07) mag within 41" and 65" apertures, respectively,
for II Zw 33 (14.79±0.06) and (14.24±0.06) mag within
17" and 80" apertures, and for II Zw 33 b
(16.55±0.45) mag within an 80" aperture.
References
- Lindner U., Einasto M., Einasto J., et al., 1996, A&A 314, 1
- Papaderos P., Loose H.-H., Thuan T.X, Fricke K.J., 1996, A&AS 120, 207
- Thuan, T.X., 1991, in 'Massive Stars in Starbursts', Leitherer C.,
Walborn N.R., Heckmann T.M., Norman C.A. (eds.), Cambridge University
Press, p. 183
- Thuan T.X., Martin G.E., 1981, ApJ 247, 823
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| First version: | 02nd | August, | 1998
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| Last update: | 30th | September, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)