P. Papaderos1, Y.I. Izotov2, N.G. Guseva2,
K.G. Noeske3, K.J. Fricke1, T.X. Thuan4
1) Universitäts-Sternwarte Göttingen, Germany, 2) Main Astronomical Observatory, Kyiv, Ukraine, 3) Lick Observatory, Univ. of California, USA, 4) Univ. of Virginia, USA
A tiny fraction of blue compact dwarf (BCD) galaxies exhibits a nearly galaxy-wide
starburst activity and no signatures of an old stellar host galaxy. The evolutionary
status and formation history of these most metal-deficient ( Z /15 ... Z /50),
extraordinarily compact (effective radius reff 0.5 kpc, exponential scale length
of the host galaxy 0.1-0.5 kpc) BCDs are still a subject of debate. Various lines of
evidence suggest, however, that these systems do not contain a substantial
population of stars older than ~1 Gyr and hence qualify as nearby young-galaxy
candidates.
!
Elaborated multiwavelength studies of these rare, young BCD candidates may
therefore provide valuable insights into the formation and starburst-driven evolution
of low-mass galaxies in the early universe. In order to assess the photometric
structure and evolutionary status of these systems it is of critical importance to
correct for the effect of extended nebular emission, as the latter may severely affect
colors and age estimates obtained therefrom.
One such example is I Zw 18 (Fig. 1a), the most metal-poor BCD galaxy known
(Z /50). This system is embedded within a filamentary low-surface brightness
(LSB) envelope, extending out to 18 ( 1.3 kpc, assuming a distance of 15 Mpc).
Papaderos et al. (2002), using broad- (BVRI) and narrow-band (H , [OIII]) HST
data, have shown that the extended LSB envelope of this young BCD candidate is
entirely due to nebular line emission: ionized gas accounts for more than 80% of
the line-of-sight emission at a galactocentric distance of 8 ( 0.65 kpc) and for up
to 50% of the total R light of I Zw 18.
Evidently, a twodimensional subtraction of ionized gas emission is indispensable
for a meaningful study of the photometric structure of such systems.
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Fig. 2: HST WFPC2 image of Pox 186 in the I band with red background galaxies depicted with circles and a
probable red foreground star marked with a cross. The horizontal bar is 1", corresponding to a linear scale of 90
pc. North is up and east to the left. The placement of the slit during the spectroscopic observations (cf. Fig. 3)
at position angles of respectively 0° and -68° is indicated.
Fig. 3: Optical spectra of the bright HII region of the ultracompact BCD Pox 186 taken with
the Multiple-Mirror Telescope (MMT) and the ESO 3.6m telescope with the emission lines
labeled. The lower spectra in either panel are the observed spectra downscaled by a factor
of 50.
Fig. 4: HST WFPC2 U-V (a) and V-I (b) colour map of Pox 186, displayed in the range -1...0.1 mag and
-0.95...-0.3 mag, respectively. Compact sources detected on unsharp-masked V and I images are shown with
contours. The compact star clusters a (axis origin) and b display blue V-I colours between -0.4 and -1.0 mag,
whereas the color of the fainter compact sources d and e is redder, 0.5 and 1.3 mag, respectively. The position
of the curved feature S, proposed by Corbin & Vacca (2002) to be an interaction-induced tidal tail is marked with
the ellipses. Note the red U-V ( 0.1...0.3 mag) along with the blue V-I ( -0.8..-1 mag) colors in the periphery of
clusters a and b. These are characteristic of extraordinarily intense ionized gas emission on scales of 100 pc
from young stellar clusters.
Fig. 5: top a&b: HST surface brightness V&I distribution of Pox 186 along the slit at PA=0°. c: (VI) color distribution. The location of extended red background sources (cf. Fig. 2) coincides with
color maxima. bottom: H and H equivalent width distribution along the slit at the position angle
-68°.
Fig. 6: Comparison of the structural properties of the stellar host galaxy of IZw18 with those of
other types of dwarf galaxies: a) extrapolated central surface brightness E,0 vs. absolute B
magnitude Mhost of the stellar host galaxy; b) exponential scale length vs. Mhost. Subtraction of
ionized gas emission shifts IZw18 by -1.6 mag and -0.31 dex in panels a and b, respectively,
moving it into the locus populated by the most compact ( ~ 100 pc) dwarf galaxies. The position
of other young BCD candidates from Fig. 1 is indicated.
(
)
&
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Fig. 1: HST WFPC2 exposures of the young BCD candidates a) I Zw 18 (D=15 Mpc), b) SBS 0335-052E
(D=54.3 Mpc), c) Tol 1214-277 (D=104 Mpc) and d) Tol 65 (D=36 Mpc).
Other examples of compact, young BCD candidates with colors significantly
affected by intense, spatially extended nebular line emission are, e.g., SBS 0335052E (Fig. 1b; Thuan et al. 1997, Papaderos et al. 1998, Izotov et al. 2001b,
Pustilnik et al. 2004), Tol 1214-277 (Fig. 1c; Fricke et al. 2001, Izotov et al. 2004)
and Tol 65 (Fig. 1d; Papaderos et al. 1999, Izotov et al. 2004).
Another example of a compact BCD with extraordinarily strong ionized gas
emission (EW(H )=906 Å, Gil de Paz et al. 2003) is Pox 186 (Fig. 2a). This
system, falling into the i0 morphological class defined by Loose & Thuan (1986),
has been regarded a nearby example of the building blocks from which larger
galaxies may have formed (Corbin & Vacca 2002).
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Recently, our group has completed a ground-based spectroscopic and HST U, V, I
photometric study of Pox 186 (Guseva et al. 2004). Our analysis indicates that the
emission of the LSB component of this system (Fig. 2) out to radii 3" (~270 pc in
linear scale) is mainly gaseous in origin. Within the latter radius the equivalent
width of the H line has been determined to be as large as 2000 Å (cf. Fig. 5). As
evident both from the U-V and V-I color maps (Fig. 4) and the simulated V-I
profile along the slit position of the spectrocopic observations with the MMT (Fig.
5, cf. Fig. 2&3), ionized gas emission severely affects broad-band colors down to a
surface brightness level ~24 V mag arcsec-2 . Moreover, H emission has been
clearly detected out to radii as large as 6 .
The surface brightness distribution in the LSB component of Pox 186 can be
approximated by an exponential law with a scale length
120 pc. This places
Pox 186 among the most compact dwarf galaxies known (cf. Fig. 6). Note that the
derived is most likely an upper limit to the scale length of the LSB component of
Pox 186 because of the appreciable flux contribution of extended nebular emission
throughout the optical size of the BCD.
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N.G.G. and Y.I.I. have been supported by DFG grants 436 UKR 17/2/02 and 436 UKR 17/22/03. P.P. and
K.J.F. acknowledge support by the Volkswagen Foundation under grant No. I/72919.
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