L.M. Cairós, N. Caon, R. amorín, B. García-Lorenzo, J.A.L. Aguerri

1. INTRODUCTION AND GOALS MKN 35 Blue Compact Dwarf (BCD) galaxies are low-luminosity and compact objects, with HII regions-like spectra (Thuan & Martin 1981), low metallicities and high star-forming rates - implying a gas exhaustion timescale shorter that the Hubble time. Initially it was hypothesized that BCDs could be truly young galaxies (Sargent & Searle 1970, Kunth & Sargent 1986). In the last years, extended, redder halos have been detected in the vast majority of them, revealing that, in most cases, BCDs are old systems with recurrent star formation episodes (Loose & Thuan 1986, Telles 1995, Papaderos et al. 1996, Cairós et al. 2001ab, 2002, 2003). We are carrying out a detailed spectrophotometric analysis of 15 BCDs, based on a strategy that combines broad- and narrow-band optical photometry with near-infrared data and long-slit spectroscopy. Our goals are: To assess the properties of the low surface brightness stellar host in BCDs (structural parameters, colors …), essential to derive their evolutionary status as well as to test those scenarios that link the different types of dwarfs and BCDs. To characterize the starburst episode (IMF, age, metallicity, star-forming rate) To obtain the star-forming history of BCDs Mrk 35 is representative of the group of BCDs populating the higher luminosity and metallicity range within the most common BCD morphological subclass, the iE type (Loose & Thuan 1986). The star-forming activity is taking place in a bar-like structure. The brighter star-forming regions (A, B1, B2) are arranged in a “heart-shaped” complex, located close to the central part of the galaxy; a tail, emerging from this core to the south-west, connects with the other two bright knots (C, D). The outer low surface brightness component is well described by elliptical isophotes. This system has been regarded as the prototype of multiple nucleus starburst galaxies. Mrk 35 = Haro 3 = NGC 3353 A B (B1 + B2) C D LINE ION F -W F -W F -W F -W 3727 [OII] 2.02 117.1 2.23 72.0 5.53 110.6 7.65 161.4 3869 [NeIII] 0.27 13.8 0.23 4.3 0.21 4.81 0.24 8.2 3889 H8, HeI 0.19 11.9 0.16 4.6 0.19 6.13 0.19 8.3 3969 [NeIII] 0.23 14.1 0.13 3.44 0.17 4.7 0.20 7.7 4101 Hδ 0.30 17.8 0.26 5.2 0.34 6.8 0.36 13.9 4340 H 0.49 36.2 0.47 12.4 0.52 15.6 0.59 23.1 C(H) 0.08 0.07 0.03 0.03 F(H) (erg cm-2 sec-1) 8.99 x 10-14 4.67 x 10-14 0.74 x 10-14 0.62 x 10 x 10-14 W(Hδ) ( Å ) 0 4.5 5 0 W(H) ( Å) 0 3 0 0 W(H) ( Å) 0 3.5 0 0 W(H) ( Å) 0 5.5 0 0 4363 [OIII] 0.02 1.6 0.03 0.7 --- --- 0.06 2.2 4471 HeI 0.04 3.0 --- --- 0.04 0.95 0.05 1.8 4861 H 1.00 90.8 1.0 31.5 1.00 40.3 1.00 54.2 4959 [OIII] 1.22 102.5 0.94 28.2 0.82 29.8 0.89 42.7 5007 [OIII] 3.60 272.4 2.71 81.6 2.40 85.8 2.60 120.1 5876 HeI 0.13 16.4 0.12 5.2 0.12 6.1 0.12 7.8 6300 [OI] 0.04 5.8 0.05 2.7 0.08 4.4 0.07 5.2 6548 [NII] 0.07 9.3 0.10 5.2 0.11 6.12 0.10 6.8 6563 H 2.88 344.0 2.76 146.8 2.9 162.9 3.0 220.8 6584 [NII] 0.22 31.1 0.31 16.2 0.33 18.0 0.32 22.8 6717 [SII] 0.20 32.1 0.30 16.5 0.38 23.0 0.35 29.6 6731 [SII] 0.16 25.9 0.23 12.7 0.28 18 0.26 22.8 SPECTROPHOTOMETRIC OBSERVATIONS OF BLUE COMPACT DWARF GALAXIES: MAPPING THE OLD AND YOUNG STARS IN MRK35 L.M. Cairós, N. Caon, R. amorín, B. García-Lorenzo, J.A.L. Aguerri Instituto de Astrofísica de Canarias Spatially resolved spectroscopy ubvri jhkand h photometry: Morphology, structure and stellar populations LONG SLIT SPECTROSCOPYwas obtained in February 2002 at the ORM, with the 4.2m WHT. We observed with the blue and red arms of ISIS. The blue spectral range was 3600 to 6920 Å, with a dispersion of 0.86 Å per pixel; in the red, the range was 8400-9900 Å, and the dispersion of 1.48 Å .We used a slit 1 arcsec wide. Two spectra were taken in two parallel positions (P.A. = 40), which pass through the five larger star-forming regions (A, B1, B2, C, D). Knots A, C, D present an almost flat spectrum, with strong emission lines, characteristic of a dominant OB population; the central knot, B, present a high continuum level, as well as pronounced absorption wings around the Balmer lines, which may indicates the presence of a substantial population of older stars. UBVRI IMAGING was carried out in January 1999 at the NOT (ORM) , with ALFOSC. The scale was 0.188 arcsec per pixel and the total field of view 6.5 x 6.5 arcmin. The seeing was < 1 arcsec. NARROW BAND IMAGES centered on the H line and continuum were obtained in March 2002 at the NOT, using ALFOSC. The seeing was around 1 arcsec. JHKs NIR IMAGING was obtained in April 2000, at the WHT with the infrared camera INGRID. The pixel scale was 0.242 arcsec per pixel, and the total field of view 4.13 x 4.13 arcmin. The morphology of the galaxy in the optical and in the NIR is similar, although optical frames reveal more distorted inner isophotes, circumstantial evidence for dust absorption. The central knot (B1 + B2), which coincides with the intensity peak in the NIR, is comparatively faint in the optical, where A is the brightest optical peak. This fact is clearly reflecting a different stellar content. The isophotes are clearly twisted from the inner regions, dominated by the starburst, to the outskirts, where they trace the light distribution of the host. In the (B-V) color map we clearly distinguish the central chain of star-forming regions from the red LSB envelope. The color distribution of the starburst is not homogeneous, with red patches visible along the central chain, coinciding exactly with the maxima in the gas emission. This is due to the contribution of the gas (in particular the [OIII]  4959, 5007 lines account for a large fraction of the flux through the V filter). The (V-K) color map presents a very complex morphology: a dust lane is crossing the center of the galaxy along the north-south direction, bending eastward to the south. The Ha equivalent width map shows an intriguing morphology: a sharp border is visible on the left side of the Ha distribution, and there is a clear gap between knots A and knots C and D. The peak of emission in the optical and NIR frames (knot B) does not emit in Ha. H equivalent width map Integrated spectra of knot B. In the blue region, the absorption wings around the Balmer lines are clearly resolved. In the red spectrum we clearly detect the CaII triplet. We have measuredlines fluxes and equivalent widths in each knot. Whenever possible, we fitted simultaneously an absorption and an emission component to the Balmer lines (in the other cases we adopt a reasonable value for the absorption equivalent width). Using the standard reddening curve, we fitted the measured lines ratio to the theoretical Balmer decrement, thus deriving the extinction coefficient. We have calculated the contribution of the emission lines to the broad-band fluxes. Table 2: Reddening corrected line intensities (normalized to H) (B-V) and (V-K) color maps of the BCD Mrk35 Table 1: Integrated Photometry The starburst population We used the FOCAS package to identify the knots in the H frame. 10 knots have been identified The colors of each knot have been measured on the image obtained subtracting out the underlying component (see next section), and corrected for the contribution of emission lines and interstellar extinction (as measured from our spectra). Ages of the knots have been derived by comparing their colors and W(H) with evolutionary synthesis models (STARBURST99). All the knots have ages < 5 Myrs Notice the significant absorption values The host galaxy In BCDs, the stellar and ionized gas emission from the starburst overshines the LSB component in its inner part, so the study of the host relies entirely on the outer regions. So far it has been common practice to fit the LSB component with an exponential function; however, many BCDs present light profiles with significant deviations from this model. Here, as a part of a larger program, we fitted the LSB profile of Mrk35 with a Sèrsic law. We fit both the average 1D profile and the 2D image (using the GALFIT program), after masking out the starburst with the help of our H images. The Sèrsic parameters determined with the two methods are all in very good agreement. For each band, the best-fit 2D model was subtracted out from the broad-band image, and magnitudes and colors of the individual knots were derived. B, The central knot Colors of the central knot (B1 + B2) indicate that it is older than 10 Myr. This knot is most probably an evolved HII region but, at this stage, we cannot rule out the possibility that it is the center (luminosity peak) of the LSB component. In the latter case, the hypothesis that the star-formation activity of the galaxy had been triggered by a merger event would be reinforced. A kinematical analysis (under way) will shed some light on this question. 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