The blue compact dwarf galaxy I Zw 18: a comparative study of its low-surface-brightness component

(abridged): Using HST and ground-based optical and NIR imaging data, we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 possesses an extended low-surface-brightness (LSB) old stellar population underlying its star-forming (SF) regions. We show that the exponential intensity decrease observed in the filamentary LSB envelope of the BCD out to 18 arcsec (1.3 kpc at a distance of 15 Mpc) is not due to an evolved stellar disc, but rather due to extended ionized gas emission. Broad-band images reveal, after subtraction of nebular line emission, a compact stellar LSB component extending slightly beyond the SF regions. This stellar host, being blue over a radius range of 5 exponential scale lengths and showing little colour contrast to the SF component, differs strikingly from the red LSB host of standard BCDs. This fact, in connection with the blue colors of component I Zw 18 C, suggests that most of the stellar mass in I Zw 18 has formed within the last 0.5 Gyr. Furthermore, we show that the exponential intensity fall-off in the filamentary ionized envelope of I Zw 18 is not particular to this system but a common property of the ionized halo of many SF dwarf galaxies on galactocentric distances of several kpc. In the absence of an appreciable underlying stellar background, extended ionized gas emission dominates in the periphery of I Zw 18, mimicking an exponential stellar disc on optical surface brightness profiles.Comment: 24 pages, 16 figures; accepted for publication in Astronomy & Astrophysics; higher resolution images available at http://alpha.uni-sw.gwdg.de/~papade/IZw18

New insights to the photometric structure of Blue Compact Dwarf Galaxies from deep Near-Infrared studies I. Observations, surface photometry and decomposition of surface brightness profiles

(shortened) We analyze deep Near Infrared (NIR) broad band images for a sample of Blue Compact Dwarf Galaxies (BCDs), allowing for the quantitative study of their extended stellar low-surface brightness (LSB) host galaxies. NIR surface brightness profiles (SBPs) of the LSB hosts agree at large galactocentric radii with those from optical studies. At small to intermediate radii, however, the NIR data reveal for more than half of our sample a significant flattening of the exponential SBP of the LSB host. Such SBPs ("type V" SBPs, Binggeli & Cameron 1991) have rarely been detected in LSB hosts of BCDs at optical wavelengths, where the relative flux contribution of the starburst is stronger than in the NIR and can hide such central intensity depressions of the LSB host. The structural properties, frequency and physical origin of type V LSB SBPs in BCDs and other dwarf galaxies have not yet been systematically studied. Nevertheless, their occurrence in a significant fraction of BCDs would impose important new constraints to the radial distribution of their stellar mass, and to the photometric fading of BCDs after the termination of star-forming activity. Both a modified exponential (Papaderos et al. 1996a) and the Sersic law give satisfactory empirical descriptions for type V SBPs. However, we argue that the practical applicability of Sersic fits to LSB SBPs of BCDs is limited by, e.g., the extreme sensitivity of the solutions to small SBP uncertainties. Most stellar LSB host galaxies in the sample show optical-NIR colors indicative of evolved stellar populations with subsolar metallicity. Unsharp-masked NIR images and optical-NIR maps reveal numerous morphological details, and cases of non-uniform dust absorption on spatial scales up to ~1 kpc.Comment: 29 pages, 17 figures; accepted for publication in Astronomy & Astrophysics; 1 typo in Table 2 corrected; higher resolution images are available at http://www.uni-sw.gwdg.de/~knoeske/PUB_LIST/noeske_BCDs_NIR.ps.g

New insights to the photometric structure of Blue Compact Dwarf Galaxies from deep Near-Infrared studies: II. The sample of northern BCDs

This paper is part of a series of publications which present a systematic study of Blue Compact Dwarf (BCD) Galaxies in the Near Infrared (NIR). Compared to the visible light, NIR data allow a better separation of the starburst emission from the light distribution of the old stellar low-surface brightness (LSB) host galaxy. We analyze deep NIR broad band images of a sample of 11 BCDs, observed with the Calar Alto 3.6m telescope. This work enlarges the samples presented in preceding papers of this study (Noeske et al. 2003, Cairos et al. 2003) by BCDs of the most common morphological type, displaying a regular elliptical LSB host galaxy. The data presented here allow the detection and quantitative study of the extended stellar LSB host galaxy in all sample BCDs. The NIR surface brightness profiles (SBPs) of the LSB host galaxies agree at large galactocentric radii with those from optical studies, showing also an exponential intensity decrease and compatible scale lengths. Similar to Noeske et al. (2003), we find centrally flattening exponential (type V) SBPs of the host galaxy for several BCDs. Such SBPs remain mostly undetected in optical bands, due to the comparatively stronger starburst emission at these wavelengths. We apply a modified exponential distribution to decompose and quantitatively analyze SBPs of LSB hosts with a type V intensity distribution. We present the results of the surface photometry and the decomposition of SBPs, and discuss individual objects with respect to morphological details of their star-forming regions.Comment: 18 pages, 11 figures; accepted for publication in Astronomy & Astrophysics; postscript file with full resolution images available at http://www.ucolick.org/~kai/PUB/noeske_nirbcds_2.ps.g

AEGIS: Extinction and Star Formation Tracers from Line Emission

Strong nebular emission lines are a sensitive probe of star formation and extinction in galaxies, and the [O II] line detects star forming populations out to z>1. However, star formation rates from emission lines depend on calibration of extinction and the [O II]/H-alpha line ratio, and separating star formation from AGN emission. We use calibrated line luminosities from the DEEP2 survey and Palomar K magnitudes to show that the behavior of emission line ratios depends on galaxy magnitude and color. For galaxies on the blue side of the color bimodality, the vast majority show emission signatures of star formation, and there are strong correlations of extinction and [O II]/H-alpha with restframe H magnitude. The conversion of [O II] to extinction-corrected H-alpha and thus to star formation rate has a significant slope with M_H, 0.23 dex/mag. Red galaxies with emission lines have a much higher scatter in their line ratios, and more than half show AGN signatures. We use 24 micron fluxes from Spitzer/MIPS to demonstrate the differing populations probed by nebular emission and by mid-IR luminosity. Although extinction is correlated with luminosity, 98% of IR-luminous galaxies at z~1 are still detected in the [O II] line. Mid-IR detected galaxies are mostly bright and intermediate color, while fainter, bluer galaxies with high [O II] luminosity are rarely detected at 24 microns.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letters AEGIS special editio

A HST study of the stellar populations in the cometary dwarf irregular galaxy NGC 2366

We present V and I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram reaches down to I~26.0 mag. It reveals not only a young population of blue main-sequence stars (age <30 Myr) but also an intermediate-age population of blue and red supergiants (20 Myr<age<100 Myr), and an older evolved populations of asymptotic giant branch (AGB) stars (age >100 Myr) and red giant branch (RGB) stars (age >1 Gyr). The measured magnitude I=23.65+/-0.10 mag of the RGB tip results in a distance modulus m-M=27.67+/-0.10, which corresponds to a distance of 3.42+/-0.15 Mpc, in agreement with previous distance determinations. The youngest stars are associated with the bright complex of HII regions NGC 2363=Mrk 71 in the southwest extremity of the galaxy. As a consequence of the diffusion and relaxation processes of stellar ensembles, the older the stellar population is, the smoother and more extended is its spatial distribution. An underlying population of older stars is found throughout the body of NGC 2366. The most notable feature of this older population is the presence of numerous relatively bright AGB stars. The number ratio of AGB to RGB stars and the average absolute brightness of AGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403, indicating a younger age of the AGB stars in NGC 2366. In addition to the present burst of age <100 Myr, there has been strong star formation activity in the past of NGC 2366, from ~100 Myr to <3 Gyr ago.Comment: 32 pages, 15 figures, accepted for publication in the Astrophysical Journa