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: 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

Pulmonary tuberculosis in the central prison of Douala, Cameroon

Objective: To determine the prevalence of and factors associated with pulmonary tuberculosis (PTB) in an urban prison in sub-Saharan Africa.Design: A cross-sectional survey. Setting: The Central Prison of Douala, Cameroon. Results: Two thousand four hundred and seventy four (87.4%) out of 2830 inmates underwent screening. Twenty seven (1.1%) of the inmates were under treatment for smear-positive PTB on commencement of the survey while 60 (2.4%) were diagnosed with smear and/or culture-positive PTB during the active case finding, resulting in a point prevalence of PTB of 3.5%. HIV seroprevalence in inmates without clinical signs of PTB was 111/1067 (10.4%) while it amounted to 6/24 (25%) in PTB patients. In multiple stepwise regression analysis, a low BMl, a prison stay of ≤12 months, and a history of previous incarceration were positively associated with PTB. Conclusion: The study results confirm the high prevalence rates of PTB in prison populations and underscore the need for urgent preventive measures. East African Medical Journal Vol. 83(1) 2006: 25-3