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Register a new userI ZW 18 -- a New Wolf-Rayet Galaxy
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Authors
Yuri I. Izotov
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We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet (W-R) bumps at the wavelengths $lambda$4650 and $lambda$5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II $lambda$4686 and C IV $lambda$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass loss rates. The WC stars in the northwest component of I Zw 18 can be responsible for the presence of the nebular He II $lambda$4686 emission line, however the observed intensity of this line is several times larger than model predictions, and other sources of ionizing radiation at wavelengths shorter than 228AA are necessary.
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Hubble Space Telescope (HST) colour - magnitude diagrams in B, V and R along with long-slit Multiple Mirror Telescope (MMT) spectrophotometric data are used to investigate the evolutionary status of the nearby blue compact dwarf (BCD) galaxy I Zw 18. We find that the distance to I Zw 18 should be as high as 20 Mpc, twice the previously accepted distance, to be consistent with existing observational data on the galaxy: colour-magnitude diagrams, the high ionization state of the gas and presence of WR stars in the main body, and the ionization state of the C component. The spectral energy distribution (SED) of the main body of I Zw 18 is consistent with that of a stellar population with age < 5 Myr. However, the presence of large-scale shells observed around the main body suggests that star formation began ~ 20 Myr at the NW end and propagated in the SE direction. Our analysis of colour-magnitude diagrams and of the spectral energy distribution of the C component implies that star formation in this component started < 100 Myr ago at the NW end, propagated to the SE and stopped ~ 15 Myr ago. Thus, I Zw 18 is likely to be one of the youngest nearby extragalactic objects.
We report the discovery of emission from Wolf-Rayet stars in a giant HII region 4.5 arcsec South of the nucleus of the IRAS barred spiral galaxy Mrk 712. The ratio of WNL to OV stars, estimated from the luminosity of the HeII 4686 line, is 0.2. By comparison with starburst and stellar evolution models, we find that this high value is only compatible with a very young starburst episode (3 - 4 Myr) and a flat initial mass function (Gamma = -1). The presence of the [ArV] line reveals that the HII region is strongly ionized by the hot Wolf-Rayet stars. The comparison with other barred Wolf-Rayet galaxies suggests that the detection of Wolf-Rayet stars depends on the dust content and orientation of the galaxy.
Ultraviolet and 21-cm observations suggest that the extremely low-metallicity galaxy, I Zw 18, is a stream-fed galaxy containing a pocket of pristine stars responsible for producing nebular He II recombination emission observed in I Zw18-NW. Far-UV spectra by Hubble/COS and the Far Ultraviolet Spectroscopic Explorer (FUSE) make this suggestion conclusive by demonstrating that the spectrum of I Zw 18-NW shows no metal lines like O VI 1032, 1038 of comparable ionization as the He II recombination emission.
We present the results of a detailed optical and near-IR study of the nearby star-forming dwarf galaxy NGC4214. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old field stars, which are used as a distance indicator. On images obtained with the Hubble Space Telescope WFPC2 and NICMOS instruments in the equivalents of the V, R, I, J and H bands, the galaxy is well resolved into stars. We achieve limiting magnitudes of F814W ~27 in the WF chips and F110W ~25 in the NIC2. The optical and near-infrared color-magnitude diagrams confirm a core-halo galaxy morphology: an inner high surface-brightness young population within ~1.5 (~1 kpc) from the center of the galaxy, where the stars are concentrated in bright complexes along a bar-like structure; and a relatively low-surface-brightness, field-star population extending out to at least 8 (7 kpc). The color-magnitude diagrams of the core region show evidence of blue and red supergiants, main-sequence stars, asymptotic giant branch stars and blue loop stars. We identify some candidate carbon stars from their extreme near-IR color. The field-star population is dominated by the red tangle, which contains the red giant branch. We use the I-band luminosity function to determine the distance based on the tip-of-the-red-giant-branch method: 2.7pm0.3 Mpc. This is much closer than the values usually assumed in the literature, and we provide revised distance dependent parameters such as physical size, luminosity, HI mass and star-formation rate.
We report the discovery of a new Wolf-Rayet star in the direction of Cygnus. The star is strongly reddened but quite bright in the infrared, with J = 9.22, H = 8.08 and K = 7.09 (2MASS). On the basis of its H + K spectrum, we have classified WR 142a a WC8 star. We have estimated its properties using as a reference those of other WC8 stars in the solar neighbourhood as well as those of WR 135, whose near-infrared spectrum is remarkably similar. We thus obtain a foreground reddening of A(V) = 8.1 mag, M(J) = -4.3, log(L/Lo) = 5.0 - 5.2, R = 0.8 Ro, T = 125,000 K, M = 7.9 - 9.7 Mo, and a mass loss of (1.4 - 2.3)e-05 Mo/yr. The derived distance modulus, DM = 11.2 +/- 0.7 mag, places it in a region occupied by several OB associations in the Cygnus arm, and particularly in the outskirts of both Cygnus OB2 and Cygnus OB9. The position in the sky alone does not allow us to unambiguously assign the star to either association, but based on the much richer massive star content of Cygnus OB2 membership in this latter association appears to be more likely.
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