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تسجيل مستخدم جديدUncovering multiple Wolf-Rayet star-clusters and the ionized ISM in Mrk178: the closest metal-poor Wolf-Rayet HII galaxy
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New integral field spectroscopy (IFS) has been obtained for the nearby metal-poor WR galaxy Mrk178 to examine the spatial correlation between its WR stars and the neighbouring ionized ISM. The strength of the broad WR features and its low metallicity make Mrk178 an intriguing object. We have detected the blue and red WR bumps in different locations across the FOV (~ 300 pc x 230 pc) in Mrk178. The study of the WR content has been extended, for the first time, beyond its brightest star-forming knot uncovering new WR star-clusters. Using SMC/LMC-template WR stars we empirically estimate a minimum of ~ 20 WR stars within the region sampled. Maps of the spatial distribution of the emission-lines and of the physical-chemical properties of the ionized ISM have been created and analyzed. Here we refine the statistical methodology by Perez-Montero et al.(2011) to probe the presence of variations in the ISM properties. An error-weighted mean of 12+log(O/H)=7.72 +/- 0.01 is taken as the representative oxygen abundance for Mrk178. A localized N and He enrichment, spatially correlated with WR stars, is suggested by this analysis. Nebular HeII4686 emission is shown to be spatially extended reaching well beyond the location of the WR stars. This spatial offset between WRs and HeII emission can be explained based on the mechanical energy input into the ISM by the WR star winds, and does not rule out WR stars as the HeII ionization source. We study systematic aperture effects on the detection and measurement of the WR features, using SDSS spectra combined with the power of IFS. In this regard, the importance of targeting low metallicity nearby systems is discussed.
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Wolf-Rayet (WR) HII galaxies are local metal-poor star-forming galaxies, observed when the most massive stars are evolving from O stars to WR stars, making them template systems to study distant starbursts. We have been performing a program to invest
igate the interplay between massive stars and gas in WR HII galaxies using IFS. Here, we highlight some results from the first 3D spectroscopic study of Mrk 178, the closest metal-poor WR HII galaxy, focusing on the origin of the nebular HeII emission and the aperture effects on the detection of WR features.
ABRIGED: Quantifying the number, type and distribution of W-R stars is a key component in the context of galaxy evolution, since they put constraints on the age of the star formation bursts. Nearby galaxies (d<5 Mpc) are particularly relevant in this
context since they fill the gap between studies in the Local Group, where individual stars can be resolved, and galaxies in the Local Volume and beyond. We intend to characterize the W-R star population in NGC625, a low-metallicity dwarf galaxy suffering a currently declining burst of star formation. Optical IFS data have been obtained with the VIMOS-IFU covering the starburst region. We estimate the number of W-R stars using a linear combination of 3 W-R templates: 1 early-type nitrogen (WN) star, 1 late-type WN star and 1 carbon-type (WC) star (or oxygen-type (WO) star). Fits using several ensembles of templates were tested. Results were confronted with: i) high spatial resolution HST photometry; ii) numbers of W-R stars in nearby galaxies; iii) model predictions. The W-R star population is spread over the main body of the galaxy, not necessarily coincident with the overall stellar distribution. Our best estimation for the number of W-R stars yields a total of 28 W-R stars in the galaxy, out of which 17 are early- type WN, 6 are late-type WN and 5 are WC stars. The width of the stellar features nicely correlates with the dominant W-R type found in each aperture. The distribution of the different types of WR in the galaxy is roughly compatible with the way star formation has propagated in the galaxy, according to previous findings using HST images. Fits using templates at the metallicity of the LMC yield more reasonable number of W-R than those using templates at the metallicity of the SMC. Given the metallicity of NGC 625, this suggests a non-linear relation between the metallicity and the luminosity of the W-R spectral features.
We present the results of an ongoing investigation to provide a detailed view of the processes by which massive stars shape the surrounding interstellar medium (ISM), from pc to kpc scales. In this paper we have focused on studying the environments o
f Wolf-Rayet (WR) stars in M31 to find evidence for WR wind-ISM interactions, through imaging ionized hydrogen nebulae surrounding these stars. We have conducted a systematic survey for HII shells surrounding 48 of the 49 known WR stars in M31. There are 17 WR stars surrounded by single shells, or shell fragments, 7 stars surrounded by concentric limb brightened shells, 20 stars where there is no clear physical association of the star with nearby H-alpha emission, and 4 stars which lack nearby H-alpha emission. For the 17+7 shells above, there are 12 which contain one or two massive stars (including a WR star) and that are <=40 pc in radius. These 12 shells may be classical WR ejecta or wind-blown shells. Further, there may be excess H-alpha point source emission associated with one of the 12 WR stars surrounded by putative ejecta or wind-blown shells. There is also evidence for excess point source emission associated with 11 other WR stars. The excess emission may arise from unresolved circumstellar shells, or within the extended outer envelopes of the stars themselves. In a few cases we find clear morphological evidence for WR shells interacting with each other. In several H-alpha images we see WR winds disrupting, or punching through, the walls of limb-brightened HII shells.
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ore the massive stars evolve into the WR phase, WR stars have been detected in several emerging massive star clusters. These detections suggest that the timescale for clusters to emerge can be at least as long as the time required to produce WR stars (a few million years), and could also indicate that WR stars may be providing the tipping point in the combined feedback processes that drive a massive star cluster to emerge. We explore the potential overlap between the emerging phase and the WR phase with an observational survey to search for WR stars in emerging massive star clusters hosting WR stars. We select candidate emerging massive star clusters from known radio continuum sources with thermal emission and obtain optical spectra with the 4m Mayall Telescope at Kitt Peak National Observatory and the 6.5m MMT. We identify 21 sources with significantly detected WR signatures, which we term emerging WR clusters. WR features are detected in $sim$50% of the radio-selected sample, and thus we find that WR stars are commonly present in massive star clusters currently emerging. The observed extinctions and ages suggest that clusters without WR detections remain embedded for longer periods of time, and may indicate that WR stars can aid, and therefore accelerate, the emergence process.
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