* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Carolina Kehrig
Space Interferometry Mission wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Theoretical astronomy wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Cassiopeia (constellation) wikipedia , lookup
Constellation wikipedia , lookup
X-ray astronomy wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
High-velocity cloud wikipedia , lookup
Planetary system wikipedia , lookup
Gamma-ray burst wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Star catalogue wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Stellar evolution wikipedia , lookup
Hubble Deep Field wikipedia , lookup
Observational astronomy wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Stellar classification wikipedia , lookup
Understanding narrow HeII emission using local metal-poor star-forming galaxies Carolina Kehrig Instituto de Astrofisica de Andalucia (IAA-CSIC) Proyecto Estallidos de Formacion Estelar Understanding Nebular Emission in High-z Galaxies - Pasadena, 13 - 17 July 2015r Why is the study of the HeII line relevant ? HeII emission: the existence of sources of hard radiation field (E ≥ 54ev) HeII line provide constraints on massive stars SEDs and on the ionization mechanism for this line HeII-emitters are observed to be more frequent among high-z galaxies than for local objects (e.g. Kehrig+2011; Cassata+2013) HeII line: one of the best tracers of PopIII-stars (the first very hot metal-free stars) (e.g., Schaerer 2003; Cassata+2013;Sobral+2015) contributed to the universe's reionization (?) (e.g., Bromm 2013) Searches for PopIII-objects is one of the main science drivers for JWST (Johnson 2010; Visbal+2015) preparation of future studies of the first SF galaxies Before interpreting high-z HeII-emitters & use HeII line to infer properties of distant starburst, it is crucial to understand the formation of HeII line in nearby metal-poor objects - local counterparts of distant HeII-emitters HeII ionization: WR stars Several mechanisms (shocks, X-ray binaries, WR stars) have been proposed to explain the HeII ionization in HII galaxies/regions (e.g., Dopita & Sutherland 1996; Schaerer 1996; Garnett+1991) SEDs of instantaneous bursts calculated from STB99 HeII HeII (Leitherer+1999) Z=0.0004 Z=0.004 HeII The appearance of WRs, 4 Myr after the burst, results in a harder UV continuum HeII Z=0.02 Z=0.008 Single-star models Shirazy & Brinchmann (2012) HeII ionizing edge at 228 Å No significant WR phase is shown for the lowest metallicity SED HeII ionization: WR stars Shirazy & Brinchmann (2012) Single-star models Predictions for emission lines using the SED generated by STB99 as an input to the photoionization code CLOUDY HeII emission is predicted only for bursts with Z of 20% Z and above, and only for ages of ~ 4-5 Myr Binary models (BPASS code): HeII emission is predicted for lower metallicites and elevated HeII emission for a longer period of time (e.g. Eldridge+2009,2011; Talks by J.Eldridge & E.Stanway) HeII ionization: WR stars What do we expect ? ★ WR stars are more common at high metallicity metallicity dependece of winds from their evolutionary precursors (e.g. Crowther 2007; Mokiem+2007) ★ Rotation is expected to increase the WR population (Meynet & Maeder 2005) but stellar evolutionary models for single (rotating/non-rotating) massive stars predict very few, if any, WRs in low-Z environments (e.g. Leitherer+2014, J.Groh talk) ★ Binary evolution models (e.g. Eldridge & Stanway 2009) predict elevated HeII for a longer period of time, but still only with WRs (J.Eldridge & E.Stanway talks) ★ HeII ionizing photons come mainy from hot WR stars nebular HeII should be seen when such hot WRs are present and is expected to be weaker/nonexistent at very low metallicity HeII ionization: The observational reality! ★ HeII line is observed to be more common at low metallicities (e.g. Schaerer 2003) ★ Nebular HeII does not appear to be always associated with WRs WRs cannot explain the HeII ionization in all cases, particularly at low metallicity z ~ 2.5 stack of narrow HeII-emitters spectra (Cassata+2013) Sobral+2015 Stellar population models including WRs do not reproduce the properties of the narrow HeII emitters. PopIII stars Alternative scenario: strong winds for very massive WNh stars (Grafener & Vink 2015) CR7 (the brightest Lyα emitter at z = 6.6): WR stars interpretation is strongly disfavoured. PopIII stars or Black Hole ? (Sobral+2015; Pallottini+2015) Stars undergo CHE (Sung-Chul Yoon talk) HeII ionization at low-redshift: The observational reality! The origin of the nebular HeII still remains difficult to understand in many cases GMOS spectroscopy of HeII nebulae in M33 (Kehrig, Oey, Crowther+2011) 2 new HeII nebulae in M33 not associated with any hot massive star! BCLMP651 HBW673 ~ 40% of the HeII-emitting HII regions in the Local Group galaxies have no obvious hot star associated Kehrig+2013 HeII ionization at low-redshift: The observational reality! HeII4686/Hβ Guseva et al. (2000) 12 out of 30 HeII-emitting SF galaxies do not show WR features in their long-slit spectra galaxies with detected and nondetected WR features are indistinguishable and other mechanisms for the origin of nebular HeII need to be invoked (see also Thuan & Izotov 2005) Log EW(Hβ) Shirazi & Brinchmann (2012) With WR features 40% of the HeII-emitting SF galaxies from SDSS do not show WR signatures lack of WR features does not seem to be a S/N issue Without WR features HeII ionization at low-redshift: The observational reality! ■ Nearby metal-poor SF galaxies using Integral Field Spectroscopy (IFS): spectral and spatial information at the same time spatial correlation between massive stars and surrounding nebular properties (e.g. Kehrig+2008,2013,2015; Perez-Montero+2011,2013) ■ “template” systems → understand the evolution and feedback from massive stars in distant starburst galaxies which cannot be studied to the same depth and to constrain models for metal-poor massive stars ■ spatially resolved nebular HeII line emission HeII ionization at low-redshift: The observational reality! Some results from IFS: Mrk178 (Z ~1/10 Z): HeII emission is extended and goes much beyond the location of WRs Hα map for IIZw70 (Z ~ 1/7 Z) WR stars Nebular HeII Kehrig,Vilchez,Telles+2008 Kehrig,Vilchez,Perez-Montero,Brinchmann+2013 Spatial separation between WR stars and the HeII-emitting zone (see also Shirazi & Brinchmann 2012) The extended HeII4686-emitting region in IZw18 unveiled: clues for peculiar ionizing sources (Kehrig,Vilchez,Perez-Montero,Iglesias-Paramo, Brinchmann, Kunth+2015, ApJL) IZw18 is a natural local counterpart of distant HeII-emitters and ideal place to study the HeII-ionization! The most metal-poor (Z~0.0004) SF galaxy and our best local analog of faraway starbursts (e.g. Searle & Sargent 1972; Vilchez & Iglesias-Paramo 1998) IZw18 16’’ x 16’’ First IFS study of IZw18! We discovered a large (D ~ 440 pc) nebular HeII4686-emitting region Hα Nebular HeII4686 Our IFU data reveal for the first time: total spatial extent and precise location of the nebular HeII region, and the corresponding total HeII-ionizing flux in IZw18! What is the main source powering nebular HeII emission in IZw18 ? Integrated “HeII-spectrum” Kehrig+2015 Total L(HeII4686)obs Q(HeII)obs ~ 1050 photon/s 1) WR stars ? based on the HeII-ionizing flux expected from “IZw18-like” WRs (Crowther & Hadfield 2006), ≥ 100 WRs is required to explain the Q(HeII)obs , but such very large WR population is not compatible with: (> 8 times) Total stellar mass of the NW cluster WR/O stars ratio at the metallicity of IZw18 (e.g. Maeder & Meynet 2012) Stellar evolutionary models for single massive stars in low-Z environments (e.g. Leitherer+2014) What is the main source powering nebular HeII emission in IZw18 ? 2) Shocks ? Spectral features of shock ionization indicate that the HeII region is unlikely to be produced by shocks 3) X-ray binaries ? CLOUDY photoionization model using as input a SED with the characteristics reported for the single X-ray binary in IZw18 (Thuan +2004) give L(HeII4686)output < 100 L(HeII4686)obs Conventional HeII-ionizing sources (WRs, shocks, X-ray binaries) cannot convincingly explain the observed nebular HeII emission in IZw18 Peculiar very hot stars in IZw18 1) very massive (300 M), metal-poor O stars ? Observations versus HeII-ionizing fluxes from radiation-driven wind models for the most massive, hottest O stars at the metallicity of IZw18 and below (Kudritzki 2002): the number of such stars needed to explain Q(HeII)obs implies a cluster mass ~ 10 - 20 x Mstar of the NW knot of IZw18 These models cannot produce the highest values of HeII4686/Hβ 300 M stellar sources are not detected in IZw18 to date Szecsi, Langer, Yoon+2015: models for fast rotating massive single stars at Z=0.0002 (~ IZw18-Z) TWUIN stars (assumptions: IMF with Mup = 500 M; 20% of the stars in IZw18, at any mass, are TWUINs from which ~ 10 with 300 M are required) Peculiar very hot stars in IZw18 2) metal-free ionizing stars (PopIII-like stars) ? Searches for PopIII-hosting galaxies have been carried out using HeII lines because of the strong UV radiation expected at (nearly) Z=0 (e.g. Schaerer 2008; Visbal+2015) Schaerer (2003): burst models cannot explain the Q(HeII)obs when metallicity ≥ 10-5 (IZw18 metallicity ~ 4x10-4) HeII emission due to stellar photoionisation is limited to very small metallicities and PopIII objects Compare the observations with HeII-ionizing fluxes from models for rotating Z=0 CHE stars (Yoon, Dierks & Langer 2012): 100 M star models ~ 13 stars are needed The harder spectra of these stellar models can explain the highest values of HeII4686/Hβ Lebouteiller+2013: metal-free gas pockets could provide the raw material for making such (nearly) metal-free stars in IZw18 Summary & Concluding Remarks There is still a lack of understanding of narrow HeII emitters even at low redshift WR features are not seen whenever narrow HeII is observed IFS spatial offset between nebular HeII-emitting zone and WR stars can be a possible explanation for the non-detection of WR features in some galaxy spectra Nearby narrow HeII emitters, specially metal-poor ones, are fundamental to better constrain models for metal-poor massive stars and understand high-z HeII emitters IFS of IZw18: reveal for the first time its total HeII-ionizing flux and conventional HeIIionizing sources (WRs, shocks, X-ray binaries) don’t fully explain the observations which challenge standard models for metal-poor massive single stars We invoke the PopIII-like stars scenario in IZw18 for the first time (Kehrig et al. 2015) Some ongoing & future work … IZw18 observations versus BPASS models (collaboration with J.Eldridge, A.Wofford et al.) UV spectra of HeII-emitting SF galaxies: observing time awarded through Cycle 23 COS/HST [collaboration with J.Brinchmann (PI) et al.]