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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
bursts calculated
from STB99
The appearance of
WRs, 4 Myr after
the burst, results in
a harder UV
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
HeII emission is
predicted only for
bursts with Z of 20%
Z and above, and
only for ages of ~ 4-5
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)
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;
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!
~ 40% of the HeII-emitting HII regions in the Local Group galaxies have no
obvious hot star associated
HeII ionization at low-redshift: The observational reality!
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
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)
16’’ x 16’’
First IFS study of IZw18!
We discovered a large (D ~ 440 pc) nebular HeII4686-emitting region
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”
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
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;
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
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.]