Transcript What the UV SED Tells us About Stellar Populations and

What the UV SED Can Tell Us
About Primitive Galaxies
Sally Heap
NASA’s Goddard Space Flight Center
Outline of Talk
1. The UV SED: introduction to b, why b is important
2. The challenge: interpreting b = f(age, Z, Fneb, dust)
3. Meeting the challenge: using the full SED to identify the
various contributors to b via case study of galaxy, I Zw 18
4. Results of case study:
• The full SED is needed to make a quantitative interpretation of b
• Improvements will be possible through:
– New stellar evolution/spectra models
– Inclusion of nebular gas & dust in model SED’s
b is the power-law index in F(l) ~ lb
Calzetti + 94
I Zw 18
The UV SED is the basis of our knowledge about
very high-redshift galaxies
ACS i’
ACS z’
Fl ~ lb
ff
bphot = 4.29(J125-H160)
= -2.77
WFC3 Y
WFC3 J
WFC3 H
Fn
(nJy)
Age < 100 Myr
Metallicity – low
Extinction – low
LFUV
SFR = 40 M☉/yr
M* = 7.8x108 M☉
lobs (mm)=8.32l
ff rest
Finkelstein + 10
b is sensitive to many factors
b is sensitive to:
• stellar age
• metallicity
• dust extinction
• nebular emission
beta_age_Z.jou
(Duration of Star Fomation)
Use the full SED to identify contributors to b
Lya
Stars
[CII]
HII Emission
Dust
Use the full SED of I Zw 18 as a test case
H II Region
HST/WFPC2
He II F469N
[OIII] F502N
Ha F656N
Young, massive stars
HST/STIS
Far-UV
H I Envelope
VLA 21-cm with optical
image superposed
I Zw 18 has been observed at all wavelengths
xray
(Chandra)
21cm
(VLA)
The spectrum reveals MXRB’s (xray), stars (UV-optical), HeIII and HII regions
(UVOIR lines & continuous emission), dust (IR), HI envelope (far-UV, 21 cm)
I Zw 18 is similar to high-redshift galaxies
Property
I Zw 18
z=7-8 Galaxies
Stellar Mass (M☉)
2:x106
108 - 109
HI Gas Mass (M☉)
2.6x107
Dynamical mass (M☉)
2.6x108
SFR (M☉/yr)
Age of young stars (Myr)
Age of older stars (Myr)
Metallicity (Z/Z☉)
Dust
Measured
b
0.1
10-100
15:
≤500? ≥1000?
<200
< 0.03
< 0.05
low
Low
-2.45
-2.13 (H160<28.5)
-3.07 (H160>28.5)
Phases of Galaxy Formation
• Birth Phase: Galaxies affected by photoionization.
Mhalo<~109 M
• Growth Phase: Star formation fueled by cold accretion,
modulated by strong, ubiquitous outflows.
Mhalo<~1012+ M
• Death Phase: Accretion quenched by AGN, growth
continues via dry mergers.
Mhalo>~1012 M
R. Dave et al. (2011) “Galaxy Evolution Across Time”
Conference: Star Formation Across Space and Time,
Tucson AZ April 2011
Evolutionary phase of I Zw 18 vs. WFC3 z=7-8 galaxies
I Zw 18 is in the “birth phase” of galaxy evolution
• Dynamical mass (halo mass) < 109 M☉
• No evidence of strong outflows
• Strong stellar ionizing radiation regulating star formation
• Huge HI cloud enveloping optical system suggesting SF in its early phase
WFC3 z=7-8 galaxies are in the “growth phase”
• Stellar mass ~ 108 M☉, so halo mass (Mstar + Mgas + DM) must be >109 M☉
• High SFR (10-100 M☉ per year)
• Large (negative) b suggests incomplete absorption of stellar ionizing radiation
➙ HI envelope is perforated, thin, or non existent
Redshift-dependent2.25differences
• Mass inflow rate ~ (1+z) (Dekel+09) so that SFR is higher in higher-z galaxies
of the same mass
• Maximum possible age of stars
Construct model SED’s to compare with observation
Geneva evolutionary tracks
Castelli+Kurucz spectral grid
iso_geneva
Z
Age
IMF
SFH (iSB vs. CSF)
Model stellar SED
Nebular geometry – spherical
Dust treatment – dust included
cloudy
Galaxy SED
Z, grains
H density (HI, HII, H2)
Inner radius
Outer radius: log NHI=21.3
Stellar Models. I. Evolutionary tracks don’t account for rotation
Rotation is a bigger factor at lower metallicity
(Maeder+2001, Meynet+2006)
• Low-Z stars are more compact, so on average are born rotating faster
• Low-Z stars retain their angular momentum since their rates of mass-loss are low
• Rotational mixing is more efficient at low Z
• Stars rotating above a certain threshold will evolve homogeneously
• Stars evolving homogeneously move toward the helium MS (higher Teff)
C&K 03
Brott et al. (2011) astro-ph 1102.0530v2
II. Spectral grids for very hot stars (Teff>50 kK) are unavailable
Teff=30 kK
Teff=50 kK
UV CMD for
Isochrones for log Z/Zsun=-1.7 (Lejeune & Schaerer 2002)
III. Spectral grids for massive stars with winds e.g.
WC stars, are unavailable
NW
RRest Wavelength (A)
HST/COS Spectrum of I Zw 18-NW
Izotov+97
CMFGEN model spectra for low-Z stars are on the way!
Comparison of model SED to observations of I Zw 18
Comparison of model UV SED to observations
Conclusions
1. The spectra of star-forming galaxies near and far are composite, with contributions
from stars, HII region, HI region, and dust.
2. The flux contributions of these components are prominent at different spectral
regions
•
•
•
Young, massive stars: UV
Nebular emission: near-IR
Dust: thermal IR
•
HI cloud: absorption (e.g. Lya) and emission lines (e.g. [CII] 158 m)
3. A robust understanding of a star-forming galaxy requires the full SED
4. Progress in our understanding of high-redshift galaxies requires
•
Evolutionary tracks & spectra of very hot stars (Teff>50,000 K) at low Z
•
Inclusion of nebular emission in model SED’s