What the UV SED Can Tell Us About Primitive Galaxies

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What the UV SED Can Tell UsAbout Primitive
Galaxies

• Sally Heap
• NASAs Goddard Space Flight Center

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Outline of Talk

• The UV SED introduction to b, why b is important
• The challenge interpreting b f(age, Z, Fneb,
  dust)
• Meeting the challenge using the full SED to
  identify the various contributors to b via case
  study of galaxy, I Zw 18
• 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 SEDs

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b is the power-law index in F(l) lb
Calzetti 94
I Zw 18
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The UV SED is the basis of our knowledge about
very high-redshift galaxies
ACS i ACS z WFC3 Y
WFC3 J WFC3 H
Fl lb bphot 4.29(J125-H160)
-2.77 Age lt 100 Myr Metallicity low Extinction
low LFUV SFR 40 M?/yr M 7.8x108
M?
ff
Finkelstein 10
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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)
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Use the full SED to identify contributors to b
Lya
CII
Stars HII Emission
Dust
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Use the full SED of I Zw 18 as a test case
H II Region Young,
massive stars H I Envelope
HST/WFPC2 He II F469N OIII F502N Ha F656N
HST/STIS Far-UV
VLA 21-cm with optical image superposed

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I Zw 18 has been observed at all wavelengths
xray

21cm
(Chandra)

(VLA)
The spectrum reveals MXRBs (xray), stars
(UV-optical), HeIII and HII regions (UVOIR lines
continuous emission), dust (IR), HI envelope
(far-UV, 21 cm)
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I Zw 18 is similar to high-redshift galaxies
Property I Zw 18 z7-8 Galaxies
Stellar Mass (M?) 2x106 108 - 109
HI Gas Mass (M?) 2.6x107
Dynamical mass (M?) 2.6x108
SFR (M?/yr) 0.1 10-100
Age of young stars (Myr) Age of older stars (Myr) 15 500? 1000? lt200
Metallicity (Z/Z?) lt 0.03 lt 0.05
Dust low Low
Measured b -2.45 -2.13 (H160lt28.5) -3.07 (H160gt28.5)
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Phases of Galaxy Formation

• Birth Phase Galaxies affected by
  photoionization.
• Mhalolt109 M?
• Growth Phase Star formation fueled by cold
  accretion, modulated by strong, ubiquitous
  outflows.
• Mhalolt1012 M?
• Death Phase Accretion quenched by AGN, growth
  continues via dry mergers.
• Mhalogt1012 M?

R. Dave et al. (2011) Galaxy Evolution Across
Time Conference Star Formation Across Space
and Time, Tucson AZ April 2011
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Evolutionary phase of I Zw 18 vs. WFC3 z7-8
galaxies

• I Zw 18 is in the birth phase of galaxy
  evolution
• Dynamical mass (halo mass) lt 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 z7-8 galaxies are in the growth phase
• Stellar mass 108 M?, so halo mass (Mstar
  Mgas DM) must be gt109 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
• Mass inflow rate (1z)2.25 (Dekel09) so that
  SFR is higher in higher-z galaxies of the same
  mass
• Maximum possible age of stars

Redshift-dependent differences
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Construct model SEDs to compare with observation
Geneva evolutionary tracks CastelliKurucz
spectral grid Nebular geometry
spherical Dust treatment dust included
Z Age IMF SFH (iSB vs. CSF) Z, grains H
density (HI, HII, H2) Inner radius Outer radius
log NHI21.3
iso_geneva

Model stellar SED

cloudy
Galaxy SED
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Stellar Models. I. Evolutionary tracks dont
account for rotation

• Rotation is a bigger factor at lower metallicity
  (Maeder2001, Meynet2006)
• 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)

CK 03
Brott et al. (2011) astro-ph 1102.0530v2
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II. Spectral grids for very hot stars (Teffgt50
kK) are unavailable
UV CMD for
Teff50 kK Teff30 kK
Isochrones for log Z/Zsun-1.7 (Lejeune
Schaerer 2002)
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III. Spectral grids for massive stars with winds
e.g. WC stars, are unavailable
NW
HST/COS Spectrum of I Zw 18-NW
Izotov97
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CMFGEN model spectra for low-Z stars are on the
way!
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Comparison of model SED to observations of I Zw 18
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Comparison of model UV SED to observations
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Conclusions

• The spectra of star-forming galaxies near and far
  are composite, with contributions from stars, HII
  region, HI region, and dust.
• 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)
• A robust understanding of a star-forming galaxy
  requires the full SED
• Progress in our understanding of high-redshift
  galaxies requires
• Evolutionary tracks spectra of very hot stars
  (Teffgt50,000 K) at low Z
• Inclusion of nebular emission in model SEDs