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What the UV SED Can Tell Us About Primitive Galaxies

What the UV SED Can Tell Us About Primitive Galaxies. Sally Heap NASA’s Goddard Space Flight Center. Outline of Talk. The UV SED: introduction to b , why b is important The challenge: interpreting b = f(age, Z, F neb , dust )

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What the UV SED Can Tell Us About Primitive Galaxies

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  1. What the UV SED Can Tell UsAbout Primitive Galaxies Sally Heap NASA’s Goddard Space Flight Center

  2. 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 SED’s

  3. bis the power-law index in F(l) ~ lb Calzetti + 94 I Zw 18

  4. The UV SED is the basis of our knowledge about very high-redshift galaxies ACS i’ ACS z’ WFC3 Y WFC3 J WFC3 H ff Fn (nJy) Fl ~ lb bphot = 4.29(J125-H160) = -2.77 Age < 100 Myr Metallicity – low Extinction – low LFUV SFR = 40 M☉/yr M* = 7.8x108M☉ lobs (mm)=8.32lrest ff Finkelstein + 10

  5. 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)

  6. Use the full SED to identify contributors to b Lya [CII] StarsHII EmissionDust

  7. 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

  8. I Zw 18 has been observed at all wavelengths xray 21cm (Chandra) (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)

  9. I Zw 18 is similar to high-redshift galaxies

  10. 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

  11. 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 • Mass inflow rate ~ (1+z)2.25 (Dekel+09) so that SFR is higher in higher-z galaxies of the same mass • Maximum possible age of stars Redshift-dependent differences

  12. Construct model SED’s to compare with observation Geneva evolutionary tracks Castelli+Kurucz 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 NHI=21.3 iso_geneva Model stellar SED cloudy Galaxy SED

  13. 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

  14. II. Spectral grids for very hot stars (Teff>50 kK) are unavailable UV CMD for Teff=50 kK Teff=30 kK Isochrones for log Z/Zsun=-1.7 (Lejeune & Schaerer 2002)

  15. 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

  16. CMFGEN model spectra for low-Z stars are on the way!

  17. Comparison of model SED to observations of I Zw 18

  18. Comparison of model UV SED to observations

  19. 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 (Teff>50,000 K) at low Z • Inclusion of nebular emission in model SED’s

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