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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