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 , why  is important 2.The challenge: interpreting  f(age, Z, F neb, dust) 3.Meeting the challenge: using the full SED to identify the various contributors to  via case study of galaxy, I Zw 18 4.Results of case study: The full SED is needed to make a quantitative interpretation of  Improvements will be possible through: – New stellar evolution/spectra models – Inclusion of nebular gas & dust in model SED’s

 is the power-law index in F(  ~  Calzetti + 94 I Zw 18

ff The UV SED is the basis of our knowledge about very high-redshift galaxies F ~   phot = 4.29(J 125 -H 160 ) = Age < 100 Myr Metallicity – low Extinction – low L FUV SFR = 40 M ☉ /yr M * = 7.8x10 8 M ☉ F ~   phot = 4.29(J 125 -H 160 ) = Age < 100 Myr Metallicity – low Extinction – low L FUV SFR = 40 M ☉ /yr M * = 7.8x10 8 M ☉ ACS i’ ACS z’ WFC3 Y WFC3 J WFC3 H ff obs (  m)=8.32 rest F (nJy) Finkelstein + 10

 is sensitive to: stellar age metallicity dust extinction nebular emission  is sensitive to: stellar age metallicity dust extinction nebular emission beta_age_Z.jou  is sensitive to many factors (Duration of Star Fomation)

Use the full SED to identify contributors to  Stars HII Emission Dust Ly  [CII]

WFPC2 HST/WFPC2 He II F469N [OIII] F502N H  F656N WFPC2 HST/WFPC2 He II F469N [OIII] F502N H  F656N HST/STIS Far-UV HST/STIS Far-UV VLA 21-cm with optical image superposed H II Region Young, massive stars H I Envelope Use the full SED of I Zw 18 as a test case

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)

PropertyI Zw 18z=7-8 Galaxies Stellar Mass (M ☉ )2:x HI Gas Mass (M ☉ )2.6x10 7 Dynamical mass (M ☉ )2.6x10 8 SFR (M ☉ /yr) Age of young stars (Myr) Age of older stars (Myr) 15: ≤500? ≥1000? <200 Metallicity (Z/Z ☉ )< 0.03< 0.05 DustlowLow Measured  (H 160 <28.5) (H 160 >28.5) I Zw 18 is similar to high-redshift galaxies

Birth Phase: Galaxies affected by photoionization. M halo <~10 9 M  Growth Phase: Star formation fueled by cold accretion, modulated by strong, ubiquitous outflows. M halo <~ M  Death Phase: Accretion quenched by AGN, growth continues via dry mergers. M halo >~10 12 M  Phases of Galaxy Formation 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) < 10 9 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 ~ 10 8 M ☉, so halo mass (M star + M gas + DM) must be >10 9 M ☉ High SFR ( M ☉ per year) Large (negative)  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 Redshift-dependent differences

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, H 2 ) Inner radius Outer radius: log N HI =21.3 Model stellar SED iso_geneva cloudy Galaxy SED Construct model SED’s to compare with observation

Stellar Models. I. Evolutionary tracks don’t account for rotation Brott et al. (2011) astro-ph v2 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 T eff ) C&K 03

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

Izotov+97 RRest Wavelength (A) NW HST/COS Spectrum of I Zw 18-NW III. Spectral grids for massive stars with winds e.g. WC stars, are unavailable

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. Ly  ) and emission lines (e.g. [CII] 158  ) 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