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The Stellar Initial Mass Function at the Epoch of Reionization (i.e. z>6)

The Stellar Initial Mass Function at the Epoch of Reionization (i.e. z>6). Ranga Ram Chary Spitzer Science Center, US Planck Center rchary@caltech.edu. Talk Outline. The evidence from gamma-ray bursts The evidence from stellar mass density in z~6 galaxies

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The Stellar Initial Mass Function at the Epoch of Reionization (i.e. z>6)

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  1. The Stellar Initial Mass Function atthe Epoch of Reionization (i.e. z>6) Ranga Ram Chary Spitzer Science Center, US Planck Center rchary@caltech.edu Chary, HMSF Townsville, Australia Sept 2010

  2. Talk Outline • The evidence from gamma-ray bursts • The evidence from stellar mass density in z~6 galaxies • Future possibilities to constrain the IMF at z>6 robustly • Halpha • Thermal radio emission Chary, HMSF Townsville, Australia Sept 2010

  3. Tanvir et al. 2009, Nature Salvaterra et al. 2009 GRB 090423 at z=8.26 • Gamma-ray burst on 23 April 2009; highest spectroscopically confirmed astrophysical object known; 600 Myr after the Big Bang. • The detection of even this one burst at z~8 is a surprise ! • - Not expected based on our estimate of SF in z>6 galaxies • Upper limit to the U-band luminosity of the host is ~8.8109L. In contrast L* at z=7.4 is ~1.8 1010L. So, as at lower redshift, the host is a sub-L* galaxy. Chary, HMSF Townsville, Australia Sept 2010

  4. WFC3 UDF (Bouwens et al) RC, Berger & Cowie 2007 • Are we missing star-forming galaxies in deep surveys e.g. UDF ? • - Is it because of dust obscuration ? • Producing GRBs more efficiently because of lower metallicity ? • Is the stellar Initial Mass Function more top-heavy ? (Chary 2008 ApJ) Chary, HMSF Townsville, Australia Sept 2010

  5. Astronomical Archaeology @ z=6 Chary, HMSF Townsville, Australia Sept 2010

  6. Deep Spitzer Observations of z~6-7 galaxies Robustly Constrain Stellar Mass Labbe et al. 2010 Observed • The stellar mass is the integral of the past star-formation history • Star-formation is what keep the Universe ionized; need 3 photons/baryon Chary, HMSF Townsville, Australia Sept 2010

  7. Data/fits from Bouwens et al. 08 • We cannot detect all galaxies, only the bright ones • So need to make a correction for the faint end • Thankfully, faint galaxies are blue i.e. young and have low M/L stellar populations Chary, HMSF Townsville, Australia Sept 2010

  8. Balancing the Ionizing Photon Budget: A Simple Low Mass Cut in the IMF is Insufficient So, a top-heavy IMF at z>6 seems necessary. And that is consistent with the GRB Rate as well Chary, HMSF Townsville, Australia Sept 2010

  9. <1 Billion Yr Old Universe M-1.6 Present-day Universe M-2.35 Nature promotes obesity!More massive stars earlier in the Universe Chary, HMSF Townsville, Australia Sept 2010

  10. Are Large Hα EW in z~5 galaxies a Clue ? Chary et al. ’05 H. Shim, RC et al. ‘10 • Hα inferred from Spitzer broadband observations • Hα excited by UV continuum shortward of Lyman limit • A hard UV continuum can arise from a top-heavy IMF Chary, HMSF Townsville, Australia Sept 2010

  11. Getting clues from the radio E. Murphy et al. 2010 Chary, HMSF Townsville, Australia Sept 2010

  12. Future High Frequency Radio Surveys N Non-thermal Radio Emission: • LνNT ∝ ν-αqsnr TopHeavy/Kroupa ~ 2.32 Thermal Radio Luminosity: • LνT ∝ Te0.45 ν-0.1Q(H0) TopHeavy/Kroupa~ 5.68 Ratio of TOTAL radio luminosity at 1.4 GHz: • L1.4 THK/L1.4K ~ 2.82 (comparable to x2 scatter in current Radio/IR relationship) Chary, HMSF Townsville, Australia Sept 2010 E. Murphy et al. 2010

  13. Conclusions • The GRB rate density suggests more massive stars forming at z>4 • Stellar mass density measured in z~6 galaxies is a x2 below what is required to account for reionization • Need more ionizing photons per unit mass of stars  Top-heavy IMF • Our best estimate is dN/dM~M-1.6+/-0.2 average for z>6 • Currently seeing evidence for unusually strong Hα • Future 30-90 GHz radio surveys will measure the free-free emission which is strongly correlated with incidence rate of ionizing photons Chary, HMSF Townsville, Australia Sept 2010

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