A Steep Faint-End Slope of the UV LF at z~2-3: Implications for the Missing Stellar Problem C. Steidel ( Caltech ) Naveen Reddy (Hubble Fellow, NOAO) Galaxies in Real Life and Simulations, Leiden, Netherlands, 16 September 2008

Are There Problems with Stellar Mass/ Star Formation Rate Measurements? SFR / stellar mass calibrations IMF evolution? Dust extinction Incompleteness Luminosity/Redshift Evolution of Dust; Incompleteness of Stellar Mass Density Measurements Reddy & Steidel (2008b)

Quantifying Number Densities via the UV Luminosity Function Advantages of UV: - direct tracer of massive star formation, modulo the effects of dust - deepest observations up to  2000 times more sensitive than those at IR and longer wavelengths - accessibility over almost the entire age of the Universe Need for Spectroscopy: - assess contamination - quantify perturbing effects of line emission and consequence for sample completeness - color corrections for IGM opacity - physical state of the ISM (stellar population; outflows)

Going Beyond the Spectroscopic Limit Reddy et al 2008a ? Use spectroscopic trends as a zeropoint for determining how galaxy properties depend on UV luminosity Preferential Scattering due to Lya: - contribution of high Ly  EW systems at UV- faint magnitudes; ~10% (>50 A) at M ~ -17 Change in mean dust attenuation: - no variance with UV luminosity? - evolution of dust with UV luminosity; UV-faint galaxies less dusty than UV-bright ones? Upshot: these systematics modulate the inferred faint-end slope of the UV LF by up to ~10%, with a tendency to steepen 

Advantages of our analysis - > 2000 spectroscopic redshifts at the bright-end - modeling of systematic effects - maximum-likelihood constraints on LF that are robust to non-uniform sources of scatter LBGs in 31 independent fields Results on the UV LF at z~2-3 Reddy et al 2008b Steep faint-end slope of  ~ -1.73, similar to that measured at z~ L* 0.07L* z=2: N(0.07L* 0.07L*)  (0.07L* 0.07L*) z=3: N(0.1L* 0.1L*)  (0.1L* 0.1L*)

Make use of UGR+JK+IRAC photometry for several hundred galaxies - spectroscopic redshifts remove a key degeneracy from this analysis! - independent monochromatic indicators constrain SFRs and reddening Stellar Population Modeling R<25.5 Reddy et al 2008b

Trends between UV luminosity and SFR with Stellar Mass Sawicki et al. 2007

Dust Corrections as a Function of UV Luminosity ~4-5 dust correction ~2 dust correction Mean Correction of ~2-3 for Total UV Luminosity Density

For >0.083L* galaxies Estimate of the Stellar Mass Function at z~2 Reddy et al 2008b

Integral of the Star Formation History z  L>0.083L*(z>2)  [L>0.083L*(z=2)] Integrated to L=0.083L*(z=6) What about contribution from massive galaxies?

Contributions to the Stellar Mass Density at z~2 For >0.083L* galaxies  (R<25.5;<10^11 M * ) ~ 3.7 +/- 0.2  crit  (R>25.5;<10^11 M * ) ~ 2.0 +/- 0.2  crit  (>10^11 M * ) ~ 1.6 +/- 0.4  crit

Comparisons with the Integrated Star Formation History >0.083L* R<25.5; M<10^11 Msun R>25.5; M<10^11 Msun M>10^11 Msun WRONG!

Redshift Evolution of the Faint-End Slope Reddy et al 2008b Slope roughly constant at z>2, with  ~ but, strong evolution in UV LF implies sub-L* galaxies at z~6 are different from sub-L* galaxies at z~2 - evolution of faint-end slope may have less to do with delayed feedback and more to do with the availability of low mass halos with cold gas below z~2

Conclusions Constraints on the faint-end slope of the UV LF: - UV LF evolves strongly between z~6 and z~2 - very steep faint-end slope of the UV LF of  ~ at z~2 and z~3, remarkably similar to those derived at higher redshifts (z~4-6) -but, faint-end population evolves between z~6 and z~2, so evolution of faint- end slope may have less to do with feedback and more to do with the availability of low mass halos with cold gas below z~2 - dust corrections depend on how far one integrates to obtain the UV LD Combining the stellar masses of star-forming galaxies at z~2-3 with the luminosity function: - significant stellar mass density in UV-faint galaxies (R>25.5) as in UV-bright ones - appears to resolve the supposed discrepancy between stellar mass density estimates and the integrated star formation history up to z~2

ADDITIONAL SLIDES

Infrared Luminosity Function at z~2 Total IR LD ~ 1.3e09 L o /Mpc 3 Total IR LD (Caputi et al. 2006) ~ 6.6e08 L o /Mpc 3 Limit of MIPS observations without prior information