The size-luminosity relation atz = 7 in CANDELS and its implication on reionization

Abstract

Context. The exploration of the relation between galaxy sizes and other physical parameters (luminosity, mass, star formation rate) has provided important clues for understanding galaxy formation, but such exploration has until recently been limited to intermediate redshift objects. Aims. We use the currently available CANDELS Deep+Wide surveys in the GOODS-South, UDS and EGS fields, complemented by data from the HUDF09 program, to address the relation between size and luminosity at z ∼ 7. Methods. The six different fields used for this study are characterized by a wide combination of depth and areal coverage, well suited for reducing the biases on the observed size-magnitude plane. From these fields, we select 153 z-band dropout galaxies. Detailed simulations have been carried out for each of these six fields, inserting simulated galaxies at different magnitudes and half light radius in the two dimensional images for all the Hubble Space Telescope (HST) bands available and recovering them as carried out for the real galaxies. These simulations allow us to derive precisely the completeness as a function of size and magnitude and to quantify measurements errors/biases, under the assumption that the 2D profile of z = 7 galaxies is well represented by an exponential disk function. Results. We find in a rather robust way that the half light radius distribution function of z ∼ 7 galaxies fainter than J = 26.6 is peaked at ≤0.1 arcsec (or equivalently 0.5 kpc proper), while at brighter magnitudes high-z galaxies are typically larger than ∼0.15 arcsec. We also find a well defined size-luminosity relation, Rh ∝ L 1/2 . We compute the luminosity function (LF) in the HUDF and P12HUDF fields, finding large spatial variation on the number density of faint galaxies. Adopting the size distribution and the size-luminosity relation found for faint galaxies at z = 7, we derive a mean slope of −1.7 ± 0.1 for the LF of LBGs at this redshift. Conclusions. Using this LF, we find that the number of ionizing photons emitted from galaxies at z ∼ 7 cannot keep the Universe re-ionized if the IGM is clumpy (C HII ≥ 3) and the Lyman continuum escape fraction of high-z LBGs is relatively low ( f esc ≤ 0.3). If these results are confirmed and strengthened by future CANDELS data, in particular by the forthcoming deep observations in GOODS-South and North and the wide field COSMOS, we can put severe limits to the role of galaxies in the reionization of the Universe.

FIRST FRONTIER FIELD CONSTRAINTS ON THE COSMIC STAR FORMATION RATE DENSITY ATz∼ 10—THE IMPACT OF LENSING SHEAR ON COMPLETENESS OF HIGH-REDSHIFT GALAXY SAMPLES

“…However, both the mass-size and the luminosity-size relations at high redshift are found to be very shallow following r M found by Huang et al (2013). These measurements are completely consistent with the surprisingly constant size scaling for late type galaxies at all redshifts z 0 3 = -seen in the CANDELS data set (r M e 0.22 µ ; see van der Wel et al 2014) and there is no convincing evidence for a change in these scaling relations at higher redshifts (but see Grazian et al 2012). This suggests that a galaxy magnified by a factor 10 m = is intrinsically only 1.7´smaller than a non-lensed galaxy observed in the field.…”

ARE ULTRA-FAINT GALAXIES ATz= 6–8 RESPONSIBLE FOR COSMIC REIONIZATION? COMBINED CONSTRAINTS FROM THE HUBBLE FRONTIER FIELDS CLUSTERS AND PARALLELS

“…As for galaxy sizes, we adopt a lognormal distribution with a mean half-light radius of 0 15 and sigma = 0 07. For consistency with previous results (Bouwens et al 2004;Ferguson et al 2004;Hathi et al 2008;Oesch et al 2010Oesch et al , 2014Grazian et al 2011Grazian et al , 2012Huang et al 2013), the distribution is based on the sizes derived from spectroscopically confirmed LBG samples at z ∼ 4 (Vanzella et al 2009), while accounting for a redshift evolution of the intrinsic physical size of galaxies with a factor of (1 + z) −1 . HST observations of dropout galaxies at z > 6 redshifts also reveal small sizes of less than 0 3 (Mosleh et al 2012;Ono et al 2013), and smaller for galaxies fainter than M UV − 21 mag.…”

The Lyman alpha reference sample

“…Half-light radius measured using elliptical apertures in the rest-frame UV image as a function of the absolute rest-frame UV magnitude. LARS measurements from this work (red diamonds) are shown together with the literature estimations by Overzier et al (2010) corresponding to the median value of their z < 0.3 LBAs, observed in the near-infrared bands, by Jiang et al (2013) for a sample of z ∼ 5.7, 6.5, 7.0 LAEs (cyan dots), by Bond et al (2012) for the stack and subsamples of LAEs at z 2−3 (black circles, magenta squares), by Grazian et al (2012) which measured SExtractor half-light radii for a sample of z-drop outs (green triangles), and by Ono et al (2013) for a sample of high signal-to-noise zand Y-drop outs detected in Hubble Ultra Deep Field, UDF12 (black stars).…”

“…These include, -Continuum-selected Lyman break galaxies (LBGs) at z ∼ 3, with and without Lyα in emission (Pentericci et al 2010); at z ∼ 1, 2, and 3 (Mosleh et al 2011), at 1.5 < z < 3.6 (Law et al 2012); at z ∼ 1.8 (Lotz et al 2004); z-drop outs at z ∼ 7 (Grazian et al 2012); high signal-to-noise z-and Y-drop outs detected in the Hubble Ultra Deep Field, UDF12 (Ono et al 2013). -Compact star-forming galaxies (cSFGs) at 2 < z < 3 (Barro et al 2013).…”