KASI Galaxy Evolution Journal Club The Morphology of Passively Evolving Galaxies at z~2 from Hubble Space Telescope/WFC3 Deep Imaging in the Hubble Ultra Deep Field - P. Cassata et al. 2010, ApJ, 714, L79 - May 19, 2011 Joon Hyeop Lee

- Most stars in today’s early-type galaxies have formed at high-z (>2; Renzini 2006), but the observations have not yet constrained how that stellar mass has assembed in the presently observed systems. - In M > M ⊙, the stellar mass function of the high-z elliptical galaxies appears to be rapidly increasing from z~3 to z~1, pointing to this epoch as one of major assembly for modern bright galaxies (e.g. Bunday et al. 2006). Bundy et al. (2006)

- Elliptical galaxies at z>1.5 are smaller, by factors ~3-5, than their local counterparts of similar stellar mass, and thus have ~30-100x higher stellar density (e.g. van Dokkum et al. 2008) – Lacks an explanation in terms of an evolutionary mechanism - Missed low-μ halos? (cosmological dimming) - Missed a relatively large fraction of very compact, yet massive early-type galaxies in the local universe ? Van Dokkum et al. (2008) Compact elliptical galaxies at = 2.3

- Most of high-redshift galaxies have only been observed in the rest- frame UV (HST/ACS) (only small samples were observed using NICMOS; e.g. Trujillo et al. 2007; Buitrago et al. 2008; Van Dokkum et al. 2008), and the dependence of the morphology of these galaxies on the wavelength has not been characterized. - In this study: unprecedented sensitivity and angular resolution of WFC3/IR images (HUDF) to study the optical rest-frame morphology of a small sample of low SSFR galaxies at z~2. Van Dokkum et al. (2008)

Data reduction - Primary imaging data: HST WFC3/IR observations (GO 11563, PI: Illingworth) in the HUDF – F105 (Y), F125W (J), F160W (H) - Independent reduction of the raw data. Final stacks reach 1σ μ fluctuations of 27.2, 26.6, 26.3 AB arcsec Drizzled the WFC3 images to 0.06” pixel scale. The size of PSF of the stacks is 0.18” in the H band. - Created a multi-wavelength source catalog. Sample Selection - BzK color selection + multi-band SED fitting: high-z (>1.3), massive (> M ⊙ ) and passive (SSFR < Gyr -1 ) galaxies. - Using Salpeter IMF and lower and upper mass limits M ⊙ - Calzetti et al. (2000) obscuration law - No detection at 24 μm down to a 1σ limit of 5 μJy.

Daddi et al. (2004) BzK selection scheme

- Final sample: 6 galaxies, 4 of which have spectroscopic redshifts - For the other 2 galaxies, derived photometric redshifts: Δz/(1+z) ~ 0.05

- Fig. 1: the observed and best-fit SED + the H-band images of the 6 sample galaxies

Morphological Analysis - GALFIT (Peng et al. 2002): fit the light profile in the z, Y, J, H bands to the Sersic law - n=1: exponential profile, n=4: de Vaucouleur profile - Bulge-dominated objects: n>2 - Disk-dominated objects: n~1 - Obtained the PSF in each band for use with GALFIT.

Morphological Properties of Galaxies at z~2 - Fig. 2: the residual images obtained by stbtracting the GALFIT best-fir models from the corresponding images + the observed and best-fit model light profiles. - All galaxies, except 24626, are very compact and symmetric

: the most extended one, isophotes deviating from the elliptical shape, position angle varies along semi-major axis – ongoing or recent interactions? - 4 objects have n>2 (19389, 22704, 23555, 24626)

: had to add a central unresolved (PSF) component (<10% of total light) to the Sersic model to obtain a reasonable fit – AGN? - Not detected in Chandra Deep Field South 2Ms catalog nor in the radio VLA maps

: inner part is barely resolved in all bands, suggesting AGN - Also detected in the Chandra image (L soft-X ~ 3.8 x erg s -1, L hard-X ~ 5.6 x erg s -1 ), but not detected in the VLA maps. - IRAC 8 μm data point: a hot dust component - Except the 8 μm data point, the SED is very similar to that of other galaxies.

: n~1.6 – probably a bulge-dominated disk galaxy. - Some passive galaxies at z>1.5 have disk morphology (e.g. Stockton et al. 2008) : n=7.42 Sersic (inner part) + n~1 (outskirts) - Both components have r e ~3 kpc – even if the lower μ were not detected, the effective radius would still be found to be ~ 3 kpc.

- Residual map analysis to investigate the presence of diffuse low- μ halos. - All galaxies have residual structure in the innermost 0.5”, which is very likely the result of the variations of the PSF across the field. - Individual stars have similar residuals. - No evidence of a halo, except for (complex residual map). - Stacked all the individual images together, except 24626: equivalent exposure time of s and reaches 1σ μ sensitivity ~ 27 mag arcsec -2 → GALFIT analysis - The stacked image is very compact. No evidence of a large-scale diffuse halo.

- Fig. 3: Sersic index and effective radius versus wavelength. - The rest-frame wavelength range extends from the rest- frame UV (3300A) to the optical (6000A). - Shows weak or no morphological k-correction. - n: constant within the errors - r e : weak, but statistically significant dependence for 3 galaxies. - The weak dependence of r e with wavelength results in a negative color gradient (bluer outskirts). - Good agreement with other studies of early-type galaxies at high-z from ACS and NICMOS.

The Mass-Size Relation at z~2 - Fig. 4: mass versus size - 4 galaxies are below the local relation for early-type galaxies, while 2 are on the local relation. - A significant fraction of these systems are 3-5 times smaller, and hence have ~ times denser central stellar density, than local counterparts: confirmed using ‘deep’ ‘rest-frame optical’ data. - Diversity of morphological properties among these galaxies.

Summary and Conclusions - Massive galaxies (M ~ M ⊙ ) with early spectral type at z~2 appear to often have a relatively smooth and regular morphology, nearly independent of the wavelength – tight correlation between spectral and morphological properties → The back hone of the Hubble sequence is already in place among bright galaxies at ~ 25% of the cosmic time. - The morphology of these galaxies depends very mildly on the wavelength (between rest-frame UV – optical).

- One is almost certainly an AGN, while another one is a candidate. → Not surprising: the similarity between the SED of obscured AGN and quiescent galaxies. → High level of contamination (33%) is observed at z~2, the cosmic era when both AGN and SF activity reach their maximum: Direct evidence of a link between the AGN and the SF cessation mechanisms? (Future works using larger sample) - Likely to bias statistical conclusions made from UV/optically selected samples of passive galaxies at z~2, unless sensitive X-ray and MIR photometry.

- 4 objects lie below the local mass-size relation for early-type galaxies times more compact, times denser than local counterparts - 2 objects : unresolved central component, the boundary of the local relation : well within the local relation. - No evdence of a low-μ halo surrounding these galaxies. - At most, only <2% of the light can be segregated in such a halo : two-component light profile - A dispersion of properties? - Needs to be studied and characterized. - Ultra-compact z~2 ellipticals evolve into the local ones? (how?) - Ultra-compact systems are also present in the local universe but have so far eluded detection?