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, 2011Joon 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 > 1011 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 <z> = 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-2.

- 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 (> 1010 M⊙) and passive (SSFR < 10-2 Gyr-1) galaxies.

- Using Salpeter IMF and lower and upper mass limits 0.1 - 100 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

- 24626: 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)

- 19389: 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

- 23495: inner part is barely resolved in all bands, suggesting AGN

- Also detected in the Chandra image (Lsoft-X ~ 3.8 x 1043 erg s-1, Lhard-X ~ 5.6 x 1043 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.

- 24279: 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)

- 24626: n=7.42 Sersic (inner part) + n~1 (outskirts)

- Both components have re~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 24626 (complex residual map).

- Stacked all the individual images together, except 24626: equivalent exposure time of 78.500s 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

- re: weak, but statistically significant dependence for 3 galaxies.

- The weak dependence of re 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 ~50-100 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 ~ 1010 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

- 3-5 times more compact, 50-100 times denser than local counterparts

- 2 objects

- 19389: unresolved central component, the boundary of the local relation

- 24626: 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.

- 24626: 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?