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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.

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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 > 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?


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