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Register a new userThe Decomposed Bulge and Disk Size-Mass Relations of Massive Galaxies at 1<z<3 in CANDELS
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We have constructed a mass-selected sample of Mstar>10^11Msolar galaxies at 1<z<3 in the CANDELS UDS and COSMOS fields and have decomposed these systems into their separate bulge and disk components according to their H(160)-band morphologies. By extending this analysis to multiple bands we have been able to conduct individual bulge and disk component SED fitting which has provided us with stellar-mass and star-formation rate estimates for the separate bulge and disk components. These have been combined with size measurements to explore the evolution of these massive high-redshift galaxies. By utilising the new decomposed stellar-mass estimates, we confirm that the bulge components display a stronger size evolution than the disks. This can be seen from both the fraction of bulge components which lie below the local relation and the median sizes of the bulge components, where the bulges are a median factor of 2.93+/-0.32 times smaller than similarly massive local galaxies at 1<z<2 and 3.41+/-0.58 smaller at 2<z<3; for the disks the corresponding factors are 1.65+/-0.14 and 1.99+/-0.25. Moreover, by splitting our sample into the passive and star-forming bulge and disk sub-populations and examining their sizes as a fraction of their present-day counter-parts, we find that the star-forming and passive bulges are equally compact, star-forming disks are larger, while the passive disks have intermediate sizes. This trend is not evident when classifying galaxy morphology on the basis of single-Sersic fits and adopting the overall star-formation rates. Finally, by evolving the star-formation histories of the passive disks back to the redshifts when the passive disks were last active, we show that the passive and star-forming disks have consistent sizes at the relevant epoch. These trends need to be reproduced by any mechanisms which attempt to explain the morphological evolution of galaxies.
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We present the results of a new and improved study of the morphological and spectral evolution of massive galaxies over the redshift range 1<z<3. Our analysis is based on a bulge-disk decomposition of 396 galaxies with Mstar>10^11 Msolar from the CANDELS WFC3/IR imaging within the COSMOS and UKIDSS UDS survey fields. We find that, by modelling the H(160) image of each galaxy with a combination of a de Vaucouleurs bulge (Sersic index n=4) and an exponential disk (n=1), we can then lock all derived morphological parameters for the bulge and disk components, and successfully reproduce the shorter-wavelength J(125), i(814), v(606) HST images simply by floating the magnitudes of the two components. This then yields sub-divided 4-band HST photometry for the bulge and disk components which, with no additional priors, is well described by spectrophotometric models of galaxy evolution. Armed with this information we are able to properly determine the masses and star-formation rates for the bulge and disk components, and find that: i) from z=3 to z=1 the galaxies move from disk-dominated to increasingly bulge-dominated, but very few galaxies are pure bulges/ellipticals by z=1; ii) while most passive galaxies are bulge-dominated, and most star-forming galaxies disk-dominated, 18+/-5% of passive galaxies are disk-dominated, and 11+/-3% of star-forming galaxies are bulge-dominated, a result which needs to be explained by any model purporting to connect star-formation quenching with morphological transformations; iii) there exists a small but significant population of pure passive disks, which are generally flatter than their star-forming counterparts (whose axial ratio distribution peaks at b/a~0.7); iv) flatter/larger disks re-emerge at the highest star-formation rates, consistent with recent studies of sub-mm galaxies, and with the concept of a maximum surface-density for star-formation activity.
Spectroscopic + photometric redshifts, stellar mass estimates, and rest-frame colors from the 3D-HST survey are combined with structural parameter measurements from CANDELS imaging to determine the galaxy size-mass distribution over the redshift range 0<z<3. Separating early- and late-type galaxies on the basis of star-formation activity, we confirm that early-type galaxies are on average smaller than late-type galaxies at all redshifts, and find a significantly different rate of average size evolution at fixed galaxy mass, with fast evolution for the early-type population, R_eff ~ (1+z)^-1.48, and moderate evolution for the late-type population, R_eff ~ (1+z)^-0.75. The large sample size and dynamic range in both galaxy mass and redshift, in combination with the high fidelity of our measurements due to the extensive use of spectroscopic data, not only fortify previous results, but also enable us to probe beyond simple average galaxy size measurements. At all redshifts the slope of the size-mass relation is shallow, R_eff ~ M_star^0.22, for late-type galaxies with stellar mass >3x10^9 M_sol, and steep, R_eff M_star^0.75, for early-type galaxies with stellar mass >2x10^10 M_sol. The intrinsic scatter is <~0.2 dex for all galaxy types and redshifts. For late-type galaxies, the logarithmic size distribution is not symmetric, but skewed toward small sizes: at all redshifts and masses a tail of small late-type galaxies exists that overlaps in size with the early-type galaxy population. The number density of massive (~10^11 M_sol), compact (R_eff < 2 kpc) early-type galaxies increases from z=3 to z=1.5-2 and then strongly decreases at later cosmic times.
We have used high-resolution, HST WFC3/IR, near-infrared imaging to conduct a detailed bulge-disk decomposition of the morphologies of ~200 of the most massive (M_star > 10^11 M_solar) galaxies at 1<z<3 in the CANDELS-UDS field. We find that, while such massive galaxies at low redshift are generally bulge-dominated, at redshifts 1<z<2 they are predominantly mixed bulge+disk systems, and by z>2 they are mostly disk-dominated. Interestingly, we find that while most of the quiescent galaxies are bulge-dominated, a significant fraction (25-40%) of the most quiescent galaxies, have disk-dominated morphologies. Thus, our results suggest that the physical mechanisms which quench star-formation activity are not simply connected to those responsible for the morphological transformation of massive galaxies.
We study the rest-frame ultra-violet sizes of massive (~0.8 x 10^11 M_Sun) galaxies at 3.4<z<4.2, selected from the FourStar Galaxy Evolution Survey (ZFOURGE), by fitting single Sersic profiles to HST/WFC3/F160W images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS). Massive quiescent galaxies are very compact, with a median circularized half-light radius r_e = 0.63 +/- 0.18 kpc. Removing 5/16 (31%) sources with signs of AGN activity does not change the result. Star-forming galaxies have r_e = 2.0 +/- 0.60 kpc, 3.2 +/- 1.3 x larger than quiescent galaxies. Quiescent galaxies at z~4 are on average 6.0 +- 0.17 x smaller than at z~0 and 1.9 +/- 0.7 x smaller than at z~2. Star-forming galaxies of the same stellar mass are 2.4 +/- 0.7 x smaller than at z~0. Overall, the size evolution at 0<z<4 is well described by a powerlaw, with r_e = 5.08 +/- 0.28 (1+z)^(-1.44+/-0.08) kpc for quiescent and r_e = 6.02 +/- 0.28 (1+z)^(-0.72+/-0.05) kpc for star-forming galaxies. Compact star-forming galaxies are rare in our sample: we find only 1/14 (7%) with r_e / (M / 10^11 M_Sun)^0.75 < 1.5, whereas 13/16 (81%) of the quiescent galaxies is compact. The number density of compact quiescent galaxies at z~4 is 1.8 +/- 0.8 x 10^-5 Mpc^-3 and increases rapidly, by >5 x, between 2<z<4. The paucity of compact star-forming galaxies at z~4 and their large rest-frame ultra-violet median sizes suggest that the formation phase of compact cores is very short and/or highly dust obscured.
[abridged] We quantify the morphological evolution of z~0 massive galaxies ($M*/M_odotsim10^{11}$) from z~3 in the 5 CANDELS fields. The progenitors are selected using abundance matching techniques to account for the mass growth. The morphologies strongly evolve from z~3. At z<1, the population matches the massive end of the Hubble sequence, with 30% of spheroids, 50% of galaxies with equally dominant disk and bulge components and 20% of disks. At z~2-3 there is a majority of irregular systems (~60-70%) with still 30% of spheroids. We then analyze the SFRs, gas fractions and structural properties for the different morphologies independently. Our results suggest two distinct channels for the growth of bulges in massive galaxies. Around 30-40% were already bulges at z~2.5, with low average SFRs and gas-fractions (10-15%), high Sersic indices (n>3-4) and small effective radii ($R_e$~1 kpc) pointing towards an early formation through gas-rich mergers or VDI. Between z~ 2.5 and z~0, they rapidly increase their size by a factor of ~4-5, become all passive but their global morphology remains unaltered. The structural evolution is independent of the gas fractions, suggesting that it is driven by ex-situ events. The remaining 60% experience a gradual morphological transformation, from clumpy disks to more regular bulge+disks systems, essentially happening at z>1. It results in the growth of a significant bulge component (n~3) for 2/3 of the systems possibly through the migration of clumps while the remaining 1/3 keeps a rather small bulge (n~1.5-2). The transition phase between disturbed and relaxed systems and the emergence of the bulge is correlated with a decrease of the star formation activity and the gas fractions. The growth of the effective radii scales roughly with $H(z)^{-1}$ and it is therefore consistent with the expected growth of disks in galaxy haloes.
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