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Register a new userClumpy Galaxies in CANDELS. II. Physical Properties of UV-bright Clumps at $0.5leq z<3$
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Studying giant star-forming clumps in distant galaxies is important to understand galaxy formation and evolution. At present, however, observers and theorists have not reached a consensus on whether the observed clumps in distant galaxies are the same phenomenon that is seen in simulations. In this paper, as a step to establish a benchmark of direct comparisons between observations and theories, we publish a sample of clumps constructed to represent the commonly observed clumps in the literature. This sample contains 3193 clumps detected from 1270 galaxies at $0.5 leq z < 3.0$. The clumps are detected from rest-frame UV images, as described in our previous paper. Their physical properties, e.g., rest-frame color, stellar mass (M*), star formation rate (SFR), age, and dust extinction, are measured by fitting the spectral energy distribution (SED) to synthetic stellar population models. We carefully test the procedures of measuring clump properties, especially the method of subtracting background fluxes from the diffuse component of galaxies. With our fiducial background subtraction, we find a radial clump U-V color variation, where clumps close to galactic centers are redder than those in outskirts. The slope of the color gradient (clump color as a function of their galactocentric distance scaled by the semi-major axis of galaxies) changes with redshift and M* of the host galaxies: at a fixed M*, the slope becomes steeper toward low redshift, and at a fixed redshift, it becomes slightly steeper with M*. Based on our SED-fitting, this observed color gradient can be explained by a combination of a negative age gradient, a negative E(B-V) gradient, and a positive specific star formation rate gradient of the clumps. We also find that the color gradients of clumps are steeper than those of intra-clump regions. [Abridged]
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Although giant clumps of stars are crucial to galaxy formation and evolution, the most basic demographics of clumps are still uncertain, mainly because the definition of clumps has not been thoroughly discussed. In this paper, we study the basic demographics of clumps in star-forming galaxies (SFGs) at 0.5<z<3, using our proposed physical definition that UV-bright clumps are discrete star-forming regions that individually contribute more than 8% of the rest-frame UV light of their galaxies. Clumps defined this way are significantly brighter than the HII regions of nearby large spiral galaxies, either individually or blended, when physical spatial resolution and cosmological dimming are considered. Under this definition, we measure the fraction of SFGs that contain at least one off-center clump (Fclumpy) and the contributions of clumps to the rest-frame UV light and star formation rate of SFGs in the CANDELS/GOODS-S and UDS fields, where our mass-complete sample consists of 3239 galaxies with axial ratio q>0.5. The redshift evolution of Fclumpy changes with the stellar mass (M*) of the galaxies. Low-mass (log(M*/Msun)<9.8) galaxies keep an almost constant Fclumpy of about 60% from z~3.0 to z~0.5. Intermediate-mass and massive galaxies drop their Fclumpy from 55% at z~3.0 to 40% and 15%, respectively, at z~0.5. We find that (1) the trend of disk stabilization predicted by violent disk instability matches the Fclumpy trend of massive galaxies; (2) minor mergers are a viable explanation of the Fclumpy trend of intermediate-mass galaxies at z<1.5, given a realistic observability timescale; and (3) major mergers are unlikely responsible for the Fclumpy trend in all masses at z<1.5. The clump contribution to the rest-frame UV light of SFGs shows a broad peak around galaxies with log(M*/Msun)~10.5 at all redshifts, possibly linked to the molecular gas fraction of the galaxies. (Abridged)
We investigate the differences in the stellar population properties, the structure, and the environment between massive compact star-forming galaxies (cSFGs) with or without active galactic nucleus (AGN) at $2<z<3$ in the five 3D-HST/CANDELS fields. In a sample of 221 massive cSFGs, we constitute the most complete AGN census so far, identifying 66 AGNs by the X-ray detection, the mid-infrared color criterion, and/or the SED fitting, while the rest (155) are non-AGNs. Further dividing these cSFGs into two redshift bins, i.e., $2<z<2.5$ and $2.5 leq z<3$, we find that in each redshift bin the cSFGs with AGNs have similar distributions of the stellar mass, the specific star formation rate, and the ratio of $L_{rm IR}$ to $L_{rm UV}$ to those without AGNs. After having performed a two-dimensional surface brightness modeling for those cSFGs with X-ray-detected AGNs (37) to correct for the influence of the central point-like X-ray AGN on measuring the structural parameters of its host galaxy, we find that in each redshift bin the cSFGs with AGNs have comparable distributions of all concerned structural parameters, i.e., the Sersic index, the 20%-light radius, the Gini coefficient, and the concentration index, to those without AGNs. With a gradual consumption of available gas and dust, the structure of cSFGs, indicated by the above structural parameters, seem to be slightly more concentrated with decreasing redshift. At $2<z<3$, the similar environment between cSFGs with and without AGNs suggests that their AGN activities are potentially triggered by internal secular processes, such as gravitational instabilities or/and dynamical friction.
We investigate the properties of the extinction curve in the rest-frame UV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5, selected from the FORS Deep Field (FDF) spectroscopic survey. A new parametric description of the rest-frame UV spectral energy distribution is adopted; its sensitivity to properties of the stellar populations or of dust attenuation is established with the use of combined stellar population and radiative transfer models. The distribution of the z ~ 2 UV-luminous FDF galaxies in several diagnostic diagrams shows that their extinction curves range between those typical of the Small and Large Magellanic Clouds (SMC and LMC, respectively). For the majority of strongly reddened objects having a UV continuum slope beta > -0.4 a significant 2175 A absorption feature is inferred, indicating an LMC-like extinction curve. On the other hand, the UV continua of the least reddened objects are mostly consistent with SMC-like extinction curves, lacking a significant 2175 A bump, as for the Calzetti et al. sample of local starbursts. Furthermore, the most opaque (beta ~ 0) UV-luminous galaxies tend to be among the most metal rich, most massive, and largest systems at z ~ 2. The presence of the UV bump does not seem to depend on the total metallicity, as given by the equivalent width (EW) of the C IV doublet. Conversely, it seems to be associated with large EWs of prominent interstellar low-ionisation absorption lines, suggesting a link between the strength of the UV bump and the topology of the interstellar medium of the most evolved UV-luminous, massive galaxies at z ~ 2.
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.
We present deep ALMA CO(5-4) observations of a main sequence, clumpy galaxy at z=1.5 in the HUDF. Thanks to the ~0.5 resolution of the ALMA data, we can link stellar population properties to the CO(5-4) emission on scales of a few kpc. We detect strong CO(5-4) emission from the nuclear region of the galaxy, consistent with the observed $L_{rm IR}$-$L^{prime}_{rm CO(5-4)}$ correlation and indicating on-going nuclear star formation. The CO(5-4) gas component appears more concentrated than other star formation tracers or the dust distribution in this galaxy. We discuss possible implications of this difference in terms of star formation efficiency and mass build-up at the galaxy centre. Conversely, we do not detect any CO(5-4) emission from the UV-bright clumps. This might imply that clumps have a high star formation efficiency (although they do not display unusually high specific star formation rates) and are not entirely gas dominated, with gas fractions no larger than that of their host galaxy (~50%). Stellar feedback and disk instability torques funnelling gas towards the galaxy centre could contribute to the relatively low gas content. Alternatively, clumps could fall in a more standard star formation efficiency regime if their actual star-formation rates are lower than generally assumed. We find that clump star-formation rates derived with several different, plausible methods can vary by up to an order of magnitude. The lowest estimates would be compatible with a CO(5-4) non-detection even for main-sequence like values of star formation efficiency and gas content.
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