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Register a new userNo More Active Galactic Nuclei in Clumpy Disks Than in Smooth Galaxies at z~2 in CANDELS / 3D-HST
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Jonathan R. Trump
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We use CANDELS imaging, 3D-HST spectroscopy, and Chandra X-ray data to investigate if active galactic nuclei (AGNs) are preferentially fueled by violent disk instabilities funneling gas into galaxy centers at 1.3<z<2.4. We select galaxies undergoing gravitational instabilities using the number of clumps and degree of patchiness as proxies. The CANDELS visual classification system is used to identify 44 clumpy disk galaxies, along with mass-matched comparison samples of smooth and intermediate morphology galaxies. We note that, despite being being mass-matched and having similar star formation rates, the smoother galaxies tend to be smaller disks with more prominent bulges compared to the clumpy galaxies. The lack of smooth extended disks is probably a general feature of the z~2 galaxy population, and means we cannot directly compare with the clumpy and smooth extended disks observed at lower redshift. We find that z~2 clumpy galaxies have slightly enhanced AGN fractions selected by integrated line ratios (in the mass-excitation method), but the spatially resolved line ratios indicate this is likely due to extended phenomena rather than nuclear AGNs. Meanwhile the X-ray data show that clumpy, smooth, and intermediate galaxies have nearly indistinguishable AGN fractions derived from both individual detections and stacked non-detections. The data demonstrate that AGN fueling modes at z~1.85 - whether violent disk instabilities or secular processes - are as efficient in smooth galaxies as they are in clumpy galaxies.
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Based on a large sample of massive ($M_{*}geq 10^{10} M_{odot}$) compact galaxies at $1.0 < z < 3.0$ in five 3D-HST/CANDELS fields, we quantify the fractional abundance and comoving number density of massive compact galaxies as a function of redshift. The samples of compact quiescent galaxies (cQGs) and compact star-forming galaxies (cSFGs) are constructed by various selection criteria of compact galaxies in literatures, and the effect of compactness definition on abundance estimate is proved to be remarkable, particularly for the cQGs and cSFGs at high redshifts. Regardless of the compactness criteria adopted, their overall redshift evolutions of fractional abundance and number density are found to be rather similar. Large samples of the cQGs exhibit a sustaining increase in number density from $z sim 3$ to 2 and a plateau at $1<z<2$. For massive cSFGs, a plateau in the number density at $2<z<3$ can be found, as well as a continuous drop from $z sim 2$ to 1. The evolutionary trends of the cQG and cSFG abundances support the scenario that the cSFGs at $z geq 2$ may have been rapidly quenched into quiescent phase via violent dissipational processes such as major merger and disk instabilities. Rarity of the cSFGs at lower redshifts ($z < 1$) can be interpreted by the decrease of gas reservoirs in dark matter halos and the consequent low efficiency of gas-rich dissipation.
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.
Recent simulation studies suggest that the compaction of star-forming galaxies (SFGs) at high redshift might be a critical process, during which the central bulge is being rapidly built, followed by quenching of the star formation. To explore dust properties of SFGs with compact morphology, we investigate the dependence of dust temperature, $T_{rm{dust}}$, on their size and star formation activity, using a sample of massive SFGs with $log (M_{ast}/M_{odot}) > 10$ at $1 < z < 3$, drawn from the 3D-{it HST}/CANDELS database in combination with deep {it Herschel} observations. $T_{rm{dust}}$ is derived via fitting the mid-to-far-infrared photometry with a mid-infrared power law and a far-infrared modified blackbody. We find that both extended and compact SFGs generally follow a similar $T_{rm{dust}}-z$ evolutionary track as that of the main-sequence galaxies. The compact SFGs seem to share similar dust temperature with extended SFGs. Despite the frequent occurrence of AGNs in compact SFGs, we do not observe any effect on dust caused by the presence of AGN in these galaxies during the compaction. Our results disfavor different ISM properties between compact and extended SFGs, suggesting that a rapid and violet compaction process might be not necessary for the formation of compact SFGs.
We present a study on structures and physical properties of massive ($M_* >10^{10} M_{sun} $) compact galaxies at $1.0<z<2.0$ in five 3D--{it HST}/CANDELS fields. Compared with the extended star-forming galaxies (eSFGs), compact star-forming galaxies (cSFGs) are found to have the lower level of star formation, and mainly distribute in the quiescent region of the {it UVJ} diagram. The distributions of dust attenuation and S{e}rsic index support that the progenitors of cQGs are cSFGs, and cSFGs are at a transitional phase between eSFGs and cQGs. The prevalence of X-ray selected AGNs ($sim 28%$) is confirmed in the cSFGs at $1<z<2$ which indicates that the violent gas-rich processes such as merger and disk instability could drive the structure to be more compact, and trigger both star formation and black hole growth in the central regions. Our results support the two-step scenario that the cSFGs at $1<z<2$ are the intermediate population after compaction but before a quick quenching. Our analysis of parametric and nonparametric morphologies shows that cQGs (eQGs) are more concentrated and have less substructures than cSFGs (eSFGs), and quenching and compactness should be associated with each other. The cSFGs at $1.5<z<2$ ($1<z<1.5$) prefer to be in higher (lower) density environment, similar as cQGs (eSFGs). It suggests that merger or strong interaction might be the main driving mechanism of compaction at higher redshifts, whereas the disk instability of individual galaxies might play a more important role on the formation of cSFGs at lower redshifts.
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)
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