arXiv subjects

Guanwen Fang

Publications and source records attributed to Guanwen Fang.

47 records · Page 3Linked to original sources

arXiv2016

The most-luminous heavily-obscured quasars have a high merger fraction: morphological study of WISE-selected hot dust-obscured galaxies

Previous studies have shown that WISE-selected hyperluminous, hot dust-obscured galaxies (Hot DOGs) are powered by highly dust-obscured, possibly Compton-thick AGNs. High obscuration provides us a good chance to study the host morphology of the most luminous AGNs directly. We analyze the host morphology of 18 Hot DOGs at $z\sim3$ using Hubble Space Telescope/WFC3 imaging. We find that Hot DOGs have a high merger fraction ($62\pm 14 \%$). By fitting the surface brightness profiles, we find that the distribution of Sérsic indices in our Hot DOG sample peaks around 2, which suggests that most of Hot DOGs have transforming morphologies. We also derive the AGN bolometric luminosity ($\sim10^{14}L_\odot$) of our Hot DOG sample by using IR SEDs decomposition. The derived merger fraction and AGN bolometric luminosity relation is well consistent with the variability-based model prediction (Hickox et al. 2014). Both the high merger fraction in IR-luminous AGN sample and relatively low merger fraction in UV/optical-selected, unobscured AGN sample can be expected in the merger-driven evolutionary model. Finally, we conclude that Hot DOGs are merger-driven and may represent a transit phase during the evolution of massive galaxies, transforming from the dusty starburst dominated phase to the unobscured QSO phase.

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arXiv2016

Structural and morphological properties of ultraluminous infrared galaxies at $1<z<3$

Using the Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) near-infrared high-resolution imaging from the 3D-HST survey, we analyze the morphology and structure of 502 ultraluminous infrared galaxies (ULIRGs; $L_{\rm IR}>10^{12}L_{\odot}$) at $1 -1.7$) and small Sérsic index ($n<2.5$), while others are elliptical-like (E/S0/Sa) morphologies with lower $M_{20}$ ($<-1.7$) and larger $n$ ($>2.5$). The morphological diversities of ULIRGs suggest that there are different formation processes for these galaxies. Merger processes between galaxies and disk instabilities play an important role in the formation and evolution of ULIRGs at high redshift. In the meantime, we also find that the evolution of the size ($r_{\rm e}$) with redshift of ULIRGs at redshift $z\sim1-3$ follows $r_{\rm e}\propto(1+z)^{-(0.96\pm0.23)}$.

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arXiv2016

The metallicity evolution of blue compact dwarf galaxies from the intermediate redshift to the local Universe

We present oxygen abundance measurements for 74 blue compact dwarf (BCD) galaxies in the redshift range in [0.2, 0.5] using the strong-line method. The spectra of these objects are taken using Hectospec on the Multiple Mirror Telescope (MMT). More than half of these BCDs had dust attenuation corrected using the Balmer decrement method. For comparison, we also selected a sample of 2023 local BCDs from the Sloan Digital Sky Survey (SDSS) database. Based on the local and intermediate-z BCD samples, we investigated the cosmic evo- lution of the metallicity, star-formation rate (SFR), and Dn (4000) index. Com- pared with local BCDs, the intermediate-z BCDs had a systematically higher R23 ratio but similar O32 ratio. Interestingly, no significant deviation in the mass-metallicity (MZ) relation was found between the intermediate-z and lo- cal BCDs. Besides the metallicity, the intermediate-z BCDs also exhibited an SFR distribution that was consistent with local BCDs, suggesting a weak de- pendence on redshift. The intermediate-z BCDs seemed to be younger than the local BCDs with lower D4000 index values. The insignificant deviation in the mass-metallicity and mass-SFR relations between intermediate-z and local BCDs indicates that the relations between the global parameters of low-mass compact galaxies may be universal. These results from low mass compact galaxies could be used to place important observational constraints on galaxy formation and evolution models.

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arXiv2015

Physical properties of distant red galaxies in the COSMOS/UltraVISTA field

We present a study on physical properties for a large distant red galaxy (DRG) sample, using the $K$-selected multi-band photometry catalog of the COSMOS/UltraVISTA field and the CANDELS NIR data. Our sample includes 4485 DRGs with $(J-K)_\mathrm{AB}>1.16$ and $K_\mathrm{AB}<$23.4 mag, and 132 DRGs have HST/WFC3 morphological measurements. The results of nonparametric measurements of DRG morphology are consistent with our rest-frame UVJ color classification: quiescent DRGs are generally compact while star-forming DRGs tend to have extended structures. We find the star formation rate (SFR) and the stellar mass of star-forming DRGs present tight "main sequence" relations in all redshift bins. Moreover, the specific SFR (sSFR) of DRGs increase with redshift in all stellar mass bins and DRGs with higher stellar masses generally have lower sSFRs, which indicates that galaxies were much more active on average in the past, and star formation contributes more to the mass growth of low-mass galaxies than to high-mass galaxies. The infrared (IR) derived SFR dominate the total SFR of DRGs which occupy the high-mass range, implying that the $J-K$ color criterion effectively selects massive and dusty galaxies. DRGs with higher $M_{*}$ generally have redder $(U-V)_\mathrm{rest}$ colors, and the $(U-V)_\mathrm{rest}$ colors of DRGs become bluer at higher redshifts, suggesting high-mass galaxies have higher internal dust extinctions or older stellar ages and they evolve with time. Finally, we find that DRGs have different overlaps with EROs, BzKs, IEROs and high-$z$ ULIRGs indicating DRGs is not a special population and they can also be selected by other color criteria.

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arXiv2015

Physical properties of compact star-forming galaxies at $z\sim2-3$

We present a study on the physical properties of compact star-forming galaxies (cSFGs) with $M_{*}\geq10^{10}M_{\odot}$ and $2\leq z\leq3$ in the COSMOS and GOODS-S fields. We find that massive cSFGs have a comoving number density of $(1.0\pm0.1)\times10^{-4}~{\rm Mpc}^{-3}$. The cSFGs are distributed at nearly the same locus on the main sequence as extended star-forming galaxies (eSFGs) and dominate the high-mass end. On the rest-frame $U-V$ vs. $V-J$ and $U-B$ vs. $M_{\rm B}$ diagrams, cSFGs are mainly distributed at the middle of eSFGs and compact quiescent galaxies (cQGs) in all colors, but are more inclined to "red sequence" than "green valley" galaxies. We also find that cSFGs have distributions similar to cQGs on the nonparametric morphology diagrams. The cQGs and cSFGs have larger $Gini$ and smaller $M_{20}$, while eSFGs have the reverse. About one-third of cSFGs show signatures of postmergers, and almost none of them can be recognized as disks. Moreover, those visually extended cSFGs all have lower $Gini$ coefficients ($Gini<0.4$), indicating that the $Gini$ coefficient could be used to clean out noncompact galaxies in a sample of candidate cSFGs. The X-ray-detected counterparts are more frequent among cSFGs than that in eSFGs and cQGs, implying that cSFGs have previously experienced violent gas-rich interactions(such as major mergers or disk instabilities), which could trigger both star formation and black hole growth in an active phase.

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arXiv2014

Structure and morphology of X-ray selected AGN hosts at 1<z<3 in CANDELS-COSMOS field

We analyze morphologies of the host galaxies of 35 X-ray selected active galactic nucleus (AGNs) at $z\sim2$ in the Cosmic Evolution Survey (COSMOS) field using Hubble Space Telescope/WFC3 imaging taken from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We build a control sample of 350 galaxies in total, by selecting ten non-active galaxies drawn from the same field with the similar stellar mass and redshift for each AGN host. By performing two dimensional fitting with GALFIT on the surface brightness profile, we find that the distribution of S$è$rsic index (n) of AGN hosts does not show a statistical difference from that of the control sample. We measure the nonparametric morphological parameters (the asymmetry index A, the Gini coefficient G, the concentration index C and the M20 index) based on point source subtracted images. All the distributions of these morphological parameters of AGN hosts are consistent with those of the control sample. We finally investigate the fraction of distorted morphologies in both samples by visual classification. Only $\sim$15% of the AGN hosts have highly distorted morphologies, possibly due to a major merger or interaction. We find there is no significant difference in the distortion fractions between the AGN host sample and control sample. We conclude that the morphologies of X-ray selected AGN hosts are similar to those of nonactive galaxies and most AGN activity is not triggered by major merger.

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arXiv2014

Selection and Mid-infrared Spectroscopy of Ultraluminous Star-Forming Galaxies at z~2

Starting from a sample of 24 \micron\ sources in the Extended Groth Strip, we use 3.6 to 8 \micron\ color criteria to select ultraluminous infrared galaxies (ULIRGs) at $z\sim2$. Spectroscopy from 20-38 \micron\ of 14 objects verifies their nature and gives their redshifts. Multi-wavelength data for these objects imply stellar masses ${>}10^{11}$ \Msun\ and star formation rates $\ge$410 \Msun yr$^{-1}$. Four objects of this sample observed at 1.6 \micron\ (rest-frame visible) with {\it HST}/WFC3 show diverse morphologies, suggesting that multiple formation processes create ULIRGs. Four of the 14 objects show signs of active galactic nuclei, but the luminosity appears to be dominated by star formation in all cases.

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arXiv2013

The structure of massive quiescent galaxies at z~3 in the CANDELS-COSMOS field

In this letter, we use a two-color (J-L) vs. (V-J) selection criteria to search massive, quiescent galaxy candidates at 2.5<z<4.0 in the CANDELS-COSMOS field. We construct a H-selected catalogue and complement it with public auxiliary data. We finally obtain 19 passive VJL-selected (hereafter pVJL) galaxies as the possible massive quiescent galaxy candidates at z~3 by several constrains. We find the sizes of our pVJL galaxies are on average 3-4 times smaller than those of local ETGs with analogous stellar mass. The compact size of these z~3 galaxies can be modelled by assuming their formation at z ~ 4-6 according to the dissipative collapse of baryons. Up to z<4, the mass-normalized size evolution can be described by $r_e\propto (1+z)^{-1.0}$. Low Sersic index and axis ratio, with median values n~1.5 and b/a~0.65 respectively, indicate most of pVJL galaxies are disk-dominated. Despite large uncertainty, the inner region of the median mass profile of our pVJL galaxies is similar to those of quiescent galaxies (QGs) at 0.5<z<2.5 and local Early-type galaxies (ETGs). It indicates local massive ETGs have been formed according to an inside-out scenario: the compact galaxies at high redshift make up the cores of local massive ETGs and then build up the outskirts according to dissipationless minor mergers.

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arXiv2012

Passive and star-forming galaxies at $1.4 \leq z \leq2.5$ in the Aegis field

Using a simple two-color selection based on $g$-, $z$-, and $K$-band photometry, we pick out 1609 star-forming galaxies (sgzKs) and 422 passively evolving galaxies (pgzKs) at z\sim2$ from a $K$-band-selected sample ($K_{\rm AB} < 22.0$) in an area of $\sim 0.44$ deg$^{2}$ of the All-wavelength Extended Groth Strip International Survey. The number counts of pgzKs\ in our sample turn over at $K_{\rm AB} \sim 21.0$, and both the number of faint and bright objects (including sgzKs\ and pgzKs) exceed the predictions of a recent semi-analytic model of galaxy formation, a more successful model is need to explain this diversity. We also find that the star formation rate (SFR) and specific SFR (sSFR) of sgzKs\ increases with redshift at all masses, implying that star-forming galaxies were much more active on average in the past. Moreover, the sSFR of massive galaxies is lower at all redshifts, suggesting that star formation contributes more to the mass growth of low-mass galaxies than to high-mass galaxies. From {\it Hubble Space Telescope} Wide Field Camera 3 near-infrared imaging data, we find that morphologies of $z\sim2$ galaxies not only have diffuse structures with lower $G$ and higher $M_{20}$ values, but also have single-object morphologies (higher $G$ and lower $M_{20}$), implying that there are morphological variety and different formation process for these galaxies at $z\sim2$. Finally, we also study the fraction of active galactic nuclei (AGNs) in the gzKs, 82 of 828 gzKs\ with four IRAC bands can be classified as AGNs ($\sim$ 10%). Most of these AGN candidates have $L_{\rm 0.5-10\ keV}>10^{41}\,\rm erg\,s^{-1}$.

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arXiv2012

CANDELS: Correlations of SEDs and Morphologies with Star-formation Status for Massive Galaxies at z ~ 2

We present a study on Spectral Energy Distributions, Morphologies, and star formation for an IRAC-selected extremely red object sample in the GOODS Chandra Deep Field-South. This work was enabled by new HST/WFC3 near-IR imaging from the CANDELS survey as well as the deepest available X-ray data from Chandra 4 Ms observations. This sample consists of 133 objects with the 3.6um limiting magnitude of [3.6] = 21.5, and is approximately complete for galaxies with M >10^{11}M_sun at 1.5 < z < 2.5. We classify this sample into two types, quiescent and star-forming galaxies, in the observed infrared color-color ([3.6]-[24] vs K-[3.6]) diagram. The further morphological study of this sample show a consistent result with the observed color classification. The classified quiescent galaxies are bulge dominated and star-forming galaxies in the sample have disk or irregular morphologies. Our observed infrared color classification is also consistent with the rest-frame color (U-V vs V-J) classification. We also found that quiescent and star-forming galaxies are well separated in the nonparametric morphology parameter (Gini vs M_{20}) diagram measuring their concentration and clumpiness: quiescent galaxies have Gini coefficient higher than 0.58 and star forming galaxies have Gini coefficient lower that 0.58. We argue that the star formation quenching process must lead to or be accompanied by the increasing galaxy concentration. One prominent morphological feature of this sample is that disks are commonly seen in this massive galaxy sample at 1.5 < z < 2.5: 30% of quiescent galaxies and 70% of star forming galaxies with M >10^{11}M_sun have disks in their rest-frame optical morphologies. The prevalence of these extended, relatively undisturbed disks challenges the merging scenario as the main mode of massive galaxy formation.

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arXiv2009

Classification of Extremely Red Objects in the COSMOS Field

We present a study of the classification of z ~1 extremely red objects (EROs), using a combination of HST/ACS, Spitzer/IRAC, and ground-based images of the COSMOS field. Our sample includes about 5300 EROs with i-Ks>2.45 (AB, equivalently I-Ks=4 in Vega) and Ks<=21.1 (AB). For EROs in our sample, we compute, using the ACS F814W images, their concentration, asymmetry, as well as their Gini coefficient and the second moment of the brightest 20% of their light. Using those morphology parameters and the Spitzer/IRAC [3.6]-[8.0] color, the spectral energy distribution (SED) fitting method, we classify EROs into two classes: old galaxies (OGs) and young, dusty starburst galaxies (DGs). We found that the fraction of OGs and DGs in our sample is similar, about 48 percentages of EROs in our sample are OGs, and 52 percentages of them are DGs. To reduce the redundancy of these three different classification methods, we performed a principal component analysis on the measurements of EROs, and find that morphology parameters and SEDs are efficient in segregating OGs and DGs. The [3.6]-[8.0] color, which depends on reddening, redshift, and photometric accuracy, is difficult to separate EROs around the discriminating line between starburst and elliptical. We investigate the dependence of the fraction of EROs on their observational properties, and the results suggest that DGs become increasingly important at fainter magnitudes, redder colors, and higher redshifts.

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