arXiv subjects

XianZhong Zheng

Publications and source records attributed to XianZhong Zheng.

14 recordsLinked to original sources

arXiv2025

Narrowband Imaging of a z=3.24 Protocluster: Insights from [O III] Emitting Galaxies

We present a narrowband imaging on a spectroscopically confirmed protocluster ``D4UD01'' at z=3.24 using CFHT/WIRCam. We identify a sample of 24 [O III] emission line galaxies in the field, which forms a large overdensity in the protocluster region. The protocluster is expected to evolve into a Virgo-like cluster by z=0. Utilizing multiwavelength data, we derive the physical properties of these [O III] emitters and find they are medium mass normal star-forming galaxies ($\sim10^{10}$M$_\odot$) roughly following the star-forming main sequence. The [O III] emitters trace an overdensity spatially offset from that of photometric-redshift and quiescent galaxies, suggesting these distinct galaxy populations may inhabit dark matter halos that formed at different epochs. A comparative analysis of [O III] emitter properties shows similar characteristics in both protocluster and field environments. This protocluster likely represents an evolved structure that has progressed beyond its peak star-formation phase, although our limited sample size may prevent detection of subtle environmental effects.

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arXiv2025

Searching for Nearby Diffuse Dwarf Galaxies in the COSMOS Field

It remains challenging to systematically survey nearby diffuse dwarf galaxies and address the formation mechanism of this population distinguishing from regular ones. We carry out a pilot search for these galaxies in the COSMOS field using the deep \textit{HST}/F814W imaging data. We report three diffuse dwarf galaxies satisfying the criteria: (1) redshift $z<0.2$, (2) effective radius $r_{\rm e}>1.0''$, and (3) central surface brightness $μ_{\rm 0}>24$ mag arcsec$^{-2}$. Two of the three galaxies, COSMOS-UDG1 and COSMOS-UDG2, are recognized as ultra-diffuse galaxies (UDGs) with redshift $z=0.130$ and $0.049$, respectively. The third galaxy, COSMOS-dw1, is spectroscopically confirmed as a dwarf galaxy at $z=0.004$. We derive the physical properties through fitting their spectral energy distributions (SEDs) extracted from deep multiwavelength observations. COSMOS-dw1 has a stellar mass of $5.6_{-2.7}^{+2.5}\times10^{6}$ M$_{\odot}$, harboring neutral hydrogen gas of mass $4.90\pm0.90\times10^{6}$ M$_{\odot}$, hinting that this galaxy may be in the nascent stages of quenching. The estimated dynamical mass of $3.4\times10^{7}\,M_{\odot}$ further suggests that COSMOS-dw1 is predominantly of dark matter. COSMOS-UDG1 and COSMOS-UDG2 exhibit comparable stellar masses of $\sim 2\times10^{8}$ M$_{\odot}$. Notably, COSMOS-UDG1 is younger and more metal-rich than COSMOS-UDG2 and COSMOS-dw1. Conversely, COSMOS-UDG2 and COSMOS-dw1 have similar stellar metallicities, yet COSMOS-UDG2 is older than COSMOS-dw1. All three galaxies adhere to the stellar mass-metallicity relation (MZR) for dwarf galaxies in the local Universe, implying they belong to the dwarf galaxy population.

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arXiv2023

The Emergence of Brightest Cluster Galaxy in a Protocluster Core at $z=2.24$

We report the detection of a pair of massive quiescent galaxies likely in the process of merging at the center of the spectroscopically confirmed, extremely massive protocluster BOSS1244 at $z=2.24\pm0.02$. These galaxies, BOSS1244-QG1 and BOSS1244-QG2, were detected with Hubble Space Telescope (HST) grism slitless spectroscopic observations. These two quiescent galaxies are among the brightest member galaxies with $z=2.223-2.255$ in BOSS1244 and reside at redshifts $z=2.244$ and $z=2.242$, with a half-light radius of $6.76\pm0.50$ and $2.72\pm0.16$ kpc, respectively. BOSS1244-QG1 and BOSS1244-QG2 are separated by a projected distance of about 70 physical kpc, implying that the two galaxies likely merge to form a massive brightest cluster galaxy (BCG) with size and mass similar to the most massive BCGs in the local Universe. We thus infer that BCG formation through dry major mergers may happen earlier than the full assembly of a cluster core, which broadens our previous understanding of the co-evolution of mature galaxy clusters and BCGs in the nearby Universe. Moreover, we find a strong density-star formation relation over a scale of $\sim18$ co-moving Mpc in BOSS1244, i.e. star formation activity decreases as density increases, implying that the quenching of star formation in BCGs and their progenitors is likely governed by environment-related processes before the virialization of the cluster core.

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arXiv2023

BayeSED-GALAXIES I. Performance test for simultaneous photometric redshift and stellar population parameter estimation of galaxies in the CSST wide-field multiband imaging survey

The forthcoming CSST wide-field multiband imaging survey will produce seven-band photometric spectral energy distributions (SEDs) for billions of galaxies. The effective extraction of astronomical information from these massive datasets of SEDs relies on the techniques of both SED synthesis (or modeling) and analysis (or fitting). We evaluate the performance of the latest version of BayeSED code combined with SED models with increasing complexity for simultaneously determining the photometric redshifts and stellar population parameters of galaxies in this survey. By using an empirical statistics-based mock galaxy sample without SED modeling errors, we show finding that the random observational errors in photometries are more important sources of errors than the parameter degeneracies and Bayesian analysis method and tool. By using a Horizon-AGN hydrodynamical simulation-based mock galaxy sample with SED modeling errors about the star formation histories (SFHs) and dust attenuation laws (DALs), the simple typical assumptions lead to significantly worse parameter estimation with CSST photometries only. The SED models with more flexible (or complicated) forms of SFH/DAL do not necessarily lead to better estimation of redshift and stellar population parameters. We discuss the selection of the best SED model by means of Bayesian model comparison in different surveys. Our results reveal that the Bayesian model comparison with Bayesian evidence may favor SED models with different complexities when using photometries from different surveys. Meanwhile, the SED model with the largest Bayesian evidence tends to give the best performance of parameter estimation, which is more clear for photometries with larger discriminative power.

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arXiv2021

The cosmic environment overtakes the local density in shaping galaxy star formation

The gas supply from the cosmic web is the key to sustain star formation in galaxies. It remains to be explored how the cosmic large-scale structure (LSS) effects on galaxy evolution at given local environments. We examine galaxy specific star formation rate as a function of local density in a LSS at $z=0.735$ in the Extended Chandra Deep Field South. The LSS is mapped by 732 galaxies with $R<24$\,mag and redshift at $0.72\le z \le 0.75$ collected from the literature and our spectroscopic observations with Magellan/IMACS, consisting of five galaxy clusters/groups and surrounding filaments over an area of $23.9 \times22.7$\,co-moving\,Mpc$^2$. The spread of spectroscopic redshifts corresponds a velocity dispersion of 494\,km\,s$^{-1}$, indicating the LSS likely to be a thin sheet with a galaxy density $\gtrsim 3.9$ times that of the general field. These clusters/groups in this LSS mostly exhibit elongated morphologies and multiple components connected with surrounding filaments. Strikingly, we find that star-forming galaxies in the LSS keep star formation at the same level as field, and show no dependence on local density but stellar mass. Meanwhile, an increasing fraction of quiescent galaxies is detected at increasing local density in both the LSS and the field, consistent with the expectation that galaxy mass and local dense environment hold the key to quench star formation. Combined together, we conclude that the cosmic environment of the LSS overtakes the local environment in remaining galaxy star formation to the level of the field.

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arXiv2021

Spectroscopic Confirmation of two Extremely Massive Protoclusters BOSS1244 and BOSS1542 at $z=2.24$

We present spectroscopic confirmation of two new massive galaxy protoclusters at $z=2.24\pm0.02$, BOSS1244 and BOSS1542, traced by groups of Coherently Strong Ly$α$ Absorption (CoSLA) systems imprinted in the absorption spectra of a number of quasars from the SDSS III and identified as overdensities of narrowband-selected H$α$ emitters (HAEs). Using MMT/MMIRS and LBT/LUCI near-infrared (NIR) spectroscopy, we confirm 46 and 36 HAEs in the BOSS1244 and BOSS1542 fields, respectively. BOSS1244 displays a South-West (SW) component at $z=2.230\pm0.002$ and another North-East (NE) component at $z=2.246\pm0.001$ with the line-of-sight velocity dispersions of $405\pm202$ km s$^{-1}$ and $377\pm99$ km s$^{-1}$, respectively. Interestingly, we find that the SW region of BOSS1244 contains two substructures in redshift space, likely merging to form a larger system. In contrast, BOSS1542 exhibits an extended filamentary structure with a low velocity dispersion of $247\pm32$ km s$^{-1}$ at $z=2.241\pm0.001$, providing a direct confirmation of a large-scale cosmic web in the early Universe. The galaxy overdensities $δ_{\rm g}$ on the scale of 15 cMpc are $22.9\pm4.9$, $10.9\pm2.5$, and $20.5\pm3.9$ for the BOSS1244 SW, BOSS1244 NE, and BOSS1542 filament, respectively. They are the most overdense galaxy protoclusters ($δ_{\rm g}>20$) discovered to date at $z>2$. These systems are expected to become virialized at $z\sim0$ with a total mass of $M_{\rm SW}=(1.59\pm0.20)\times10^{15}$ $M_{\odot}$, $M_{\rm NE} =(0.83\pm0.11)\times10^{15}$ $M_{\odot}$ and $M_{\rm filament}=(1.42\pm0.18)\times10^{15}$ $M_{\odot}$, respectively. Together with BOSS1441 described in Cai et al. (2017a), these extremely massive overdensities at $z=2-3$ exhibit different morphologies, indicating that they are in different assembly stages in the formation of early galaxy clusters.

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arXiv2021

A Lyman-α protocluster at redshift 6.9

Protoclusters, the progenitors of the most massive structures in the Universe, have been identified at redshifts of up to 6.6. Besides exploring early structure formation, searching for protoclusters at even higher redshifts is particularly useful to probe the reionization. Here we report the discovery of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was only 770 million years old and could be experiencing rapid evolution of the neutral hydrogen fraction in the intergalactic medium. The protocluster is identified by an overdensity of 6 times the average galaxy density, and with 21 narrowband selected Lyman-$α$ galaxies, among which 16 have been spectroscopically confirmed. At redshifts similar to or above this record, smaller protogroups with fewer members have been reported. LAGER-z7OD1 shows an elongated shape and consists of two subprotoclusters, which would have merged into one massive cluster with a present-day mass of $3.7 \times 10^{15}$ solar masses. The total volume of the ionized bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionized. LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process.

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arXiv2020

MAMMOTH: Confirmation of Two Massive Galaxy Overdensities at $z=2.24$ with H$α$ Emitters

Massive galaxy overdensities at the peak epoch of cosmic star formation provide ideal testbeds for the formation theories of galaxies and large-scale structure. We report the confirmation of two massive galaxy overdensities at $z=2.24$, BOSS1244 and BOSS1542, selected from the MAMMOTH project using Ly$α$ absorption from the intergalactic medium over the scales of 15$-$30 $h^{-1}$ Mpc imprinted on the quasar spectra. We use H$α$ emitters (HAEs) as the density tracer and identify them using deep narrowband $H_2S1$ and broadband $K_{\rm s}$ imaging data obtained with CFHT/WIRCam. In total, 244 and 223 line emitters are detected in these two fields, and $196\pm 2$ and $175\pm 2$ are expected to be HAEs with an H$α$ flux of $> 2.5\times 10^{-17}$ erg s$^{-1}$ cm$^{-2}$ (corresponding to an SFR of $>$5 M$_\odot$ yr$^{-1}$). The detection rate of HAE candidates suggests an overdensity factor of $δ_{\rm gal}=5.6\pm0.3$ and $4.9\pm0.3$ over the volume of $54\times32\times32$ cMpc$^3$. The overdensity factor increases $2-3$ times when focusing on the high-density regions of scales $10-15$ cMpc. Interestingly, the HAE density maps reveal that BOSS1244 contains a dominant structure, while BOSS1542 manifests as a giant filamentary structure. We measure the H$α$ luminosity functions (HLF), finding that BOSS1244's HLF is nearly identical to that of the general field at the same epoch, while BOSS1542 shows an excess of HAEs with high H$α$ luminosity, indicating the presence of enhanced star formation or AGN activity. We conclude that the two massive MAMMOTH overdensities are undergoing a rapid galaxy mass assembly.

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arXiv2019

The Ly$α$ Luminosity Function and Cosmic Reionization at $z \sim$ 7.0: a Tale of Two LAGER Fields

We present the largest-ever sample of 79 Ly$α$ emitters (LAEs) at $z\sim$ 7.0 selected in the COSMOS and CDFS fields of the LAGER project (the Lyman Alpha Galaxies in the Epoch of Reionization). Our newly amassed ultradeep narrowband exposure and deeper/wider broadband images have more than doubled the number of LAEs in COSMOS, and we have selected 30 LAEs in the second field CDFS. We detect two large-scale LAE-overdense regions in the COSMOS that are likely protoclusters at the highest redshift to date. We perform injection and recovery simulations to derive the sample incompleteness. We show significant incompleteness comes from blending with foreground sources, which however has not been corrected in LAE luminosity functions in {the} literature. The bright end bump in the Ly$α$ luminosity function in COSMOS is confirmed with 6 (2 newly selected) luminous LAEs (L$_{Lyα}$ $>$ 10$^{43.3}$ erg s$^{-1}$). Interestingly, the bump is absent in CDFS, in which only one luminous LAE is detected. Meanwhile, the faint end luminosity functions from the two fields well agree with each other. The 6 luminous LAEs in COSMOS coincide with 2 LAE-overdense regions, while such regions are not seen in CDFS. The bright-end luminosity function bump could be attributed to ionized bubbles in a patchy reionization. It appears associated with cosmic overdensities, thus supports an inside-out reionization topology at $z$ $\sim$ 7.0, i.e., the high density peaks were ionized earlier compared to the voids. An average neutral hydrogen fraction of $x_{HI}$ $\sim$ 0.2 -- 0.4 is derived at $z\sim$ 7.0 based on the cosmic evolution of the Ly$α$ luminosity function.

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arXiv2019

Design for the First Narrowband Filter for the Dark Energy Camera: Optimizing the LAGER Survey for z ~ 7 Galaxies

We present the design for the first narrowband filter NB964 for the Dark Energy Camera (DECam), which is operated on the 4m Blanco Telescope at the Cerro Tololo Inter-American Observatory. The NB964 filter profile is essentially defined by maximizing the power of searching for Lyman alpha emitting galaxies (LAEs) in the epoch of reionization, with the consideration of the night sky background in the near-infrared and the DECam quantum efficiency. The NB964 filter was manufactured by Materion in 2015. It has a central wavelength of 964.2 nm and a full width at half maximum (FWHM) of 9.2 nm. An NB964 survey named LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) has been ongoing since December 2015. Here we report results of lab tests, on-site tests and observations with the NB964 filter. The excellent performances of this filter ensure that the LAGER project is able to detect LAEs at z~7 with a high efficiency.

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arXiv2017

First Spectroscopic Confirmations of z ~ 7.0 Lya Emitting Galaxies in the LAGER Survey

Narrowband imaging is a highly successful approach for finding large numbers of high redshift Lya emitting galaxies (LAEs) up to z~6.6. However, at z>~7 there are as yet only 3 narrowband selected LAEs with spectroscopic confirmations (two at z~6.9-7.0, one at z~7.3), which hinders extensive studies on cosmic reionization and galaxy evolution at this key epoch. We have selected 23 candidate z~6.9 LAEs in COSMOS field with the large area narrowband survey LAGER (Lyman-Alpha Galaxies at the End of Reionization). In this work we present spectroscopic followup observations of 12 candidates using IMACS on Magellan. For 9 of these, the observations are sufficiently deep to detect the expected lines. Lya emission lines are identified in six sources (yielding a success rate of 2/3), including 3 luminous LAEs with Lya luminosities of L(Lya) ~ 10^{43.5} erg/s, the highest among known spectroscopically confirmed galaxies at >~7.0. This triples the sample size of spectroscopically confirmed narrowband selected LAEs at z>~7, and confirms the bright end bump in the Lya luminosity function we previously derived based on the photometric sample, supporting a patchy reionization scenario. Two luminous LAEs appear physically linked with projected distance of 1.1 pMpc and velocity difference of ~ 170 km/s. They likely sit in a common ionized bubble produced by themselves or with close neighbors, which reduces the IGM attenuation of Lya. A tentative narrow NV$λ$1240 line is seen in one source, hinting at activity of a central massive black hole with metal rich line emitting gas.

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arXiv2016

Measuring Structural Parameters Through Stacking Galaxy Images

It remains challenging to detect the low surface brightness structures of faint high-z galaxies, which is key to understanding the structural evolution of galaxies. The technique of image stacking allows us to measure the averaged light profile beneath the detection limit and probe the extended structure of a group of galaxies. We carry out simulations to examine the recovery of the averaged surface brightness profile through stacking model HST/ACS images of a set of galaxies as functions of Sersic index (n), effective radius (Re) and axis ratio (AR). The Sersic profile best fitting the radial profile of the stacked image is taken as the recovered profile, in comparison with the intrinsic mean profile of the model galaxies. Our results show that, in general, the structural parameters of the mean profile can be properly determined through stacking, although systematic biases need to be corrected when spreads of Re and AR are counted. We find that Sersic index is slightly overestimated and Re is underestimated at AR < 0.5 as the stacked image appears to be more compact due to the presence of inclined galaxies; the spread of Re biases the stacked profile to have a higher Sersic index. We stress that the measurements of structural parameters through stacking should take these biases into account. We estimate the biases in the recovered structural parameters from stacks of galaxies when the samples have distributions of Re, AR and n seen in local galaxies.

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arXiv2016

The Flattening of Dust Attenuation Curve to z=2.5

We examine the evolution of dust attenuation curve using a sample of 9504 disk star-forming galaxies (SFGs) selected from the CANDELS and 3D-HST surveys and a new technique relying on the fact that disk SFGs of similar stellar masses at the same cosmic epoch are statistically identical in stellar populations. We attribute the discrepancy in median magnitude between face-on (b/a>0.7) and edge-on (b/a<=0.4) subsamples solely to dust attenuation, and obtain the average attenuation in the rest-frame UV and optical as functions of stellar mass and redshift out to z=2.5. Our results show that the attenuation curve becomes remarkably flatter at increasing redshift for both massive and low-mass disk SFGs, and remains likely unchanged with galaxy stellar mass at a fixed epoch within uncertainties. Compared with the Calzetti law, our dust attenuation curves appear to be slightly steeper at 0.5<z<1.4 and remarkably flatter at 1.4<z<2.5. Our findings are consistent with a picture in which the evolution of dust grain size distribution is mainly responsible for the evolution of the dust attenuation curve in SFGs; dust shattering becomes a dominant process at z<~1.4, resulting in an enrichment of small dust grains and consequently a steeper attenuation curve. We stress that extinction correction for high-z galaxies should be done using mass- and redshift-dependent attenuation curves.

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arXiv2004

Galaxy formation and evolution since z=1

Determination of the star formation rate can be done using mid-IR photometry or Balmer line luminosity after a proper correction for extinction effects. Both methods show convergent results while those based on UV or on [OII]3727 luminosities underestimate the SFR by factors ranging from 5 to 40 for starbursts and for luminous IR galaxies, respectively. Most of the evolution of the cosmic star formation density is related to the evolution of luminous compact galaxies and to luminous IR galaxies. Because they were metal deficient and were forming stars at very high rates (40 to 100 solar mass per year), it is probable that these (massive) galaxies were actively forming the bulk of their stellar/metal content at z < 1.

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