arXiv subjects

Yi-Kuan Chiang

Publications and source records attributed to Yi-Kuan Chiang.

48 records · Page 3Linked to original sources

arXiv2018

Extragalactic Imprints in Galactic Dust Maps

Extragalactic astronomy relies on the accurate estimation of source photometry corrected for Milky Way dust extinction. This has motivated the creation of a number of "Galactic" dust maps. We investigate whether these maps are contaminated by extragalactic signals using the clustering-redshift technique, i.e., by measuring a set of angular cross-correlations with spectroscopic objects as a function of redshift. Our tomographic analysis reveals imprints of extragalactic large-scale structure patterns in nine out of 10 Galactic dust maps, including all infrared-based maps as well as "stellar" reddening maps. When such maps are used for extinction corrections, this extragalactic contamination introduces redshift- and scale-dependent biases in photometric estimates at the millimagnitude level. It can affect both object-based analyses, such as the estimation of the Hubble diagram with supernovae, as well as spatial statistics. The bias can be appreciable when measuring angular correlation functions with low amplitudes, such as lensing-induced correlations or angular correlations for sources distributed over a broad redshift range. As expected, we do not detect any extragalactic contamination for the dust map inferred from 21cm HI observations. Such a map provides an alternative to widely used infrared-based maps but relies on the assumption of a constant dust-to-gas ratio. We note that, using the WISE 12 micron map sensitive to polycyclic aromatic hydrocarbons (PAH), an indirect dust tracer, we detect the diffuse extragalactic PAH background up to $z\sim2$. Finally, we provide a procedure to minimize the level of biased magnitude corrections in maps with extragalactic imprints.

astro-ph.GA↗

arXiv2018

A Giant Protocluster of Galaxies at Redshift 5.7

Galaxy clusters trace the largest structures of the Universe and provide ideal laboratories for studying galaxy evolution and cosmology. Clusters with extended X-ray emission have been discovered at redshifts up to z ~ 2.5. Meanwhile, there has been growing interest in hunting for protoclusters, the progenitors of clusters, at higher redshifts. It is, however, very challenging to find the largest protoclusters at early times when they start to assemble. Here we report a giant protocluster of galaxies at redshift z = 5.7, when the Universe was only one billion years old. This protocluster occupies a volume of about 35x35x35 cubic co-moving megaparsecs. It is embedded in an even larger overdense region with at least 41 spectroscopically confirmed, luminous Lyman-alpha emitting galaxies (Lyman-alpha Emitters, or LAEs), including several previously reported LAEs. Its LAE density is 6.6 times the average density at z ~ 5.7. It is the only one of its kind in an LAE survey in four square degrees on the sky. Such a large structure is also rarely seen in current cosmological simulations. This protocluster will collapse into a galaxy cluster with a mass of (3.6+/-0.9) x 10^{15} solar masses, comparable to those of the most massive clusters or protoclusters known to date.

astro-ph.GA↗

arXiv2018

SILVERRUSH. VII. Subaru/HSC Identifications of 42 Protocluster Candidates at z~6-7 with the Spectroscopic Redshifts up to z=6.574: Implications for Cosmic Reionization

We report fourteen and twenty-eight protocluster candidates at z=5.7 and 6.6 over 14 and 19 deg^2 areas, respectively, selected from 2,230 (259) Lya emitters (LAEs) photometrically (spectroscopically) identified with Subaru/Hyper Suprime-Cam (HSC) deep images (Keck, Subaru, and Magellan spectra and the literature data). Six out of the 42 protocluster candidates include 1-12 spectroscopically confirmed LAEs at redshifts up to z=6.574. By the comparisons with the cosmological Lya radiative transfer (RT) model reproducing LAEs with the reionization effects, we find that more than a half of these protocluster candidates are progenitors of the present-day clusters with a mass of > 10^14 M_sun. We then investigate the correlation between LAE overdensity delta and Lya rest-frame equivalent width EW_Lya^rest, because the cosmological Lya RT model suggests that a slope of EW_Lya^rest-delta relation is steepened towards the epoch of cosmic reionization (EoR), due to the existence of the ionized bubbles around galaxy overdensities easing the escape of Lya emission from the partly neutral intergalactic medium (IGM). The available HSC data suggest that the slope of the EW_Lya^rest-delta correlation does not evolve from the post-reionization epoch z=5.7 to the EoR z=6.6 beyond the moderately large statistical errors. There is a possibility that we would detect the evolution of the EW_Lya^rest - delta relation from z=5.7 to 7.3 by the upcoming HSC observations providing large samples of LAEs at z=6.6-7.3.

astro-ph.GA↗

arXiv2017

Luminous Quasars Do Not Live in the Most Overdense Regions of Galaxies at z~4

We present the cross-correlation between 151 luminous quasars ($M_{ \mathrm{UV}} < -26$) and 179 protocluster candidates at $z \sim 3.8$, extracted from the Wide imaging survey ($ \sim 121~ $deg$^2$) performed with a part of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We find that only two out of 151 quasars reside in regions that are more overdense compared to the average field at $ > 4 σ$. The distributions of the distance between quasars and the nearest protoclusters and the significance of the overdensity at the position of quasars are statistically identical to those found for $g$-dropout galaxies, suggesting that quasars tend to reside in almost the same environment as star-forming galaxies at this redshift. Using stacking analysis, we find that the average density of $g$-dropout galaxies around quasars is slightly higher than that around $g$-dropout galaxies on $1.0 - 2.5$ pMpc scales, while at $ < 0.5$ pMpc that around quasars tends to be lower. We also find that quasars with higher UV-luminosity or with more massive black holes tend to avoid the most overdense regions, and that the quasar near zone sizes are anti-correlated with overdensity. These findings are consistent with a scenario in which the luminous quasar at $z \sim4 $ resides in structures that are less massive than those expected for the progenitors of today's rich clusters of galaxies, and possibly that luminous quasars may be suppressing star formation in their close vicinity.

astro-ph.GA↗

arXiv2017

Galaxy Protoclusters as Drivers of Cosmic Star-Formation History in the First 2 Gyr

Present-day clusters are massive halos containing mostly quiescent galaxies, while distant protoclusters are extended structures containing numerous star-forming galaxies. We investigate the implications of this fundamental change in a cosmological context using a set of N-body simulations and semi-analytic models. We find that the fraction of the cosmic volume occupied by all (proto)clusters increases by nearly three orders of magnitude from z=0 to z=7. We show that (proto)cluster galaxies are an important, and even dominant population at high redshift, as their expected contribution to the cosmic star-formation rate density rises (from 1% at z=0) to 20% at z=2 and 50% at z=10. Protoclusters thus provide a significant fraction of the cosmic ionizing photons, and may have been crucial in driving the timing and topology of cosmic reionization. Internally, the average history of cluster formation can be described by three distinct phases: at z~10-5, galaxy growth in protoclusters proceeded in an inside-out manner, with centrally dominant halos that are among the most active regions in the Universe; at z~5-1.5, rapid star formation occurred within the entire 10-20 Mpc structures, forming most of their present-day stellar mass; at z<~1.5, violent gravitational collapse drove these stellar contents into single cluster halos, largely erasing the details of cluster galaxy formation due to relaxation and virialization. Our results motivate observations of distant protoclusters in order to understand the rapid, extended stellar growth during Cosmic Noon, and their connection to reionization during Cosmic Dawn.

astro-ph.GA↗

arXiv2017

Cosmic Galaxy-IGM HI Relation at ${\it{z}}\sim 2-3$ Probed in the COSMOS/UltraVISTA $1.6$ deg$^2$ Field

We present spatial correlations of galaxies and IGM HI in the COSMOS/UltraVISTA 1.62 deg$^2$ field. Our data consist of 13,415 photo-$z$ galaxies at $z\sim2-3$ with $K_s<23.4$ and the Ly$α$ forest absorptions in the background quasar spectra selected from SDSS data with no signature of damped Ly$α$ system contamination. We estimate a galaxy overdensity $δ_{gal}$ in an impact parameter of 2.5 pMpc, and calculate the Ly$α$ forest fluctuations $δ_{\langle F\rangle}$ whose negative values correspond to the strong Ly$α$ forest absorptions. We identify weak evidence of an anti-correlation between $δ_{gal}$ and $δ_{\langle F\rangle}$ with a Spearman's rank correlation coefficient of $-0.39$ suggesting that the galaxy overdensities and the Ly$α$ forest absorptions positively correlate in space at the $\sim90\%$ confidence level. This positive correlation indicates that high-$z$ galaxies exist around an excess of HI gas in the Ly$α$ forest. We find four cosmic volumes, dubbed $A_{obs}$-$D_{obs}$, that have extremely large (small) values of $δ_{gal} \simeq0.8$ ($-1$) and $δ_{\langle F\rangle}$ $\simeq0.1$ ($-0.4$), three out of which, $B_{obs}$-$D_{obs}$, significantly depart from the correlation, and weaken the correlation signal. We perform cosmological hydrodynamical simulations, and compare with our observational results. Our simulations reproduce the correlation, agreeing with the observational results. Moreover, our simulations have model counterparts of $A_{obs}$-$D_{obs}$, and suggest that the observations pinpoint, by chance, a galaxy overdensity like a proto-cluster, gas filaments lying on the sightline, a large void, and orthogonal low-density filaments. Our simulations indicate that the significant departures of $B_{obs}$-$D_{obs}$ are produced by the filamentary large-scale structures and the observation sightline effects.

astro-ph.GA↗

arXiv2016

Large scale structure around a z=2.1 cluster

The most prodigious starburst galaxies are absent in massive galaxy clusters today, but their connection with large scale environments is less clear at $z\gtrsim2$. We present a search of large scale structure around a galaxy cluster core at $z=2.095$ using a set of spectroscopically confirmed galaxies. We find that both color-selected star-forming galaxies (SFGs) and dusty star-forming galaxies (DSFGs) show significant overdensities around the $z=2.095$ cluster. A total of 8 DSFGs (including 3 X-ray luminous active galactic nuclei, AGNs) and 34 SFGs are found within a 10 arcmin radius (corresponds to $\sim$15 cMpc at $z\sim2.1$) from the cluster center and within a redshift range of $Δz=0.02$, which leads to galaxy overdensities of $δ_{\rm DSFG}\sim12.3$ and $δ_{\rm SFG}\sim2.8$. The cluster core and the extended DSFG- and SFG-rich structure together demonstrate an active cluster formation phase, in which the cluster is accreting a significant amount of material from large scale structure while the more mature core may begin to virialize. Our finding of this DSFG-rich structure, along with a number of other protoclusters with excess DSFGs and AGNs found to date, suggest that the overdensities of these rare sources indeed trace significant mass overdensities. However, it remains puzzling how these intense star formers are triggered concurrently. Although an increased probability of galaxy interactions and/or enhanced gas supply can trigger the excess of DSFGs, our stacking analysis based on 850 $μ$m images and morphological analysis based on rest-frame optical imaging do not show such enhancements of merger fraction and gas content in this structure.

astro-ph.GA↗

arXiv2015

HST Emission Line Galaxies at z ~ 2: Comparing Physical Properties of Lyman Alpha and Optical Emission Line Selected Galaxies

We compare the physical and morphological properties of z ~ 2 Lyman-alpha emitting galaxies (LAEs) identified in the HETDEX Pilot Survey and narrow band studies with those of z ~ 2 optical emission line galaxies (oELGs) identified via HST WFC3 infrared grism spectroscopy. Both sets of galaxies extend over the same range in stellar mass (7.5 < logM < 10.5), size (0.5 < R < 3.0 kpc), and star-formation rate (~1 < SFR < 100). Remarkably, a comparison of the most commonly used physical and morphological parameters -- stellar mass, half-light radius, UV slope, star formation rate, ellipticity, nearest neighbor distance, star formation surface density, specific star formation rate, [O III] luminosity, and [O III] equivalent width -- reveals no statistically significant differences between the populations. This suggests that the processes and conditions which regulate the escape of Ly-alpha from a z ~ 2 star-forming galaxy do not depend on these quantities. In particular, the lack of dependence on the UV slope suggests that Ly-alpha emission is not being significantly modulated by diffuse dust in the interstellar medium. We develop a simple model of Ly-alpha emission that connects LAEs to all high-redshift star forming galaxies where the escape of Ly-alpha depends on the sightline through the galaxy. Using this model, we find that mean solid angle for Ly-alpha escape is 2.4+/-0.8 steradians; this value is consistent with those calculated from other studies.

astro-ph.GA↗

arXiv2015

Surveying Galaxy Proto-clusters in Emission: A Large-scale Structure at z=2.44 and the Outlook for HETDEX

Galaxy proto-clusters at z >~ 2 provide a direct probe of the rapid mass assembly and galaxy growth of present day massive clusters. Because of the need of precise galaxy redshifts for density mapping and the prevalence of star formation before quenching, nearly all the proto-clusters known to date were confirmed by spectroscopy of galaxies with strong emission lines. Therefore, large emission-line galaxy surveys provide an efficient way to identify proto-clusters directly. Here we report the discovery of a large-scale structure at z = 2.44 in the HETDEX Pilot Survey. On a scale of a few tens of Mpc comoving, this structure shows a complex overdensity of Lya emitters (LAE), which coincides with broad-band selected galaxies in the COSMOS/UltraVISTA photometric and zCOSMOS spectroscopic catalogs, as well as overdensities of intergalactic gas revealed in the Lya absorption maps of Lee et al. (2014). We construct mock LAE catalogs to predict the cosmic evolution of this structure. We find that such an overdensity should have already broken away from the Hubble flow, and part of the structure will collapse to form a galaxy cluster with 10^14.5 +- 0.4 M_sun by z = 0. The structure contains a higher median stellar mass of broad-band selected galaxies, a boost of extended Lya nebulae, and a marginal excess of active galactic nuclei relative to the field, supporting a scenario of accelerated galaxy evolution in cluster progenitors. Based on the correlation between galaxy overdensity and the z = 0 descendant halo mass calibrated in the simulation, we predict that several hundred 1.9 < z < 3.5 proto-clusters with z = 0 mass of > 10^14.5 M_sun will be discovered in the 8.5 Gpc^3 of space surveyed by the Hobby Eberly Telescope Dark Energy Experiment.

astro-ph.CO↗

arXiv2013

Ancient Light from Young Cosmic Cities: Physical and Observational Signatures of Galaxy Proto-Clusters

For a complete picture of galaxy cluster formation, it is important that we start probing the early epoch of z~2-7 during which clusters and their galaxies first began to form. Because the study of these so-called "proto-clusters" is currently limited by small number statistics, widely varying selection techniques and assumptions, we have performed a systematic study of cluster formation utilizing cosmological simulations. We use the Millennium Simulations to track the evolution of dark matter and galaxies in ~3,000 clusters from the earliest times to z=0. We define an effective radius R_e for proto-clusters and characterize their growth in size and mass. We show that the progenitor regions of galaxy clusters (M>10^14 M_sun/h) can already be identified at least up to z~5, provided that the galaxy overdensities, delta_gal, are measured on a sufficiently large scale (R_e~5-10 cMpc). We present the overdensities in matter, DM halos, and galaxies as functions of present-day cluster mass, redshift, bias, and window size that can be used to interpret the structures found in real surveys. We derive the probability that a structure having a delta_gal, defined by a set of observational selection criteria, is indeed a proto-cluster, and show how their z=0 masses can already be estimated long before virialization. Galaxy overdensity profiles as a function of radius are presented. We further show how their projected surface overdensities decrease as the uncertainties in redshift measurements increase. We provide a table of proto-cluster candidates selected from the literature, and discuss their properties in the light of our simulations predictions. This work provides the general framework that will allow us to extend the study of cluster formation out to much higher redshifts using the large number of proto-clusters that are expected to be discovered in, e.g., the upcoming HETDEX and HSC surveys.

astro-ph.CO↗

arXiv2013

Discovery of a large number of candidate proto-clusters traced by ~15 Mpc-scale galaxy overdensities in COSMOS

To demonstrate the feasibility of studying the epoch of massive galaxy cluster formation in a more systematic manner using current and future galaxy surveys, we report the discovery of a large sample of proto-cluster candidates in the 1.62 deg^2 COSMOS/UltraVISTA field traced by optical/IR selected galaxies using photometric redshifts. By comparing properly smoothed 3D galaxy density maps of the observations and a set of matched simulations incorporating the dominant observational effects (galaxy selection and photometric redshift uncertainties), we first confirm that the observed ~15 comoving Mpc scale galaxy clustering is consistent with LCDM models. Using further the relation between high-z overdensity and the present day cluster mass calibrated in these matched simulations, we found 36 candidate structures at 1.6<z<3.1, showing overdensities consistent with the progenitors of M_z=0 ~10^15 M_sun clusters. Taking into account the significant upward scattering of lower mass structures, the probabilities for the candidates to have at least M_z=0 ~10^14 M_sun are ~70%. For each structure, about 15%-40% of photometric galaxy candidates are expected to be true proto-cluster members that will merge into a cluster-scale halo by z=0. With solely photometric redshifts, we successfully rediscover two spectroscopically confirmed structures in this field, suggesting that our algorithm is robust. This work generates a large sample of uniformly-selected proto-cluster candidates, providing rich targets for spectroscopic follow-up and subsequent studies of cluster formation. Meanwhile, it demonstrates the potential for probing early cluster formation with upcoming redshift surveys such as the Hobby-Eberly Telescope Dark Energy Experiment and the Subaru Prime Focus Spectrograph survey.

astro-ph.CO↗

arXiv2011

The Long-term Variability of the X-ray Sources in M82

We investigate the long-term variability exhibited by the X-ray point sources in the starburst galaxy M82. By combining 9 Chandra observations taken between 1999 and 2007, we detect 58 X-ray point sources within the D25 isophote of M82 down to a luminosity of ~ 10^37 ergs/s. Of these 58 sources, we identify 3 supernova remnant candidates and one supersoft source. Twenty-six sources in M82 exhibit long-term (i.e., days to years) flux variability and 3 show long-term spectral variability. Furthermore, we classify 26 sources as variables and 10 as persistent sources. Among the total 26 variables, 17 varied by a flux ratio of > 3 and 6 are transient candidates. By comparing with other nearby galaxies, M82 shows extremely strong long-term X-ray variability that 47% of the X-ray sources are variables with a flux ratio of > 3. The strong X-ray variability of M82 suggests that the population is dominated by X-ray binaries.

astro-ph.HE↗