arXiv subjects

Guangtun Zhu

Publications and source records attributed to Guangtun Zhu.

At least 19 recordsLinked to original sources

arXiv2023

Circumgalactic Medium at High Halo Masses -- Signatures of Cold Gas Depletion in Luminous Red Galaxies

We study ultraviolet HI and metal line transitions in the circumgalactic medium (CGM) of 15 massive, quenched luminous red galaxies (LRGs) at redshift $z\sim 0.5$ and with impact parameters up to 400 kpc. We selected 8 of LRG-CGM systems to study general properties of the CGM around LRGs, while the other 7 are already known to contain cool CGM gas from MgII optical studies (MgII-LRGs). In the general LRGs population, we detect HI in 4 of 8 LRGs, in all cases with $N_{HI} < 10^{16.7} {\rm cm^{-2}}$. In contrast, all MgII-LRGs show HI; for four LRGs the HI column density is $N_{HI} \gtrsim 10^{18} {\rm cm^{-2}}$. The CGM of LRGs also shows low and intermediate ionized lines (such as CIII, CII, SiIII, SiII) and highly ionized lines of OVI (we detect OVI around 5 of 7 MgII-LRGs and 1 of 8 in the random sample). Next, we combine our sample with literature LRGs and $\lesssim L^{*}$ galaxies and we find that while for $\lesssim L^{*}$ galaxies CGM HI Ly$α$ absorption is stronger as galaxies are more massive, the cool CGM traced by HI Ly$α$ is suppressed above stellar masses of $M* \sim 10^{11.5}$ $M_{\odot}$. While most LRG CGM systems show weak or non-detectable OVI (equivalent width less than 0.2 Å), a few LRG CGM systems show strong OVI 1031, which in most cases likely originates from groups containing both a LRG and a blue star-forming neighboring galaxy.

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arXiv2020

A systematic study of galactic outflows via fluorescence emission: implications for their size and structure

Galactic outflows play a major role in the evolution of galaxies, but the underlying physical processes are poorly understood. This is mainly because we have little information about the outflow structure, especially on large scales. In this paper, we probe the structure of galactic outflows in low-$z$ starburst by using a combination of ultra-violet spectroscopy and imaging of the fluorescence emission lines (associated with transitions to excited fine-structure levels) and spectroscopy of the corresponding strongly blue-shifted resonance absorption lines. We find that in the majority of cases the observed fluorescence emission lines are much weaker and narrower than the absorption lines, originating in the star-forming interstellar medium and/or the slowest-moving part of the inner outflow. In a minority of cases, the outflowing absorbing material does make a significant contribution to the fluorescence emission. These latter systems are characterized by both strong Ly$α$ emission lines and weak low-ionization absorption lines (both known to be empirical signs of Lyman-continuum leakage). We argue that the observed weakness of emission from the outflow seen in the majority of cases is due to the missing emission arising on scales larger than those encompassed by the aperture of the {\it{Hubble Space Telescope}}. This implies shallow radial density profiles in these outflows, and suggests that most of the observed absorbing material must be created/injected at radii much larger than that of the starburst. This has important implications for our understanding of both the physics of galactic outflows and for our estimation of their principal properties.

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arXiv2019

The Sloan Digital Sky Survey Reverberation Mapping Project: Sample Characterization

We present a detailed characterization of the 849 broad-line quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Our quasar sample covers a redshift range of 0.1<z<4.5 and is flux-limited to i_PSF<21.7 without any other cuts on quasar properties. The main sample characterization includes: 1) spectral measurements of the continuum and broad emission lines for individual objects from the coadded first-season spectroscopy in 2014; 2) identification of broad and narrow absorption lines in the spectra; 3) optical variability properties for continuum and broad lines from multi-epoch spectroscopy. We provide improved systemic redshift estimates for all quasars, and demonstrate the effects of signal-to-noise ratio on the spectral measurements. We compile measured properties for all 849 quasars along with supplemental multi-wavelength data for subsets of our sample from other surveys. The SDSS-RM sample probes a diverse range in quasar properties, and shows well detected continuum and broad-line variability for many objects from first-season monitoring data. The compiled properties serve as the benchmark for follow-up work based on SDSS-RM data. The spectral fitting tools are made public along with this work.

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arXiv2018

Extreme circumgalactic HI and CIII absorption around the most massive, quenched galaxies

Luminous red galaxies (LRGs) are the most massive galaxies at $z\sim 0.5$ and, by selection, have negligible star formation. These objects have halo masses between those of $L_{*}$ galaxies, whose circumgalactic media (CGM) are observed to have large masses of cold gas, and clusters of galaxies, which primarily contain hot gas. Here we report detections of strong and extended metal (CIII 977) and HI lines in the CGM of two LRGs. The CIII lines have equivalent widths of $\sim 1.8$ Å and $\sim 1.2$ Å , and velocity spreads of $\sim 796$ km s$^{-1}$ and $\sim 1245$ km s$^{-1}$, exceeding all such measurements from local $\sim L_{*}$ galaxies (maximal CIII equivalent widths $\sim 1$ Å). The data demonstrate that a subset of halos hosting very massive, quenched galaxies contain significant complexes of cold gas. Possible scenarios to explain our observations include that the LRGs' CGM originate from past activity (e.g., star formation or active galactic nuclei driven outflows) or from the CGM of galaxies in overlapping subhalos. We favor the latter scenario, in which the properties of the CGM are more tightly linked to the underlying dark matter halo than properties of the targeted galaxies (e.g., star formation).

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arXiv2017

A Local Leaky-box Model for the Local Stellar Surface Density - Gas Surface Density - Gas Phase Metallicity Relation

We revisit the relation between the stellar surface density, the gas surface density, and the gas-phase metallicity of typical disk galaxies in the local Universe with the SDSS-IV/MaNGA survey, using the star formation rate surface density as an indicator for the gas surface density. We show that these three local parameters form a tight relationship, confirming previous works (e.g., by the PINGS and CALIFA surveys), but with a larger sample. We present a new local leaky-box model, assuming star formation history and chemical evolution is localized except for outflowing materials. We derive closed-form solutions for the evolution of stellar surface density, gas surface density and gas-phase metallicity, and show that these parameters form a tight relation independent of initial gas density and time. We show that, with canonical values of model parameters, this predicted relation match the observed one well. In addition, we briefly describe a pathway to improving the current semi-analytic models of galaxy formation by incorporating the local leaky-box model in the cosmological context, which can potentially explain simultaneously multiple properties of Milky Way-type disk galaxies, such as the size growth and the global stellar mass-gas metallicity relation.

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arXiv2017

Clustering of MgII absorption line systems around massive galaxies: an important constraint on feedback processes in galaxy formation

We use the latest version of the metal line absorption catalogue of Zhu & Ménard (2013) to study the clustering of MgII absorbers around massive galaxies (~10^11.5 M_sun), quasars and radio-loud AGN with redshifts between 0.4 and 0.75. Clustering is evaluated in two dimensions, by binning absorbers both in projected radius and in velocity separation. Excess MgII is detected around massive galaxies out to R_p=20 Mpc. At projected radii less than 800 kpc, the excess extends out to velocity separations of 10,000 km/s. The extent of the high velocity tail within this radius is independent of the mean stellar age of the galaxy and whether or not it harbours an active galactic nucleus. We interpret our results using the publicly available Illustris and Millennium simulations. Models where the MgII absorbers trace the dark matter particle or subhalo distributions do not fit the data. They overpredict the clustering on small scales and do not reproduce the excess high velocity separation MgII absorbers seen within the virial radius of the halo. The Illustris simulations which include thermal, but not mechanical feedback from AGN, also do not provide an adequate fit to the properties of the cool halo gas within the virial radius. We propose that the large velocity separation MgII absorbers trace gas that has been pushed out of the dark matter halos, possibly by multiple episodes of AGN-driven mechanical feedback acting over long timescales.

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arXiv2016

Nonnegative Matrix Factorization (NMF) with Heteroscedastic Uncertainties and Missing data

Dimensionality reduction and matrix factorization techniques are important and useful machine-learning techniques in many fields. Nonnegative matrix factorization (NMF) is particularly useful for spectral analysis and image processing in astronomy. I present the vectorized update rules and an independent proof of their convergence for NMF with heteroscedastic measurements and missing data. I release a Python implementation of the rules and use an optical spectroscopic dataset of extragalactic sources as an example for demonstration. A future paper will present results of applying the technique to image processing of planetary disks.

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arXiv2016

Hydrogen Emission from the Ionized Gaseous Halos of Low Redshift Galaxies

Using a sample of nearly half million galaxies, intersected by over 7 million lines of sight from the Sloan Digital Sky Survey Data Release 12, we trace H$α$ + [N{\small II}] emission from a galactocentric projected radius, $r_p$, of 5 kpc to more than 100 kpc. The emission flux surface brightness is $\propto r_p^{-1.9 \pm 0.4}$. We obtain consistent results using only the H$α$ or [N{\small II}] flux. We measure a stronger signal for the bluer half of the target sample than for the redder half on small scales, $r_p <$ 20 kpc. We obtain a $3σ$ detection of H$α$ + [N{\small II}] emission in the 50 to 100 kpc $r_p$ bin. The mean emission flux within this bin is $(1.10 \pm 0.35) \times 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ Å$^{-1}$, which corresponds to $1.87 \times 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ arcsec$^{-2}$ or 0.0033 Rayleigh. This detection is 34 times fainter than a previous strict limit obtained using deep narrow-band imaging. The faintness of the signal demonstrates why it has been so difficult to trace recombination radiation out to large radii around galaxies. This signal, combined with published estimates of n$_{\rm H}$, lead us to estimate the temperature of the gas to be 12,000 K, consistent with independent empirical estimates based on metal ion absorption lines and expectations from numerical simulations.

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arXiv2016

The evolution of the [OII], Hβ and [OIII] emission-line luminosity functions over the last nine billions years

Emission line galaxies are one of the main tracers of the large-scale structure to be targeted by the next-generation dark energy surveys. To provide a better understanding of the properties and statistics of these galaxies, we have collected spectroscopic data from the VVDS and DEEP2 deep surveys and estimated the galaxy luminosity functions (LFs) of three distinct emission lines, [OII] ($0.5 < z < 1.3$), Hβ ($0.3 < z < 0.8$) and [OIII] ($0.3 < z < 0.8$). Our measurements are based on 35,639 emission line galaxies and cover a volume of $\sim10^7$Mpc$^3$. We present the first measurement of the Hβ LF at these redshifts. We have also compiled LFs from the literature that were based on independent data or covered different redshift ranges, and we fit the entire set over the whole redshift range with analytic Schechter and Saunders models, assuming a natural redshift dependence of the parameters. We find that the characteristic luminosity ($L_*$) and density ($ϕ_*$) of all LFs increase with redshift. Using the Schechter model over the redshift ranges considered, we find that, for [OII] emitters, the characteristic luminosity $L_*(z=0.5)=3.2\times10^{41}$ erg/s increases by a factor of $2.7 \pm 0.2$ from z=0.5 to 1.3; for Hβ emitters $L_*(z=0.3)=1.3\times10^{41}$ erg/s increases by a factor of $2.0 \pm 0.2$ from z=0.3 to 0.8; and for [OIII] emitters $L_*(z=0.3)=7.3\times10^{41}$ erg/s increases by a factor of $3.5 \pm 0.4$ from z=0.3 to 0.8.

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arXiv2016

A New View of Classification in Astronomy with the Archetype Technique: An Astronomical Case of the NP-complete Set Cover Problem

We introduce a new generic Archetype technique for source classification and identification, based on the NP-complete set cover problem (SCP) in computer science and operations research (OR). We have developed a new heuristic SCP solver, by combining the greedy algorithm and the Lagrangian Relaxation (LR) approximation method. We test the performance of our code on the test cases from Beasley's OR Library and show that our SCP solver can efficiently yield solutions that are on average 99% optimal in terms of the cost. We discuss how to adopt SCP for classification purposes and put forward a new Archetype technique. We use an optical spectroscopic dataset of extragalactic sources from the Sloan Digital Sky Survey (SDSS) as an example to illustrate the steps of the technique. We show how the technique naturally selects a basis set of physically-motivated archetypal systems to represent all the extragalactic sources in the sample. We discuss several key aspects in the technique and in any general classification scheme, including distance metric, dimensionality, and measurement uncertainties. We briefly discuss the relationships between the Archetype technique and other machine-learning techniques, such as the $k$-means clustering method. Finally, our code is publicly available and the technique is generic and easy to use and expand. We expect that it can help maximize the potential for astrophysical sciences of the low-S/N spectroscopic data from future dark-energy surveys, and can find applications in many fields of astronomy, including the formation and evolution of a variety of astrophysical systems, such as galaxies, stars and planets.

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arXiv2016

PRIMUS + DEEP2: Clustering of X-ray, Radio and IR-AGN at z~0.7

We measure the clustering of X-ray, radio, and mid-IR-selected active galactic nuclei (AGN) at 0.2 < z < 1.2 using multi-wavelength imaging and spectroscopic redshifts from the PRIMUS and DEEP2 redshift surveys, covering 7 separate fields spanning ~10 square degrees. Using the cross-correlation of AGN with dense galaxy samples, we measure the clustering scale length and slope, as well as the bias, of AGN selected at different wavelengths. Similar to previous studies, we find that X-ray and radio AGN are more clustered than mid-IR-selected AGN. We further compare the clustering of each AGN sample with matched galaxy samples designed to have the same stellar mass, star formation rate, and redshift distributions as the AGN host galaxies and find no significant differences between their clustering properties. The observed differences in the clustering of AGN selected at different wavelengths can therefore be explained by the clustering differences of their host populations, which have different distributions in both stellar mass and star formation rate. Selection biases inherent in AGN selection, therefore, determine the clustering of observed AGN samples. We further find no significant difference between the clustering of obscured and unobscured AGN, using IRAC or WISE colors or X-ray hardness ratio.

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arXiv2015

PRIMUS: The Effect of Physical Scale on the Luminosity-Dependence of Galaxy Clustering via Cross-Correlations

We report small-scale clustering measurements from the PRIMUS spectroscopic redshift survey as a function of color and luminosity. We measure the real-space cross-correlations between 62,106 primary galaxies with PRIMUS redshifts and a tracer population of 545,000 photometric galaxies over redshifts from z=0.2 to z=1. We separately fit a power-law model in redshift and luminosity to each of three independent color-selected samples of galaxies. We report clustering amplitudes at fiducial values of z=0.5 and L=1.5 L*. The clustering of the red galaxies is ~3 times as strong as that of the blue galaxies and ~1.5 as strong as that of the green galaxies. We also find that the luminosity dependence of the clustering is strongly dependent on physical scale, with greater luminosity dependence being found between r=0.0625 Mpc/h and r=0.25 Mpc/h, compared to the r=0.5 Mpc/h to r=2 Mpc/h range. Moreover, over a range of two orders of magnitude in luminosity, a single power-law fit to the luminosity dependence is not sufficient to explain the increase in clustering at both the bright and faint ends at the smaller scales. We argue that luminosity-dependent clustering at small scales is a necessary component of galaxy-halo occupation models for blue, star-forming galaxies as well as for red, quenched galaxies.

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arXiv2015

Distribution of Faint Atomic Gas in Hickson Compact Groups

We present 21cm HI observations of four Hickson Compact Groups with evidence for a substantial intragroup medium using the Robert C. Byrd Green Bank Telescope (GBT). By mapping H I emission in a region of 25$^{\prime}\times$25$^{\prime}$ (140-650 kpc) surrounding each HCG, these observations provide better estimates of HI masses. In particular, we detected 65% more \HI than that detected in the Karl G. Jansky Very Large Array (VLA) imaging of HCG92. We also identify if the diffuse gas has the same spatial distribution as the high-surface brightness (HSB) HI features detected in the VLA maps of these groups by comparing the HI strengths between the observed and modeled masses based on VLA maps. We found that the HI observed with the GBT to have a similar spatial distribution as the HSB structures in HCGs 31 and 68. Conversely, the observed HI distributions in HCGs44 and 92 were extended and showed significant offsets from the modeled masses. Most of the faint gas in HCG44 lies to the Northeast-Southwest region and in HCG 92 lies in the Northwest region of their respective groups. The spatial and dynamical similarities between the total (faint+HSB) and the HSB HI indicate that the faint gas is of tidal origin. We found that the gas will survive ionization by the cosmic UV background and the escaping ionizing photons from the star forming regions and stay primarily neutral for at least 500 Myrs.

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arXiv2015

Exploring the diffuse interstellar bands with the Sloan Digital Sky Survey

We use star, galaxy and quasar spectra taken by the Sloan Digital Sky Survey to map out the distribution of diffuse interstellar bands (DIBs) induced by the Milky Way. We show that, after carefully removing the intrinsic spectral energy distribution of each source, it is possible to measure statistical flux fluctuations at the 1e-3 level, detect more than 20 DIBs and measure their strength as a function of position on the sky. We create a map of DIB absorption covering about 5000 square degrees and measure correlations with various tracers of the interstellar medium: atomic and molecular hydrogen, dust and polycyclic aromatic hydrocarbons (PAHs). After recovering known correlations, we show that each DIB has a different dependence on atomic and molecular hydrogen: while they are all positively correlated with N(HI), they exhibit a range of behaviours with N(H2) showing positive, negative or no correlation. We show that a simple parametrization involving only N(HI) and N(H2) applied to all the DIBs is sufficient to reproduce a large collection of observational results reported in the literature: it allows us to naturally describe the relations between DIB strength and dust reddening (including the so-called skin effect), the related scatter, DIB pair-wise correlations & families, the affinity for $σ/ζ$-type environments and other correlations related to molecules. Our approach allows us to characterize DIB dependencies in a simple manner and provides us with a metric to characterize the similarity between different DIBs.

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arXiv2015

Dark Matter Halo Models of Stellar Mass-Dependent Galaxy Clustering in PRIMUS+DEEP2 at 0.2<z<1.2

We utilize $Λ$CDM halo occupation models of galaxy clustering to investigate the evolving stellar mass dependent clustering of galaxies in the PRIsm MUlti-object Survey (PRIMUS) and DEEP2 Redshift Survey over the past eight billion years of cosmic time, between $0.2<z<1.2$. These clustering measurements provide new constraints on the connections between dark matter halo properties and galaxy properties in the context of the evolving large-scale structure of the universe. Using both an analytic model and a set of mock galaxy catalogs, we find a strong correlation between central galaxy stellar mass and dark matter halo mass over the range $M_\mathrm{halo}\sim10^{11}$-$10^{13}~h^{-1}M_\odot$, approximately consistent with previous observations and theoretical predictions. However, the stellar-to-halo mass relation (SHMR) and the mass scale where star formation efficiency reaches a maximum appear to evolve more strongly than predicted by other models, including models based primarily on abundance-matching constraints. We find that the fraction of satellite galaxies in haloes of a given mass decreases significantly from $z\sim0.5$ to $z\sim0.9$, partly due to the fact that haloes at fixed mass are rarer at higher redshift and have lower abundances. We also find that the $M_1/M_\mathrm{min}$ ratio, a model parameter that quantifies the critical mass above which haloes host at least one satellite, decreases from $\approx20$ at $z\sim0$ to $\approx13$ at $z\sim0.9$. Considering the evolution of the subhalo mass function vis-à-vis satellite abundances, this trend has implications for relations between satellite galaxies and halo substructures and for intracluster mass, which we argue has grown due to stripped and disrupted satellites between $z\sim0.9$ and $z\sim0.5$.

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arXiv2015

Calcium H & K and sodium D absorption induced by the interstellar and circumgalactic media of the Milky Way

We map out calcium II & sodium I absorption (Fraunhofer H, K & D lines) induced by both the interstellar medium and the circumgalactic medium of the Milky Way. Our measurements cover more than $9000$ deg$^2$ and make use of about $300,000$ extragalactic spectra from the Sloan Digital Sky Survey. We present absorption maps for these two species and then compare their distributions to those of neutral hydrogen and dust. We show that the abundance of Na I with respect to neutral hydrogen stays roughly constant in different environments, while that of Ca II decreases with hydrogen column density. Studying how these tracers vary as a function of velocity, we show that, on average, the N(Na I)/N(Ca II) ratio decreases at higher velocity with respect to the local standard of rest, similar to the local Routly-Spitzer effect but seen on Galactic scale. We show that it is likely caused by higher gas/dust density at lower velocity. Finally, we show that Galactic Ca II and Na I absorption needs to be taken into account for precision photometry and, more importantly, for photometric redshift estimation with star forming galaxies. Our maps of Ca II and Na I absorption are publicly available.

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arXiv2015

Using Galaxy Pairs to Probe Star Formation During Major Halo Mergers

Currently-proposed galaxy quenching mechanisms predict very different behaviours during major halo mergers, ranging from significant quenching enhancement (e.g., clump-induced gravitational heating models) to significant star formation enhancement (e.g., gas starvation models). To test real galaxies' behaviour, we present an observational galaxy pair method for selecting galaxies whose host haloes are preferentially undergoing major mergers. Applying the method to central L* (10^10 Msun < M_* < 10^10.5 Msun) galaxies in the Sloan Digital Sky Survey (SDSS) at z<0.06, we find that major halo mergers can at most modestly reduce the star-forming fraction, from 59% to 47%. Consistent with past research, however, mergers accompany enhanced specific star formation rates for star-forming L* centrals: ~10% when a paired galaxy is within 200 kpc (approximately the host halo's virial radius), climbing to ~70% when a paired galaxy is within 30 kpc. No evidence is seen for even extremely close pairs (<30 kpc separation) rejuvenating star formation in quenched galaxies. For galaxy formation models, our results suggest: (1) quenching in L* galaxies likely begins due to decoupling of the galaxy from existing hot and cold gas reservoirs, rather than a lack of available gas or gravitational heating from infalling clumps, (2) state-of-the-art semi-analytic models currently over-predict the effect of major halo mergers on quenching, and (3) major halo mergers can trigger enhanced star formation in non-quenched central galaxies.

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arXiv2015

PRIMUS: The relationship between Star formation and AGN accretion

We study the evidence for a connection between active galactic nuclei (AGN) fueling and star formation by investigating the relationship between the X-ray luminosities of AGN and the star formation rates (SFRs) of their host galaxies. We identify a sample of 309 AGN with $10^{41}<L_\mathrm{X}<10^{44} $ erg s$^{-1}$ at $0.2 < z < 1.2$ in the PRIMUS redshift survey. We find AGN in galaxies with a wide range of SFR at a given $L_X$. We do not find a significant correlation between SFR and the observed instantaneous $L_X$ for star forming AGN host galaxies. However, there is a weak but significant correlation between the mean $L_\mathrm{X}$ and SFR of detected AGN in star forming galaxies, which likely reflects that $L_\mathrm{X}$ varies on shorter timescales than SFR. We find no correlation between stellar mass and $L_\mathrm{X}$ within the AGN population. Within both populations of star forming and quiescent galaxies, we find a similar power-law distribution in the probability of hosting an AGN as a function of specific accretion rate. Furthermore, at a given stellar mass, we find a star forming galaxy $\sim2-3$ more likely than a quiescent galaxy to host an AGN of a given specific accretion rate. The probability of a galaxy hosting an AGN is constant across the main sequence of star formation. These results indicate that there is an underlying connection between star formation and the presence of AGN, but AGN are often hosted by quiescent galaxies.

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