arXiv subjects

Song Huang

Publications and source records attributed to Song Huang.

At least 91 records · Page 5Linked to original sources

arXiv2017

First Data Release of the Hyper Suprime-Cam Subaru Strategic Program

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layered imaging survey aimed at addressing some of the most outstanding questions in astronomy today, including the nature of dark matter and dark energy. The survey has been awarded 300 nights of observing time at the Subaru Telescope and it started in March 2014. This paper presents the first public data release of HSC-SSP. This release includes data taken in the first 1.7 years of observations (61.5 nights) and each of the Wide, Deep, and UltraDeep layers covers about 108, 26, and 4 square degrees down to depths of i~26.4, ~26.5, and ~27.0 mag, respectively (5sigma for point sources). All the layers are observed in five broad bands (grizy), and the Deep and UltraDeep layers are observed in narrow bands as well. We achieve an impressive image quality of 0.6 arcsec in the i-band in the Wide layer. We show that we achieve 1-2 per cent PSF photometry (rms) both internally and externally (against Pan-STARRS1), and ~10 mas and 40 mas internal and external astrometric accuracy, respectively. Both the calibrated images and catalogs are made available to the community through dedicated user interfaces and database servers. In addition to the pipeline products, we also provide value-added products such as photometric redshifts and a collection of public spectroscopic redshifts. Detailed descriptions of all the data can be found online. The data release website is https://hsc-release.mtk.nao.ac.jp/.

astro-ph.IM↗

arXiv2017

Individual Stellar Halos of Massive Galaxies Measured to 100 kpc at $0.3<z<0.5$ using Hyper Suprime-Cam

Massive galaxies display extended light profiles that can reach several hundreds of kilo parsecs. These stellar halos provide a fossil record of galaxy assembly histories. Using data that is both wide (~100 square degree) and deep (i>28.5 mag/arcsec^2 in i-band), we present a systematic study of the stellar halos of a sample of more than 3000 galaxies at 0.3 < z < 0.5 with $\log M_{\star}/M_{\odot} > 11.4$. Our study is based on high-quality (0.6 arcsec seeing) imaging data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP), which enables us to individually estimate surface mass density profiles to 100 kpc without stacking. As in previous work, we find that more massive galaxies exhibit more extended outer profiles. When this extended light is not properly accounted for as a result of shallow imaging or inadequate profile modeling, the derived stellar mass function can be significantly underestimated at the highest masses. Across our sample, the ellipticity of outer light profiles increases substantially as we probe larger radii. We show for the first time that these ellipticity gradients steepen dramatically as a function of galaxy mass, but we detect no mass-dependence in outer color gradients. Our results support the two-phase formation scenario for massive galaxies in which outer envelopes are built up at late times from a series of merging events. We provide surface mass surface mass density profiles in a convenient tabulated format to facilitate comparisons with predictions from numerical simulations of galaxy formation.

astro-ph.GA↗

arXiv2017

The Hyper Suprime-Cam Software Pipeline

In this paper, we describe the optical imaging data processing pipeline developed for the Subaru Telescope's Hyper Suprime-Cam (HSC) instrument. The HSC Pipeline builds on the prototype pipeline being developed by the Large Synoptic Survey Telescope's Data Management system, adding customizations for HSC, large-scale processing capabilities, and novel algorithms that have since been reincorporated into the LSST codebase. While designed primarily to reduce HSC Subaru Strategic Program (SSP) data, it is also the recommended pipeline for reducing general-observer HSC data. The HSC pipeline includes high level processing steps that generate coadded images and science-ready catalogs as well as low-level detrending and image characterizations.

astro-ph.IM↗

arXiv2017

Characterization and Photometric Performance of the Hyper Suprime-Cam Software Pipeline

The Subaru Strategic Program (SSP) is an ambitious multi-band survey using the Hyper Suprime-Cam (HSC) on the Subaru telescope. The Wide layer of the SSP is both wide and deep, reaching a detection limit of i~26.0 mag. At these depths, it is challenging to achieve accurate, unbiased, and consistent photometry across all five bands. The HSC data are reduced using a pipeline that builds on the prototype pipeline for the Large Synoptic Survey Telescope. We have developed a Python-based, flexible framework to inject synthetic galaxies into real HSC images called SynPipe. Here we explain the design and implementation of SynPipe and generate a sample of synthetic galaxies to examine the photometric performance of the HSC pipeline. For stars, we achieve 1% photometric precision at i~19.0 mag and 6% precision at i~25.0 in the i-band. For synthetic galaxies with single-Sersic profiles, forced CModel photometry achieves 13% photometric precision at i~20.0 mag and 18% precision at i~25.0 in the i-band. We show that both forced PSF and CModel photometry yield unbiased color estimates that are robust to seeing conditions. We identify several caveats that apply to the version of HSC pipeline used for the first public HSC data release (DR1) that need to be taking into consideration. First, the degree to which an object is blended with other objects impacts the overall photometric performance. This is especially true for point sources. Highly blended objects tend to have larger photometric uncertainties, systematically underestimated fluxes and slightly biased colors. Second, >20% of stars at 22.5< i < 25.0 mag can be misclassified as extended objects. Third, the current CModel algorithm tends to strongly underestimate the half-light radius and ellipticity of galaxy with i>21.5 mag.

astro-ph.IM↗

arXiv2017

Sumo Puff: Tidal Debris or Disturbed Ultra-Diffuse Galaxy?

We report the discovery of a diffuse stellar cloud with an angular extent $\gtrsim30^{\prime\prime}$, which we term "Sumo Puff", in data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). While we do not have a redshift for this object, it is in close angular proximity to a post-merger galaxy at redshift $z=0.0431$ and is projected within a few virial radii (assuming similar redshifts) of two other ${\sim}L_\star$ galaxies, which we use to bracket a potential redshift range of $0.0055 < z < 0.0431$. The object's light distribution is flat, as characterized by a low Sersic index ($n\sim0.3$). It has a low central $g$-band surface brightness of ${\sim}26.4$ mag arcsec$^{-2}$, large effective radius of ${\sim}13^{\prime\prime}$ (${\sim}11$ kpc at $z=0.0431$ and ${\sim}1.5$ kpc at $z=0.0055$), and an elongated morphology ($b/a\sim0.4$). Its red color ($g-i\sim1$) is consistent with a passively evolving stellar population and similar to the nearby post-merger galaxy, and we may see tidal material connecting Sumo Puff with this galaxy. We offer two possible interpretations for the nature of this object: (1) it is an extreme, galaxy-size tidal feature associated with a recent merger event, or (2) it is a foreground dwarf galaxy with properties consistent with a quenched, disturbed ultra-diffuse galaxy. We present a qualitative comparison with simulations that demonstrates the feasibility of forming a structure similar to this object in a merger event. Follow-up spectroscopy and/or deeper imaging to confirm the presence of the bridge of tidal material will be necessary to reveal the true nature of this object.

astro-ph.GA↗

arXiv2017

SDSS IV MaNGA: Discovery of an $H_α$ blob associated with a dry galaxy pair -- ejected gas or a `dark' galaxy candidate?

We report the discovery of a mysterious giant $H_α$ blob that is $\sim 8$ kpc away from the main MaNGA target 1-24145, one component of a dry galaxy merger, identified in the first-year SDSS-IV MaNGA data. The size of the $H_α$ blob is $\sim$ 3-4 kpc in radius, and the $H_α$ distribution is centrally concentrated. However, there is no optical continuum counterpart in deep broadband images reaching $\sim$26.9 mag arcsec$^{-2}$ in surface brightness. We estimate that the masses of ionized and cold gases are $3.3 \times 10^{5}$ $\rm M_{\odot}$ and $< 1.3 \times 10^{9}$ $\rm M_{\odot}$, respectively. The emission-line ratios indicate that the $H_α$ blob is photoionized by a combination of massive young stars and AGN. Furthermore, the ionization line ratio decreases from MaNGA 1-24145 to the $H_α$ blob, suggesting that the primary ionizing source may come from MaNGA 1-24145, likely a low-activity AGN. Possible explanations of this $H_α$ blob include AGN outflow, the gas remnant being tidally or ram-pressure stripped from MaNGA 1-24145, or an extremely low surface brightness (LSB) galaxy. However, the stripping scenario is less favoured according to galaxy merger simulations and the morphology of the $H_α$ blob. With the current data, we can not distinguish whether this $H_α$ blob is ejected gas due to a past AGN outburst, or a special category of `ultra-diffuse galaxy' (UDG) interacting with MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA 1-24145.

astro-ph.GA↗

arXiv2016

The growth of the central region by acquisition of counter-rotating gas in star-forming galaxies

Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Based on new measurements with integral field spectroscopy of a large representative galaxy sample, we find an appreciable fraction of counter-rotators among blue galaxies (9 out of 489 galaxies). The central regions of blue counter-rotators show younger stellar populations and more intense, ongoing star formation than their outer parts, indicating ongoing growth of the central regions. The result offers observational evidence that the acquisition of external gas in blue galaxies is possible; the interaction with pre-existing gas funnels the gas into nuclear regions (< 1 kpc) to form new stars.

astro-ph.GA↗

arXiv2016

SDSS-IV MaNGA: Properties of galaxies with kinematically decoupled stellar and gaseous components

We study the properties of 66 galaxies with kinematically misaligned gas and stars from MaNGA survey. The fraction of kinematically misaligned galaxies varies with galaxy physical parameters, i.e. M*, SFR and sSFR. According to their sSFR, we further classify these 66 galaxies into three categories, 10 star-forming, 26 "Green Valley" and 30 quiescent ones. The properties of different types of kinematically misaligned galaxies are different in that the star-forming ones have positive gradient in D4000 and higher gas-phase metallicity, while the green valley/quiescent ones have negative D4000 gradients and lower gas-phase metallicity on average. There is evidence that all types of the kinematically misaligned galaxies tend to live in more isolated environment. Based on all these observational results, we propose a scenario for the formation of star forming galaxies with kinematically misaligned gas and stars - the progenitor accretes misaligned gas from a gas-rich dwarf or cosmic web, the cancellation of angular momentum from gas-gas collisions between the pre-existing gas and the accreted gas largely accelerates gas inflow, leading to fast centrally-concentrated star-formation. The higher metallicity is due to enrichment from this star formation. For the kinematically misaligned green valley and quiescent galaxies, they might be formed through gas-poor progenitors accreting kinematically misaligned gas from satellites which are smaller in mass.

astro-ph.GA↗

arXiv2016

SDSS-IV MaNGA: A serendipitous observation of a potential gas accretion event

The nature of warm, ionized gas outside of galaxies may illuminate several key galaxy evolutionary processes. A serendipitous observation by the MaNGA survey has revealed a large, asymmetric H$α$ complex with no optical counterpart that extends $\approx8"$ ($\approx6.3$ kpc) beyond the effective radius of a dusty, starbursting galaxy. This H$α$ extension is approximately three times the effective radius of the host galaxy and displays a tail-like morphology. We analyze its gas-phase metallicities, gaseous kinematics, and emission-line ratios, and discuss whether this H$α$ extension could be diffuse ionized gas, a gas accretion event, or something else. We find that this warm, ionized gas structure is most consistent with gas accretion through recycled wind material, which could be an important process that regulates the low-mass end of the galaxy stellar mass function.

astro-ph.GA↗

arXiv2016

The Carnegie-Irvine Galaxy Survey. IV. A Method to Determine the Average Mass Ratio of Mergers That Built Massive Elliptical Galaxies

Many recent observations and numerical simulations suggest that nearby massive, early-type galaxies were formed through a "two-phase" process. In the proposed second phase, the extended stellar envelope was accumulated through many dry mergers. However, details of the past merger history of present-day ellipticals, such as the typical merger mass ratio, are difficult to constrain observationally. Within the context and assumptions of the two-phase formation scenario, we propose a straightforward method, using photometric data alone, to estimate the average mass ratio of mergers that contributed to the build-up of massive elliptical galaxies. We study a sample of nearby massive elliptical galaxies selected from the Carnegie-Irvine Galaxy Survey, using two-dimensional analysis to decompose their light distribution into an inner, denser component plus an extended, outer envelope, each having a different optical color. The combination of these two substructures accurately recovers the negative color gradient exhibited by the galaxy as whole. The color difference between the two components (<Δ(B-V)> ~ 0.10 mag; <Δ(B-R)> ~ 0.14 mag), based on the slope of the M_stellar-color relation for nearby early-type galaxies, can be translated into an estimate of the average mass ratio of the mergers. The rough estimate, 1:5 to 1:10, is consistent with the expectation of the two-phase formation scenario, suggesting that minor mergers were largely responsible for building up to the outer stellar envelope of present-day massive ellipticals. With the help of accurate photometry, large sample size, and more choices of colors promised by ongoing and future surveys, the approach proposed here can reveal more insights into the growth of massive galaxies during the last few Gyr.

astro-ph.GA↗

arXiv2015

The Stripe 82 Massive Galaxy Project II: Stellar Mass Completeness of Spectroscopic Galaxy Samples from the Baryon Oscillation Spectroscopic Survey

The Baryon Oscillation Spectroscopic Survey (BOSS) has collected spectra for over one million galaxies at $0.15 11.6$ only in the narrow redshift range $z=[0.51,0.61]$. The low redshift LOWZ sample is 80% complete at $\log_{10}(M_*/M_{\odot}) >11.6$ for $z=[0.15,0.43]$. To construct mass complete samples at lower masses, spectroscopic samples need to be significantly supplemented by photometric redshifts. This work will enable future studies to better utilize the BOSS samples for galaxy-formation science.

astro-ph.GA↗

arXiv2014

How Robust Are the Size Measurements of High-redshift Compact Galaxies?

Massive quiescent galaxies at $z \approx 2$ are apparently much more compact than galaxies of comparable mass today. How robust are these size measurements? We perform comprehensive simulations to determine possible biases and uncertainties in fitting single-component light distributions to real galaxies. In particular, we examine the robustness of the measurements of the luminosity, size, and other structural parameters. We devise simulations with increasing realism to systematically disentangle effects due to the technique (specifically using GALFIT) and the intrinsic structures of the galaxies. By accurately capturing the detailed substructures of nearby elliptical galaxies and then rescaling their sizes and signal-to-noise to mimic galaxies at different redshifts, we confirm that the massive quiescent galaxies at $z \approx 2$ are significantly more compact intrinsically than their local counterparts. Their observed compactness is not a result of missing faint outer light due to systematic errors in modeling. In fact, we find that fitting multi-component galaxies with a single Sérsic profile, the procedure most commonly adopted in the literature, biases the inferred sizes higher by up to 10% - 20%, which accentuates the amount of size evolution required. If the sky estimation has been done robustly and the model for the point-spread function is fairly accurate, GALFIT can retrieve the properties of single-component galaxies over a wide range of signal-to-noise ratios without introducing any systematic errors.

astro-ph.GA↗

arXiv2014

Color-Magnitude Distribution of Face-on Nearby Galaxies in SDSS DR7

We have analyzed the distributions in the color-magnitude diagram (CMD) of a large sample of face-on galaxies to minimize the effect of dust extinctions on galaxy color. About 300 thousand galaxies with $log(a/b) < $ 0.2 and redshift $z < 0.2$ are selected from the SDSS DR7 catalog. Two methods are employed to investigate the distributions of galaxies in the CMD including 1-D Gaussian fitting to the distributions in individual magnitude bins and 2-D Gaussian mixture model (GMM) fitting to galaxies as a whole. We find that in the 1-D fitting only two Gaussians are not enough to fit galaxies with the excess present between the blue cloud and the red sequence. The fitting to this excess defines the centre of the green-valley in the local universe to be $(u-r)_{0.1} = -0.121M_{r,0.1}-0.061$. The fraction of blue cloud and red sequence galaxies turns over around $M_{r,0.1} \sim -20.1$ mag, corresponding to stellar mass of $3\times10^{10}M_\odot$. For the 2-D GMM fitting, a total of four Gaussians are required, one for the blue cloud, one for the red sequence and the additional two for the green valley. The fact that two Gaussians are needed to describe the distributions of galaxies in the green valley is consistent with some models that argue for two different evolutionary paths from the blue cloud to the red sequence.

astro-ph.GA↗

arXiv2014

Characterizing Workload of Web Applications on Virtualized Servers

With the ever increasing demands of cloud computing services, planning and management of cloud resources has become a more and more important issue which directed affects the resource utilization and SLA and customer satisfaction. But before any management strategy is made, a good understanding of applications' workload in virtualized environment is the basic fact and principle to the resource management methods. Unfortunately, little work has been focused on this area. Lack of raw data could be one reason; another reason is that people still use the traditional models or methods shared under non-virtualized environment. The study of applications' workload in virtualized environment should take on some of its peculiar features comparing to the non-virtualized environment. In this paper, we are open to analyze the workload demands that reflect applications' behavior and the impact of virtualization. The results are obtained from an experimental cloud testbed running web applications, specifically the RUBiS benchmark application. We profile the workload dynamics on both virtualized and non-virtualized environments and compare the findings. The experimental results are valuable for us to estimate the performance of applications on computer architectures, to predict SLA compliance or violation based on the projected application workload and to guide the decision making to support applications with the right hardware.

cs.PF↗

arXiv2013

A Novel Approach to Constrain the Mass Ratio of Minor Mergers in Elliptical Galaxies: Application to NGC 4889, the Brightest Cluster Galaxy in Coma

Minor mergers are thought to be important for the build-up and structural evolution of massive elliptical galaxies. In this work, we report the discovery of a system of four shell features in NGC 4889, one of the brightest members of the Coma cluster, using optical images taken with the Hubble Space Telescope and the Sloan Digital Sky Survey. The shells are well aligned with the major axis of the host and are likely to have been formed by the accretion of a small satellite galaxy. We have performed a detailed two-dimensional photometric decomposition of NGC 4889 and of the many overlapping nearby galaxies in its vicinity. This comprehensive model allows us not only to firmly detect the low-surface brightness shells, but, crucially, also to accurately measure their luminosities and colors. The shells are bluer than the underlying stars at the same radius in the main galaxy. We make use of the colors of the shells and the color-magnitude relation of the Coma cluster to infer the luminosity (or mass) of the progenitor galaxy. The shells in NGC 4889 appear to have been produced by the minor merger of a moderate-luminosity (M_I ~ -18.7 mag) disk (S0 or spiral) galaxy with a luminosity (mass) ratio of ~ 90:1 with respect to the primary galaxy. The novel methodology presented in this work can be exploited to decode the fossil record imprinted in the photometric substructure of other nearby early-type galaxies.

astro-ph.CO↗

arXiv2013

Fossil Evidence for the Two-phase Formation of Elliptical Galaxies

Massive early-type galaxies have undergone dramatic structural evolution over the last 10 Gyr. A companion paper shows that nearby elliptical galaxies with M*>1.3x10^{11} M_sun generically contain three photometric subcomponents: a compact inner component with effective radius Re<1 kpc, an intermediate-scale middle component with Re~2.5 kpc, and an extended outer envelope with $R_e \approx 10$ kpc. Here we attempt to relate these substructures with the properties of early-type galaxies observed at higher redshifts. We find that a hypothetical structure formed from combining the inner plus the middle components of local ellipticals follows a strikingly tight stellar mass-size relation, one that resembles the distribution of early-type galaxies at z~1.5. Outside of the central kpc, the median stellar mass surface density profiles of this composite structure agree closest with those of massive galaxies that have similar cumulative number density at 1.5<z<2.0 within the uncertainty. We propose that the central substructures in nearby ellipticals are the evolutionary descendants of the "red nuggets" formed under highly dissipative ("wet") conditions at high redshifts, as envisioned in the initial stages of the two-phase formation scenario recently advocated for massive galaxies. Subsequent accretion, plausibly through dissipationless ("dry") minor mergers, builds the outer regions of the galaxy identified as the outer envelope in our decomposition. The large scatter exhibited by this component on the stellar mass-size plane testifies to the stochastic nature of the accretion events.

astro-ph.CO↗

arXiv2012

The Carnegie-Irvine Galaxy Survey. III. The Three-Component Structure of Nearby Elliptical Galaxies

Motivated by recent developments in our understanding of the formation and evolution of massive galaxies, we explore the detailed photometric structure of a representative sample of 94 bright, nearby elliptical galaxies, using high-quality optical images from the Carnegie-Irvine Galaxy Survey. The sample spans a range of environments and stellar masses, from M* = 10^{10.2} to 10^{12.0} solar mass. We exploit the unique capabilities of two-dimensional image decomposition to explore the possibility that local elliptical galaxies may contain photometrically distinct substructure that can shed light on their evolutionary history. Compared with the traditional one-dimensional approach, these two-dimensional models are capable of consistently recovering the surface brightness distribution and the systematic radial variation of geometric information at the same time. Contrary to conventional perception, we find that the global light distribution of the majority (>75%) of elliptical galaxies is not well described by a single Sersic function. Instead, we propose that local elliptical galaxies generically contain three subcomponents: a compact (R_e < 1 kpc) inner component with luminosity fraction f ~ 0.1-0.15; an intermediate-scale (R_e ~ 2.5 kpc) middle component with f ~ 0.2-0.25; and a dominant (f = 0.6), extended (R_e ~ 10 kpc) outer envelope. All subcomponents have average Sersic indices n ~ 1-2, significantly lower than the values typically obtained from single-component fits. The individual subcomponents follow well-defined photometric scaling relations and the stellar mass-size relation. We discuss the physical nature of the substructures and their implications for the formation of massive elliptical galaxies.

astro-ph.CO↗

arXiv2010

Surface Photometry and Radial Color Gradients of Nearby Luminous Early-type Galaxies in SDSS Stripe 82

We make use of the images from the Sloan Digital Sky Survey Stripe 82 to present an analysis of r band surface brightness profiles and radial color gradients (g - r, u - r) in 111 nearby early-type galaxies (ETGs). With Stripe 82 images, we are able to pay special attentions to the low-surface-brightness areas (LSB areas) of the galaxies. The LSB areas make a difference to the Sersic fittings and concentration indices, making both the indices less than the typical values for ETGs. There are about 60% negative color gradients (red-core) within 1.5Re , much more than the approximately 10% positive ones (blue-core) within the same radius. However, taking into account of the LSB areas, we find that the color gradients are not necessarily monotonic: about one third of the red-core (or blue-core) galaxies have positive (or negative) color gradients in the outer areas. So LSB areas not only make ETGs' Sersic profiles deviate from de Vaucouleur ones and shift to the disk end, but also reveal that quite a number of ETGs have opposite color gradients in inner and outer areas. These outcomes remind us the necessity of double-Sersic fitting. These LSB phenomena may be interpreted by mergers and thus different metallicity in the outer areas. Isophotal parameters are also discussed briefly in this paper: more disky nearby ETGs are spotted than boxy ones.

astro-ph.CO↗