Song Huang

E315, (New) Physics Building,

Tsinghua University

Beijing 100084, China

shuang [at] tsinghua.edu.cn

This website is like my career. It is still (and probably will always be) under construction!!

My name is Song Huang (黄崧). I am an associate professor (but I am still not tenured…fingers crossed) at the Department of Astronomy at Tsinghua University, China. You can find my CV here or check out my Github profile.

My primary research interest is the observational study of galaxy evolution and galaxy-halo connection. Most of my works take advantage of today’s multi-band imaging data, spectroscopic surveys, and hydrodynamic simulations to probe the structure assembly and star formation history of different classes of galaxies. I have a group of excellent students at Tsinghua now, and we welcome collaborations!

My recent works focus on the most massive (elliptical) and most abundant (dwarf galaxies) galaxies in our universe. Using weak gravitational lensing and statistical modeling techniques, I aspire to deepen our understanding of galaxy-halo connection models and explore their applications in observational cosmology.

I am a member of the Subaru Strategic Program (SSP) using the Hyper-Suprime Camera (HSC) on the 8.2-meter Subaru Telescope and the Merian Survey - an intermediate-band survey of the HSC footprint using the Dark Energy Camera (DECam). Recently, I have also joined the Dark Energy Spectroscopic Instrument (DESI) spectroscopic survey project.

At Tsinghua, we are building an ambitious 6.5-meter telescope called MUltiplexed Survey Telescope (MUST) for the next-generation spectroscopic survey. I am currently the co-project scientist helping organize MUST’s scientific collaboration.

I am always happy to discuss science and collaborate with people. Feel free to contact me if you are interested in my research or have any questions.

Highlight Publications

2022

MNRAS

Reaching for the Edge I: probing the outskirts of massive galaxies with HSC, DECaLS, SDSS, and Dragonfly

Jiaxuan Li, Song Huang, Alexie Leauthaud, and 9 more authors

MNRAS, Oct 2022

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The outer light (stellar haloes) of massive galaxies has recently emerged as a possible low scatter tracer of dark matter halo mass. To test the robustness of outer light measurements across different data sets, we compare the 1D azimuthally averaged surface brightness profiles of massive galaxies using four independent data sets: the Hyper Suprime-Cam survey (HSC), the Dark Energy Camera Legacy Survey (DECaLS), the Sloan Digital Sky Survey (SDSS), and the Dragonfly Wide Field Survey (Dragonfly). We test the sky subtraction and proposed corrections for HSC and DECaLS. For galaxies at z < 0.05, Dragonfly has the best control of systematics, reaching surface brightness levels of μr ≈ 30 mag arcsec-2. At 0.19 < z < 0.50, HSC can reliably recover individual surface brightness profiles to μr ≈ 28.5 mag arcsec-2 (R = 100-150 kpc in semimajor axis). In a statistical sense, DECaLS agrees with HSC to R > 200 kpc. DECaLS and HSC measurements of the stellar mass contained within 100 kpc agree within 0.05 dex. Finally, we use weak lensing to show that measurements of outer light with DECaLS at 0.19 < z < 0.50 show a similar promise as HSC as a low scatter proxy of halo mass. The tests and results from this paper represent an important step forward for accurate measurements of the outer light of massive galaxies and demonstrate that outer light measurements from DECam imaging will be a promising method for finding galaxy clusters.
MNRAS

The outer stellar mass of massive galaxies: a simple tracer of halo mass with scatter comparable to richness and reduced projection effects

Song Huang, Alexie Leauthaud, Christopher Bradshaw, and 7 more authors

MNRAS, Oct 2022

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Using the weak gravitational lensing data from the Hyper Suprime-Cam Subaru Strategic Program (HSC survey), we study the potential of different stellar mass estimates in tracing halo mass. We consider galaxies with log10(M⋆/M⊙) > 11.5 at 0.2 < z < 0.5 with carefully measured light profiles, and clusters from the redMaPPer and CAMIRA richness-based algorithms. We devise a method (the ’Top-N test’) to evaluate the scatter in the halo mass-observable relation for different tracers, and to inter-compare halo mass proxies in four number density bins using stacked galaxy-galaxy lensing profiles. This test reveals three key findings. Stellar masses based on CModel photometry and aperture luminosity within R <30 kpc are poor proxies of halo mass. In contrast, the stellar mass of the outer envelope is an excellent halo mass proxy. The stellar mass within R = [50, 100] kpc, M⋆, [50, 100], has performance comparable to the state-of-the-art richness-based cluster finders at log10Mvir ≳ 14.0 and could be a better halo mass tracer at lower halo masses. Finally, using N-body simulations, we find that the lensing profiles of massive haloes selected by M⋆, [50, 100] are consistent with the expectation for a sample without projection or mis-centring effects. Richness-selected clusters, on the other hand, display an excess at R 1 Mpc in their lensing profiles, which may suggest a more significant impact from selection biases. These results suggest that M⋆-based tracers have distinct advantages in identifying massive haloes, which could open up new avenues for cluster cosmology.

2020

MNRAS

Weak lensing reveals a tight connection between dark matter halo mass and the distribution of stellar mass in massive galaxies

Song Huang, Alexie Leauthaud, Andrew Hearin, and 9 more authors

MNRAS, Mar 2020

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Using deep images from the Hyper Suprime-Cam (HSC) survey and taking advantage of its unprecedented weak lensing capabilities, we reveal a remarkably tight connection between the stellar mass distribution of massive central galaxies and their host dark matter halo mass. Massive galaxies with more extended stellar mass distributions tend to live in more massive dark matter haloes. We explain this connection with a phenomenological model that assumes, (1) a tight relation between the halo mass and the total stellar content in the halo, (2) that the fraction of in situ and ex situ mass at r <10 kpc depends on halo mass. This model provides an excellent description of the stellar mass functions (SMFs) of total stellar mass (M_⋆^max) and stellar mass within inner 10 kpc (M_⋆^10) and also reproduces the HSC weak lensing signals of massive galaxies with different stellar mass distributions. The best-fitting model shows that halo mass varies significantly at fixed total stellar mass (as much as 0.4 dex) with a clear dependence on M_⋆^10. Our two-parameter M_⋆^max-M_⋆^10 description provides a more accurate picture of the galaxy-halo connection at the high-mass end than the simple stellar-halo mass relation (SHMR) and opens a new window to connect the assembly history of haloes with those of central galaxies. The model also predicts that the ex situ component dominates the mass profiles of galaxies at r < 10 kpc for log M⋆ ≥ 11.7.

2018

MNRAS

A detection of the environmental dependence of the sizes and stellar haloes of massive central galaxies

Song Huang, Alexie Leauthaud, Jenny Greene, and 6 more authors

MNRAS, Oct 2018

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We use ∼100 deg2 of deep (>28.5 mag arcsec-2 in i band), high-quality (median 0.6 arcsec seeing) imaging data from the Hyper Suprime-Cam (HSC) survey to reveal the halo mass dependence of the surface mass density profiles and outer stellar envelopes of massive galaxies. The i-band images from the HSC survey reach ∼4 mag deeper than Sloan Digital Sky Survey and enable us to directly trace stellar mass distributions to 100 kpc without requiring stacking. We conclusively show that, at fixed stellar mass, the stellar profiles of massive galaxies depend on the masses of their dark matter haloes. On average, massive central galaxies with log10(M⋆, 100 kpc/M⊙)>11.6 in more massive haloes at 0.3 < z < 0.5 have shallower inner stellar mass density profiles (within ∼10-20 kpc) and more prominent outer envelopes. These differences translate into a halo mass dependence of the mass-size relation. Central galaxies in haloes with log10(M200b/M⊙)>14.0 are ∼20 per cent larger in R50 at fixed M⋆, 100 kpc. Such dependence is also reflected in the relationship between the stellar mass within 10 and 100 kpc. Comparing to the mass-size relation, the M⋆, 100 kpc-M⋆, 10 kpc relation avoids the ambiguity in the definition of size, and can be straightforwardly compared with simulations. Our results demonstrate that, with deep images from HSC, we can quantify the connection between halo mass and the outer stellar halo, which may provide new constraints on the formation and assembly of massive central galaxies.
MNRAS

Individual stellar haloes of massive galaxies measured to 100 kpc at 0.3 < z < 0.5 using Hyper Suprime-Cam

Song Huang, Alexie Leauthaud, Jenny E. Greene, and 5 more authors

MNRAS, Apr 2018

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Massive galaxies display extended light profiles that can reach several hundreds of kiloparsecs. We use data from the Hyper Suprime-Cam (HSC) survey that is simultaneously wide (∼100 deg2) and deep (>28.5 mag arcsec-2 in i band) to study the stellar haloes of a sample of ∼7000 massive galaxies at z ∼ 0.4. The depth of the HSC data enables us to measure surface mass density profiles to 100 kpc for individual galaxies without stacking. As in previous work, we find that more massive galaxies exhibit more extended outer profiles than smaller galaxies. When this extended light is not properly accounted for (because of shallow imaging and/or inadequate profile modelling), the derived stellar mass function can be significantly underestimated at the high-mass end. Across our sample, the ellipticity of outer light profile increases substantially with radius. 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 colour gradients. Our results support the two-phase formation scenario for massive galaxies in which outer envelopes are built up at a later time from a series of merging events. We provide surface mass density profiles in a convenient tabulated format to facilitate comparisons with predictions from numerical simulations of galaxy formation.
Characterization and photometric performance of the Hyper Suprime-Cam Software Pipeline

Song Huang, Alexie Leauthaud, Ryoma Murata, and 9 more authors

PASJ, Jan 2018

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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 (corresponding to statistical scatters of ∼0.01 and ∼0.06 mag respectively). For synthetic galaxies with single-Sérsic profiles, forced CModel photometry achieves 13% photometric precision at i ∼ 20.0 mag and 18% precision at i ∼ 25.0 in the i band (corresponding to statistical scatters of ∼0.15 and ∼0.22 mag respectively). We show that both forced point spread function 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. Secondly, >20% of stars at 22.5 < i < 25.0 mag can be misclassified as extended objects. Thirdly, the current CModel algorithm tends to strongly underestimate the half-light radius and ellipticity of galaxy with i > 21.5 mag.

2013

ApJL

Fossil Evidence for the Two-phase Formation of Elliptical Galaxies

Song Huang, Luis C. Ho, Chien Y. Peng, and 2 more authors

ApJl, May 2013

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Massive early-type galaxies (ETGs) have undergone dramatic structural evolution over the last 10 Gyr. A companion paper shows that nearby elliptical galaxies with M * >= 1.3 × 1011 M ⊙ 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 Re ≈ 10 kpc. Here we attempt to relate these substructures with the properties of ETGs observed at higher redshifts. We find that a hypothetical structure formed from combining the inner and middle components of local ellipticals follows a strikingly tight stellar mass-size relation, one that resembles the distribution of ETGs at z ≈ 1. 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.
ApJ

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

Song Huang, Luis C. Ho, Chien Y. Peng, and 2 more authors

ApJ, Mar 2013

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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 * = 1010.2 to 1012.0 M ⊙. 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 Sérsic function. Instead, we propose that local elliptical galaxies generically contain three subcomponents: a compact (Re < 1 kpc) inner component with luminosity fraction f ≈ 0.1-0.15; an intermediate-scale (Re ≈ 2.5 kpc) middle component with f ≈ 0.2-0.25; and a dominant (f = 0.6), extended (Re ≈ 10 kpc) outer envelope. All subcomponents have average Sérsic 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.