arXiv subjects

Sukyoung K. Yi

Publications and source records attributed to Sukyoung K. Yi.

At least 73 records · Page 4Linked to original sources

arXiv2018

YZiCS: Preprocessing of dark halos in the hydrodynamic zoom-in simulation of clusters

To understand the galaxy population in clusters today, we should also consider the impact of previous environments prior to cluster infall, namely preprocessing. We use YZiCS, a hydrodynamic high-resolution zoom-in simulation of 15 clusters, and focus on the tidal stripping suffered by the dark matter halos of cluster members due to preprocessing. We find ~48% of today's cluster members were once satellites of other hosts. This is slightly higher than previous estimates, in part, because we consider not just group-mass hosts, but hosts of all masses also. Thus, we find the preprocessed fraction is poorly correlated with cluster mass and is instead related to each cluster's recent mass growth rate. Hosts less massive than groups are significant contributors, providing more than one-third of the total preprocessed fraction. We find that halo mass loss is a clear function of the time spent in hosts. However, two factors can increase the mass loss rate considerably; the mass ratio of a satellite to its host, and the cosmological epoch when the satellite was hosted. The latter means we may have previously underestimated the role of high redshift groups. From a sample of heavily tidally stripped members in clusters today, nearly three quarters were previously in a host. Thus, visibly disturbed cluster members are more likely to have experienced preprocessing. Being hosted before cluster infall enables cluster members to experience tidal stripping for extended durations compared to direct cluster infall and at earlier epochs when hosts were more destructive.

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arXiv2018

On the origin of gas-poor galaxies in galaxy clusters using cosmological hydrodynamic simulations

The environmental effect is commonly used to explain the excess of gas-poor galaxies in galaxy clusters. Meanwhile, the presence of gas-poor galaxies at cluster outskirts, where galaxies have not spent enough time to feel the cluster environmental effect, hints for the presence of pre-processing. Using cosmological hydrodynamic simulations on 16 clusters, we investigate the mechanisms of gas depletion of galaxies found inside clusters. The gas depletion mechanisms can be categorized into three channels based on where and when they took place. First, 34$\%$ of our galaxies are gas poor before entering clusters (`pre-processing'). They are mainly satellites that have undergone the environmental effect inside group halos. Second, 43$\%$ of the sample became quickly gas deficient in clusters before the first pericentric pass (`fast cluster processing'). Some of them were group satellites that are low in gas at the time of cluster entry compared to the galaxies directly coming from the field. Even the galaxies with large gas fractions take this channel if they fall into massive clusters ($> 10^{14.5}\, \rm M_{\odot}$) or approach cluster centers through radial orbits. Third, 24$\%$ of our sample retain gas even after their first pericentric pass (`slow cluster processing') as they fall into the less massive clusters and/or have circular orbits. The relative importance of each channel varies with a cluster's mass, while the exact degree of significance is subject to large uncertainties. Group pre-processing accounts for a third of the total gas depletion; but it also determines the gas fraction of galaxies at their cluster entry which in turn determines whether a galaxy should take the fast or the slow cluster processing.

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arXiv2018

Cosmic CARNage II: the evolution of the galaxy stellar mass function in observations and galaxy formation models

We present a comparison of the observed evolving galaxy stellar mass functions with the predictions of eight semi-analytic models and one halo occupation distribution model. While most models are able to fit the data at low redshift, some of them struggle to simultaneously fit observations at high redshift. We separate the galaxies into 'passive' and 'star-forming' classes and find that several of the models produce too many low-mass star-forming galaxies at high redshift compared to observations, in some cases by nearly a factor of 10 in the redshift range $2.5 < z < 3.0$. We also find important differences in the implied mass of the dark matter haloes the galaxies inhabit, by comparing with halo masses inferred from observations. Galaxies at high redshift in the models are in lower mass haloes than suggested by observations, and the star formation efficiency in low-mass haloes is higher than observed. We conclude that many of the models require a physical prescription that acts to dissociate the growth of low-mass galaxies from the growth of their dark matter haloes at high redshift.

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arXiv2018

Wobbling galaxy spin axes in dense environments

The orientation of galaxy spin vectors within the large scale structure has been considered an important test of our understanding of structure formation. We investigate the angular changes of galaxy spin vectors in clusters - denser environments than are normally focused upon, using hydrodynamic zoomed simulations of 17 clusters YZiCS and a set of complementary controlled simulations. The magnitude by which galaxies change their spin vector is found to be a function of their rotational support with larger cumulative angular changes of spin vectors when they have initially lower $V_θ/σ$. We find that both mergers and tidal perturbations can significantly swing spin vectors, with larger changes in spin vector for smaller pericentre distances. Strong tidal perturbations are also correlated with the changes in stellar mass and specific angular momentum of satellite galaxies. However, changes in spin vector can often result in a canceling out of previous changes. As a result, the integrated angular change is always much larger than the angular change measured at any instant. Also, overall the majority of satellite galaxies do not undergo mergers or sufficiently strong tidal perturbation after infall into clusters, and thus they end up suffering little change to their spin vectors. Taken as a whole, these results suggest that any signatures of spin alignment from the large scale structure will be preserved in the cluster environment for many gigayears.

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arXiv2018

KYDISC: Galaxy Morphology, Quenching, and Mergers in the Cluster Environment

We present the KASI-Yonsei Deep Imaging Survey of Clusters (KYDISC) targeting 14 clusters at 0.015 < z < 0.144 using the Inamori Magellan Areal Camera and Spectrograph on the 6.5-meter Magellan Baade telescope and the MegaCam on the 3.6-meter Canada-France-Hawaii Telescope. We provide a catalog of cluster galaxies that lists magnitudes, redshifts, morphologies, bulge-to-total ratios, and local density. Based on the 1409 spectroscopically-confirmed cluster galaxies brighter than -19.8 in the r-band, we study galaxy morphology, color, and visual features generated by galaxy mergers. We see a clear trend between morphological content and cluster velocity dispersion, which was not presented by previous studies using local clusters. Passive spirals are preferentially found in a highly dense region (i.e., cluster center), indicating that they have gone through the environmental quenching. In deep images (mu_r ~ 27 mag/"^2), 20% of our sample show signatures of recent mergers, which is not expected from theoretical predictions and a low frequency of ongoing mergers in our sample (~ 4%). Such a high fraction of recent mergers in the cluster environment supports a scenario that the merger events that made the features have preceded the galaxy accretion into the cluster environment. We conclude that mergers affect a cluster population mainly through the pre-processing on recently accreted galaxies.

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arXiv2018

Early-type galaxy spin evolution in the Horizon-AGN simulation

Using the Horizon-AGN simulation data, we study the relative role of mergers and environmental effects in shaping the spin of early-type galaxies (ETGs) after $z \simeq 1$. We follow the spin evolution of 10,037 color-selected ETGs more massive than 10$^{10} \rm \, M_{\odot}$ that are divided into four groups: cluster centrals (3%), cluster satellites (33%), group centrals (5%), and field ETGs (59%). We find a strong mass dependence of the slow rotator fraction, $f_{\rm SR}$, and the mean spin of massive ETGs. Although we do not find a clear environmental dependence of $f_{\rm SR}$, a weak trend is seen in the mean value of spin parameter driven by the satellite ETGs as they gradually lose their spin as their environment becomes denser. Galaxy mergers appear to be the main cause of total spin changes in 94% of central ETGs of halos with $M_{vir} > 10^{12.5}\rm M_{\odot}$, but only 22% of satellite and field ETGs. We find that non-merger induced tidal perturbations better correlate with the galaxy spin-down in satellite ETGs than mergers. Given that the majority of ETGs are not central in dense environments, we conclude that non-merger tidal perturbation effects played a key role in the spin evolution of ETGs observed in the local ($z < 1$) universe.

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arXiv2017

Cosmic CARNage I: on the calibration of galaxy formation models

We present a comparison of nine galaxy formation models, eight semi-analytical and one halo occupation distribution model, run on the same underlying cold dark matter simulation (cosmological box of co-moving width 125$h^{-1}$ Mpc, with a dark-matter particle mass of $1.24\times 10^9 h^{-1}$ Msun) and the same merger trees. While their free parameters have been calibrated to the same observational data sets using two approaches, they nevertheless retain some 'memory' of any previous calibration that served as the starting point (especially for the manually-tuned models). For the first calibration, models reproduce the observed z = 0 galaxy stellar mass function (SMF) within 3-σ. The second calibration extended the observational data to include the z = 2 SMF alongside the z~0 star formation rate function, cold gas mass and the black hole-bulge mass relation. Encapsulating the observed evolution of the SMF from z = 2 to z = 0 is found to be very hard within the context of the physics currently included in the models. We finally use our calibrated models to study the evolution of the stellar-to-halo mass (SHM) ratio. For all models we find that the peak value of the SHM relation decreases with redshift. However, the trends seen for the evolution of the peak position as well as the mean scatter in the SHM relation are rather weak and strongly model dependent. Both the calibration data sets and model results are publicly available.

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arXiv2017

On the Structure of the AGN Torus through the Fraction of Optically-Selected Type 1 AGNs

The ratio in number between unobscured (type 1) and obscured (type 2) AGNs is often used to explore the structure of the torus in the unified scheme for active galactic nuclei (AGNs). Oh et al. (2015) investigated the type 1 AGN fraction on two-dimensional space in terms of black hole mass ($M_{\rm BH}$) and bolometric luminosity ($L_{\rm bol}$) and found that the fraction changes depending on both $M_{\rm BH}$ and $L_{\rm bol}$, forming a ridge-shaped distribution. In this study, based on the up-to-date type 1 AGN catalog of Oh et al. (2015), we examine how the trend of the type 1 AGN fraction in the $M_{\rm BH}-L_{\rm bol}$ plane is affected by the different methods used to derive $M_{\rm BH}$ and $L_{\rm bol}$, and suggest an analytic model to explain the observations. We use galaxies from the Sloan Digital Sky Survey Data Release 7 in the redshift range of $0.01 \leq z \leq 0.2$. In estimating $L_{\rm bol}$, we employ two different methods using [O III] and/or [O I] emission lines, and find that the $L_{\rm bol}$ values obtained from the two methods agree well. We consider the $M_{\rm BH}-σ_*$ relation, the $M_{\rm BH}-L_{\rm bulge}$ relation and the single-epoch H$α$-based $M_{\rm BH}$ estimate in calculating $M_{\rm BH}$. We find that the trends of the type 1 AGN fraction with respect to $M_{\rm BH}$ and $L_{\rm bol}$ are similar for the different derivation methods for $L_{\rm bol}$ but different when using different methods to derive $M_{\rm BH}$. We present a model based on the clumpy-torus scheme that reproduces the ridge-shaped distribution of the fraction parallel to the iso-Eddington ratio lines.

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arXiv2017

The SAMI Galaxy Survey: Global stellar populations on the size-mass plane

We present an analysis of the global stellar populations of galaxies in the SAMI Galaxy Survey. Our sample consists of 1319 galaxies spanning four orders of magnitude in stellar mass and includes all morphologies and environments. We derive luminosity-weighted, single stellar population equivalent stellar ages, metallicities and alpha enhancements from spectra integrated within one effective radius apertures. Variations in galaxy size explain the majority of the scatter in the age--mass and metallicity--mass relations. Stellar populations vary systematically in the plane of galaxy size and stellar mass, such that galaxies with high stellar surface mass density are older, more metal-rich and alpha-enhanced than less dense galaxies. Galaxies with high surface mass densities have a very narrow range of metallicities, however, at fixed mass, the spread in metallicity increases substantially with increasing galaxy size (decreasing density). We identify residual correlations with morphology and environment. At fixed mass and size, galaxies with late-type morphologies, small bulges and low Sersic n are younger than early-type, high n, high bulge-to-total galaxies. Age and metallicity both show small residual correlations with environment; at fixed mass and size, galaxies in denser environments or more massive halos are older and somewhat more metal rich than those in less dense environments. We connect these trends to evolutionary tracks within the size--mass plane.

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arXiv2017

Star Formation of Merging Disk Galaxies with AGN Feedback Effects

Using numerical hydrodynamics code, we perform various idealized galaxy merger simulations to study the star formation (SF) of two merging disk galaxies. Our simulations include gas accretion onto supermassive black holes and active galactic nucleus (AGN) feedback. By comparing AGN simulations with those without AGNs, we attempt to understand when the AGN feedback effect is significant. With ~70 simulations, we investigated the SF with the AGN effect in mergers with variety of mass ratios, inclinations, orbits, galaxy structures and morphologies. Using these merger simulations with AGN feedback, we measure merger-driven SF using the burst efficiency parameter introduced by Cox et al. We confirm the previous studies that, in galaxy mergers, AGN suppresses SF more efficiently than in isolated galaxies. However, we additionally find that the effect of AGNs on SF is larger in major mergers than in minor mergers. In minor merger simulations with different primary bulge-to-total ratios, the effect of bulge fraction on the merger-driven SF decreases due to AGN feedback. We create models of Sa, Sb and Sc type galaxies and compare their SF properties while undergoing mergers. With the current AGN prescriptions, the difference in merger-driven SF is not as pronounced as that in the recent observational study of Kaviraj. We discuss the implications of this discrepancy.

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arXiv2017

The SAMI Galaxy Survey: Revising the Fraction of Slow Rotators in IFS Galaxy Surveys

The fraction of galaxies supported by internal rotation compared to galaxies stabilized by internal pressure provides a strong constraint on galaxy formation models. In integral field spectroscopy surveys, this fraction is biased because survey instruments typically only trace the inner parts of the most massive galaxies. We present aperture corrections for the two most widely used stellar kinematic quantities $V/σ$ and $λ_{R}$. Our demonstration involves integral field data from the SAMI Galaxy Survey and the ATLAS$^{\rm{3D}}$ Survey. We find a tight relation for both $V/σ$ and $λ_{R}$ when measured in different apertures that can be used as a linear transformation as a function of radius, i.e., a first-order aperture correction. We find that $V/σ$ and $λ_{R}$ radial growth curves are well approximated by second order polynomials. By only fitting the inner profile (0.5$R_{\rm{e}}$), we successfully recover the profile out to one $R_{\rm{e}}$ if a constraint between the linear and quadratic parameter in the fit is applied. However, the aperture corrections for $V/σ$ and $λ_{R}$ derived by extrapolating the profiles perform as well as applying a first-order correction. With our aperture-corrected $λ_{R}$ measurements, we find that the fraction of slow rotating galaxies increases with stellar mass. For galaxies with $\log M_{*}/M_{\odot}>$ 11, the fraction of slow rotators is $35.9\pm4.3$ percent, but is underestimated if galaxies without coverage beyond one $R_{\rm{e}}$ are not included in the sample ($24.2\pm5.3$ percent). With measurements out to the largest aperture radius the slow rotator fraction is similar as compared to using aperture corrected values ($38.3\pm4.4$ percent). Thus, aperture effects can significantly bias stellar kinematic IFS studies, but this bias can now be removed with the method outlined here.

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arXiv2017

Pixel Color-Magnitude Diagram Analysis of the Brightest Cluster Galaxies in Dynamically Young and Old Clusters, Abell 1139 and Abell 2589

As a case study to understand the coevolution of Brightest Cluster Galaxies (BCGs) and their host clusters, we investigate the BCGs in dynamically young and old clusters, Abell 1139 (A1139) and Abell 2589 (A2589). We analyze the pixel color-magnitude diagrams (pCMDs) using deep g- and r-band images, obtained from the Canada-France-Hawaii Telescope observations. After masking foreground/background objects and smoothing pixels in consideration of the observational seeing size, detailed pCMD features are compared between the two BCGs. (1) While the overall shapes of the pCMDs are similar to those of typical early-type galaxies, the A2589-BCG tends to have redder mean pixel color and smaller pixel color deviation at given surface brightness than the A1139-BCG. (2) The mean pixel color distribution as a function of pixel surface brightness (pCMD backbone) indicates that the A2589-BCG formed a larger central body (~ 2.0 kpc in radius) by major dry mergers at an early epoch than the A1139-BCG (a central body ~ 1.3 kpc in radius), while they have grown commonly by subsequent minor mergers. (3) The spatial distributions of the pCMD outliers reveal that the A1139-BCG experienced considerable tidal events more recently than the A2589-BCG, whereas the A2589-BCG has an asymmetric compact core possibly resulting from major dry merger at an early epoch. (4) The A2589-BCG shows a very large faint-to-bright pixel number ratio compared to early-type non-BCGs, whereas the ratio for the A1139-BCG is not distinctively large. These results are consistent with the idea that the BCG in the dynamically older cluster (A2589) formed earlier and is relaxed better.

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arXiv2017

The SAMI Galaxy Survey: mass as the driver of the kinematic morphology - density relation in clusters

We examine the kinematic morphology of early-type galaxies (ETGs) in eight galaxy clusters in the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. The clusters cover a mass range of 14.2<log(M_200/M_odot)<15.2 and we measure spatially-resolved stellar kinematics for 315 member galaxies with stellar masses 10.0<log(M_*/M_odot)<11.7 within 1R_200 of the cluster centers. We calculate the spin parameter, lambda_R and use that to classify the kinematic morphology of the galaxies as fast or slow rotators. The total fraction of slow rotators in the early-type galaxy population, F_SR=0.14+/-0.02 and does not depend on host cluster mass. Across the eight clusters, the fraction of slow rotators increases with increasing local overdensity. We also find that the slow-rotator fraction increases at small clustercentric radii (R_cl<0.3R_200), and note that there is also an increase in slow-rotator fraction at R_cl~0.6R_200. The slow rotators at these larger radii reside in cluster substructure. We find the strongest increase in slow-rotator fraction occurs with increasing stellar mass. After accounting for the strong correlation with stellar mass, we find no significant relationship between spin parameter and local overdensity in the cluster environment. We conclude that the primary driver for the kinematic morphology--density relationship in galaxy clusters is the changing distribution of galaxy stellar mass with local environment. The presence of slow rotators in substructure suggests that the cluster kinematic morphology--density relationship is a result of mass segregation of slow-rotating galaxies forming in groups that later merge with clusters and sink to the cluster center via dynamical friction.

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arXiv2017

Discovery of ram-pressure stripped gas around an elliptical galaxy in Abell 2670

Studies of cluster galaxies are increasingly finding galaxies with spectacular one-sided tails of gas and young stars, suggestive of intense ram-pressure stripping. These so-called "jellyfish" galaxies typically have late-type morphology. In this paper, we present MUSE observations of an elliptical galaxy in Abell 2670 with long tails of material visible in the optical spectra, as well as blobs with tadpole-like morphology. The spectra in the central part of the galaxy reveals a stellar component as well as ionized gas. The stellar component does not have significant rotation, while the ionized gas defines a clear star-forming gas disk. We argue, based on deep optical images of the galaxy, that the gas was most likely acquired during a past wet merger. It is possible that the star-forming blobs are also remnants of the merger. In addition, the direction and kinematics of the one-sided ionized tails, combined with the tadpole morphology of the star-forming blobs, strongly suggests that the system is undergoing ram pressure from the intracluster medium. In summary, this paper presents the discovery of a post-merger elliptical galaxy undergoing ram pressure stripping.

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arXiv2017

nIFTy Cosmology: the clustering consistency of galaxy formation models

We present a clustering comparison of 12 galaxy formation models (including Semi-Analytic Models (SAMs) and Halo Occupation Distribution (HOD) models) all run on halo catalogues and merger trees extracted from a single ΛCDM N-body simulation. We compare the results of the measurements of the mean halo occupation numbers, the radial distribution of galaxies in haloes and the 2-Point Correlation Functions (2PCF). We also study the implications of the different treatments of orphan (galaxies not assigned to any dark matter subhalo) and non-orphan galaxies in these measurements. Our main result is that the galaxy formation models generally agree in their clustering predictions but they disagree significantly between HOD and SAMs for the orphan satellites. Although there is a very good agreement between the models on the 2PCF of central galaxies, the scatter between the models when orphan satellites are included can be larger than a factor of 2 for scales smaller than 1 Mpc/h. We also show that galaxy formation models that do not include orphan satellite galaxies have a significantly lower 2PCF on small scales, consistent with previous studies. Finally, we show that the 2PCF of orphan satellites is remarkably different between SAMs and HOD models. Orphan satellites in SAMs present a higher clustering than in HOD models because they tend to occupy more massive haloes. We conclude that orphan satellites have an important role on galaxy clustering and they are the main cause of the differences in the clustering between HOD models and SAMs.

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arXiv2017

Phase-space Analysis in the Group and Cluster environment: Time since Infall and Tidal Mass Loss

Using the latest cosmological hydrodynamic N-body simulations of groups and clusters, we study how location in phase-space coordinates at $z$$=$$0$ can provide information on environmental effects acting in clusters. We confirm the results of previous authors showing that galaxies tend to follow a typical path in phase-space as they settle into the cluster potential. As such, different regions of phase-space can be associated with different times since first infalling into the cluster. However, in addition, we see a clear trend between total mass loss due to cluster tides, and time since infall. Thus we find location in phase-space provides information on both infall time, and tidal mass loss. We find the predictive power of phase-space diagrams remains even when projected quantities are used (i.e. line-of-sight velocities, and projected distances from the cluster). We provide figures that can be directly compared with observed samples of cluster galaxies and we also provide the data used to make them as supplementary data, in order to encourage the use of phase-space diagrams as a tool to understand cluster environmental effects. We find that our results depend very weakly on galaxy mass or host mass, so the predictions in our phase-space diagrams can be applied to groups or clusters alike, or to galaxy populations from dwarfs up to giants.

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arXiv2017

Formation and assembly history of stellar components in galaxies as a function of stellar and halo mass

Galaxy mass assembly is an end product of structure formation in the $Λ$CDM cosmology. As an extension of Lee \& Yi (2013), we investigate the assembly history of stellar components in galaxies as a function of halo environments and stellar mass using semi-analytic approaches. In our fiducial model, halo mass intrinsically determines the formation and assembly of the stellar mass. Overall, the ex situ fraction slowly increases in central galaxies with increasing halo mass but sharply increases for $\log M_{*}/M_{\odot}\gtrsim11$. A similar trend is also found in satellite galaxies, which implies that mergers are essential to build stellar masses above $\log M_{*}/M_{\odot}\sim11$. We also examine the time evolution of the contribution of mass growth channels. Mergers become the primary channel in the mass growth of central galaxies when their host halo mass begins to exceed $\log M_{200}/M_{\odot}\sim13$. However, satellite galaxies seldom reach the merger-dominant phase despite their reduced star formation activities due to environmental effects.

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arXiv2016

The SAMI Galaxy Survey: Revisiting Galaxy Classification Through High-Order Stellar Kinematics

Recent cosmological hydrodynamical simulations suggest that integral field spectroscopy can connect the high-order stellar kinematic moments h3 (~skewness) and h4 (~kurtosis) in galaxies to their cosmological assembly history. Here, we assess these results by measuring the stellar kinematics on a sample of 315 galaxies, without a morphological selection, using 2D integral field data from the SAMI Galaxy Survey. A proxy for the spin parameter ($λ_{R_e}$) and ellipticity ($ε_e$) are used to separate fast and slow rotators; there exists a good correspondence to regular and non-regular rotators, respectively, as also seen in earlier studies. We confirm that regular rotators show a strong h3 versus $V/σ$ anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h3 and $V/σ$. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h3 versus $V/σ$ signatures. We identify five classes of high-order stellar kinematic signatures using Gaussian mixture models. Class 1 corresponds to slow rotators, whereas Classes 2-5 correspond to fast rotators. We find that galaxies with similar $λ_{R_e}-ε_e$ values can show distinctly different h3-$V/σ$ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h3 versus $V/σ$ anti-correlation. These objects are predicted to be disk-less galaxies formed by gas-poor mergers. From morphological examination, however, there is evidence for large stellar disks. Instead, Class 5 objects are more likely disturbed galaxies, have counter-rotating bulges, or bars in edge-on galaxies. Finally, we interpret the strong anti-correlation in h3 versus $V/σ$ as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators.

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