Evolution of the Brightest Cluster Galaxies

In this thesis, I explore the evolution of the brightest cluster galaxies (BCGs) over the last 10 billion years through detailed studies of both local BCGs from SDSS and their high-z progenitors from CANDELS UDS. First, I study a large sample of local BCGs and link their morphologies to their structural properties. We derive visual morphologies for these BCGs and find that ~57% of local BCGs are cD galaxies, ~13% are ellipticals, and ~21% belong to intermediate classes, mostly between E and cD. There is a continuous distribution in the properties of the BCG's envelopes, ranging from undetected (elliptical BCGs) to clearly detected (cD galaxies), with intermediate classes showing increasing degrees of the envelope presence. A minority (~7%) of BCGs have disk morphologies, with spirals and S0s in similar proportions, and the rest (~2%) are mergers. After carefully fitting the galaxy light distributions using Sersic models, I find a clear link between BCG morphology and structure, such that cD galaxies are typically larger than elliptical BCGs, and the visually extended envelope of cD galaxies is a distinct structure differing from the central bulge. Based on this BCG morphology--structure correlation, I develop a statistically robust way to separate cD from non-cD BCGs, by which cD galaxies can be selected with reasonably high completeness and low contamination. Next, I investigate the effect of environment on the properties of local BCGs by studying the relationship between the BCG's internal properties (stellar mass, structure and morphology) and their environment (local density and cluster halo mass). I find that the size of BCGs is determined by the intrinsic BCG stellar mass, with a weak correlation with the cluster environment. Additionally, more massive BCGs tend to inhabit denser regions and more massive clusters than lower mass BCGs. The growth of the BCGs seems to be linked to the hierarchical growth of the structures they inhabit: as the groups and clusters became denser and more massive, the BCGs at their centres also grew. Moreover, I demonstrate that cD galaxies are ~40% more massive than elliptical BCGs, and prefer denser regions and more massive haloes. My results, together with the findings of previous studies, suggest an evolutionary link between elliptical and cD BCGs. I propose that most present-day cDs started their life as ellipticals at z~1, which subsequently grew in stellar mass and size due to mergers. In this process, the cD envelope developed. This process is nearing completion since the majority of the local BCGs have cD morphology. However, the presence of BCGs with intermediate morphological classes suggests that the growth and morphological transformation of BCGs is still ongoing. Finally, I present a new method for tracing the evolution of BCGs from z~2 to z~0. I conclude, on the basis of semi-analytical models, that the best method to select BCG progenitors at z~2 is a hybrid environmental density and stellar mass ranking approach. Ultimately I am able to retrieve 45% of BCG progenitors. Although the selected high-z progenitor sample is a mixture of BCG and non-BCG progenitors, I demonstrate that their properties can be used to trace BCG evolution. Applying this method to the CANDELS UDS data, I construct an observational BCG progenitor sample at z~2. A local BCG comparison sample is constructed using the SDSS data, taking into account the likely contamination from non-BCGs to ensure a fair comparison between high-z and low-z samples. Using these samples I demonstrate that BCG sizes have grown by a factor of ~3.2 since z~2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early-type local counterparts. I also find that BCG progenitors have more disturbed morphologies, while local BCGs have much smoother profiles. Moreover, I find that the stellar masses of BCGs have grown by a factor of ~2.5 since z~2, and the SFR of BCG progenitors has a median value of ~14 Msun/yr, much higher than their quiescent local descendants. I demonstrate that at 1<z<2 star formation and merging contribute approximately equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z<1. I also find that BCG progenitors at high-z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z<2

Evolution of the Brightest Cluster Galaxies

In this thesis, I explore the evolution of the brightest cluster galaxies (BCGs) over the last 10 billion years through detailed studies of both local BCGs from SDSS and their high-z progenitors from CANDELS UDS. First, I study a large sample of local BCGs and link their morphologies to their structural properties. We derive visual morphologies for these BCGs and find that ~57% of local BCGs are cD galaxies, ~13% are ellipticals, and ~21% belong to intermediate classes, mostly between E and cD. There is a continuous distribution in the properties of the BCG's envelopes, ranging from undetected (elliptical BCGs) to clearly detected (cD galaxies), with intermediate classes showing increasing degrees of the envelope presence. A minority (~7%) of BCGs have disk morphologies, with spirals and S0s in similar proportions, and the rest (~2%) are mergers. After carefully fitting the galaxy light distributions using Sersic models, I find a clear link between BCG morphology and structure, such that cD galaxies are typically larger than elliptical BCGs, and the visually extended envelope of cD galaxies is a distinct structure differing from the central bulge. Based on this BCG morphology--structure correlation, I develop a statistically robust way to separate cD from non-cD BCGs, by which cD galaxies can be selected with reasonably high completeness and low contamination. Next, I investigate the effect of environment on the properties of local BCGs by studying the relationship between the BCG's internal properties (stellar mass, structure and morphology) and their environment (local density and cluster halo mass). I find that the size of BCGs is determined by the intrinsic BCG stellar mass, with a weak correlation with the cluster environment. Additionally, more massive BCGs tend to inhabit denser regions and more massive clusters than lower mass BCGs. The growth of the BCGs seems to be linked to the hierarchical growth of the structures they inhabit: as the groups and clusters became denser and more massive, the BCGs at their centres also grew. Moreover, I demonstrate that cD galaxies are ~40% more massive than elliptical BCGs, and prefer denser regions and more massive haloes. My results, together with the findings of previous studies, suggest an evolutionary link between elliptical and cD BCGs. I propose that most present-day cDs started their life as ellipticals at z~1, which subsequently grew in stellar mass and size due to mergers. In this process, the cD envelope developed. This process is nearing completion since the majority of the local BCGs have cD morphology. However, the presence of BCGs with intermediate morphological classes suggests that the growth and morphological transformation of BCGs is still ongoing. Finally, I present a new method for tracing the evolution of BCGs from z~2 to z~0. I conclude, on the basis of semi-analytical models, that the best method to select BCG progenitors at z~2 is a hybrid environmental density and stellar mass ranking approach. Ultimately I am able to retrieve 45% of BCG progenitors. Although the selected high-z progenitor sample is a mixture of BCG and non-BCG progenitors, I demonstrate that their properties can be used to trace BCG evolution. Applying this method to the CANDELS UDS data, I construct an observational BCG progenitor sample at z~2. A local BCG comparison sample is constructed using the SDSS data, taking into account the likely contamination from non-BCGs to ensure a fair comparison between high-z and low-z samples. Using these samples I demonstrate that BCG sizes have grown by a factor of ~3.2 since z~2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early-type local counterparts. I also find that BCG progenitors have more disturbed morphologies, while local BCGs have much smoother profiles. Moreover, I find that the stellar masses of BCGs have grown by a factor of ~2.5 since z~2, and the SFR of BCG progenitors has a median value of ~14 Msun/yr, much higher than their quiescent local descendants. I demonstrate that at 1<z<2 star formation and merging contribute approximately equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z<1. I also find that BCG progenitors at high-z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z<2

Evolution of the brightest cluster galaxies: the influence of morphology, stellar mass and environment

Using a sample of 425 nearby brightest cluster galaxies (BCGs) from von der Linden et al., we study the relationship between their internal properties (stellar masses, structural parameters and morphologies) and their environment. More massive BCGs tend to inhabit denser regions and more massive clusters than lower mass BCGs. Furthermore, cDs, which are BCGs with particularly extended envelopes, seem to prefer marginally denser regions and tend to be hosted by more massive haloes than elliptical BCGs. cD and elliptical BCGs show parallel positive correlations between their stellar masses and environmental densities. However, at a fixed environmental density, cDs are, on average,∼ 40 per cent more massive. Our results, together with the findings of previous studies, suggest an evolutionary link between elliptical and cD BCGs. We suggest that most present-day cDs started their life as ellipticals, which subsequently grew in stellar mass and size due to mergers. In this process, the cD envelope developed. The large scatter in the stellar masses and sizes of the cDs reflects their different merger histories. The growth of the BCGs in mass and size seems to be linked to the hierarchical growth of the structures they inhabit: as the groups and clusters became denser and more massive, the BCGs at their centres also grew. This process is nearing completion since the majority (∼60 per cent)of the BCGs in the local Universe have cD morphology. However, the presence of galaxies with intermediate morphological classes (between ellipticals and cDs) suggests that the growth and morphological transformation of some BCGs is still ongoing

Kinematic decomposition of IllustrisTNG disk galaxies: morphology and relation with morphological structures

We recently developed an automated method, auto-GMM to decompose simulated galaxies. It extracts kinematic structures in an accurate, efficient, and unsupervised way. We use auto-GMM to study the stellar kinematic structures of disk galaxies from the TNG100 run of IllustrisTNG. We identify four to five structures that are commonly present among the diverse galaxy population. Structures having strong to moderate rotation are defined as cold and warm disks, respectively. Spheroidal structures dominated by random motions are classified as bulges or stellar halos, depending on how tightly bound they are. Disky bulges are structures that have moderate rotation but compact morphology. Across all disky galaxies and accounting for the stellar mass within 3 half-mass radii, the kinematic spheroidal structures, obtained by summing up stars of bulges and halos, contribute ~45% of the total stellar mass, while the disky structures constitute 55%. This study also provides important insights about the relationship between kinematically and morphologically derived galactic structures. Comparing the morphology of kinematic structures with that of traditional bulge+disk decomposition, we conclude: (1) the morphologically decomposed bulges are composite structures comprised of a slowly rotating bulge, an inner halo, and a disky bulge; (2) kinematically disky bulges, akin to what are commonly called pseudo bulges in observations, are compact disk-like components that have rotation similar to warm disks; (3) halos contribute almost 30% of the surface density of the outer part of morphological disks when viewed face-on; and (4) both cold and warm disks are often truncated in central regions.Comment: 20 pages, 14 figures. Accepted for publication in ApJ. The mass fraction catalogue and images of the kinematically derived galactic structures are publicly available (https://www.tng-project.org/data/docs/specifications/#sec5m

A New Channel of Bulge Formation via the Destruction of Short Bars

Short (inner) bars of subkiloparsec radius have been hypothesized to be an important mechanism for driving gas inflows to small scales, thus feeding central black holes (BHs). Recent numerical simulations have shown that the growth of central BHs in galaxies can destroy short bars, when the BH reaches a mass of ∼0.1% of the total stellar mass of the galaxy. We study N-body simulations of galaxies with single and double bars to track the long-term evolution of the central stellar mass distribution. We find that the destruction of the short bar contributes significantly to the growth of the bulge. The final bulge mass is roughly equal to the sum of the masses of the initial pseudo bulge and short bar. The initially boxy/peanut-shaped bulge of Sérsic index n1 is transformed into a more massive, compact structure that bears many similarities to a classical bulge, in terms of its morphology (n≈2), kinematics (dispersion-dominated, isotropic), and location on standard scaling relations (Kormendy relation, mass-size relation, and correlations between BH mass and bulge stellar mass and velocity dispersion). Our proposed channel for forming classical bulges relies solely on the destruction of short bars without any reliance on mergers. We suggest that some of the less massive, less compact classical bulges were formed in this manner

Integrated microRNA-mRNA analysis provides new insights into gonad coloration in the sea urchin Strongylocentrotus intermedius

Comparative microRNA (miRNA) and mRNA transcriptome analyses were performed on Strongylocentrotus intermedius of the same sex with significant gonadal color differences. The results indicated that 1) the color of female gonads was generally superior to that of males. 2) Comparative and integrated miRNA and mRNA transcriptome analyses identified differentially expressed genes (DEGs) and differentially expressed miRNAs (DEMs) in female and male individuals with significant gonadal color differences. Common and sex-specific DEGs and “DEM-DEG” pairs involved in carotenoid absorption, accumulation, and transformation were identified as candidates correlated with gonad coloration in S. intermedius. Collectively, the results from this study have enriched our knowledge of the process of sea urchin gonad coloration and should provide additional clues for increasing the gonad quality of commercial sea urchins from molecular and metabolic aspects

Exploring the progenitors of brightest cluster galaxies at z ∼ 2

We present a new method for tracing the evolution of brightest cluster galaxies (BCGs) from z ∼ 2 to z ∼ 0. We conclude on the basis of semi-analytical models that the best method to select BCG progenitors at z ∼ 2 is a hybrid environmental density and stellar mass ranking approach. Ultimately, we are able to retrieve 45 per cent of BCG progenitors. We apply this method on the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Ultra Deep Survey data to construct a progenitor sample at high redshift. We furthermore populate the comparisons in local Universe by using Sloan Digital Sky Survey data with statistically likely contamination to ensure a fair comparison between high and low redshifts. Using these samples we demonstrate that the BCG sizes have grown by a factor of ∼3.2 since z ∼ 2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early type local counterparts. We find that BCG progenitors have more disturbed morphologies. In contrast, local BCGs have much smoother profiles. Moreover, we find that the stellar masses of BCGs have grown by a factor of ∼2.5 since z ∼ 2, and the star formation rate of BCG progenitors has a median value of 13.5 Mʘ yr‾¹, much higher than their quiescent local descendants. We demonstrate that over z = 1–2 star formation and merging contribute equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z ≲ 1. We also find that BCG progenitors at high z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z ≲ 2

The link between morphology and structure of brightest cluster galaxies: automatic identification of cDs

We study a large sample of 625 low-redshift brightest cluster galaxies (BCGs) and link their morphologies to their structural properties. We derive visual morphologies and find that ∼57 per cent of the BCGs are cD galaxies, ∼13 per cent are ellipticals, and ∼21 per cent belong to the intermediate classes mostly between E and cD. There is a continuous distribution in the properties of the BCG's envelopes, ranging from undetected (E class) to clearly detected (cD class), with intermediate classes (E/cD and cD/E) showing the increasing degrees of the envelope presence. A minority (∼7 per cent) of BCGs have disc morphologies, with spirals and S0s in similar proportions, and the rest (∼2 per cent) are mergers. After carefully fitting the galaxies light distributions by using one-component (Sérsic) and two-component (Sérsic+Exponential) models, we find a clear link between the BCGs morphologies and their structures and conclude that a combination of the best-fitting parameters derived from the fits can be used to separate cD galaxies from non-cD BCGs. In particular, cDs and non-cDs show very different distributions in the Re–RFF plane, where Re is the effective radius and RFF (the residual flux fraction) measures the proportion of the galaxy flux present in the residual images after subtracting the models. In general, cDs have larger Re and RFF values than ellipticals. Therefore we find, in a statistically robust way, a boundary separating cD and non-cD BCGs in this parameter space. BCGs with cD morphology can be selected with reasonably high completeness (∼75 per cent) and low contamination (∼20 per cent). This automatic and objective technique can be applied to any current or future BCG sample with good-quality images