EAYAM, Ishigaki Japan, 14 November 2017

Exploring the Evolution of Brightest Cluster Galaxies
since z~2
Dongyao Zhao
Christopher Conselice; Alfonso Aragón-Salamanca
School of Physics & Astronomy
University of Nottingham

Brightest Cluster Galaxies (BCGs)❖The most massive and luminous galaxies in the nearby universe❖

Reside in the centre of galaxy clusters/groups❖

Unique properties and special environment make BCG evolution
important to better understand the evolution of massive galaxies
and galaxy clusters

BCG in Abell 370 (HST ACS)

Motivation❖Selecting a statistically large sample of BCG progenitors at z~2in observations❖Studying the structure (size and shape), morphology, stellar
mass and SFR/sSFR of our selected BCG progenitors❖

Comparing with local BCGs, and discussing the BCG evolution
since z~2

Observational Data❖❖

High redshifts❖

CANDELS UDS galaxies over z=1~3❖

CANDELS UDS images

Local universe❖

Descendants are selected from BCG catalogue of von derLinden+07 and SDSS DR7 galaxy catalogue over 0.02 ≤ z ≤ 0.10❖

SDSS DR7 images, which are moved to z=2 and observed byCANDELS UDS. This is done by using FERENGI code (Barden+08)

EvolutionEvolutionof BCGs ofsinceBCGsz ∼since2 5z ∼ 25eters which are measured directly from the original SDSSimages and the original CANDELS UDS images is unreliable to study the size growth.
In order to explore the intrinsic linked structural evolution of BCGs, the images of SDSS and CANDELS UDS
need to be calibrated to allow comparisons of similar quantities at all redshift. This can be achieved by the code FER❖ Assumption1: Progenitors
areofthemostof massive
ENGI (Fullofandlocal
EﬃcientBCGsRedshifting
Ensembles
NearbyGalaxy Images; Barden et al. 2008). This code takes intogalaxies in the densest
regions
account
the cosmological corrections for size, surface brightness and bandpass shifting when simulating low redshift❖ Assumption2: Constant number density of 10-4 h3Mpc-3
galaxies to high redshift. Simulated images are produced
when the input galaxy images are converted to higher-z im❖ 38 progenitors can beagesselectedusing the output
instrument
properties.
as theredshift
mostandmassivegalaxyin the topThe FERENGI code is primarily created for the conversion
38 densest regions fromCANDELS
UDS
surveyfrom SDSS
to GEMS andCOSMOSACS, but it can be used
4 Zhao et al.
for other data sources. Full description about the code see
Barden et al. (2008).
eters which are measured directly from the original SDSS
By applying our
propsedprogenitor
imagesand theBCG
original
CANDELS selection
UDS images(deis unreli❖
Q:intoWhatof the true
study
the fraction
size
growth.
bution of the 38 progenitorsprogenitors
selected bytailed description ableisSection3) onthe CANDELSUDS BCGorder to explore
linked redshift
structural evomassive galaxies in the densest environdata, the selected 38 In
progenitors
at zthe=intrinsic[1, 3] haveprogenitors
canlikely
be selected?
lutionof BCGs, the imagesof SDSSand CANDELS
UDS
ift of this distribution is z = 2.04+0.54−0.64 .
distribution as showninFigure
1.
The
SDSS
imagesareneed to be calibrated to allow comparisons of similar quantherefore shifted totitiesz =at[1,following
thedistribuall 3]
redshift.
This canberedshiftachieved by
the code FERENGI(Full andintoEﬃcient
Redshifting
Ensemblesof Nearby
A: taking
44%oftheselectedprogenitors
bytion of Figure 1❖ withaccountthe ofK-correction
Images; Barden et al. 2008). This code takes intohown that this method of selectin the FERENGI Galaxy
code. Tobe eﬃcient when running FERaccountthecosmologicalcorrections
forBCGsize, surface brightour method
aretruel advantages. Leja et al. (2013) hasENGI, we only simulate
SDSS
g-bandimagestoCANDELS
ness and bandpass shifting when simulating low redshiftnique is robust at directly tracking
high redshift.
Simulatedimagesarenon-BCG
produced
UDS H160 -band atgalaxies
z = 2.to Thisavoidsthe K-correction
beprogenitors,and56%
areitor galaxies over cosmic time whengalaxyconverted towavehigher-z imcause the g-band whenat z the∼ input
0 is intheimages
samearerest-frame
agesusing the output (contaminants)
redshift and instrument properties.
ic models. Mundy et al. (2015) has
progenitors
length as the H160The-band
atz=2.
Figureillustrates
FERENGI code is primarily1 created
for thethat
conversion
nstant number density selected samz ∼ 2 is the medianof ourselected
high-zprofromredshift
SDSS to GEMS
and38
COSMOSACS,
but it canbe used3−3< log ND[h Mpc ] < −3.0) is supefor otherdata sources.
about thecode seegenitors and ∼ 90%of them
are at Fullz <description
2.5, implyingthat
al. (2008).
mass selected sample, and can trace
the K-correction isBarden
not aetsignificant
factor in the simulation.
By
applying
ourpropsed
BCG progenitor selection (deerage stellar mass and average SFR
Testing on a small number of galaxies we demonstrate that

Selecting BCG ProgenitorsEffect of Contaminants at High-z❖

The differences are ≤ 0.1 dex
from contaminants for stellar
mass and size.

❖Contaminants make SFR/sSFR
larger by a factor of ~0.4 dex.

Descendants at z=0❖

470 descendants with number density of 10-4 h3Mpc-3❖

Within 470 local descendants, 44% (207) are BCGs and 56% (263)
are non-BCGs❖❖

207 BCGs are selected from BCG catalogue of von derLinden+07❖

263 non-BCGs are selected from SDSS DR7 galaxy catalogue
according to the mass distribution

The contaminations are no more than 0.1 dex from non-BCGs
for structure, stellar mass and SFR/sSFRBCG Evolution since z~2
1. size and shape evolution❖Size of BCGs has grown by a factor of ~3.2 since z~2.

❖

BCG progenitors are late-type galaxies with Sersic index n~2.3, while
their local descendants are early-type galaxies with Sersic index n~4.5.

14

Zhao et al.

BCG Evolution since z~2
2. morphology evolution❖30% BCG progenitors at z~2 are disturbed, while all local BCGs
have smoother profile.

BCG Evolution since z~2
3. stellar mass evolution❖Stellar mass of BCGs has grown by a factor of ~2.5 since z~2❖

al. at z~2 than z~0
SFR is ~70 times (sSFR is18~166 Zhaotimes)etlarger

Mechanisms driving BCG Mass Growth❖At high redshifts (z=1~2):❖❖❖

Star formation contributes ～18% of the stellar mass of local BCGs
Merging contributes ~ 12% of the stellar mass of local BCGs by using
merger rate of Hopkins+10

At low redshifts (z<1):❖Our results show that by z ∼ 1 the BCG stellar mass will be no more than
70% of the total local mass, suggesting that there has to be an additional
mass build-up in BCGs after z ∼ 1. The BCG mass will increase by a factorof no less than 1.4 at z=1-0.

❖

Little contribution from star formation.

❖Merger is the dominant process.

Take Home Messages❖

At z~2, less than 50% of the most massive galaxies in the densest
environments are the true BCG progenitors.

❖Our selected observational sample can be employed to study BCG
evolution since the uncertainties introduced by contaminants arerelatively small.

❖Since z~2, the size of BCG has grown by a factor of ~3.2 and stellar
mass by a factor of ~2.5. The BCG progenitors are less concentrated andmore disturbed while the local BCGs are smoother early-type galaxies.

❖Merger and star formation contribute equally for BCG mass growth athigh redshift (z=1~2). At lower redshift (z<1), merger is the dominantprocess.

❖Dongyao Zhao, et al. 2017, MNRAS, 464, 1393

Thank you