Challenge to the Most Distant Evolved Galaxies:

Hint on Star-formation Activity
during the First 500 Million Yearsof the Cosmic History

Ken Mawatari (ICRR Fellow)

18/6/19

ICRR seminar

1Background“Passive galaxy” in this work:
galaxy experiencing > 200Myr after stopping star-formation
Stellar Mass Density

Stars of z~6 passive galaxies should havebeen formed at much higher redshift (z>10!)

Star Formation
Rate Density?
passivefraction?

?

@z>10
(Madau & Dickinson 2014)

Motivation(1) Search for passive galaxies at z > 5 （2）To constrain SFRD at z > 10
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ICRR seminar2Balmer/4000A breakKey spectral feature of passive galaxies
Balmer Breakbecomes stronger
with older age orhigher metallicity.

Balmer Break Galaxy (BBG) = Passive galaxy
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Color selection of passive galaxies
Observed near-infrared (2 – 5um) colors of model galaxies (BruzualK, Spitzer-3.6um, and 4.5um bands& Charlot 2003)（１）Passive model@z~6（２）Star-forming（３）Star-forming＋Dust

Red around Balmer BreakBlue at longer wavelengthBlue around Balmer BreakBlue at longer wavelength

Red around Balmer BreakRed at longer wavelength

BalmerBreak

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ICRR seminar

4Color selection of passive galaxies
Three types of galaxies with red
K-[3.6] color.
(1) Balmer Break Galaxy (BBG)
age >200Myr(2) Dusty Star-forming galaxy(DSFG)(3) Dusty Nebular-line emitter(DNLE)
Extremely young star-forming
galaxy has very strong emission
lines

Models are composite of stellar continuum(Bruzual&Charlot03), Nebular emissions (Inoue+11)

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ICRR seminar

Criteria for BBG at 5<z<8
K - [3.6] > 1.75
K – [3.6] > 2.4([3.6]-[4.5])+1.05
c.f. Mawatari+16b5

Data
All the imaging data available in the UVISTA UD stripes 1〜3.

Used area

Key data

Depth of K〜25 & [3.6]〜24
mag enable us to detectpassive galaxies down to M* ~3x1010 Msun.
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ICRR seminar6

z〜6 BBG selectionl Balmer Break Galaxy (BBG) selection
・We focus on onlyisolated objects in the K and 3.6um image
for photometric accuracy.
N〜37000

3.6um

K・The Color selection
K - [3.6] > 1.75
K – [3.6] > 2.4([3.6]-[4.5])+1.05
N = 23・Non-detection in
optical, MIR(λ>10um), FIR, radio, and X-ray
to removestrong line emitters, extremely dusty galaxies,and AGNs.

8 BBG candidates at z〜6
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ICRR seminar

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StackinglStacking of the 8 BBG candidates
Detections in the H, K, 3.6um, 4.5um, and very deep constraints in other bands.
x 〜sqrt(8) improvement in photometry

Det bands

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z~6 BBGs’ color

dusty starforming

passive- Photometric selection of 8 passive galaxies with the strong Balmer break at z ~ 6Open circles: the 8 BBG candidates
Filled violet pentagon: stacked BBG
18/6/19

ICRR seminar

9SED fitting- Model template fitting for the stacked Spectral Energy Distribution (SED) results inthe best-fit by an old galaxy model at z 〜 6 with 0.7 Gyr passive phase.

10M ,Best-fitby Old
modelparameterat z =6.3; values
M*=4.4x10
Sorry, but
the best-fit
are secrettillsunpaper
is accepted
Age=0.7Gyr, Bursty
SFH(τ=30Myr)※z~3 dusty model isNOT selected inthe MC simulation※ We forbidden extremely dustypassive parameter space where theKennicutt-Schdmit law is completelybroken:
Av > 4xSFR0.3

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z prob dist is
secret tillpaper is
accepted

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Stellar Mass Density of z~6 BBGsAssumption: The 8 candidates are all BBGs
at z = 5 – 8,with M* = 4x1010Msun

Star-formingAfter correcting the detection
incompleteness, stellar mass density(SMD) of z~6 BBGs is
4 M /Mpc3
ρ* =Sorry,
5.7butx 10
the SMDvalue is secret till paper issun
accepted・consistent with the trend at z = 0 - 4・passive fraction〜3％

NEWFormation and quenching of firstmassive galaxies in the universe wasproceeded by z = 6 !
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ICRR seminar11

Progenitors of z~6 BBGs
lStar-forming progenitors
・Best-fit model
suggests
the star-forming progenitor (age=τ)at z 〜18 with SFR 〜 900Msun/yr

SF phase(0<age<2τ)※Our model SFH is
SFR(t) = SFR(0) x exp(-τ/t)

0.2GyrBBG phase・We also investigated SFHs of all
best-fit models in the MC runs.

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ICRR seminar

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Star Formation Rate Density at z〜20
SFRD of the BBG progenitors is estimated via

SFRDprog = SFR(t = τ ) × n prog × fobs
Number density of the BBG progenitors= that of the BBGs at z~6
Typical SFR of the BBG progenitors

TSF
2 ×τfobs ==TBBG ageBBG − 2 × τ − 0.2Gyr: Fraction of observable timescale

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Star Formation Rate Density at z〜20

Sorry, but theSFRD result issecret till paper is
accepted

SFRD of the BBG progenitors is regarded as a lower-limit of the cosmic SFRD.

We constrain the SFRD at z>10 for the first time.
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Summary- Photometric selection of 8 passive galaxies with the strong Balmer break at z ~ 6- Their SEDs are well fit by old galaxy model at z 〜 6 with > 0.5 Gyr passive (nonstar-forming) phase.
- Their expected star-formation epoch is z > 15!
- Follow-up observation by ALMA is proceeding now.
- Spectroscopic confirmation is very important. (=> JWST)Star-formingThisworkz prob dist is
secret tillpaper is
accepted

Madau(2014 & Dickinso)
nSorry, but the SFRD result
is secret till paper isacceptedThis work

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