銀河の環境効果、形状と色
岡本崇 (国立天文台)
Yet another semi-analytic model• Okamoto & Nagashima 2001
• Okamoto & Nagashima 2003
• νGC についてちょっとMorphology-Density Relation

FieldClusters

ES0Spirals

Dressler 1980

Nature or Nurture?
• Nature? Elliptical galaxies only form in
protoclusters at high redshift. Rest ofpopulation is due to infall.
• or Nurture? Galaxy evolution proceeds
along a different path within denseenvironments.
– If this is true in groups and clusters, thenenvironment could be the driving force ofrecent galaxy evolution…

Merger hypothesis• Bulges form through major mergers.
Major merger

Bulge formation

merging
starburst
A galaxy merger destroys
discs and leads to bulge
formation

All gas is used up inthe burst.

Further cooling forms
an additional disc.

N-body + SA simulation• Need positions and luminosities of cluster
galaxies• High-resolution cosmological simulation of
a cluster formation (Ω0 = 1 and 0.3)
• Follow galaxy evolution within sub-halos
using semi-analytic model.
• Can the MD-relation be reproduced bymerger driven bulge formation scenario?

Density map of the simulated SCDM cluster

TO & Habe 1999

SCDM

OCDM

FOF halos
Centralgalaxies
Sub-halosdsx
Centralgalaxies ofsub-halos
Satellite galaxies
Naked• Construct sub-halo merger trees from N-body simulations• Sub-halos must be FOF-halos in the past.
• Trace position of naked galaxies using the most bound particles

Semi-analytic model
FOF halo

Shock-heated gas in each FOF-halos
with virial temperature

hot gasRadiative cooling−α hot⎛V ⎞SN feedback M& reheat = β (Vc ) M& * , β (Vc ) = ⎜⎜ c ⎟⎟⎝ Vhot ⎠cold gas
M cold0 ⎛ Vc ⎞&, τ * = τ * ⎜⎜ ⎟⎟Star formation M * =τ*⎝ V* ⎠DiskStarsGalaxy
Major merger& star burst

Bulge starsα*Model parameters

E+S0 fractionMorphology-density relation• The observed elliptical fraction
is well reproduced.
• Too few S0s (too manyspirals)• The observed trend of S0
distribution cannot be
reproduced only by mergers.
• Based on the merger
hypothesis galaxies aredivided into two populations,i.e. almost pure disc andalmost pure bulge galaxies.

Morphology-density: evolution

NS0/NE

Number of galaxies

Low redshift

Z~0.5Redshift
Dressler et al. 1997; Couch et al. 1994; 1998
Fasano et al. 2000
Wide field HST: Treu et al. 2003
Log surface density

SFR-density relation

Lewis+’02

SF is suppressed within 2 Rv

colour-density relation @ z=0.41

この辺がぁゃιぃ

Kodama+’01

S to S0 transformation?
Kenney et al. 2003
Vollmer et al. 2004

• Ram pressure stripping of the disk
could transform a spiral into a S0
(Gunn & Gott 1972; Solanes &Salvador-Solé 2001)• Strangulation may lead to anemic orpassive spiral galaxies (Shiyoa et al.
2002)

Non-SF spiral galaxies from SDSS (Goto et al. 2003)
First noted by Poggianti et al. (1999) in z~0.5 clustersAdditional physics?
• Strangulation (Larson+1980)– Long time-scale ~ t*

• Ram-pressure stripping (Gunn & Gott 1972)– Short time-scale• Minor merger– Minor mergers may also induce small starbursts• Harassment (Moor+1996)– Cumulative effect of high-speed encounters between
galaxies causes starbursts and morphological change.

Ram-pressure stripping
1. Calculate orbits of galaxies in the main cluster
assuming NFW-profile.

2. Instantaneous removal of cold gas fromgalaxies

when Pram > Fgrav, cold @ effective radius, all thecold gas is stripped from a galaxy

Minor burst• Minor merger– fminor < Msat/Mcen < fmajor– fmajor = 0.25 and fminor = 0.1 fmajor• fburst Mcold is consumed in the minor burst
and added to the bulge.

Sub-halos in a ΛCDM cluster

Parameters and models• Standard– Strangulation + major
merger• Ram-pressure– Standard + RPS• Minor-burst– Standard + minorburst• Low-SFR– Standard but with 4
times longer τ*Field galaxies

LFs of simulated cluster galaxies

Colour-magnituderelation• Early-type galaxies
are always on thered sequence.

• Standard, RPS,and Minor-burst
models look thesame on thisplane.

SFR-radiusrelation• Effects of the RPS on SFR
is only seen in the LOWSFR model:
the strangulation
sufficiently suppresses theSFR in the cluster.

Colour-radius relation• With the standard
parameter set, theeffects of the RPS isnegligible in the core.
• Inefficientstrangulation makes
the signal of the RPS
detectable, but fieldgalaxies are too blue.

Morphology-radiusrelation• Consistent with what
was found by TO &Nagashima ’01.
• The RPS alone can’t
increase theintermediate fraction.
• The Minor-burst model
reproduces themorphological fraction
reasonably well.

Bimodality in morphology!!
• seems common insemi-analytic
models but notseen in the realuniverse.
• Minor bursts makethe gap narrower.
• RPS can’t help.

Bimodal colour distribution: commentson νGC and Mitaka modelColour-magnitude relation
CMR for spiral galaxies
also observed (e.g.
Chester & Roberts 1964;
Visvanathan 1981; Tully,Mould & Aaronson
1982)
SDSS allows full
distribution to bequantified with highprecision ( Baldry et al.
2003; Hogg et al. 2003;
Blanton et al. 2003)

Sloan DSS data• Red population
always exists.
• Blue population
dominates infaint galaxies.

u-r

Baldry+’04

• 24346 galaxies fromSDSS
DR1.
magnitude limitedwith z<0.08• density
estimates
based on Mr<-20

Balogh et al. 2004

Hα distribution• Hα distribution shows abimodality: mean/median ofwhole distribution can bemisleading

Balogh et al. 2004

• lp211_V14• solid – with dust
• dashed –without dust
• Blue population onlyexists in brightgalaxies without
dust…

• lp221_V14• Fix it!!

Other models I• Except for thebrightest galaxies,
the bimodality iswell reproduced.

Menci+’04

Other models II• Virtually two populations

Croton+’06

SF in Menci et al.
mcrdm& * =, where τ d = ,qτ dvd
andrd = rv g (vc ).
from Mo + '98

λjdg (vc ) ≈f c (vc )2 (mc / m)
Authors insist this is the key!

−12f R (v )Summary• To my knowledge, GALFORM and Mitaka
model fail to reproduce the bimodal colour
distribution.
• Morphology of galaxies in SA models often
shows bimodality that we don’t want.
• At least some SA models successfully
reproduce the bimodality in colour.