Rich cluster [M ~ 1015 Msun]

Large groups (Small clusters)[M ~ 1013 - 1014 Msun]

Environmental effects on neutral and molecular hydrogen of galaxies

Small/compact groups[M < 1013 Msun]

Kenji Bekki (ICRAR at UWA, Australia)

(From APOD)

Outline: five key physical processes in HI/H2 evolution of galaxies.

The relative importance of these processes is different in different environments, so, how do these influence the HI/H2

evolution of galaxies ?

[1012-1013 Msun]

Mass-scaleSize-scale

[1014 Msun] [1015 Msun]

1.Galaxy interaction (e.g., Noguchi 1987)

2.Galaxy Merging (e.g., Barnes & Hernquist 1992)

3. Ram pressure stripping (e.g., Gunn & Gott 1971)

4.Group tide (e.g., Icke 1985)

5. Galaxy harassment (e.g., Moore et al. 1996)

Evolution of Rmol (=M(H2)/M(HI)), HI/H2 disk sizes, and HI-to-star-mass ratios.

Why is Rmol so different in different environments ?

(Boselli et al. 2014)

HI-deficient galaxy

HI-normal galaxy

H2 formation on dust grains and formation/destruction processes of dust in

galaxy-scale simulations

(1) H2 formation on dust grains

H H2

(2) Dust growth and destruction

Dust particle

Growth DestructionFormation

SNeMetal

Simulation

(Bekki 2014)

(Gould & Salpeter 1963)

(1)The roles of galaxy interaction/merging in HI/H2 evolution of small/compact groups.

• How does galaxy interaction and merging

change the HI/H2 contents and their spatial distributions ?

(Stephan’s Quintet from APOD)

Evolution of a small group of galaxies

(Cosmic Front:NHK 2014 based on my simulations)

The roles of galaxy interaction inHI/H2 evolution.

• Enhancement of collisions of giant molecular clouds (GMCs) Induced starburst (a factor of ~ 5) in gas-rich disks (Noguchi & Ishibashi 1986; Mihos et al. 1993)

• Gas inflow to the central regions of disk galaxies by gravitational torque (bar) Central concentration of gas.

(Noguchi & Ishibashi 1986)

SFR

Time

Companion

Gas disk

Enhancement of H2 formation in interacting disk galaxies.

(Bekki 2014)

Companion

Gas disk

StellarDisk size

T=0 (Gyr) 0.3

0.8

0.6

1.1 1.4

M(H2)

Rmol

210 kpc

A factor of 3-5 enhancement depending on orbital configurations and mass-ratios of two galaxies.

Evolution of HI/H2 in interacting galaxies.

HI

H2

HI/H2 distributions in interacting disks.

T=0.3 Gyr 0.6

1.11.10.8

0.6T=0.3 Gyr

0.8

HI H2

(Bekki 2014)

More clumpy distribution

H2 formation in tidal arms

Isolated H2 clouds and tidal dwarf galaxies (TDGs) in multiple mergers.

(Bekki et al. 2014)

T=2.1 Gyr

H2

Newstars

T=2.1 Gyr

2.8

2.8 (1) H2-rich TDGs ?(2) Isolated H2 clouds in collapsing groups ?

TDG

ESp

H2 in TDGs

No TDGs

140 kpc

5 Sp E + SP E(Rmol =0.1 0.5-1)

Implications

• Molecular fractions should be significantly higher in small groups with interacting/merging galaxies.

• TDGs should be much more H2-rich than normal dwarfs ( ALMA targets ?).

• Intra-group Isolated H2 clouds may exist ( ALMA targets?).

• Abundant dust can be found in intra-group gas.

Dust distribution ina gas-rich group

E

Sp

2.8 Gyr

140 kpc

(2) Ram pressure stripping (RPS) and tidal fields in large group & small clusters.

.

• Ram pressure stripping of halo and disk gas can be effective in groups A mechanism for `strangulation’ (e.g., Kawata & Mulchaey 2008)

• Group tide can be responsible for morphological transformation of small dwarfs (e.g., Meyer et al. 2001).

Gas stripping in groups

• Gas stripping Gradual SF suppression (Larson et al. 1980, Balogh et al. 2000, Bekki et al. 2002; McCarthy et al. 2008)

(Kawata & Mulchaey 2008)

Disk (Face-on)

Disk (Edge-on)

DM distribution in a group (M=8*1012 Mo)

Gas density contour

Z=0.51 Z=0

SF enhancement in the stripped clumps

(Roediger et al. 2014)

Gas SFR

83 Myr

183 Myr

• Stripping of diffuse gas: The stripping efficiency depends on Vrel and IGM (Abadi et al. 1999; Quills et al. 2000; Vollmer et al. 2006; Tonnesen & Bryan 2008).

• Enhancement of SF formation (e.g., Bekki & Couch 2003; Kronberger et al. 2008; Mastropietro et al. 2009).

• How do the two ram pressure effects (i.e., stripping + SF enhancement) depend on galaxy masses ?

• How does ram pressure influence the HI and H2 fractions of galaxies in groups and clusters ?

Two key questions:

2.1 Outside-in truncation of SF in disks under ram pressure stripping (RPS).

(Bekki 2014)

Gas distribution

70 kpc

SFR density

Group (IGM)

Disk

Mgr=1014 Msun,Rvir=1.2 Mpc,T=2.6*107K,FIGM=0.15,Mdisk=6*1010 Msun,MW-type galaxy

IGM

RPS effects dependent on galaxy mass

• RPS is more effective in less massive disk galaxies: SF is more likely to be quenched in dwarf disks.

M=1011 Msun,M33-type

M=1010 Msun,SMC-type 15 kpc

33 kpc

70 kpc

M=1012 Msun,MW-type

For the same environment,orbits,inclination….

Stellar disk size

Enhancement of H2 formation in disks under moderately strong ram pressure.

• High-density gaseous regions can be formed in the inner disks owing to compression of gas by ram pressure of IGM, but only for a short timescale.

M(H2)Isolated

RPS

(Bekki et al. 2014)

H2

HI

Rmol(=M(H2)/M(HI)) increase: 0.10.3

HI clumps

StrippedH2 clump

53 kpc

HI

H2

T=0.4 Gyr T=1.4 Gyr

H2 distributions look clumpier than HI before and during RPS.

IGM flow

Significant Rmol increase in disks after RPS.

• Main reason is that the outer HI-rich gas disk can be preferentially stripped (while the inner H2-rich gas remains intact).

(Bekki et al. 2014)

Evolutionary direction

For 16 disk modelsafter ~ 3 Gyr evolution(under RPS) in groups/ small clusters

Initial disk (Field)

A correlation between HI-to-star-size ratios and M(HI)/Ms?

• Selective stripping of outer HI gas by RPS Outside-in truncation of SF in massive groups/small clusters.

For 16 disk modelsafter ~ 3 Gyr evolution(under RPS) in groups/ small clusters

Initial disk (Field)

Evolutionary direction

(Bekki et al.2014)

Implications• The observed flat HI mass function for less

massive galaxies in groups (e.g., Kilborn et al. 2009) can be understood in terms of `selective stripping’ of HI gas from less massive galaxies. However we need to estimate more quantitatively the evolution of the HI mass function in future theoretical studies.

• H2 formation from HI on dust grains can not continue to be efficient owing to the HI-deficient disks HI-deficient galaxies are likely to be H2-deficent too.

• The projected distributions of SF regions in disk galaxies can be diverse after RPS.

Characteristic 2D distributions of H

(e.g., Broken ring, crescent-shape, arcs etc... in disks under RPS)

(Bekki 2014)

(2b) Group tide (+slow galaxy encounter)S0 formation via repetitive slow galaxy interaction.

(Bekki & Couch 2011)

Group member galaxy

Mgr=2*1013 Msun, Rvir=0.54 Mpc, Rperi=4rs, Ngal=87.

S0 formation via repetitive slow galaxy interaction.

(Bekki & Couch 2011)

(2b) Group tide (+slow encounter)

• Basic roles: Morphological transformation from spirals into S0s (Bekki & Couch 2011) and disk thickening (Villalobos et al. 2012).

• Gas stripping and intra-group HI formation.• Enhancement of H2 formation (this work).• Unlike RPS, group tide (+interaction) can not

cause rather small RHI/Rs (<1), because both stars and gas can be efficiently stripped.

(3a) Rich cluster environment: High-speed

encounter• Galaxy harassment (multiple high-speed

encounters + cluster tide) can transform low-mass late-type disks (Sd/Im) into dwarf spheroids and ellipticals (Moore et al. 1996, 1994).

• What is the HI/H2 evolution in harassed galaxies ?

(Moore et al .1996)

Simulation Observation

Time sequence

Sporadic enhancement of H2 formation in dwarf disk galaxies.

M(H2)

Rmol

(A) These epochs correspond to(i) when a galaxy passes through its orbital pericenter or (ii) when it interacts with a more massivegalaxy.

Isolated

Tide & interaction

Mcluster=1015 Msun,Rvir=2.6 Mpc, rapo=0.8 MpcNgalaxy~1000Mdisk=6*108 Msun,SMC-type galaxy

(B) Rmol is significantly lower than MW-type disk galaxies owing to low dust abundances. Still gas within the disk !

Complete stripping of HI and H2 gas by ram pressure in rich clusters

• If dwarf disk galaxies are located within 0.3-0.5Rvir of a rich cluster, then they will lose all gas within 1-2 Gyr (before morphological transformation).

Implications

• Although galaxy interaction/cluster tide can be responsible for the morphological transformation from Sd/Im into dE/dSph, they alone can not completely truncate SF. Ram pressure stripping would be required to explain `dead’ dE/dSph.

• Some low-mass disks first become red/dead disks (`passive spirals’) owing to gas stripping by ram pressure, and then are transformed into dE/dSph (or S0s) via high-speed galaxy interaction/cluster tide: Color evolution first, morphological transformation second.

(3b) Suppression of disk-rebuilding in early-type galaxies.

(6 Gyr Evolution of merging pairs: Mihos 2003, 2004)

Post-merger evolution in fields and clusters:Stripping of tidal debris by cluster tide Suppression of disk rebuilding ?

Field Cluster0.9 Mpc

Ram pressure stripping of cold HI streams around E/S0s.

• Gaseous tidal streams around E/S0s (formed from merging) can be converted into numerous `high-velocity’ clouds.

T=0 Gyr1.1

2.8

0.6

1.7 2.3

Field

Cluster

IGM

Gas clumps

140 kpc

IGME

Rvir

2.8Gyrevolution

Effect of RPS of groups on cold gas streams around an elliptical galaxy

2.8Gyrevolution

Effect of RPS of the MW-type galaxy on a BCD-like dwarf that is infalling onto the galaxy after strong tidal galaxy interaction.

Implications

• Masses, sizes, morphologies, kinematics, and detection probabilities of low-density hydrogen gas around E/S0s can be quite different in different environment (in particular, kinematical differences should be investigated, e.g., by SAMI etc).

• HI gas streams of satellite galaxies around MW and M31 can be broken into many high-velocity clouds via ram pressure effects, if the HI streams form before their accretion onto the Local Group.

Conclusions

• Galaxy interaction and merging in small/compact groups can enhance (at least temporarily) the H2 formation efficiency thus the fraction of molecular hydrogen (Rmol) in disk galaxies.

• Ram pressure stripping can significantly increase Rmol due largely to the HI gas stripping from the outer parts of galactic gas disks.

• Environmental effects can cause the changes of galaxy locations on the Rmol-MHI/Ms and Rmol-RHI/Rs diagrams.

Conclusions

Observation (Boselli et al. 2014)

Simulated disks under RPS

Zone of avoidance (?)

(Bekki et al.2014)

Too high to beconsistent withsimulations

Conclusions

• Group/cluster environments can suppress the rebuilding process of low-density gas disks around early-type galaxies (owing to tidal effects and ram pressure stripping).

• The cold HI streams around merger remnants and interacting galaxies can be transformed into

numerous compact (high-velocity) clouds in groups/clusters.