Rich cluster [M ~ M sun ] Large groups (Small clusters) [M ~ M sun ] Environmental effects on neutral and molecular hydrogen of galaxies Small/compact groups [M < M sun ] Kenji Bekki (ICRAR at UWA, Australia) (From APOD)

Outline: five key physical processes in HI/H 2 evolution of galaxies. The relative importance of these processes is different in different environments, so, how do these influence the HI/H 2 evolution of galaxies ? [ M sun ] Mass-scale Size-scale [10 14 M sun ][10 15 M sun ] 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 R mol (=M(H 2 )/M(HI)), HI/H 2 disk sizes, and HI-to-star-mass ratios. Why is R mol so different in different environments ? (Boselli et al. 2014) HI-deficient galaxy HI-normal galaxy

H 2 formation on dust grains and formation/destruction processes of dust in galaxy-scale simulations (1) H 2 formation on dust grains H H2H2 (2) Dust growth and destruction Dust particle Growth Destruction Formation SNe Metal Simulation (Bekki 2014) (Gould & Salpeter 1963)

(1)The roles of galaxy interaction/merging in HI/H 2 evolution of small/compact groups. How does galaxy interaction and merging change the HI/H 2 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 in HI/H 2 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 H 2 formation in interacting disk galaxies. (Bekki 2014) Companion Gas disk Stellar Disk size T=0 (Gyr) M(H 2 ) R mol 210 kpc A factor of 3-5 enhancement depending on orbital configurations and mass-ratios of two galaxies.

Evolution of HI/H 2 in interacting galaxies. HI H2H2

HI/H 2 distributions in interacting disks. T=0.3 Gyr T=0.3 Gyr 0.8 HIH2H2 (Bekki 2014) More clumpy distribution H 2 formation in tidal arms

Isolated H 2 clouds and tidal dwarf galaxies (TDGs) in multiple mergers. (Bekki et al. 2014) T=2.1 Gyr H2H2 New stars T=2.1 Gyr 2.8 (1) H 2 -rich TDGs ? (2) Isolated H 2 clouds in collapsing groups ? TDG E Sp H 2 in TDGs No TDGs 140 kpc 5 Sp  E + SP  E (R mol =0.1  0.5-1)

Implications Molecular fractions should be significantly higher in small groups with interacting/merging galaxies. TDGs should be much more H 2 - rich than normal dwarfs (  ALMA targets ?). Intra-group Isolated H 2 clouds may exist (  ALMA targets?). Abundant dust can be found in intra-group gas. Dust distribution in a 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*10 12 M o ) Gas density contour Z=0.51Z=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 V rel 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 H 2 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 M gr =10 14 M sun, R vir =1.2 Mpc, T=2.6*10 7 K, F IGM =0.15, M disk =6*10 10 M sun, 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=10 11 M sun, M33-type M=10 10 M sun, SMC-type 15 kpc 33 kpc 70 kpc M=10 12 M sun, MW-type For the same environment, orbits, inclination…. Stellar disk size

Enhancement of H 2 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(H 2 ) Isolated RPS (Bekki et al. 2014)

H2H2 HI

R mol (=M(H 2 )/M(HI)) increase: 0.1  0.3 HI clumps Stripped H 2 clump 53 kpc HI H2H2 T=0.4 Gyr T=1.4 Gyr H 2 distributions look clumpier than HI before and during RPS. IGM flow

Significant R mol increase in disks after RPS. Main reason is that the outer HI-rich gas disk can be preferentially stripped (while the inner H 2 - rich gas remains intact). (Bekki et al. 2014) Evolutionary direction For 16 disk models after ~ 3 Gyr evolution (under RPS) in groups/ small clusters Initial disk (Field)

A correlation between HI-to-star-size ratios and M(HI)/M s ? Selective stripping of outer HI gas by RPS  Outside-in truncation of SF in massive groups/small clusters. For 16 disk models after ~ 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. H 2 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 H 2 - 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 M gr =2*10 13 M sun, R vir =0.54 Mpc, R peri =4r s, N gal =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 H 2 formation (this work). Unlike RPS, group tide (+interaction) can not cause rather small R HI /R s (<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/H 2 evolution in harassed galaxies ? (Moore et al.1996) SimulationObservation Time sequence

Sporadic enhancement of H 2 formation in dwarf disk galaxies. M(H 2 ) R mol (A) These epochs correspond to (i) when a galaxy passes through its orbital pericenter or (ii) when it interacts with a more massive galaxy. Isolated Tide & interaction M cluster =10 15 M sun, R vir =2.6 Mpc, r apo =0.8 Mpc N galaxy ~1000 M disk =6*10 8 M sun, SMC-type galaxy (B) R mol is significantly lower than MW-type disk galaxies owing to low dust abundances. Still gas within the disk !

Complete stripping of HI and H 2 gas by ram pressure in rich clusters If dwarf disk galaxies are located within R vir 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 ? FieldCluster 0.9 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 Gyr Field Cluster IGM Gas clumps 140 kpc IGM E R vir

2.8Gyr evolution Effect of RPS of groups on cold gas streams around an elliptical galaxy

2.8Gyr evolution 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 H 2 formation efficiency thus the fraction of molecular hydrogen (R mol ) in disk galaxies. Ram pressure stripping can significantly increase R mol 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 R mol -M HI /M s and R mol - R HI /R s diagrams.

Conclusions Observation (Boselli et al. 2014) Simulated disks under RPS Zone of avoidance (?) (Bekki et al.2014) Too high to be consistent with simulations

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.