Image Credit: NASA,ESA and the Hubble SM4 ERO Team Michelle Cluver

Collaborators Philip Appleton (NHSC/Caltech) Patrick Ogle (SSC/Caltech) Jesper Rasmussen (Dark Cosmology Centre, Copenhagen) Thomas Jarrett (IPAC/Caltech) Ute Lisenfeld (Universidad de Granada) Pierre Guillard (SSC/Caltech) Francois Boulanger (IAS, Orsay) Kevin Xu (NHSC/Caltech) Min Yun (UMass-Amherst) Lourdes Verdes-Montenegro (IAA, Granada) Thodoris Bitsakis (U. Crete) Vassilis Charmandaris (U. Crete)

Galaxy Evolution How do galaxies move from the blue sequence to the red cloud? What are the characteristics of galaxies with intermediate colours and what role does environment play? Credit: T. Gonclaves

How do Galaxies Transform? Schiminovich et al. (2007)

Transformation within Group Environments S0 evolution significantly more dramatic in groups than in clusters (Wilman et al. 2009; Just et al. 2010) ; ram pressure stripping not dominant mechanism Recent simulations show spirals in group environments strongly influenced by repetitive slow encounters, increasing mass of bulges and transforming into S0’s % of stars and gas stripped during this process (Bekki and Couch 2011)

Shapley supercluster (Haines et al. 2011) quenching in S0/a before they reach dense core  late-types are being transformed; quenching occurs before and after transformation to lenticular HI-deficient, disturbed with inefficient ram- pressure stripping seen in NGC 2563 group (Rasmussen et al. 2012) and Pegasus I cluster(Rose et al. 2010)

Why Compact Groups? Evolution is aggressive, but not too much Compact Groups are high density environments, galaxies are strongly interacting Relatively shallow gravitational potential well prolongs gravitational interactions (probe evolution in dense environment)

Transformation in Compact Groups Hickson Compact Groups (HCGs); Hickson et al. (1982), 4+ members, median z ~ 0.03, median σ ~ 200 km/s Galaxies are HI deficient: tidal interactions and ISM stripping lead to gas-poor systems (Verdes-Montenegro et al. 2001) Negligible ram-pressure stripping from hot, tenuous medium: in most HI-deficient groups, diffuse X-rays detected in only 50%, insufficient to remove gas significantly Galaxies appear to be undergoing rapid evolution onto the red sequence (Johnson et al. 2007, Walker et al. 2010)

(also Walker et al – 174 galaxies in 37 HCGs) Only similar in distribution to Coma Infall region Bimodality of dusty/gas-rich and dust- free/gas-poor; suggests rapid evolution Spitzer IRAC colours show tight trend correlating with evolutionary stage Johnson et al. (2007), Walker et al. (2010) 42 galaxies in 12 HCGs Transformation in Compact Groups

Warm Molecular Hydrogen Emission Mid-IR emission from pure rotational H 2 direct detection of H 2 associated with starbursts, (U)LIRGs, AGN Genzel et al. 1998; Rigopoulou et al. 2002; Lutz et al Mechanisms: Far-UV induced pumping and/or collisional heating (PDRs associated with HII regions) hard X-rays heating regions in molecular clouds, H 2 excited through collisions collisional excitation due to acceleration produced by shocks

Stephan’s Quintet: An HCG with dramatic H 2 Line-Cooling High velocity (~ 800 km/s) collision of NGC 7318b with intragroup medium: intergalactic shock wave (~35 kpc) 17.03μm: MJy/sr Powerful, widespread shock- excited H 2 emission (Cluver et al. 2010a)

Stephan’s Quintet: An HCG with dramatic H 2 Line-Cooling Dominates in mid-IR H 2 fits in gap in HI distribution : implies HI converted into hot plasma + H 2 (Cluver et al. 2010a)

Optical (CFHT/Coelum) + X-ray (NASA/CXC/CfA/E.O’Sullivan)

Hubble WFC3 (comp) + Spitzer S(1) H 2 (blue) Image credit: Robert Hurt, Michelle Cluver (SSC)

Origin of H 2 and X-ray emission High-speed collision with a multi-phase medium creates multiple shocks (velocities) Low density HI  hot plasma (X-rays) Denser clumps of HI  forms H 2 Slow MHD shocks (5-20 km/s) excite H 2 Clouds of H 2 are heated by turbulence in the hot gas i.e. the kinetic energy of shock fuels H 2 emission. Molecular gas is continuously excited by supersonic turbulence See model of Guillard et al. (2009)

Ares I-X – bow shock forms collar of water droplets

Is Stephan’s Quintet unusual or just extreme? Spitzer IRS low res spectroscopy (and photometry) of 23 HCGs Intermediate HI depletion with visible signs of tidal interaction in 2+ galaxies  dynamically active Probe evolutionary sequence + connection of SQ Sample covers 74 group members HCG 40 Cluver et al. (2012, in prep)

IRAC Colour Evolution 74 galaxies in 23 groups H 2 enhanced (above star formation) Star Forming Early Types

Molecular Hydrogen Emission Galaxies (MOHEGs) defined using H 2 divided by star formation indicator (Ogle et al. 2010) 9/13 are S0 (pec) type, 2 Sab (pec), 1 Sm

HCG 57A (Sb)– Disk spectrum

H 2 Relative to Warm Dust Emission (24mm) Trend confirms H 2 /PAH result and indicates limited AGN contamination

What is exciting H 2 ? Star formation – ruled out X-rays – ruled out Cosmic Rays – ruled out Shocks What produces shock excitation? AGN jets? Stochastic collisions: Accretion? Viscous Stripping?

Recent GBT + VLA observations reveal extended, faint emission; galaxies with largest HI deficiencies have more massive, diffuse HI component (Borthakur et al. 2010) Protracted gravitational interactions  sea of material + disrupted disks Galaxies pass through debris  stochastic heating + viscous stripping Enhanced, excited H 2 could be result of shock excitation as ISM interacts with tidal material – less energetic version of what we see in Stephan’s Quintet Death by Debris?

Specific Star Formation H 2 -enhancement occurs at intermediate/low specific star formation IRAC colour acts as proxy for sSFR

A Green Valley Connection In dynamically “old” groups ~40% of late-type and ~50% of early-type lie in so-called “green valley” Bitsakis et al. (2010) In “dynamically old” groups >70% of early-types are S0’s

How do you make an S0? Ram Pressure Stripping (e.g. Gunn and Gott 1972) Truncation of gas replenishment (e.g. Bekki 2002) Tidal Encounters (e.g. Icke 1985) Minor merging (e.g. Bekki 1998) Slow encounters in groups  builds bulge mass + gas stripping (Bekki and Couch 2011)

Compact Groups may be key To what extent are galaxies pre-processed in a group environment through: Building bulge-dominated disks Intragroup HI stripping/heating

Group Environment Other H 2 -enhanced show similar location in mid-IR colour Interacting pair, triple, cluster or compact group shown as hollow circles SINGS

Intragroup HI interacting with group galaxies could be common mechanism Disrupted star formation/accretion could produce accelerated evolution (seen in colour-colour plane) Similar to ESO in Norma Cluster? H 2 tail due to ram-pressure stripping (Sivanandam et al. 2010)

GAMA ASKAP -- WALLABY The tidal and dynamical processes influencing the evolution of galaxies in a group environment will likely be key to understanding the role of environment in driving the evolution of galaxies since z > 1. K. Bekki

25B (Sa) 6B (Sab) 15D (S0)

Morphology, Activity 9/13 are S0 (pec) type, 2 Sab (pec), 1 Sm 1 SF spectrum with SF colours (68C) 1 AGN-dominated spectrum (56B) Cluver et al. (2012) 68% HCG galaxies host AGN (Martinez et al. 2010) BUT, low power low-luminosity LINERs or Sy2 (Coziol et al. 2004) ~3% broad-to-narrow-line AGN

Shock excitation could be from gas falling back onto galaxies Velocity dispersion of gas/galaxies? No enhancement in SFR (Iglesias-Paramo+ Vilchez 1999) overall relatively low (Bitsakis et al. 2011) Truncation of SF in early-types (de la Rosa et al. 2007)

H 2 in the IGM HCG 40 HCG 91

Significant Cooling Pathway! Power in shock-driven molecular hydrogen line cooling has implications for models of galaxy mergers gas accretion onto galaxies accretion onto massive halos in early structure formation starburst driven winds (outflows) SNR (U)LIRGS AGN (jet interactions with ISM)

Strong warm H 2 emission systems +/- 30% local 3CR radio galaxies have dominant MIR H 2 (often coupled with weak thermal continuum) - MOHEGS. Mechanical heating driven by jet interaction with host ISM (Ogle et al. 2007, 2010) Seen in central cluster galaxies (Egami et al. 2006, Donahue et al. 2011) Also in filaments in clusters (Johnstone et al. 2007) – “cooling flows” Elliptical galaxies (Kaneda et al. 2008) Taffy Galaxies (Peterson et al., ApJ in press)

LVL: Dale et al. (2009) < 11 Mpc 258 galaxies (dominated by spiral and irregular) HCG: Bitsakis et al. (2011) 135 galaxies in 32 HCGs

HCG 56 HCG 40 HCG 68 HCG 57 HCG 25 HCG 95