Cinthya Herrera (NAOJ) ASTE/ALMA Development Workshop 2014, June 18th, 2014
Galaxies interactions... Key in hierarchical model of galaxy formation and evolution (e.g., Kauffmann et al. 1993) Most massive galaxies have experienced a major merger They trigger bursts of star formation (IR luminous sources, Sanders & Mirabel 1996)
Galaxies interactions... Key in hierarchical model of galaxy formation and evolution (e.g., Kauffmann et al. 1993) Most massive galaxies have experienced a major merger They trigger bursts of star formation (IR luminous sources, Sanders & Mirabel 1996) Spitzer (IRAC)
Galaxies interactions... Most of the star formation occurs in extreme form: super star clusters (SSCs, HST observations, Whitmore 2003). R136 in 30Dor SSCs are likely the progenitors of globular clusters (e.g., Ashman & Zepf 2001): they are compact (a few parsec size) and massive (>10 5 M ), enough to lead to the formation of globular clusters.
The Antennae galaxy merger: well studied template @22 Mpc Overlap region: Where the 2 galaxies collide! Age < 6 Myr 6 Myr < Age < 9 Myr 9 Myr < Age < 13 Myr 13 Myr < Age Mengel et al. 2005
Galaxies interactions... Most of the star formation occurs in extreme form: super star clusters (SSCs, HST observations, Whitmore 2003). R136 in 30Dor SSCs are likely the progenitors of globular clusters (e.g., Ashman & Zepf 2001): they are compact (a few parsec size) and massive (>10 5 M ), enough to lead to the formation of globular clusters. It is not clear yet how galaxy interaction triggers gas collapse and the early phases of star formation
Nearby galaxy mergers are ideal sources to understand this process. Bulk of the molecular gas: CO line emission Single dish observations Probe the total flux, and the gas distribution ASTE and kinematics at large scale structures. ~20 beam: 2kpc at 20Mpc 16 @NRO Kaneko et al. 2013 Interferometric observations ALMA Probe giant molecular clouds and substructures within them ALMA Cycle 2: Band 3 CO(1-0) ~0.4. 40pc at 20 Mpc Band 6 CO(2-1) ~0.2. 20pc Band 7 CO(3-2) ~0.1. 10pc
In the prototypical Antennae galaxy merger: At large scales, CO gas is found in Super Giant Molecular Complexes (few 10 8 M ) CO(1-0) At higher resolutions, CO gas is clumpy (few 10 7 M ) CO(3-2) OVRO Wilson et al. 2000 SMA Ueda et al. 2012 Ωbeam = 3.3x4.9.. (340x520 pc) Ωbeam = 1.4x1.1.. (150x120 pc) rms= 55 mjy/beam rms = 38 mjy/beam CO gas is correlated with youngest and more massive star clusters.
The Antennae Galaxies seen by ALMA CO(3-2) OVRO Wilson et al. (2000) Science Verification Cycle 0 PI. B. Whitmore Ω beam = 1.08x0.62.. Ω beam = 0.65x0.38.. rms=3.3 mjy/beam rms=1.3mjy/beam
Formation of SGMCs Image: H2 VLT/ SINFONI Contours: CO(3-2) ALMA SV Herrera et al. (2012) Combine tracers of gas mass and turbulent energy dissipation CO traces the molecular mass and kinematics in galaxies Most of the H 2 in the Antennae overlap region is shock-excited and traces the dissipation of mechanical energy (Herrera et al. 2011)
Formation of SGMCs H2 CO Similar global kinematics SGMCs are not virialized (α vir = 2 EK/Eg a factor of a few) Formation of turbulent SGMCs is driven by the large scale dynamics. SGMC 1 SGMC 2 SGMC 6 SGMC 1 SGMC 2 SGMC 6 SGMC 4/5 SGMC 4/5 For SGMC 2 Herrera et al. (2012) L H2 = 5x10 6 L and FWHM = 150 km s -1 L H2 =3/2 f H2 Ṁ 2 v Ṁ = 20 (0.25/f H2 ) M yr 1 The gas accretion rate has the required magnitude to drive turbulence and form SGMC 2 on a time in agreement with the time scale (10 Myr) over which the large scale dynamics has been driving the convergent flow (Renaud et al. 2008)
Formation of SSCs Theory: Massive clusters form in mergers because of shocks and high turbulent pressure favors the formation of bound clusters (Elmegreen & Efremov 1997) Numerical simulations: Large-scale compression of the gas can trigger gravitational instabilities and the formation of molecular clouds (Teyssier et al. 2010). The tidal forces in the Antennae are compressive (Renaud et al. 2008, 2009). Observations: Two populations of GMCs in the Antennae. Smaller, less massive clouds reside in more quiescent areas, while larger, more massive clouds cluster around regions of intense star formation where pressure is high (Wei et al. 2012). Only ALMA gives us the required angular resolution to identify the primordial molecular clouds
Formation of SSCs SGMC2 1350 1540 km/s Located in the SGMC2, where the velocity gradient is the steepest, at the interface of blue and red shifted gas Localized mechanical energy dissipation Image: H2 Contours: CO 1540 1750 km/s Contours: H2 1-0 S(1) Blue and red CO shifted gas H 2 and CO lines have similar line profiles. Herrera et al. 2012 Source is compact (~50 pc), and massive (~5x10 6 M ) Very bright in H 2, L H2 = 3 10 6 L No star formation (no Brγ nor radio continuum counterpart) The cloud luminosity can be accounted for with a gas accretion rate of 10 M /yr, yielding a formation time scale of ~1 Myr (a crossing time).
Caveat with H2 observations: high dust extinction and seeing limited. Another proxy to trace the energy dissipation? Other shock tracers observable with ALMA! IC342 SiO is observed as shock tracers in molecular cloud in nearby galaxies (Garcia-Burillo et al. 2000, Usero et al. 2006). We can extend this to mergers! (Flower& Pineau des Forets 2003) MHD shock models show that SiO and H 2 are correlated The finding of SiO (or HNCO or CH 3 OH) will be indubitable evidence of shocks at scales of SGMCs and substructures within them
Summary Molecular gas is correlated with young and massive SSCs Observed molecular clouds have enough mass to form a SSC How do the large-scale dynamics of galaxy interactions trigger star formation on much smaller scales? Large scale dynamics triggers large scale gas compression and drives the formation of turbulent star-forming SGMCs. Star formation occurs in giant molecular complexes where the gas turbulent kinetic energy is being dissipated allowing the gas to form bound clouds. We need more observations of the molecular gas (CO J-higher transitions, denser gas, etc) in order to better characterize (and model) the molecular gas at scales of GMCs. We need to increase statistics on the formation of SSCs. ALMA is today the only instrument which gives us the needed angular resolution to identify the primordial clouds of SSCs Several ALMA proposals in Cycles 0, 1 and 2: upcoming science!
Need to probe SGMC and SSC formation by looking other sources First step: single dish observations to study the distribution and kinematics of SGMCs Goals for single dish observations: 1.- Determine if cold molecular gas is more turbulent in the overlap than in nuclei 2.- Velocity gradient along the interaction? 3.- Is the gas dense enough to form SGMCs? South: ASTE North: NRO45m