Molecules at high z, Perspectives SAAS-FEE Lecture 8 Françoise COMBES

2 Current state of the art CO emission the 1st detection: Faint IRAS source F at z=2.3 (Brown & van den Bout 92, Solomon et al 92) drove the search for high-z molecules Downes et al 95

3 Then several quasars or hyper-luminous objects in FIR or submm, mostly with high gravitational amplification, followed z between 2.2 and 2.8 (cloverleaf z=2.56) Two objects at z=4.5 BR (Omont et al 96, Ohta et al 96) BR (Guilloteau et al 97) likely to be lensed also (disputed) Objects at 93% of the age of the Universe ==> enrichment occurs at very early epochs All objects first detected in dust emission submm bolometer arrays SCUBA-JCMT, MAMBO-IRAM

4 High J CO lines Advantage with respect to HI Larger flux at high J, if dense and hot medium Difficulty in knowing the line ratios The gravitational amplification might not be the same for all lines!

5 Strategy: search for continuum much easier to detect due to K-correction Even better in front of a rich cluster for amplification SMM sources (Frayer et al 98) Abell 370, z=0.37 (arc at z=0.73)

6 CO lines 2000 update 15 objects 1< z < 5

7 High-z (full dots), ULIRGS (open pentagons, Solomon et al 97) normal galxies in Coma (Casoli et al 96) Lines are 1σ detection with IRAM-30m, with the various receivers (3mm, 2mm, 1.3mm)

8 Extremely Red Objects EROs EROS R-K > 5 identified in searches for primeval galaxies (Elston et al 88) A fraction (10%) of the submm could be EROs (Smail et al 99) Prototypical ERO at z=1.44 HR10 submm detected (Cimatti et al 98) CO lines (Andreani et al 00) CO21 CO54

9 SED of HR10 stellar pop PEGASE Extinction of Calzetti et al (00) Absorbed flux ==> FIR dust model Désert et al (90) from Melchior et al 2001) SED observed for HR10 Dey et al 99 (rest-frame)

10 VLA detection of CO(1-0) ==> times more mass? APM , z=3.91 Papadopoulos et al (2001) SMM objects could be evolving in present day bright E-gal (same comoving density) 1-2 L* Frayer et al 99 dots: local galaxies blue: disk (MW) green: closed-box E-gal red: starburst merger

11 Modelisation of a starburst Common characteristics Very high molecular masses Mo High temperatures of dust: 30-50K up to 100K (200K if QSO?) If demagnified, similar to the ULIRGs at z=0.1 (Solomon et al 97) Very small sizes: below 1kpc (300pc disks) Interferometric mapping (cf Arp220) In these conditions, the average column density is around cm -2 and the dust becomes optically thick at λ < 150μ cf black-body model

12 These high central condensations are supposed to be formed during mergers, when the gas is driven in by gravitational torques Up to 50% of the dynamical mass could be in the form of H 2 in mergers (Scoville et al 1991) The average density is 10 4 cm -3, 100 times denser than in normal disks Clouds themselves are denser, as in Galactic centers (tidal stresses) The medium must be very clumpy, since high-density tracers indicate even higher densities and the ratio of HCN/CO even increases for these objects SED peaks at 100μ with a flux of 1-3 Jy at z=0.1 at z=4-5, a few mJy at 1.2mm (Omont et al 96)

13 Simple two components model The central starburst can be considered as the accumulation of very active star-forming complexes, such as Orion (typically clouds of 700M o each) Total mass M o in 1kpc diameter Two components, both with low filling factor the dense and hot component: star forming cores 10 6 cm -3, 90K each embedded in a cloud 10 4 cm -3, 30K Individual velocity dispersion of 10km/s Embedded in the rotational gradient of the galaxy, 300km/s

14 LVG approximation To compute the excitation of the CO molecules, and the population of the various J levels => very simple approx the turbulence inside the clouds are equivalent to a velocity gradient (LVG) Most of the clouds (which are individually optically thick in the CO lines) are not overlapping on the l.o.s. at a given velocity ==> filling factor fv in velocity space In fact, there might be some overlap, to be taken into account For comparison: also a model of homogeneous medium, T = 50K, density 10 3 cm -3, N(H 2 ) = cm -2

15 Assume same energy coming from stars Black-body T dust 4 - T bg 4 = cste or optically thin dust τ ~ ν 2 T dust 6 - T bg 6 = cste

16 Model Results With the two-component models, almost LTE (high excitation) Not the case for the homogeneous sphere

17 Homogeneous sphere T dust 6 - T bg 6 = cste CO lines Submm continuum

18 T dust 6 - T bg 6 = csteT dust 4 - T bg 4 = cste 2 components 30 & 90K

19 Same for integrated flux more relevant for detection ƒS ν dν (slope in ν 3 ) S ~2 kT /λ 2 dν = ν dV/c Continuum/total CO ratio (full) Continuum/CO(1-0) (dash) index= CO/H 2 abundance dotted: M/3 h: homogeneous model squares: ULIRGs Solomon et al 97

20 Simulations of low metallicity and low CO abundance Then the CO lines become optically thin About 2 orders of magnitude lower fluxes Same density, and excitation necessary to excite the high J Also lower fluxes in continuum (less dust) CO/H 2 =10 -6

21 Comparison with continuum The CO emission has no negative K-correction, in part because of optical thickness ==> much more difficult to observe than dust emission No need to observe at high ν selection rules reduce the observed frequency for T=90K, the dust peaks at ~60μ, and the CO at ~600μ The increase of the background temperature does not help in detecting the CO lines, in spite of the higher degree of excitation Molecular gas at T=T bg always undetectable T A * = (f(T ex )-f(T bg )) (1 - e -τ )

22 Panorama in mm & sub-mm IRAM-30m 707m 2 1mm 10" IRAM-PdB 6x15m=1060m 2 1mm 0.5" NRO 6x10m=509m 2 1mm 0.5" OVRO 6x10m= 509m 2 1mm 0.5" BIMA 10x6m=282m 2 1mm 0.5" => CARMA (in project) 791m 2 1mm 0.5" SMA 7x6m=200m 2 0.3mm 0.1" GBT 100m=7854m 2 2.6mm 7" In project LMT 50m=1963m 2 1mm 6" ALMA 64x12m=7238m 2 0.3mm " EVLA 35x25m=17200m 2 6mm 0.004"

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24 Best strategy to observe a high-z galaxy Depends on excitation Could be at low frequency T=50K, 10 3 cm -3 N(CO)= cm -2

25 HR10, z=1.44 Papadopoulos & Ivison (2001) CO J=5-4 is of low excitation in HR10 R 54 /R 21 = 0.15 Would be difficult to observe at higher z

26 Prediction of source counts Hierarchical theory of galaxy formation Einstein-de Sitter model Ω = 1 (Ho = 75km/s/Mpc, qo=0.5) or Ω =0.3 Λ=0.7 Number of mergers (z) from Press-Schechter assuming self-similarity for P(halo merging) dN/dM ~ M -2 (M/M*) γ/2 exp[ -(M/M*) γ ] Merger rate is the sum of the evolution (derivative) of this function + term maximum at equal mass merger (Blain & Longair 93) => number of mergers at each epoch but efficiency of star formation must also vary considerably with redshift with a peak at z=2

27 Integration over z should equal COBE background To fit source counts: life-time of merger much shorter at high z Once the counts are fit to the submm observations, the model indicates what must be the contribution of the various redshifts to the counts Star formation begins before z=6 The bulk of the contribution is 2 < z < 5 Results depend on the shape of the SF efficiency ==> Observations of redshift distribution should therefore bring a lot of insight in the physics of protogalaxies

28 SFR CIBR Z>5 Z<2 Z>5 Z<2 Data on source counts:Barger et al 99, Blain et al 00 Carilli et al 01

29 Z<2 Z>5 N(>F) F At 2mm wavelength, the dominant contribution is from 2 < z < 5 At 5mm, the dominant is from z > 5

30 Conclusions CO lines will be observed at high z easily with the next generation of the mm instruments Much more information than the continuum, in giving the mass of H 2 in the galaxy, the efficiency of star formation as a function of redshift, the kinematics More unbiased information than in the optical, where the line width does not reflect the total mass (outflows, extinction..) The fraction of gas mass should increase with redshift as well as the fraction of starburst and their efficiency (dynamical time shorter)