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2.1. Emission lines

The detection of high-redhifted (z > 2) millimeter CO lines in the hyperluminous object IRAS 10214+4724 (z = 2.28, Brown & Vanden Bout 1992, Solomon et al. 1992), has opened a new way of research to tackle the star formation history of the Universe. Although the object turned out to be highly gravitationally amplified, it revealed however that galaxies at this epoch could have large amounts of molecular gas, excited by an important starburst, and sufficiently metal-enriched to emit detectable CO emission lines. The latter bring fundamental information about the cold gas component in high-z objects and therefore about the physical conditions of the formation of galaxies and the first generations of stars. At high enough redshifts, most of the galaxy mass could be molecular. The main problem to detect this molecular component could be its low metallicity, but theoretical calculations have shown that in a violent starburst, the metallicity could reach solar values very quickly (Elbaz et al. 1992).

After the first discovery, many searches for other candidates took place, but they were harder than expected, and only a few, often gravitationally amplified, objects have been detected: the lensed Cloverleaf quasar H 1413+117 at z = 2.558 (Barvainis et al. 1994), the lensed radiogalaxy MG0414+0534 at z = 2.639 (Barvainis et al. 1998), the possibly magnified object BR1202-0725 at z = 4.69 (Ohta et al. 1996, Omont et al. 1996a), the amplified submillimeter-selected hyperluminous galaxies SMM02399-0136 at z = 2.808 (Frayer et al. 1998), and SMM 14011+0252 at 2.565 (Frayer et al. 1999), and the magnified BAL quasar APM08279+5255, at z = 3.911, where the gas temperature derived from the CO lines is ~ 200K, maybe excited by the quasar (Downes et al. 1999a). Recently Scoville et al. (1997b) reported the detection of the first non-lensed object at z = 2.394, the weak radio galaxy 53W002, and Guilloteau et al. (1997) the radio-quiet quasar BRI 1335-0417, at z = 4.407, which has no direct indication of lensing. If the non-amplification is confirmed, these objects would contain the largest molecular contents known (8 - 10 x 1010 Msun with a standard CO/H2 conversion ratio, and even more if the metallicity is low). The derived molecular masses are so high that H2 would constitute between 30 to 80% of the total dynamical mass (according to the unknown inclination), if the standard CO/H2 conversion ratio was adopted. The application of this conversion ratio is however doubtful, and it is possible that the involved H2 masses are 3-4 times lower (Solomon et al. 1997).

Figure 1

Figure 1. H2 masses for the CO-detected objects at high redshift (stars), compared to the ultra-luminous-IR sample of Solomon et al. (1997, open pentagons), to the Coma supercluster sample from Casoli et al (1996, filled triangles), and to the quasar 3c48, marked as a filled dot (Scoville et al. 1993, Wink et al. 1997). The curve indicates the 3sigma detection limit of I(CO) = 1 K km/s at the IRAM-30m telescope (equivalent to an rms of 1mK, with an assumed DeltaV = 300km/s). Note the absence of detected objects between 0.36 and 2.2 in redshift, where the sensitivity is insufficient, and the gravitational lenses maybe not yet frequent enough to compensate. The points at high z can be detected well below the sensitivity limit, since they are gravitationally amplified.

The CO line detections at high z up to now are summarized in Table 1, and the molecular masses as a function of redshift are displayed in Fig. 1 It is clear from this figure that our present sensitivity prevents detection of CO lines above a redshift of 0.4, unless the objects are lensed; but this will rapidly change with the new millimeter instruments planned over the world (the Green-Bank-100m of NRAO, the LMT-50m of UMass-INAOE, the ALMA (Europe/USA) and the LMSA (Japan) interferometers). It is therefore interesting to predict with simple models the detection capabilities, as a function of redshift, metallicity or physical conditions in the high-z objects. In particular, it would be highly interesting to detect not only the few exceptional amplified monsters in the sky, but also the widely spread normal galaxy population of the young universe. A previous study of galaxies at very high redshift (up to z = 30) by Silk & Spaans (1997) concluded that CO lines could be even more easy to detect than the continuum; The models presented in section 4 do not agree with this conclusion.

Today galaxies are detected in the optical up to z = 6, when the age of the universe is about 5% of its age, or 1010 yr in a standard flat universe model. For larger redshifts, it is likely that the total amount of cumulated star formation is not a significant fraction of the total (e.g. Madau et al. 1996). However, it is of overwhelming interest to trace the first star-forming structures, as early as possible to constrain theories of galaxy formation.

Table 1. CO data for high redshift objects

Source z CO S DeltaV MH2 Ref
    line mJy km/s 1010 Msun  

F10214+4724 2.285 3-2 18 230 2* 1
53W002 2.394 3-2 3 540 7 2
H 1413+117 2.558 3-2 23 330 2-6 * 3
SMM 14011+0252 2.565 3-2 13 200 5* 4
MG 0414+0534 2.639 3-2 4 580 5* 5
SMM 02399-0136 2.808 3-2 4 710 8* 6
8C1909+722 3.532 4-3 2 530 4.5 7
4C60.07 3.791 4-3 1.7 1000 8 7
APM 08279+5255 3.911 4-3 6 400 0.3* 8
BR 1335-0417 4.407 5-4 7 420 10 9
BR 0952-0115 4.434 5-4 4 230 0.3* 10
BR 1202-0725 4.690 5-4 8 320 10 11

* corrected for magnification, when estimated
Masses have been rescaled to H0 = 75km/s/Mpc. When multiple images are resolved, the flux corresponds to their sum
(1) Solomon et al. (1992), Downes et al. (1995); (2) Scoville et al. (1997b); (3) Barvainis et al. (1994, 1998); (4) Frayer et al. (1999); (5) Barvainis et al. (1998); (6) Frayer et al. (1998); (7) Papadopoulos et al. (1999); (8) Downes et al. (1999a); (9) Guilloteau et al. (1997); (10) Guilloteau et al. (1999); (11) Omont et al. (1996a)

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