4.3. Discussion and conclusions

The implication of the HST/PC components of 53W002 are discussed here in combination with clues from other recently published ground-based and HST data.

4.3.1. Nature of the alignment effect in 53W002

A recent interferometric OVRO image of 53W002 in redshifted CO (J 3-2) - which has 3" FWHM - provides an important clue to the nature of the alignment effect (Scoville et al. 1997; S97). CO was detected ~ 2-3" away on both sides of 53W002's AGN, and in the same direction as both blue HST clouds and the extended 8.4 GHz radio source, but not perpendicular to this direction. Since the CO extends further in both directions than the two aligned blue clouds and the currently visible extended radio source (see Fig. 3 of S97), the CO was likely deposited there by physical processes related to the jet - possibly through a previous incarnation of the jet. Since carbon and oxygen had to be formed in massive stars, jet-induced star-formation thus likely played a role at some stage in the evolution of 53W002. Its overall r^1/4-like stellar population is extended in the same direction as the radio source, so that the jet possibly triggered a non-negligible fraction of 53W002's mass to form stars in these two directions. As long as this all happened within a few x 10⁸ years, there would have been just enough time for the stellar population to settle into a r^1/4-like profile (vA82). Together with the continuum and Ly morphology of the blue clouds (4.2.4 & Fig. 8), it thus appears that both reflection cones from an AGN and jet-induced star-formation are responsible for the alignment effect in 53W002.

The structure of 53W002 may be compared to the HST images of powerful radio galaxies of Lo95 and B97. From a set of eight 3CR radio galaxies at z 1, they suggest that the morphologies change systematically with (projected) radio source size, and interpret this as evidence for jet-induced star formation in the aligned component. The weak radio galaxy 53W002 shares both properties with these powerful 3CR sources - a compact component identified with the AGN in the center of an extended starlight distribution, and a pair of clouds aligned with the projected radio axis. Contrary to many of these luminous 3CR sources, we can already see a fairly relaxed symmetric distribution of starlight centered around 53W002's nucleus. In hindsight, it is perhaps surprising to find the aligned component to be important even at these low radio powers, but the continuity of structure with the powerful radio galaxies is striking, and required the extra resolving power of the WFPC2/PC in the B-band to observe in detail.

4.3.2. 53W002's gas+dust content, its surrounding cluster, and possible evolution

The measured Ly fluxes and broad-band UBVRIgriJHK colors of 53W002 constrain its dust absorption (A_V 0.2 mag) and star formation rate (SFR; W91, W94b). Similar arguments have been made by P96b for the other 17 surrounding blue z 2.4 candidates. The SFR of 53W002 is of order ~ 100 M / year (W91), and 5-10x less for the other z 2.40 candidates (P96b). The total stellar mass of 53W002 - integrated over its assumed exponentially declining SFR - is 1.8 x 10¹¹ M (W91). The r^1/4-like light-profile of 53W002 suggests that at z = 2.39 the object had already converted a non-negligible fraction of its gas mass into stars - rather efficiently on a ~ 0.4 Gyr time-scale - suggesting a young early-type galaxy. The OVRO CO-flux of 1.5 Jy/km s^-1 implies ~ 2.1 x 10¹¹ M in gas around 53W002 alone (S97). The velocity widths of the CO clouds are ~ 250 km s^-1(HWHM), extending ~ 1".5 or ~ 13 kpc on each side of 53W002 in the direction of the blue clouds and of the extended radio source, and possibly indicating a forming rotation curve (S97). These numbers imply an enclosed Keplerian mass of 1.5-3.8 x 10¹¹ M, consistent with its total stellar mass above (W91). Taken together, this means that 53W002's H₂(+CO) gas-mass could be ~ 30-60% of its total (luminous+gas+dark) matter.

What could this mean for the evolution of 53W002? If all this gas settled into disk stars within a few free-fall times (~ 1 Gyr), 53W002 could evolve into an mid-type spiral galaxy today (with B/D-ratio ~ 0.5), or into an earlier-type galaxy (B/D 1) if most of the gas was used up during the initial starburst, and/or if a substantial fraction of the gas remained neutral (as seen in some nearby ellipticals and merger remnants, cf. H95a). The small velocity dispersion ( 300 km s^-1) in P96b's group of z 2.4 candidates with measured redshifts, and the small area ( 1 Mpc²) over which the 17 sub-galactic z 2.4 candidates surrounding 53W002 are seen, suggests that many of these objects will likely merge into a few larger galaxies during the next half Hubble time after z = 2.4. Hence, while 53W002 may have formed as a r^1/4-dominated galaxy during a relatively quick and sudden collapse that started at z 3 (or ~ 0.4 Gyr before z = 2.4) - possibly induced by star-formation along its radio jet - it appears to be also developing a massive disk at z 2.4 (S97). This disk may have completely settled ~ 1-2 Gyrs later (or at z 1.5), but possibly be destroyed again during future mergers (at z 1.5) with the surrounding sub-galactic sized z 2.4 candidates (P96b), so that 53W002 may end up as a massive early-type galaxy today. 53W002 may thus provide important clues as to how the luminous nearby early-type (radio) galaxies could have formed and evolved.