4 Robustness of Q$\mathsf{_{b}}$ in the high-redshift universe

Block et al. (2001) have suggested that a physically meaningful classification system for high-redshift galaxies may be more easily devised at rest-frame infrared wavelengths, rather than in the optical regime. Sub-mm observations indicate that at least some of these systems are heavily obscured by dust (Sanders 1999). Lessons from our local Universe are that optical morphologies can be radically different from near-infrared ones; some optically flocculent galaxies, for example, may have beautiful grand design stellar disks when examined at K. The decoupling of gaseous and stellar disks can be dramatic (see e.g., Puerari et al. 2000; Elmegreen et al. 1999; Grosbøl & Patsis 1998; Block et al. 1994). Block et al. (2001) show that Fourier spectra may be generated on simulated Next Generation Space Telescope (NGST) rest-frame Kpost-stamp FITS images which may be as small as 1'' on a side.

Figure 9: A simulated NGST image of the galaxy NGC 922 as it would appear at a redshift of z = 1.2, in its rest-frame K-band. The square post-stamp FITS image measures $\sim$2'' on a side. The image is recreated from a groundbased K-band image obtained at the NASA Infrared Telescope Facility, following the simulation methodology of Takamiya (1999). At a redshift of z = 1.2, the characteristic "bar signature'' (indicated by the pattern of "four dots'' as in Fig. 2) is still found in a robust manner, even though by eye the bar region has suffered considerable degradation as a result of limited spatial resolution. In recreating this simulated NGST M-band (4.7 $\mu$m) image, an exposure time of one-hour is assumed. For further details, see Block et al. (2001), where a groundbased K-band image of NGC 922 may also be found.

With the tremendous importance in attempting to bridge the low and high-redshift universe from a morphological point of view, it is natural to enquire how robust our bar torque method is as we degrade the quality of the images. First indications are that $Q_{\rm b}$ is surprisingly robust in the presence of noise and limited spatial resolution.

Recreated rest-frame K-band NGST images of the galaxies NGC 2997 and NGC 922 when moved out to redshifts of z = 0.7 and z = 1.2, are presented in Block et al. (2001). We have applied our $Q_{\rm b}$ method to NGC 922, which may serve as an excellent morphological interface between the low and high redshift universe (see Block et al. 2001 for full details).

The results are indicated in Fig. 9. Even in the presence of significant image degradation of NGC 922 when this galaxy is moved out to a redshift of z=1.2 (the eye now cannot easily distinguish the boundaries of a bar, see Fig. 9) - the four locations where the ratio of the tangential force to the mean axisymmetric radial force reaches a maximum (in modulus) may readily be identified. NGC 922 in the groundbased as well as in the simulated image (Fig. 9) belongs to bar class 2. Further work on moving each galaxy in Table 1 out to $z\sim 1$ (L and M band imaging) as well as to $z\sim 3$(simulated N-band imaging) at rest-frame K-band, with NGST, are in progress. This would yield a statistically complete sample upon which to test the robustness in preservation of bar torque class with increasing z.