C 368 z1132

Figure 8.6. This montage of Hubble images shows radio galaxies at a range of redshift, all to the same angular scale and all in the emitted near-ultraviolet. Low-redshift objects are normal ellipticals, while high-redshift examples are less symmetric and preferentially aligned with the radio source axes. (Data from the NASA/ESA Hubble Space Telescope archive, with observations originally obtained by P. Best and M. Longair.)

might suspect this process in the early Universe, so that AGN would help start galaxy growth if not formation, and it has been proposed to explain the alignment effect seen in radio galaxies at high redshift.

In the local Universe we do not see any particular alignment between the direction of radio jets emerging from active galaxies and the apparent shape outlined by the galaxies' starlight. This changes as we look to high redshifts (Figure 8.6), so that for z > 0.4 the radio and optical axes are more often nearly parallel (hence "alignment effect"). At the highest redshifts, radio galaxies can be very elongated and lumpy as seen in the continuum, again well aligned with the radio source. This effect seems to mix several causes. One is purely observational. At high redshifts, we are often observing at shorter emitted wavelengths, into the ultraviolet, where recent star formation and scattering by dust grains are both more important than in visible light. Indeed, local radio galaxies can appear more "aligned" when seen in the ultraviolet. Still, we find that the ultraviolet light from radio galaxies is better aligned at high redshifts, even when these biases are taken into account. Possible reasons might be that the radio source induces star formation as it emerges and continues to emanate

Figure 8.7. Hubble image of Minkowski's Object (left), in which star formation is apparently being triggered by interaction with a jet from the radio galaxy NGC 541 (left). Numerous bright star clusters appear in the disrupted gas-rich system; the radio jet is deflected and loses collimation at this same point. (Data from the NASA/ESA Hubble Space Telescope archive, originally obtained by Stefi Baum.)

Figure 8.7. Hubble image of Minkowski's Object (left), in which star formation is apparently being triggered by interaction with a jet from the radio galaxy NGC 541 (left). Numerous bright star clusters appear in the disrupted gas-rich system; the radio jet is deflected and loses collimation at this same point. (Data from the NASA/ESA Hubble Space Telescope archive, originally obtained by Stefi Baum.)

from the inner regions, that the radio source is most easily produced along a particular axis of a nonspherical young galaxy, or that we're not seeing starlight but light from the active nucleus, scattered from surrounding dust and seen where the radiation emerges most easily along the axis of escaping radio plasma. Triggered star formation remains a possibility in many instances of this effect.

While there has been considerable speculation on the role of AGN in triggering starbirth, there is less in the way of systematic data. There are a few local examples in which the process seems to be ongoing as we watch. In the nearest moderately powerful radio galaxy, Centaurus A or NGC 5128, we see supergiant stars and ionized gas tracing one side of the radio jet as it leaves the dense part of the galaxy, things that are not seen elsewhere so far from the center (even given that there is plenty of gas and star formation in the prominent dust lane). The association certainly suggests that the radio jet has something to do with these short-lived stars. On a grander scale, the radio source in NGC 541 (a radio galaxy in the cluster Abell 194) points directly to a bright blue galaxy known as Minkowski's Object. This object, oddly for a cluster member, shows a high rate of star formation and irregular morphology (Figure 8.7). Furthermore, the radio source seems to know about Minkowski's Object, being deflected and broadened on passing it. These observations have led several groups to suggest that star formation in this case was brought on as the radio jet reached the interstellar medium of a dwarf galaxy, compressing much of its gas into gravitational collapse.

0 0

Post a comment