Some interesting galaxies

It is now well established that star formation appears to be the dominant energy source in most (70%-80%) luminous infrared galaxies [40,13,29, for example], while the remaining cases would require implausibly luminous radio supernovae to explain the emission through a starburst model [95], and are most likely to be powered by an AGN source instead. Indeed, as many as 50% of such galaxies may contain both AGN and circumnuclear starburst sources [40]. Highly radio luminous starburst galaxies are also likely to be interacting systems, the interaction triggering the burst of star formation [94]. A commonly invoked mechanism for explaining this scenario is angular momentum loss during galaxy interactions which drives gas towards the nucleus, there concentrating and compressing it to initiate a circumnuclear starburst [5]. This is followed by further gas transport into the region of the accretion disk of a central massive black hole, to power an AGN [74,92].

To identify new objects for further study into this relationship and other details of the luminous starburst phenomenon, the most luminous PDS galaxies identified as being possible starbursts were investigated.

Luminous starbursts in the PDS

From the sample of radio sources with known redshifts, the galaxies with radio luminosities greater than 10^23.5 WHz^-1 were examined more closely. For galaxies whose source of radio luminosity is a result solely of a starburst, a radio luminosity this high is extremely bright. There are 44 galaxies (out of the 133 with measured redshifts) with L_1.4>10^23.5 WHz^-1. The majority of these, unsurprisingly, are either quasars (in fact all 9 objects established to be quasars are in this group) or absorption line galaxies (giant ellipticals), and hence their radio luminosity is most likely derived from an AGN. Only 13 of these 44 galaxies show narrow emission lines, indicating the presence of a starburst or Seyfert 2 activity. These are listed in Table 6.1.

A selection of five of these thirteen emission line galaxies are shown (in decreasing order of radio luminosity) in Figures 6.1 to 6.5. These Figures are radio contour images, the contour levels being given in the Figure captions, overlayed on grey-scale optical CCD frames. In each case the coordinates of the relevant object are given in the Figure title, and the PDS catalogue number (see Table A) in the caption.

The spectra for these galaxies are presented in Figures 6.6 to 6.10. The Figures indicate the redshift of the object and also the location of some emission lines. Night sky lines, still present after poor sky subtraction, are indicated by "NS". Note the presence of [O II] and [O III] emission, which are strong indicators of star formation. All five of these objects show [O II] emission, and three show [O III] lines as well. This alone does not confirm the objects are starbursts, as AGNs may show narrow emission lines from ionised oxygen. In fact, the radio morphology indicates that perhaps at least two of these may have their radio emission derived from AGN processes. PDS1001 shows an extended radio tail, typical of an AGN. There is a nearby galaxy within the radio contours of this object, possibly an interacting system. PDS1018 also shows extended radio emission, the warping visible possibly due to radio jets distorted by the passage of the galaxy through the intergalactic medium. However, PDS0946, PDS0729 and PDS0679 all show fairly compact radio sources and nearby optical galaxies. Where there is any distortion of the radio contours it is in the direction of these companions, indicating a possible interaction between the galaxies [19]. Such interactions are associated with initiating starbursts [51] and AGN phenomena in galaxies, so it is possible that this is the situation here. The prodigious radio emission may be due either to the combination of emission from starbursts in both objects, or to an AGN together with a starburst [96]. initiated by the interaction. PDS0679 and PDS1001 are the only two of these five galaxies which appear in the line-ratio diagnostic diagrams, (Figure 5.5). Their line ratios indicate that PDS0679 is possibly a starburst object, while PDS1001 is probably an AGN (since it lies close to PDS0840 and PDS0544 which are likely to be AGNs by virtue of their line ratios presented in the other graphs in this Figure). It is possible that the other compact radio sources are starbursts involved in interactions, but more detailed observations and analysis of the individual systems are required to confirm this hypothesis.

**Table 6.1:** The 13 narrow-line objects with L_1.4>10^23.5 WHz^-1. These objects are the brightest radio sources whose radio emission may derive solely from starburst mechanisms. The first column is the PDS catalogue number (see Appendix A). The last column gives all (uncorrected) measured line ratios.
	$\log(L_{1.4})$	M_R	M_V	M_{B_J}	class	z	Q	type
0409	23.51	-22.83	-20.80	-21.43	g	0.427	1	A	[O II]/H $\beta$ = 1.65
0505	23.55	-22.70	-21.80	-20.93	g	0.445	1	A	[O II]/H $\beta$ = 1.13
									[O III]/H $\beta$ = 1.52
0672	24.55	-21.73	-20.54	-20.22	s	0.655	3	AB
0679	23.52	-23.42		-23.05	g	0.459	1	A	[O II]/H $\beta$ = 1.11
									[O III]/H $\beta$ = 2.35
0728	23.56	-22.36		-21.69	g	0.333	1	A	[O II]/H $\beta$ = 1.50
									[O III]/H $\beta$ = 0.84
0729	23.66	-22.75		-22.80	g	0.225	1	AB	[N II]/H $\alpha$ = 0.48
									[S II]/H $\alpha$ = 0.39
0847	23.56	-22.49		-26.29	g	0.564	1	AB
0856	24.00	-22.38		-19.55	g	0.419	1	AB
0895	24.15	-22.15			g	0.370	2	AB
0946	24.13	-22.86		-22.01	s	0.527	1	AB
1001	24.27	-22.90		-21.16	g	0.371	1	A	[O II]/H $\beta$ = 0.50
									[O III]/H $\beta$ = 7.24
1018	23.77	-22.97	-22.00	-20.97	g	0.378	1	AB
1019	23.90	-21.91		-19.83	g	0.563	2	AB

Some interesting galaxies

The most luminous starburst galaxies

Luminous starbursts in the PDS