Search Radius
We next took the radio core emission AGN described in Chapter 3.2 as the detected
sample and performed a search around a much wider, 60′′
radius, to identify any extended emission (at positions > 4′′
from the optical AGN) that may be associated
with them. The 60′′
search radius was chosen due to the fact that the largest known FR IIs are on the scale of a Mpc (Saripalli & Subrahmanyan 2009 and references therein), and at the sample redshift limit ofz = 0.35, a 60′′
search radius corresponds to nearly 1 Mpc in diameter. All extended sources were visually inspected in the SDSS and FIRST images to give us confidence that the radio emission is associated. This
does not mean that any clearly associated emission out past 60′′
was not included,
but that any associated emission out past 60′′
was added to the total by hand after visual inspection of the FIRST images.
In order to confirm the association of extended emission with the optical counter- part, it is first necessary to make sure that the extended emission is not associated with another optical source in the field. The majority of cases where this takes place
is when there is a core source within 4′′
and a second source within 60′′
, where the second source is at the same position as another galaxy in the SDSS image. Therefore, any FIRST sources found in the extended search with obvious optical counterparts in the SDSS images were eliminated as possible extended source matches (e.g., SDSS J094603.94+013923.6 is a BL AGN misclassified as a star in SDSS DR7 with a re- solved spiral galaxy to the north that is more likely the source of the extended radio emission). The criteria used to confirm the association of the extended emission to the central optical source are illustrated in Figure 3.5, where the center of each image is the SDSS optical AGN position and the linear scale is given below each image. In
Figure 3.5 the sources a−e were all found to have core emission in the original 4′′
search described in Chapter 3.2. In Figure 3.5a (top left) we show the radio map
of SDSS J170013.70+400855.6, which shows the core-source with a nearby (∼ 35
kpc projected distance) knot of radio emission. There is also a possible lobe to the
south−west that is below the flux limit of FIRST. The association is based on the
source at or in the vicinity of the extended emission. The sources that had only two emission regions turned out to be the most numerous, and most were associated in this fashion. In Figure 3.5b we show the radio map of SDSS J122011.89+020342.2. Here the association is based on the physical connection of several emission knots in
the eastern jet to the core-source, and to a somewhat distant (∼ 275 kpc projected
distance) faint lobe to the west. In Figure 3.5c we show the radio map of SDSS J132834.14-012917.6. The association is based on the alignment of the very distant
lobes (both are∼500 kpc projected distance) with the radio core emission along with
clear trails of radio emission back to the core. There are several variations of this
type, such as those having small bending angles (usually less than ∼ 15◦
) between the distant lobes, as shown in Figure 3.5d. In Figure 3.5e we show the radio map of SDSS J091401.76+050750.6. The association is based on the distant southern lobe
(∼ 400 kpc projected distance) having a hot spot and lobe emission structure that
points back to the core radio emission. This object may in fact have an additional lobe source to the north that is just outside of the image. However, this was not added to the total radio emission due to the fact that association at that distance is not guaranteed without the other criteria being met. In this case, the exclusion of this ‘could-be lobe’ has very little effect on the conclusions due to the fact that it is very dim and the added emission would have been only 4% of the total. After visual inspection of all possible matches, we find that all AGNs with 3 or more radio sources
outside 4′′
, but inside 60′′
, are all positively associated when the search is done using the original radio core sources.
Figure 3.5: Images of FIRST extended sources with the optical source at the center of each frame, north is up and east is to the left. The SDSS name and projected physical size is given beneath each image. a: The 1.2′
× 1.2′
image shows a double source with one component on the optical core and one offset from it. b: The 3.0′
×3.0′
image has a strong jet and lobe to the east and a weaker lobe to the west. c: The 6.0′
× 6.0′
image shows a giant FR II where the lobe to the NE is aligned with the radio core and lobe to the SW.d: The 3.0′×
3.0′
image shows a distant lobe to the east, a radio core, and a lobe to the west slightly misaligned. e: The 3.0′
×3.0′
image shows a giant radio lobe with multiple sources to the south that all point back to the radio core. Not shown in this image is a more distant and slightly misaligned source to the north that may be an associated lobe, but with very low flux. f: The 3.0′
×3.0′
image shows two lobes that are roughly aligned with the optical center, but with no detected radio core.
We find 51 (6% of the original radio core emission sample and 0.6% of the total sample) AGN with extended emission that must be taken into account when calcu-
lating the total AGN radio luminosity. Of these 51, we find varying ranges in the amount of extra emission that is picked up. Some objects have a brighter core and
one dimmer lobe (∼10%-50% in added radio emission), but we also find huge, bright
FR IIs that have total integrated fluxes in the 1000 mJy range (100%-6000% in added radio emission). In order to characterize the amount of flux added due to extended emission, we show in Figure 3.6 the fraction of extended flux added with respect to the initial core emission. About half of the sources lie in the 0.01 – 2 range, showing that nearly half of the sources add only a small fraction to twice of the core flux to the total, while the other half of the sample at least doubles the amount of flux added to the core, and the brightest source adds nearly 70 times more emission when compared to the core.
A second search was done using the entire optical sample to find possible FR II types in which radio emission is only seen from lobes but there is weak (below the 1 mJy flux limit of FIRST) or no core radio emission. The largest group found in this
search has just one single radio source that is within 60′′
. After visual inspection, usually there was another optical sources at the extended radio source. Even when there is no such alternative optical identification it is not possible to claim an associa- tion since there is no discernible jet to lobe connection or double lobe symmetry that would be excellent indications of association. Most of these were rejected outright. We do however find an additional 12 objects (these are not included in the 51 AGN
discussed above) that have significant flux inside the 60′′
Figure 3.6: Histogram characterizing the amount of extended radio flux detected for the 51 multi-components sources with core emission. Nearly half (25) of the sources add only a fraction and up to two times the core flux. The inset shows that the largest increase in total flux due to extended emission is nearly 70 times the core flux.
emission inside 4′′
that can be positively associated with the optical source. All of these were visually inspected to ensure that the radio emission was not associated
with another optical source in the field and that any emission out past 60′′
was taken into account and added by hand to the total radio flux. The criteria for establishing
association for these objects is the alignment of emission out past 4′′
with the optical source (having no radio core), as shown in Figure 3.5f for SDSS J091519.56+563837.8. We do note that any sources with radio lobes that have significant bending angles would not satisfy our alignment criteria, and some sources may be excluded due to this effect.
Table B.2 lists the SDSS name of all 63 AGN with extended radio emission along with their redshifts, projected physical extent and a ‘by eye’ classification of the radio morphology based on FIRST images. There are 22 sources with no previous radio identification in the NASA extragalactic database (NED) from other radio surveys and they therefore have no radio catalogue source name in Table 1. The radio classi- fication column makes use of the ‘giant radio galaxy’ (GRG) classification, where the total projected linear extent exceeds 750 kpc (Saripalli & Subrahmanyan 2009 and references therein), and the hybrid morphology radio sources (HYMORS) classifica- tion, where an FR II lobe is seen on one side and an FR I jet is seen on the other side of the central source (Gopal-Krishna & Wiita 2000). We also classify X-shaped radio sources (e.g., Gopal-Krishna et al. 2003), where a possible reorientation of the jets has taken place to feed two individual sets of lobes, and the double-double morphology where interruption of the jets can cause two distinct sets of lobes to form throughout the lifetime of the AGN, where the first and older set is at a larger distance than the second, younger pair (e.g., Schoenmakers et al. 2000). We summarize these results here in Table 3.1.
Table 3.1: Summary of Morphologies for the Extended Radio Sample
Radio Morphology Number
FR I 10 FR II 25 FR II/GRG 6 DDRG 2 X−shaped 1 HYMORS 2 Indeterminate 17
We use the usual flux-luminosity relation with the same cosmology used by Greene
& Ho (2007) from Spergel et al. (2003) (H0 = 71 km s−1 Mpc−1, Ωm = 0.27, and
ΩΛ = 0.73) to calculate the total radio luminosity for each source. From these new
data, we update the R values of the radio detected sample of Chapter 3.2, including
all associated extended emission.
3.3.1 Sample Subsets Using the Extended and Core-only Sources
We again break this sample into subsamples as described in Chapter 3.2. We make use of the ‘detected sample’ which consists of all the AGN with radio emission and contains the core-only sources and the extended sources. The 63 extended sources
are all RL (i.e., all have R > 10) with the exception of one (R = 2.39) that is a
face-on spiral galaxy (SDSS J220233.84-073225.0) with a modest LHα = 1042.01 ergs
s−1
, but a very low flux radio core (Fint = 2.36 mJy) and an even fainter ‘lobe’ (Fint =
0.97 mJy) offset by 10.5′′
. We find that of the 793 remaining core-only sources, 383
are RL and 410 are RQ based on R. We also use the ‘flux limited sample’ which
is explicitly defined in Chapter 3.2 and has 5485 total objects, using an upper limit of 1 mJy at 1.4 GHz for all AGN without radio detections as an optical flux cutoff. With extended emission taken into account for the flux limited sample we find that 4.9% (270/5485) of the AGN are RL, compared to the 4.7% (259/5485) found when extended emission was not taken into account.
Analysis of the Core-only Sample
In this chapter we show our analysis based on different subsamples. The detected
sample is useful when plotting R as a function of MBH and Lbol/LEdd, since R is
uniquely determined for all AGN in this subsample. It is however highly biased when calculating the radio-loud fraction (RLF), in that the number of RQ AGN is significantly underestimated due to the FIRST flux limit. The flux limited sample is not as affected by this bias since many of the RQ AGN still make the optical flux cut and remain in the subsample, and is therefore used to determine the RLF. The statistical errors in the RLF plots are calculated from Poisson statistics.