PKS 1413+135 ( z = 0.24671 ) is one of very few radio-loud AGN with an apparent spiral host galaxy .
Previous authors have attributed its nearly exponential infrared cutoff to heavy absorption but have been unable to place tight limits on the absorber or its location in the optical galaxy .
In addition , doubts remain about the relationship of the AGN to the optical galaxy given the observed lack of re-emitted radiation .
We present new HST , ASCA and VLBA observations which throw significant new light on these issues .
The HST observations reveal that the active nucleus of PKS 1413+135 has an extremely red color : V - H = 6.9 mag , requiring both a spectral turnover at a few microns due to synchrotron aging and a giant molecular cloud-sized absorbing region .
Combining constraints from the HST and ASCA data we derive an intrinsic column N _ { H } = 4.6 ^ { +2.1 } _ { -1.6 } \times 10 ^ { 22 } { ~ { } cm ^ { -2 } } and covering fraction f = 0.12 ^ { +0.07 } _ { -0.05 } .
The spin temperature of the molecular absorption lines found by previous authors suggests that the cloud is located in the disk of the optical galaxy , making our sightline rather unlikely ( P \sim 2 \times 10 ^ { -4 } ) .
The properties of this region appear typical of large giant molecular clouds in our own galaxy .
The HI absorber appears centered 25 milliarcseconds away from the nucleus , while the X-ray and nearly all of the molecular absorbers must cover the nucleus , implying a rather complicated geometry and cloud structure , in particular requiring a molecular core along our line of sight to the nucleus .
Interestingly , the HST /NICMOS data require the AGN to be decentered relative to the optical galaxy by 13 \pm 4 milliarcseconds .
This could be interpreted as suggestive of an AGN location far in the background compared to the optical galaxy , but it can also be explained by obscuration and/or nuclear structure , which is more consistent with the observed lack of multiple images .