arXiv subjects

John Biretta

Publications and source records attributed to John Biretta.

6 recordsLinked to original sources

arXiv2017

Proper Motions of Jets on the Kiloparsec Scale: New Results with HST

The Hubble Space Telescope recently celebrated 25 years of operation. Some of the first images of extragalactic optical jets were taken by HST in the mid-1990s; with time baselines on the order of 20 years and state-of-the-art astrometry techniques, we are now able to reach accuracies in proper-motion measurements on the order of a tenth of a milliarcsecond per year. We present the results of a recent HST program to measure the kiloparsec-scale proper motions of eleven nearby optical jets with Hubble, the first sample of its kind. When paired with VLBI proper-motion measurements on the parsec scale, we are now able to map the full velocity profile of these jets from near the black hole to the final deceleration as they extend out into and beyond the host galaxy. We see convincing evidence that weak-flavor jets (i.e., FR Is) have a slowly increasing jet speed up to 100 pc from the core, where superluminal components are first seen.

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arXiv2016

An HST Proper-Motion Study of the Large-scale Jet of 3C273

The radio galaxy 3C 273 hosts one of the nearest and best-studied powerful quasar jets. Having been imaged repeatedly by the Hubble Space Telescope (HST) over the past twenty years, it was chosen for an HST program to measure proper motions in the kiloparsec-scale resolved jets of nearby radio-loud active galaxies. The jet in 3C 273 is highly relativistic on sub-parsec scales, with apparent proper motions up to 15$c$ observed by VLBI (Lister et al., 2013). In contrast, we find that the kpc-scale knots are compatible with being stationary, with a mean speed of $-$0.2$\pm$0.5$c$ over the whole jet. Assuming the knots are packets of moving plasma, an upper limit of 1c implies a bulk Lorentz factor $Γ<$2.9. This suggests that the jet has either decelerated significantly by the time it reaches the kpc scale, or that the knots in the jet are standing shock features. The second scenario is incompatible with the inverse Compton off the Cosmic Microwave Background (IC/CMB) model for the X-ray emission of these knots, which requires the knots to be in motion, but IC/CMB is also disfavored in the first scenario due to energetic considerations, in agreement with the recent finding of Meyer & Georganopoulos (2014) which ruled out the IC/CMB model for the X-ray emission of 3C 273 via gamma-ray upper limits.

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arXiv2001

Optical Structure and Physics of the M87 Jet

We summarize HST observations of the M87 jet, concentrating on polarimetry and spectral index maps, and compare its optical and radio structures. The evidence now supports a stratified model for the structure of the jet, whereby high-energy, optical synchrotron emitting particles occupy physically different regions of the jet, closer to the jet axis, with different magnetic field configurations. It is in these regions where the shocks that produce the knots in the inner jet appear to originate. Knot regions have optical spectra which are much flatter than average for the jet, with the flattest-spectrum regions coinciding with flux maxima of knots. These same regions are preceded by regions where perpendicular magnetic fields are seen. Thus not only do we see all the necessary ingredients for {\it in situ} particle acceleration in the knots, but there is now fairly direct evidence for it as well. By tracking the changes in radio-optical and optical spectral index in the knot regions, we can comment on acceleration and cooling timescales in each knot.

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arXiv2000

The Optical-Near-IR Spectrum of the M87 Jet From HST Observations

We present 1998 HST observations of M87 which yield the first single-epoch optical and radio-optical spectral index images of the jet at $0.15''$ resolution. We find $<α_{ro} > \approx 0.67$, comparable to previous measurements, and $<α_o > \approx 0.9$ ($F_ν\propto ν^{-α}$), slightly flatter than previous workers. Reasons for this discrepancy are discussed. These observations reveal a large variety of spectral slopes. Bright knots exhibit flatter spectra than interknot regions. The flattest spectra ($α_o \sim 0.5-0.6$; comparable to or flatter than $α_{ro}$) are found in two inner jet knots (D-East and HST-1) which contain the fastest superluminal components. In knots A, B and C, $α_o$ and $α_{ro}$ are essentially anti-correlated. Near the flux maxima of knots HST-1 and F, changes in $α_{ro}$ lag changes in $α_o$, but in knots D and E, the opposite relationship is observed. This is further evidence that radio and optical emissions in the M87 jet come from substantially different physical regions. The delays observed in the inner jet are consistent with localized particle acceleration, with $t_{acc} << t_{cool}$ for optically emitting electrons in knots HST-1 and F, and $t_{acc} \sim t_{cool}$ for optically emitting electrons in knots D and E. Synchrotron models yield $ν_B \gsim 10^{16}$ Hz for knots D, A and B, and somewhat lower values, $ν_B \sim 10^{15}- 10^{16}$ Hz, in other regions. If X-ray emissions from knots A, B and D are co-spatial with optical and radio emission, we can strongly rule out the ``continuous injection'' model. Because of the short lifetimes of X-ray synchrotron emitting particles, the X-ray emission likely fills volumes much smaller than the optical emission regions.

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arXiv2000

Face-on Dust Disks in Galaxies with Optical Jets

The presence of optical synchrotron jets in radio galaxies is relatively rare. Here, we show that of the nearest five FR-I 3CR radio galaxies showing optical jets, {\it four} show evidence for almost circular, presumably face-on, dust disks. This is strong support for the two-fold idea that (\romannumeral1) jets emerge close to perpendicular to inner gas disks and (\romannumeral2) optical non-thermal synchrotron emission is seen only when the jet points towards the observer. The implied critical angle to the line-of-sight is approximately 30 - $40^{\circ}$, i.e. if the angle of the jet to the line-of-sight is less than about $40^{\circ}$ we see an optical jet. The corresponding relativisitic $γ$ factor is $\approx 1.5$ which is consistent with current observations of jet proper motion that show a range up to $γ\sim 6$ for M87. The relatively low speeds implied by $γ\approx 1.5$ may be due to a global deceleration of the jet as in unified theories, or else to stratification within the jet. Unresolved nuclei are common in the optical. Their luminosities are also consistent with the beaming concept when compared to inclination inferred from the dust lanes. The disk sizes are typically several hundred parsecs, to kiloparsec size. The galaxy with an optical jet that does not show a face-on disk, M87, instead has more complex radial dust and ionized gas filaments.

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arXiv1999

HST Snapshot Survey of 3CR Quasars: The Data

We present images taken with the Wide Field Planetary Camera (WFPC-2) on the Hubble Space Telescope of 43 quasars selected from the 3CR radio catalog. The redshift range of the targets is large --- 0.3 < z < 2. These data were taken in the course of a large program that imaged 267 3CR radio galaxies and quasars using the HST in "snapshot mode". Each quasar was centered on the Planetary Camera (PC1) and was imaged through the F702W filter (bandpass similar to Cousins R), typically for 5 and 10 minutes. Our analysis suggests that the quasar fuzz contributes from <5% to nearly 100% in the most extreme case (about 20% being typical) of the total light from the quasar, with 16 of the quasars (~40%) being unresolved according to the analysis of their light profiles (with only 7 being considered unresolved determined by PSF subtraction of the quasar images). Most of the host galaxies show twisted, asymmetric, or distorted isophotes. About 1/4 of the quasar hosts have close (within a few arc seconds) companions seen in projection and about 1/10 show obvious signs of tidal interactions with a close companion. We find that these sources exhibit a tendency for the principal axes of the radio and optical emission to align similar but perhaps weaker than that observed for radio galaxies.

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