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Unusual Morphologies from the HST Snapshot Survey of BL Lac Objects Three Optical Jets and

Unusual Morphologies from the HST Snapshot Survey of BL Lac

Objects:

Three Optical Jets and A Possible Einstein Ring

C.Megan Urry&Riccardo Scarpa

Space Telescope Science Institute,3700San Martin Dr.,Baltimore,MD21218,USA

(scarpa@https://www.sodocs.net/doc/3410881976.html,,cmu@https://www.sodocs.net/doc/3410881976.html,)

Tom Broadhurst

Department of astronomy,601Campbell Hall,University of California at Berkeley,CA,USA

(tjb@https://www.sodocs.net/doc/3410881976.html,)

Renato Falomo

Osservatorio Astronomico di Padova,Vicolo dell’Osservatorio5,I-35122Padova,Italy

(falomo@astrpd.pd.astro.it)

Jochen Heidt

Landessternwarte Heidelberg,K¨o nigstuhl,D-69117Heidelberg,Germany

(J.Heidt@lsw.uni-heidelberg.de)

Aldo Treves

Universit′a dell’Insubria,via Lucini3,I-22100,Como,Italy

(treves@uni.mi.astro.it)

Rachel Webster&Matthew O’Dowd

Melbourne University,Parkville,Victoria,Australia,3052

(rwebster@https://www.sodocs.net/doc/3410881976.html,.au,modowd@https://www.sodocs.net/doc/3410881976.html,.au)

Aimo Sillanp¨a a&Leo Takalo

Tuorla Observatory,Turku,Finland

(aimosill@deneb.astro.utu.?,takalo@deneb.astro.utu.?)

1The HST Snapshot Survey of BL Lac Objects

We have carried out an HST snapshot survey of108BL Lac objects,at redshifts from z=0.051to z>~1 (Urry et al.1999,Scarpa et al.1999b).Observations were made with the WFPC2,mostly in the broad F702W?lter.Host galaxies were detected in nearly all BL Lacs at z<0.5,and in about1/4of those at higher redshift.In addition to the host galaxies,the high resolution HST images have revealed several unusual morphologies.Here we describe three optical jets and a possible Einstein ring.(H0=50km/s/Mpc and q0=0are used.)

2Optical Jets

Three optical jets were detected in our survey,one newly discovered,PKS2201+044,and two previously known,PKS0521–365and3C371.At HST spatial resolution the jets are well resolved(Figures1-3),with knotty morphologies closely matching the radio morphologies,suggesting the optical emission is probably synchrotron radiation.The radio-to-optical spectral index(αro~0.8)and the derived intensity of the

equipartition magnetic?eld(B eq~10?4G)are approximately constant along the jet,suggesting the electron energy distribution does not evolve signi?cantly along the jet(Scarpa&Urry1999).

Jets are relatively rare in this BL Lac survey,a factor of3-4less common than in the HST survey of3C radio sources(Martel et al.1998,Sparks et al.1995),which has comparable depth and higher mean redshift. Since BL Lacs are clearly beamed(Urry&Padovani1995),this suggests their jets may be at smaller angles than the low-frequency-selected,higher luminosity3C sample.

3A Possible Einstein Ring Surrounding1517+656

The HST image of the BL Lac object1517+656shows three concentric arcs centered on the otherwise unresolved BL Lac nucleus(z>0.7).The arcs describe an almost perfect ring of radius2.4arcsec(Fig.4). The most plausible explanation is that the BL Lac object(including host galaxy)has lensed one or two background galaxies into arcs approximating an Einstein ring(Scarpa et al.1999a).If con?rmed,this would be the?rst case of a BL Lac acting as a lens rather than being itself lensed.

BL Lac objects are often in poor groups or clusters,which would contribute to the lensing and might account for the large diameter of the ring(lens mass~a few times1012M⊙,depending on source/lens redshifts and cosmology).Surrounding galaxies should be visible in a deeper HST image and may already have been seen in the two resolved spots.In order to establish whether these arcs indeed constitute an Einstein ring,deeper images with~0.1arcsec resolution are required.Multicolor images would also constrain the redshift of the arcs,if indeed they represent stellar light.

Figure Caption:

Fig.1:The optical jet of nearby BL Lac object PKS0521-365(z=0.055).The jet has total magnitude m R=19.9±0.2mag,is9.3kpc long,and orthogonal to the axis has nearly constant width of 1′′.5.

Upper panel:The WPFC2F702W gray-scale image of the jet,after subtracting the central point source and the best-?t elliptical galaxy model,smoothed with a Gaussian?lter withσ=3pixels(e?ective resolution 0′′.14).The bright spot in the lower left,previously called“hot spot C”,is actually a resolved galaxy with a de Vaucouleurs pro?le(Scarpa et al.1999a).

Lower panel:Contour plot of the HST image,with knots labeled.The big cross at the upper right indicates the position of the(subtracted)central point source.Isophotes are0.25magnitudes apart,starting from μR=21.9mag/arcsec2.

Fig.2:Comparison of the HST image(gray scale)with with the2cm VLA map(contours;Keel1986). Radio and optical morphologies agree very well in shape and extent.The radio-to-optical spectral index and equipartition magnetic?eld in four distinct knots(or averaged over the whole jet)are<αro>~?0.7and ~6×10?5G,respectively.

Fig.3:The optical jet of nearby BL Lac object3C371(z=0.0508).The jet has total magnitude m V=20.9±0.2mag,is5.5kpc long,and width at knot“A”of1′′.15.

Upper panel:The WFPC2F555W image(gray-scale)after subtracting the central point source and the best-?t elliptical galaxy model,smoothed with a Gaussian?lter withσ=3pixels(e?ective resolution 0.14arcsec),over-plotted(contours)with the1.6GHz radio map(Akujor et al.1994).

Lower panel:Contour plot of the above HST image.The big cross indicates the position of the(subtracted) central point source.Isophotes are0.25magnitudes apart,starting fromμV=23.3mag/arcsec2.The radio-to-optical spectral indices of the knots,and hence the derived equipartition magnetic?eld strengths,

are approximately constant along the jet,<αro>~?0.75and~6.5×10?5G,respectively.

Fig.4:Discovery of an optical jet in the nearby BL Lac object PKS2201+044(z=0.028). Only the brightest radio knot is clearly detected in the optical;its magnitude is m R=24.2±0.1mag.The jet structure is complex and extends over a region of~0.5×0.9arcsec2.

Upper panel:The WFPC2F702W image after subtracting only the best-?t elliptical galaxy;the central point source(“Nucleus”)is not very bright and so does not swamp the jet,although4di?raction spikes are clearly seen.The image was smoothed with a Gaussian?lter withσ=1pixel.

Lower panel:Contour plot of the above image.Knot“A”is2′′.12from the nucleus.Isophotes are0.5 magnitudes apart,starting fromμR=23.2mag/arcsec2.

Fig5:Comparison of the HST image(gray scale)with with the5GHz radio map(contours;Laurent-Muehleisen et al.1993),which has lower resolution than the HST data.The radio-to-optical spectral index for knot“A”,αro=?0.85,is consistent with values found in other optical jets(Crane et al.1993;Scarpa &Urry1999),as is the equipartition magnetic?eld,B eq=8×10?5G.

Fig.6:Upper panel:Center of the WFPC2F720W PC image of the BL Lac object1517+656,smoothed with a Gaussian ofσ=0.8pixels.The three narrow arcs at position angles2?,145?,and275?,describe an almost perfect ring around the unresolved BL Lac nucleus.The surface brightnesses of the arcs are approximately constant,μR~22.4mag arcsec?2,near the limit of detectability in this relatively short (320-second)exposure.A deeper image might well be?lled in with fainter arcs.The arcs are resolved

radially,having width~0.2arcsec.Two bright spots at position angles126?and260?are also resolved, and have magnitudes m R=23.6mag and23.8mag,respectively.The arrow points north and is1.38arcsec (30pixels)long.

Lower left:Deeper R-band image of1517+656taken with the Nordic Optical Telescope(1440-sec exposure), deconvolved using the PSF of a nearby single star.The arcs are readily apparent although the structure visible in the HST image is not resolved.

Lower right:Simple lensing model for the1517+656system,consisting of an elliptical mass distribution lensing both a background galaxy(yellow spot)lying across the tangential caustic(white diamond shape) and a hot spot(e.g.,HII region;white dot)just outside the caustic.The lensed images appear as three arcs with bright spots,similar to those observed in1517+656.The derived center of mass of the lens coincides with the BL Lac object,making it an unusual case of BL Lac as lens rather than lensed object.This is one possible?t to the observations;more precise”back-modeling”would be possible with higher signal-to-noise observations.

References

Martel,A.R.,Sparks,W.B.,Macchetto,D.,Biretta,J.A.,Baum,S.A.,Golombek,D.,McCarthy,P.J., De Ko?S.,&Miley,G.K.1998,ApJ,496,203

Scarpa,R.,&Urry,C.M.1999,ApJ,submitted

Scarpa,R.,Urry,C.M.,Falomo,R.,Pesce,J.E.,Webster,R.,O’Dowd,M.,&Treves,A.1999a,ApJ,in press(astroph9902333)

Scarpa,R.,Urry,C.M.,et al.1999b,in preparation

Sparks,W.B.,Golombek,D.,Baum,S.A.,Biretta,J.A.,De Ko?S.,Macchetto,D.,McCarthy,P.J.,& Miley,G.K.1995,ApJ,450,L55

Urry,C.M.,&Padovani,P.1995,PASP,107,803

Urry,C.M.,Scarpa,R.,et al.1999,in preparation

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