The Galaxy Zoo survey for giant AGN-ionized clouds: past

27 Pages·2011·3.1 MB·English

Mon.Not.R.Astron.Soc.000,1–21(2011) Printed6October 2011 (MNLATEXstylefilev2.2)
The Galaxy Zoo survey for giant AGN-ionized clouds: past
and present black-hole accretion events
William C. Keel1,2,3⋆, S. Drew Chojnowski2,3,4, Vardha N. Bennert5,6,
Kevin Schawinski7,8,9, Chris J. Lintott10,11, Stuart Lynn2,10, Anna Pancoast5,
Chelsea Harris5, A.M. Nierenberg5, Alessandro Sonnenfeld5, & Richard Proctor12
1Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487, USA
2Visiting Astronomer, Kitt Peak National Observatory,operated by AURA, Inc. under contract to the US National
Science Foundation.
3 SARA Observatory
4TexasChristian University, Forth Worth, TX 76129 USA
5 Department of Physics, Universityof California, Santa Barbara, CA 93106 USA
6 PhysicsDepartment, California Polytechnic State University,San Luis Obispo, CA 93407, USA7Department of Physics, Yale University,NewHaven,
8Yale Centerfor Astronomy and Astrophysics, Yale University,P.O.Box 208121, NewHaven, CT06520, USA
9EinsteinFellow
10Astrophysics, Oxford University
11Adler Planetarium, 1300 S. Lakeshore Drive,Chicago, IL 60605
12Waveney Consulting, Wimborne, Dorset BH21 3QY
ABSTRACT
Some active galactic nuclei (AGN) are surrounded by extended emission-line regions
(EELRs), which trace both the illumination pattern of escaping radiation and its
history over the light-travel time from the AGN to the gas. From a new set of such
EELRs,wepresentevidencethattheAGNinmanySeyfertgalaxiesundergoluminous
episodes 0.2–2×105 years in duration. Motivated by the discovery of the spectacular
nebula known as Hanny’s Voorwerp, ionized by a powerful AGN which has appar-
ently faded dramatically within ≈105 years, Galaxy Zoo volunteers have carried out
both targeted and serendipitous searches for similar emission-line clouds around low-
redshiftgalaxies.Wepresenttheresultinglistofcandidatesanddescribespectroscopy
identifying 19 galaxies with AGN-ionized regions at projected radii r > 10 kpc.
proj
This searchrecoveredknown EELRs(such as Mkn 78, Mkn 266,and NGC 5252)and
identified additional previously unknown cases, one with detected emission to r =37
kpc.OnenewSy2wasidentified.Atleast14/19areininteractingormergingsystems,
suggestingthat tidal tails are a prime source ofdistant gas out of the galaxy plane to
be ionized by an AGN. We see a mix of one- and two-sided structures, with observed
◦
cone angles from23–112 . We consider the energy balance in the ionized clouds, with
lower and upper bounds on ionizing luminosity from recombination and ionization-
parameter arguments, and estimate the luminosity of the core from the far-infrared
data. The implied ratio of ionizing radiation seen by the clouds to that emitted by
the nucleus, on the assumption of a nonvariable nuclear source, ranges from 0.02 to
>12; 7/19 exceed unity. Small values fit well with a heavily obscured AGN in which
only a small fraction of the ionizing output escapes to be traced by surrounding gas.
However, large values may require that the AGN has faded over tens of thousands of
years,giving us severalexamples of systems in which such dramatic long-periodvari-
ation has occurred; this is the only current technique for addressing these timescales
in AGN history. The relative numbers of faded and non-faded objects we infer, and
the projected extents of the ionized regions, give our estimate (0.2–2×105 years ) for
the length of individual bright phases.
Key words: galaxies: Seyfert — galaxies: ISM — galaxies: active
⋆ E-mail:[email protected]
(cid:13)c 2011RAS
2 W.C. Keel et al.
1 INTRODUCTION catalogued AGN, since the most interesting objects - those
which have faded dramatically - may no longer appear as
The compact sizes of the central engines of active galactic
spectroscopically classified AGN.
nuclei (AGN) have long driven study of their distant sur-
Hanny’s Voorwerp was first noted by Dutch teacher
roundings for clues to their geometry and interaction with
Hanny van Arkel in the course of the Galaxy Zoo project
thesurrounding galaxy. Observationsof gas seen many kpc
(Lintott et al. 2008), on the basis of its unusual structure
from the AGN itself have proven fruitful in offering views
and colour. In view of the interest of similar ionized clouds
of the core from different angles, and implicitly at different
for studyof both thehistory and obscuration of AGN,par-
times.
ticipantsintheGalaxy Zooproject havecarried out awide
Narrowbandimagesrevealedextendedemission-linere- search for such clouds using data from the Sloan Digital
gions (EELRs) around some luminous AGN, particularly SkySurvey(SDSS).TheyexaminedbothknownAGNhosts
radio-loud QSOs as well as radio galaxies, as reviewed by and galaxies not known to have AGN, using the distinc-
Stockton,Fu,&Canalizo(2006).Similarstructuresinlower- tive colour of highly-ionized regions across the SDSS gri
luminosity Seyfert galaxies often appearas single or double filtersasafirstselection criterion.Wepresenttheresultsof
trianglesinprojection(Ungeretal.1987,Tadhunter&Tsve- further analysis of the SDSSimages, narrow-band imaging,
tanov1989),generallyinterpretedasionizationcones.When and spectroscopy, yielding a list of 19 galaxies with AGN-
small-scale radiojetsarepresent,theyliewithintheioniza- photoionized clouds detected to beyond 10 kpc from the
tioncones.However,inmanycases,thegasmustbeionized nuclei (many of which are newly identified). We consider
by radiation from the nucleus rather than direct interac- constraints on changes in ionizing luminosity for these, and
tion with a jet or outflow, as seen from narrow linewidths identifyseveralasthemostlikelycandidatesforthekindof
and(particularlydiagnostic)modestelectrontemperatures, long-term fading seen in IC 2497 and Hanny’sVoorwerp.
bothofwhichwouldbemuchlargerinthepresenceofshocks
fast enough tomatch theobserved ionization levels. This is
particularly true for very large EELRs, where interaction
withtheradiojetoranorigininoutflowsalonebecomeless
2 SEARCHES FOR EMISSION-LINE CLOUDS
andless likely.Infact, thebest-definedionization conesare
seen in radio-quiet objects (Wilson 1996).
The Galaxy Zoo search for giant AGN-ionized clouds com-
This is one line of evidence linking large-scale struc- bined both targeted and serendipitous approaches, to com-
turestothesmall-scaleobscuringregions(“tori”)impliedby bineacomplete examination of knownAGNhostswith the
otherargumentsfor aunification scheme(Antonucci1993), possibility of finding ionized clouds around AGN which are
in which Seyferts of types 1 and 2 are part of a single par- yet unknown or in fact optically unseen. In the targeted
entpopulation,appearingdifferentbasedonhowourlineof search, we formed a sample of potential AGN at z < 0.1.
sight passes this torus. The emission-line structures can be This combined all galaxies whose SDSS pipeline emission-
largeandwell-resolved,offeringawaytomeasuretheopen- lineratiosputthemineithertheAGNorcompositeregions
ingangleoverwhichionizingradiationescapes.Someprevi- oftheBaldwin-Phillips-Terlevich (BPT)diagram (Baldwin,
ous studies have also noted that these emission-line clouds Phillips, & Terlevich 1981, asrevised byKewley et al. 2001
provideaviewtotheimmediatepastoftheAGN,vialight- andKauffmannetal.2003)using[OIII]/Hβand[NII]/Hα,
travel time to the cloud and then toward us (Dadina et al. andalladditionalobjectslistedintheVeron-Cetty&Veron
2010). catalog (Veron-Cetty & Veron 2010) at z < 0.1 falling
Using extended emission-line clouds as probes of AGN within the SDSS data release 7 (DR7) area. This addition
history came of age with the discovery of Hanny’s Voor- accounted for AGN with no SDSS nuclear spectrum, either
werp, a high-ionization region extending 45 kpc in projec- becausetheyarerelativelybrightor,moreoften,becausefi-
tion from the LINER IC 2497 at z = 0.05 (Lintott et al. brecollisions orsampling rulespreventedtheirselection for
2009). Linewidths and electron temperature indicate that spectra, and type 1 AGN where the pipeline spectroscopic
the gas is photoionized rather than shock-excited, while a classificationislessreliablethanfornarrow-lineobjects.The
combination of ionization-parameter and recombination ar- merged AGN sample, designed to err on the side of inclu-
guments bound the required nuclear ionizing luminosity to sion in borderline cases, included 18,116 objects. With a
be1−4×1045 ergs−1.However,X-rayspectroscopyshows web interface designed by RP, 199 participants examined
the nucleus of IC 2497 to be only modestly absorbed, with all of these within a 6-week period in 2009, marking each
ionizing luminosity only ≈ 1042 erg s−1 (Schawinski et al. ascertain,possible,orlackinganextendedemission region.
2010a).Itisdifficulttoavoidtheconclusionthatthenucleus These emission regions have distinctive signatures in both
of IC 2497 was in fact a QSO (thenearest known luminous morphology and colour from the SDSS data. They do not
QSO) until roughly 105 years before our current view, and followtheusualspiralorannulardistributionsofstarforma-
hasfadeddramaticallyintheinterim;radioandHSTobser- tion in disc galaxies. Such regions show unusual colours in
vations offerhintsthat some of its energy output may have the SDSS composite images, which map gri bands to blue,
switched to kinetic forms over this timespan (Josza et al. green,andred(Luptonetal. 2004). Hencestrong[OIII]at
2009,Rampadarathetal.2010,Schawinskietal.2010a,Keel low redshift is rendered as a pure blue, as in the discovery
et al. 2011). The unlikeliness of the nearest QSO showing of Hanny’s Voorwerp. A combination of strong [O III] and
highlyunusualbehavioursuggeststhatsuchvariationsmay significantHα+[NII]appearspurple;beyondaboutz =0.1,
be common among AGN, prompting us to re-examine the [OIII]falls inthegap betweeng andr filters,sooursearch
incidence and properties of extended ionized clouds around techniquelosesutilityuntil[OIII]iswellwithintherband,
nearbyAGN.Suchanexaminationshouldnotbeconfinedto when the galaxies have much smaller angular sizes. This
(cid:13)c 2011RAS,MNRAS000,1–21
Giant AGN clouds 3
subproject was known as the “voorwerpje hunt”, using the
Dutchdiminutiveform of Voorwerp.
Eachgalaxy wasexaminedbyatleasttenparticipants;
199 Zoo volunteers participated in this program, seven of g r g-0.75r
whom examined the entire sample. The final average num-
berofvoteswas11.2perobject.Afterthisscreeningprocess,
a straightforward ranking was by relative numbers of “yes”
(weight=1), “maybe” (weight=0.5), and “no” (weighted
zero) votes.
The most interesting results of such a search would be
galaxies with prominent AGN-ionized clouds in which we
don’t see the AGN, either because of strong obscuration or
dramatic variability during the light-travel time from the
nucleus to the clouds. These would not be found by tar-
geting known AGN,and neitherwould cloudsaround AGN
whichdonothavecatalogued spectralinformation.Accord-
ingly, we also posted a request on the Galaxy Zoo discus-
sion forum, with examples of confirmed AGN clouds and
Figure 1. Linear combination of SDSS images to iso-
various kinds of similar-appearing image artifacts. Partici-
late candidate [O III] emission regions, shown with SDSS
pants were invited to post possible cases from among the
J143029.88+133912.0(theTeacupAGN).Theregionshownspans
galaxies they saw in the ordinary course of the Galaxy Zoo
64×128SDSSpixels,or25.3×50.6”,withnorthatthetop.Each
classification programs (Lintott et al. 2008), and some ac-
image is displayed with a logarithmic intensity mapping, with a
tiveusers reposted examples from other discussion threads. smallzeropointoffsettoreducetheeffects ofnoisearoundzero.
The resulting followups1 provided an additional sample for At the redshiftz =0.085 of this galaxy, Hα and [N II] emission
investigation;WCKalsocheckedallthethreadswithimage fallredwardoftherfilterband(response0.2%ofpeak)sother
discussionformoresuchobjects,earlyexamplesofwhichin- imageisusedasacontinuum estimate.
stigatedthissearchonthefirstplace.Toreducethenumber
offalsepositivescausedbyextendedstar-formingregionsor
starburstwinds,objectswereremovedfromconsideration if galaxy image are more vulnerable to this artifact than nor-
an SDSS spectrum shows emission lines characteristic of a mal galactic nuclei. Since many candidates (includingsome
starburst.Remainingcandidateswereexaminedfirstonthe withspectroscopic confirmation) have“purplehaze”on the
SDSS composite images for appropriate colour and geome- SDSSimages,whichcouldeitherbegenuinelyextendedand
try, and the most promising ones were carried forward for somewhat amorphous [O III] and Hα or an artifact, this
furtheranalysis. was a helpful step. We adopted a tomographic approach,
Both targeted and serendipitous lists overlap for many taking one of the SDSS bands free of strong emission lines
objects with bright emission-line structures, and recover (r or i, depending on redshift) as an estimate of the struc-
suchwell-studiedcasesfromtheliteratureasMkn266,NGC tureofstarlightinthegalaxy.Thiswasscaledtomatchthe
5252 and Mkn 78; we observed these so as to have a con- largest part of the g structure, iteratively when necessary.
sistent set of spectra for comparison. The entire list of can- This is illustrated in Fig. 1, isolating the emission-line loop
didates is given in Table 8. In the Survey column, S or T in SDSS 1430+13 (nicknamed the Teacup AGN because of
denotes whether the object was found in the serendipitous thisstructure).ChojnowskiandKeelinspectedthebestsub-
survey, the targeted survey of known AGN, or both. The traction among various scalings (often a compromise, due
typeofnuclearopticalspectrumislisted asSy1/1.5/1.9/2, to colour gradients within the galaxy) to assess the reality
LINER, SB for starburst, or nonAGN for an ordinary stel- of extended emission-line features not associated with clear
lar population. The final column indicates which Galaxy spiralarmsorstellarrings.Theseresultsletusrankthecan-
Zoo participant (by user name) first posted objects in the didatelistsfrombothtargetedandserendipitoussearchesin
serendipitous survey. orderofsignificanceoftheemission-linestructuresbasedon
theSDSSimages themselves.Weusedtheseresultstolimit
thenumberofcandidatesfromthetargetedsearchtothetop
2.1 SDSS image analysis and new images 50;belowthistherewerenoconvincingcandidatesbasedon
more detailed analysis of the SDSSimages.
Forbothsubsamples,furtherwinnowinghadthesamesteps.
Where appropriate filters were available for [O III] or
Mostimportantly,wereanalyzedtheSDSSimages,toverify
Hα at a galaxy’s redshift, some candidates were imaged at
that the features do not have continuum counterparts, and
theremote SARA1m (Kitt Peak) and 0.6m (Cerro Tololo)
eliminate artifacts caused by imperfect registration of the telescopes. For [O III], we used a filter centered at 5100 ˚A
imageswhenformingthecolourcomposites.Thiseffectisof withhalf-transmission width100˚A,useablefortheredshift
particularconcernforSeyfert1nuclei,wherethePSFofthe rangez=0.009−0.025.AtHα,bothtelescopeshavestepped
bright nucleus can produce a decentered colour signature if sets of filters 75 ˚A apart with FWHM ∼75 ˚A . Continuum
one of theconstituent images is slightly misregistered; Sy 1
was taken from V,R or g,r, appropriately scaled for sub-
tractiontoshownetemission-linestructures.Thesedataare
particularlyhelpfulintracingtheemission-linestructuresof
1 inhttp://www.galaxyzooforum.org/index.php?topic=275014.0 UGC 7342 (Fig. 2) and SDSS2201+11 (Fig. 3).
(cid:13)c 2011RAS,MNRAS000,1–21
4 W.C. Keel et al.
run, so that the 3m spectra could be concentrated on the
most interesting galaxies. Total exposures ranged from 30
minutes,for initial reconnaissance to see whether an object
might host AGN clouds, to 2 hours for weaker lines in con-
firmed targets. Either night-sky line or interspersed lamp
observations were used to track flexure, as needed. Reduc-
tion used the longslit package in IRAF2 (Tody 1986), and
included rebinning to a linear wavelength scale, sky sub-
traction,andfluxcalibration. Spectrophotometricstandard
starswereobservedtosetthefluxscale;inafewcaseswhere
passingcloudswereanissue,thespectrawerescaledsothat
the nucleus within a 2×3” region matched the flux of the
SDSSspectra.
Our identification of these extended regions as being
photoionized by AGN rests on three results - location in
thestrong-lineBPTdiagram,strengthofthehigh-ionization
species He II and [Ne V], and electron temperature consis-
tentwithphotoionizationbutnotwithshockionization.We
Figure 2.TheextendedcloudsinUGC7342.Left,thestarlight classify emission regions based on the “BPT” line-ratio di-
continuum in a band at 6450 ˚A from the SARA 1m telescope. agrams pioneered for galactic nuclei by Baldwin, Phillips,
Center,anestimated[OIII]imagefromtheSDSSdataasinFig. & Terlevich (1981) and refined by Veilleux & Osterbrock
1. Right, continuum-subtracted Hα image from the SARA 1m. (1987), with caution based on the possibility that some of
North is at the top and east to the left; the field shown spans
the external gas could have much lower metal abundances
97×150arcseconds.
thanfoundingalactic nuclei(asseen inHanny’sVoorwerp;
Lintott et al. 2009). Abundance effects in gas photoionized
by AGN, as manifested in the BPT diagrams, have been
considered in calculations by Bennert et al. (2006a). The
largest effect is higher equilibrium temperature at lower O
abundance, since it is an important coolant, which drives
stronger forbidden lines and higher ionization levels until
verylowlevels(0.1solar)arereached.Inanycase,theabun-
dance changes are not large enough to move these clouds
across the empirical AGN/starburst ionization boundary.
Furthermore, in the galaxies where we have data covering
the red emission lines, the clouds’ locations in the (essen-
tiallyabundance-independent)auxiliaryBPTdiagramof[O
III]/Hβversus[OI]/Hαalsoindicatedphotoionizationbyan
AGN continuum.The various BPT diagrams are compared
for Points along the slit in each of the clouds we classify as
AGN-ionizedinFig. 4.Thisclassification isexamined more
closely in the context of its radial behavior in the next sec-
tion.
Independent of these line ratios, strong He II λ4686 or
Figure3.TheextendedcloudsinSDSS2201+11.Left,agimage [Ne V] λλ3346,3426 indicate photoionization by a harder
fromtheSARA-S0.6mtelescope,showingthedustydisk.Right, continuumthanprovidedbyyoungstars,andresolvedemis-
continuum-subtracted [O III] image from the SARA-N 1m tele- sion from these species is immediately diagnostic of AGN
scope, smoothed by a Gaussian of 2.0” FWHM. North is at the photoionizationinthiscontext.Forsomeobjects,wedonot
topandeasttotheleft;thefieldshownspans64×84arcseconds.
havereddata;inthese,weclassifythecloudasAGN-ionized
based on the presence of the high-ionization lines or conti-
nuity of line ratios with the nucleus. Line ratios in the ex-
2.2 Spectroscopy
treme blue may be affected in subtle ways by atmospheric
ToconfirmthatregionsareinfactionizedbyAGN,andde- dispersion(Filippenko1982);theschedulingofourobserva-
rivediagnosticemission-lineproperties,wecarriedoutlong- tionsforcedustoobservemosttargetsathourangleswhich
slit spectroscopy for thehighest-priority candidates. Obser- did not allow us to put the slit simultaneously along the
vations used the GoldCam spectrograph at the 2.1m tele- structuresofinterestandclosetotheparallacticangle.The
scopeofKittPeakNationalObservatoryandtheKastdou- extended regions we observe are generally wider than the
ble spectrograph at the 3m Shane telescope of Lick Obser-
vatory. Table 1 compares the setups used for each session.
The slit width was set at 2” for all these observations, and 2 IRAF is distributed by the National Optical Astronomy Ob-
thespectrographswererotatedtosamplethemostextended servatory, which is operated by the Association of Universities
known structures of each galaxy. Scheduling allowed us to forResearchinAstronomy(AURA)undercooperativeagreement
reduce the Kitt Peak data before the first Lick observing withtheNationalScienceFoundation.
(cid:13)c 2011RAS,MNRAS000,1–21
Giant AGN clouds 5
Figure 4. Summary Baldwin-Phillips-Terlevich (BPT) diagrams for the Lick spectra, where we measured the requisite red emission
lines. Circles indicate points along the slit for extended clouds classified as AGN-ionized and the host nuclei. Gray-scale background
showsthedensityofpointsfromalllow-redshiftSDSSgalacticnuclei,asinSchawinskietal.(2010b).Thedividinglinesbetweenregions
photoionized byAGNandbyhotstarsareshownasgivenbyKewleyetal.(2001; Ke01)andbyKauffmannetal.(2003;Ka03).
slit; to first order line intensities are not affected by atmo- remnants (including some with lower ionization levels than
sphericrefraction, since we calibrate with standard stars at in these clouds) range from 20,000-69,000 K (e.g., Fesen et
lowairmass.SomeoftheLickbluespectrahaveatmospheric al. 1982, Wallerstein & Balick 1990, Morse et al. 1995). In
dispersioncontributingasmuchas3”ofoffsetalongtheslit addition,veryhighshockvelocities≈400kms−1areneeded
from red to blue ends of the spectrum, important only for toproducesignificant[NeV]emission Dopita&Sutherland
thenuclei and corrected in extracting their spectra. (1996). This is far in excess of the local velocity ranges we
observe (section 6); even though we would not necessarily
The BPT diagrams are designed to separate common observe material on both sides of a shock in the same ion,
sourcesofphotoionizationingalaxies;temperatureandkine- it is difficult to envision a situation with large-scale shocks
maticdataarealsoimportanttounderstandwhethershocks ofthisvelocity without observablevelocity widthsor struc-
pay a significant role. In a few cases, the [O III] λ4363 line turesexceeding 100 k s−1.
was measured in theextended clouds with sufficient signal-
to-noise ratio for a measurement of the electron tempera- While not the main thrust of our survey, it is worth
tureviaitsratiotothestrongλλ4959,5007lines.Usingthe noting that we find a few instances of either double AGN
IRAFtasktemden,whichimplementsthealgorithmofShaw in interacting systems, or AGN in the fainter member of
&Dufour(2007),andconsideringne <100cm−3,wefindTe a close pair (Mkn 177, Was 49, possibly SDSS J111100.60-
valuesof18,600±1000 intheSDSS2201+11, 13,300±300 005334.9 and SDSS J142522.28+141126.5). These may be
forMkn266,and15,400±500fortheTeacupsystem.These worth deeper spectroscopy in the context of mapping AGN
confirmthatthegasisphotoionizedratherthanshocked;for obscuration; if a high-ionization component can be isolated
comparison, temperatures in the [O III] zone of supernova in the gas of the other galaxy, its distribution could show
(cid:13)c 2011RAS,MNRAS000,1–21
6 W.C. Keel et al.
where ionizing radiation escapes any circumnuclearabsorb- 3 ENERGY BUDGET IN EXTENDED
ingstructure.Thisoffersadistinctwayoftracingtheioniz- CLOUDS: OBSCURATION VERSUS
ing radiation even in the absence of extensive gaseous tidal VARIABILITY
features, in an approach that has been discussed for Was
49ab by Moran et al. (1992). Seeing the effects of radiation from an AGN on gas tens of
kpcfromthenucleusallowsusthepossibilityoftracingdra-
matic changes in core luminosity. One straightforward way
to approach this question is a simple energy balance. The
Table4liststheresultsofourspectroscopy.Confirmed,
spectragiveusupperandlowerboundsontherequiredion-
resolvedcloudsionizedbytheAGNareseparatedfromother
izing luminosity. To probe themost extreme conditions, we
results(unresolvedAGNemission,extendedstar-formingre-
analyzegalaxiesinwhichwedetectionizedgasatprojected
gionsdenotedasHII,andsoon).Theinstrumentsusedare
distances r>10 kpc.For all distances and luminosities, we
denotedbyGCam(KittPeakGoldCam)andLick(Lick3m
use the WMAP “consensus” cosmological parameters, with
with Kast spectrograph). New redshifts and spectral classi-
H =72 km s−1 Mpc−1 (Spergel et al. 2007).
fications are marked with asterisks. We separate the AGN 0
clouds of most interest based on the detected extent of [O Thelowerboundcomesfromthehighestrecombination-
III]λ5007;ourspectrahavealowerdetectionthresholdthan line surface brightness we observe; the central source must
ourimagesforthis,roughly10−16 ergcm−2s−1arcsec−2 for provideatleastenoughionizingphotonstosustainthisover
emission regionsafewarcsecondsinsize.Spectraofthenu- periodslongerthantherecombinationtimescale(whichmay
clei and representative cloud regions are shown in Figs. 5 be as long as 104 years at these low densities). This is a
and 6. Table 5 lists emission-line ratios and selected fluxes lowerlimit,sincetheactualemission-linesurfacebrightness
forthesameregionsplottedinthesefigures.Fluxesaregiven of some regions may be smeared out by seeing, and we do
bothfor[OIII]λ5007andHα,sincethesewereusuallymea- notknowthatagivenfeatureisopticallythickattheLyman
sure with different gratings and detectors. For some of the limit. This limit depends only very weakly on the slope of
nuclei, correction of the Hβ flux for underlying absorption the ionizing continuum, since helium will generally absorb
inthestellarpopulationwassignificant;wehaveappliedan mostoftheradiationshortwardofitsionizationedgeleaving
approximatecorrectionbasedontypicalvaluesforsynthetic onlythe13.6-54.4 eVrangetoconsiderforhydrogenioniza-
stellar populations from Keel (1983). tion.Webaseourboundsonthehighestimpliedluminosity
among structures at various projected radii in a given sys-
tem, with no correction for projection effects. This makes
The upper part in Table 4, with AGN-ionized gas de- our limits conservative, since a given cloud will always lie
tectedmorethan10kpcfromthenucleus,formsthesample farther from the nucleus than our projected measurement.
for our subsequent analysis. As a sign of completeness, of Inessence, thisargumentisbased onthesurfacebrightness
these 19, 14 were found in both targeted and serendipitous inarecombinationline;weuseHβ sincewehavethesedata
searches. SDSS J095559.88+395446.9 was newly identified for the whole sample. In a simple approximation, we take
asatype2Seyfertinourspectrum,afterhavingbeenfound the surface brightness in the brightest portion of a cloud,
in the “blind” search of galaxies independent of prior clas- assuming this to be constant across the slit. We take the
sification as an AGN host (so it was not included in the region sampled in this way to be circular in cross-section
targeted sample). Of the remainder, Mkn 78 and Mkn 463 as seen from the nucleus, so its solid angle is derived from
were selected in thetargeted AGNsample, whileMkn 1498 the region subtended by the slit. We then see this region
and UGC 11185 were recognized only in the serendipitous occupying a small angle α = 2arctan(slit halfwidth/r) as
survey. It may be relevant that both Mkn 78 and Mkn 463 seen projected at angular distance r from theAGN,there-
have ionized regions with relatively small projected extent, quiredionizingluminosity isgivenfrom observedquantities
easilylost against thegalaxystarlight(whichinMkn463is as Lion = 1.3×1064z2F(Hβ)/α2 for α in degrees. The de-
morphologically complex). rivedvaluesarelistedinTable6,alongwithcomplementary
quantitiesrelatedtothenuclearluminosity(ascollectedbe-
low).Thederivedionizingluminositiesarelowerlimits,since
The [S II] λ6717,6731 lines are particularly important, theremaybeunresolvedregionsofhighersurfacebrightness,
tracing electron densities and thereby providing one esti- andwedonotknowwhetheragivencloudisopticallythick
mate of the intensity of the impinging ionizing radiation. in the Lyman continuum. Higher-resolution imaging in the
Since the densities in these extended clouds are low, and emission lines could this increase these values.
the ratio is generally near its low-density limit, where the Upper limits to the incident ionizing flux come from
mapping from line ratio to density is highly nonlinear, we a complementary analysis using the ionization parameter
haveexaminedtheerrorsinmeasuringthelineratioclosely. (U, the ratio of ionizing photons to particles), since these
We generated multiple realizations of pixel-to-pixel noise, emission-linefeaturesallhave[SII]lineratiosnearthelow-
and each was scaled to four representative fractions of the density limit. Our density results from the λ6717/λ6731 [S
strongerlinepeak.Thiswasaddedtolinepairs,modeledto II] line ratio are given in Table 7. Values are listed only for
match the line widths and pixel separation of the red Lick objectswithusefulmeasuresfarfromthecore.Ineachcase,
data. Gaussian fitting of the lines with added noise gave a weevaluatedthedensityatatypicaltemperatureof104 K,
relation between the peak signal-to-noise and error of the andatthehighertemperature1.3×104 KfoundinHanny’s
fitted ratio which we adopted; we use ±2σ error bounds to Voorwerp(Lintottet al.2009) andinourdataforMkn266
deriveboundsonthedensity.Densityvalueswerecalculated and SDSS 2201+11, as set by thermal equilibrium for sub-
using theIRAFtask temden. stantiallysubsolaroxygenabundance.Wequotetheextreme
(cid:13)c 2011RAS,MNRAS000,1–21
Giant AGN clouds 7
[Ne V] 3346[Ne V] 3426 [O II] 3727 [Ne III] 3869 [Ne III] 3968[S II] 40Hδ6 94102 Hγ 434[1O III] 4363 [He II] 4686 Hβ 4861 [O III] 4[95O 9III] 5007[O I] 6300[O I] 6364 Hα 65[6N3 II] 6584 [S II] [6S 7I1I]6 3675300
800 nucleus cloud cloud: 6.3" (6.6 kpc) wsw nucleus Mkn 78 300
600 (Lick - Kast) 250
400 200
200 110500
0 50
−200 0
3500 3700 3900 4100 4300 4500 4700 4900 5100 6600 6800 7000
11802000 nucleus cloud cloud: 21.9" (12.5 kpc) north nucleus M(Lickk n- K 2as6t)6 111024000
60 80
40 60
20 40
0 20
−20 0
3500 3700 3900 4100 4300 4500 4700 4900 5100 6500 6700 6900
235000 nucleus cloud cloud: 8.4" (6.4 kpc) east nucleus Mkn 883 2350
200 (Lick - Kast) 20
150
100 15
50 10
172(10erg/cm/s/)x Å− −−−2111145422068024500000000000000 35003600 nnuucclleeuuss37030cc8ll0oouu0dd 39040000 cc4lloo10uu402dd0::0 71.40."0 "( 7(.884.3 5k04 04pk0cp0)c )n nnnww 45406000 47nn408uu00cc0lleeuuss 49500000 51502000 MN((LLiiG6cck67kk6 0nC--00 KK 0 1aa5ss4tt9))9782 66980000 717000000501234511100000024000 Cloud Flux (10erg/−17
Flu 100 6800 cm
Nucleus 3520005 3500 nucleus 37cl0o0ud 3900 clou4d10: 0 5.7" (5.6 k4p3c00) sse 4500 nu47c0l0eus 4900 5100 6S50D0SS 09556700 6900 0242000 Å/s/)2
20 (Lick - Kast)
15 15
150 10
0 5
−5
0
3600 3800 4000 4200 4400 4600 4800 5000 5200 6600 6800 7000
40 40
30 nucleus cloud cloud: 4.2" (4.3 kpc) ene nucleus S(LDickS - SKa s1t)005 3305
20 25
20
10 15
0 510
−10 0
3600 3800 4000 4200 4400 4600 4800 5000 5200 6700 6900 7100
100 50
6800 nucleus cloud cloud: 11.7" (7.0 kpc) nne nucleus S(LDickS - SKa s2t)201 3400
40
20 20
0 10
−20 0
3500 3700 3900 4100 4300 4500 4700 4900 5100 6500 6700 6900
Observed Wavelength (Å)
Figure 5. Lick spectra for nuclei and associated AGN-ionized clouds. Small insets at left show the Hβ+[O III] region for nuclei and
clouds,scaledtothepeakofλ5007emission.Panelsontherightshowthe[OI]-[SII]regionatthesamefluxscaleasthebluespectra.
He II and [Ne V] emission,especially important as indicators of a hard ionizingradiation field, are indicated by red dotted lines when
clearlydetectedinclouds.Nuclearspectrarepresent2×3.1-arcsecondareas,andcloudspectraaresummedover2×6.2-arcsecondareas.
Distances and directions ofcloudrelative tonuclei areindicated asshown. Threespectra havegaps inthe blueregion,sincethey were
takenwiththedichroicsplittingredandblueoptical trainsnear4600˚A.
rangeofdensityvaluesbetweenthesetwocases(allowingin aregiveninTable7.Itisreassuringthattheupperlimitsto
theTeacup an upperbound on theelectron densityas high ionizingluminosityderivedfromU andne alwaysfallabove
as240cm−3,andinsomecaseslimits <10cm−3),sincethe thelower limits from recombination balance.
temperature-sensitive[O III]λ4959+5007/λ4363 lineratio
The lower limits from recombination-balance are inde-
isnot well-enough measured in most of theseobjects touse
individualTe values. WederiveU from the[O II]λ3727/[O pendent of assumptions about thelocal density ne, making
it more robust than ionization-parameter arguments when
III]λ5007ratiousingthepower-lawcontinuummodelsfrom
we have no independent tracer at these low densities. Fig.
Komossa&Schulz(1997),andtheanalyticfitsfromBennert
7 shows several of our objects in one of the “BPT” dia-
(2005) as interpolation tools. For fully ionized hydrogen at
grams, going beyond their initial use to classify the gas as
a distance d from the AGN, thephoton flux in theionizing
continuumisQ=4πd2neU/c.Forobjects withredspectra, AGN-ionized to examine changes with projected distance
from the nuclei. Some of these, such as Mkn 1498 and the
givingdensitiesfromthe[SII]lines,limitstotheluminosity
Teacup 1430+13, show a phenomenon remarked earlier in,
(cid:13)c 2011RAS,MNRAS000,1–21
8 W.C. Keel et al.
[Ne V] 3346[Ne V] 3426 [O II] 3727 [Ne III] 3869[Ne III] 3968[S II] 4H0δ6 94102 Hγ 43[41O III] 4363 [He II] 4686 Hβ 4861 [O III] [49O 5I9II] 5007[O I] 630[0O I] 6364 Hα 6[56N 3II] 6584 [S II][ S6 I7I1] 66730
50
120 nucleus cloud cloud: 6.6" (11.3 kpc) ne nucleus Teacup
100 40
(Lick - Kast)
80 30
60
ux 40 20 C
Fl 20 10 lo
us 0 0 ud
ucle 150 3700 nucleus 39c0l0oud 4100 cl4o3u00d: 6.8" (545.600 kpc) eas4t700 490n0ucleus 5100 5300 UG690C0 111857100 7300 50 Flu
N 100 (Lick - Kast) 40 x
30
50
20
0 10
−50 0
3500 3700 3900 4100 4300 4500 4700 4900 5100 6600 6800 7000
Observed Wavelength (Å)
Figure 5–continued
for example, NGC 5252 (Dadina et al. 2010) - the ioniza- tween 42-122µ. Numerically,
tniaotnurbaalllyanecxepslatianyesdriofutghhelychcaornascttaenrtistwicithdernasditiyusn,ew∝hicrh−2is. FIR(W m−2)=1.26×10−14(2.58f60+f100)
This might occur naturally for gas in the host galaxy; tidal forIRASfluxesinthe60and100µbandsgiveninJy(mul-
streamsofgaswouldnotbelikelytomatchtheextrapolated tipliedby107foraresultinergscm−2s−1).IRASdatawere
behavior of gas within the galaxy and indeed we see some supplemented,where possible, by Akari data (Murakami et
cases (Mkn 266, NGC 5972, SDSS 2201+11) with substan- al. 2007, Kawada et al. 2007, Yamamura et al. 2010) of
tial radial changes in U. However, for Seyfert narrow-line higher accuracy. The positions of all these galaxies were
regions, Bennert etal. (2006b) findashallower densitygra- covered in the IRAS survey,so we can assign typical upper
dient ne ∝ R−1.1, which would imply U ∝ R−0.9 for gas limitstonondetectionsdependingoneclipticlatitude;Akari
which is optically thin (or has a small covering fraction). addedtwoadditionaldetectionsnotfoundintheIRASdata,
Theseobjectshaveheterogenousbehavior;Intheionization using only quality 1 (confirmed detection) fluxes. For non-
coneofNGC7212, Cracco etal.(2011) findnoradialtrend ULIRG objects (LFIR < 1045 erg s−1), we can reproduce
of ne. the IRAS FIR parameter from Akari 90µ fluxes and mean
Similar conclusions come from the more limited blue- colours via
line diagram also considered by Baldwin, Phillips, & Ter- FIR(W m−2)≈5.0×10−14f
levich (1981), which we can apply to the objects for which 90
we have only blue-light spectra from KPNO. Some of ob- with 30% accuracy (±0.15 dex), and we use this to fill in
jects in this diagram as well as in Fig. 7 show systematic FIR luminosities for the objects detected only by Akari.
changes in ionization level with radius, manifested as off- Theinputvaluesand results of thisenergy-balancetest are
setsfromupperleft(higherionization)tolowerright(lower showninTable6.Withinoursample,Mkn273andMkn266
ionization). We show this behaviorin Fig. 8. are classic ultraluminous infrared galaxies (ULIRGs), with
We use far-IR data to estimate (or limit, for nondetec- LFIR = 1−5×1045 erg s−1. Some of the others remain
tions) the amount of AGN radiation absorbed (and reradi- undetectedin both the IRASand Akari surveys,leading to
ated)bynearbydensematerial,whetherinanAGN“torus” limits typically < 1044 erg s−1. An index of whether the
or in the inner parts of the host interstellar medium. The extendedcloudscanbeionizedbyanobscuredAGNispro-
FIR luminosity is conservatively high as an estimate of vided by the ratio of required ionizing luminosity to FIR
thepotentialobscured AGN luminosity,since theremay be luminosity,tabulatedinTable??.Thesevaluesarealllower
a nontrivial contribution from star formation in the host limits,sincetheionizingluminosityisalowerlimit Thisra-
galaxy as well as the AGN, and in some cases companion tio ranges from 0.02 to values > 12 (FIg. 9). Low values
galaxies might blend with the target in the FIR beam. In clearly represent AGN which are strongly obscured along
a simple picture where a fraction f of the AGN radiation ourlineofsightbutnottowardtheEELRclouds.Largera-
is absorbed by nearby dust and reradiated, the FIR lumi- tiosindicatelong-term fadingof theAGN,aspectral shape
nosity will beof order Lionf/4π, with an additional scaling strongly peaked in theionizing UV, or very specific geome-
factorofafewtoaccountfornon-ionizingradiationheating tryforobscuringmaterial,andthusindicateobjectsworthy
the grains (which we omit at this point for the sake of a of close attention.
conservative calculation). For convenience, we approximate Arguments for long-timescale variations in the central
thetotalfar-IRoutputbytheFIRparameterintroducedfor sources here depend on our having estimates for their total
IRASpoint-sourcecatalogdata(Fullmer&Lonsdale1989), luminosity as seen directly, which could in principle fail ei-
alinearcombinationoffluxvaluesinthe60and100µbands theriftheirionizingradiationwerecollimatedbysomething
whichgivesareasonableapproximationtothetotalfluxbe- other than obscuration, or the spectral shapes in the deep
(cid:13)c 2011RAS,MNRAS000,1–21
Giant AGN clouds 9
[Ne V] 3426 [O II] 3727 [Ne III] 386[9Ne III] 3968[S II] 4Hδ0 649102 Hγ 4[34O 1III] 4363 [He II] 4686 Hβ 4861[O III] [4O 9I5II9] 5007 [Ne V] 3426 [O II] 3727 [Ne III] 386[9Ne III] 3968[S II] 4Hδ0 649102 Hγ 4[34O 1III] 4363 [He II] 4686 Hβ 4861[O III] [4O 9I5II9] 5007
250 200 120 80
IC 2637 cloud: 5.6" (3.3 kpc) sw nucleus 100 Mkn 273 cloud: 9.8" (7.6 kpc) sw nucleus 70
200
(KPNO - GCam) 150 (KPNO - GCam) 60
80
150 50
100 60 40
100 40 30
50 50 20 20
10
0
0 0 0
3700 3900 4100 4300 4500 4700 4900 5100 3600 3800 4000 4200 4400 4600 4800 5000 5200
100 140 100
800
Mkn 463 cloud: 10.9" (11.0 kpc) south nucleus 120 Mkn 739 cloud: 10.6" (6.4 kpc) nnw nucleus
600 (KPNO - GCam) 80 100 (KPNO - GCam) 80
)Å 400 60 80 60 C
172(10erg/cm/s/us Flux − −12250000000000 N(K3G7P0NC0O - 4GC3a8m8)3900 clo41u0d0: 12.9" (243.40 0kpc) ssw4500 4700nucleus4900 5100 024681100000200 2231140520605000000000 N3(6K0GP0NCO - 5GC2a5m23)800 c4lo0u00d: 14.4" (462.070 kpc) sou4t4h00 4600nucleus4800 5000 02446810000000loud Flux (10erg/cm/−172
Nucle 0 2400 500 20 Ås/)
−500 −50
0 0
3500 3700 3900 4100 4300 4500 4700 4900 3600 3800 4000 4200 4400 4600 4800 5000
60 40 50
50 SDSS 1510 cloud: 7.3" (6.8 kpc) south nucleus 35 60 SDSS 1524 cloud: 12.8" (9.6 kpc) sse nucleus 40
40 (KPNO - GCam) 30 (KPNO - GCam)
30 25 40 30
20 20
10 15 20 20
−100 510 0 10
0 0
3700 3900 4100 4300 4500 4700 4900 5100 3600 3800 4000 4200 4400 4600 4800 5000 5200
50
Observed Wavelength (Å)
150 UGC 7342 cloud: 19.5" (18.7 kpc) sw nucleus
40
(KPNO - GCam)
100 30
50 20
0 10
0
3700 3900 4100 4300 4500 4700 4900 5100
Observed Wavelength (Å)
Figure 6.KPNOGoldCamspectrasamplingnucleiandassociatedAGN-ionizedclouds.Spectraarescaledtoshow[OIII]λ4959,with
nuclearspectrarepresenting2×3.1-arcsecondareasandcloudspectrarepresenting2×6.2-arcsecondareas.HeIIand[NeV]emission,
especially important as indicators of a hard ionizing radiation field, are indicated by red dotted lines when clearly detected in clouds.
Distancesanddirectionsofcloudrelativetonucleiareindicatedasshown.
ultravioletdifferfromourexpectationsbasedontheUVand (Condon et al. 1998). All but two objects (SDSS 1510+07
X-raybehavioroffamiliar AGN.Collimation byrelativistic and1005+28)weredetectedabovethe2.5mJysurveylimit;
beamingdoesnotfitthecombinationsofopeningangleand thesource luminosity L(1.4 GHz) ranges from <1.3×1022
flux ratio is required (as found for Hanny’s Voorwerp; Keel WHz−1 to2.0×1024,thelatterforthedoublesourceasso-
et al. 2011). A spectral solution to the behavior would re- ciated with NGC 5972 and comprising 94% of the galaxy’s
quireanextreme-ultraviolet’bump”dominatingtheionizing total flux. Eight of the galaxies qualify as radio-loud if one
flux from 13.6-54 eV by more than an order of magnitude. uses a simple, representative division at 1023 W Hz−1, and
However, known AGN do not provide evident examples of onlyoneliesabove1024.Thisone-NGC5972-representsan
either solution; the most straightforward interpretation of interesting departurefrom theusualalignment of emission-
the data suggests that some of these clouds are ionized by line and radio structure(section 7).
AGNwhichhavefadedoverthedifferentiallight-traveltime
between ourviews of theclouds and nuclei.
5 HOST AND CLOUD MORPHOLOGY
The examples of Hanny’s Voorwerp (Josza et al. 2009) and
4 NUCLEAR AND EXTENDED RADIO
NGC5252(Prieto&Freudlng1996)suggestthatacommon
EMISSION
source of extraplanar gas at large radii is tidal debris. The
To further characterize the AGN in these galaxies, we col- host morphologies of the galaxies where we find extended
lectedradiofluxesat1.4GHzfromtheNVSSsourcecatalog ionized clouds support this notion. Table 8 lists morpho-
(cid:13)c 2011RAS,MNRAS000,1–21
10 W.C. Keel et al.
Figure 7.Baldwin-Phillips-Terlevich(BPT)diagramsforspatialslicesinLickdata. EachrowshowsthethreeclassicalBPTdiagrams
foreach object, highlightingradial ionizationbehaviors. Thenucleus isindicated bycrosshairs,withcolors changing fromwhitetored
with increasing distance from the nucleus. The greyscale background and dividing lines are the same as in Figure 4; these show only
the region around the AGN loci in each case for discriminationof detail. Nearlyall measurements liefirmlyin the AGN domain, with
possible exceptions in some regions of SDSS 1005 and SDSS 0955. The starburst/AGN ionization boundary from Kewley et al. (2001)
is shownas the redfullcurve, whilethe boundary fromKauffmann et al. (2003) isthe black dashed curve. Allthese measurements lie
firmlyinthe AGN domain, with the possible exception of two regions inSDSS 1005 and the nucleus of Mkn 883, whose red spectrum
showsabroad-lineregionandstrong[OI].Thegreyscaleshowthedensityofpointsrepresentinglow-redshiftgalacticnucleiintheSDSS,
fromSchawinskietal.(2010b), whichwealsofollowinadoptingthestraightlineastheLINER/Seyfertboundary
(cid:13)c 2011RAS,MNRAS000,1–21