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The [OIII] Emission-line Nebula of the z=3.594 Radio Galaxy 4C +19.71

The [OIII] Emission-line Nebula of the z=3.594 Radio Galaxy 4C +19.71
The [OIII] Emission-line Nebula of the z=3.594 Radio Galaxy 4C +19.71

a r X i v :a s t r o -p h /9709292v 1 29 S e p 1997The [OIII]Emission-line Nebula of the z =3.594Radio Galaxy 4C

+19.71?

L.Armus 1,B.T.Soifer,T.W.Murphy Jr.,G.Neugebauer,A.S.Evans &K.Matthews

Palomar Observatory,Caltech,Pasadena,CA 911251Current Address,Infrared Processing and Analysis Center,Caltech 100-22,Pasadena,CA 91125?Based on observations at the W.M.Keck Observatory,which is operated by the California Institute of Technology and the University of California Received

ABSTRACT

We have imaged the z=3.594radio galaxy4C+19.71in the light of the redshifted [OIII]5007?A emission line,using a narrow-band?lter centered at2.3μm with the Near Infrared Camera on the Keck Telescope.The[OIII]nebula of4C+19.71has a size of74×9kpc,and a luminosity of L5007~3×1037W.The rest frame equivalent width of the5007?A line,averaged over the entire nebula,is560?A.The length of the major axis of the[OIII]emission is nearly identical to the separation of the radio lobes seen at1465MHz(Rottgering,et al.1994),and the position angle of the nebula is the same as that of the two radio lobes.In addition,4C+19.71follows the optical emission line vs.radio power correlation seen in other powerful radio galaxies.The [OIII]and Lyαemission-line luminosities suggest that the ionized gas mass lies in the range of2×108?109M⊙.The O/H ratio in the nebula is at least a few tenths solar, and may be as high as a factor of three above solar,indicating a previous phase of star formation in4C+19.71.Thirty?ve percent of the total K-band?ux is contributed by the5007?A emission line,and the continuum of4C+19.71has a K~19.6mag.This places4C+19.71along the K-z relation found for other radio galaxies and radio loud quasars.If the continuum is dominated by starlight,the host galaxy has a rest frame visual luminosity of about40L?.There are no candidate emission-line objects at the redshift of4C+19.71having[OIII]rest frame equivalent widths of more than about 2%that of the radio galaxy itself within a volume of212Mpc3.

Subject headings:Galaxies:General,Galaxies:Individual-4C19.71,Galaxies: photometry-infrared

1.Introduction

A ubiquitous feature in the spectra of powerful radio galaxies is the presence of bright ultraviolet and optical emission lines.These lines can be quite strong,with rest frame equivalent widths of several hundred angstroms.The strength of the emission lines,coupled with the large sky coverage of radio surveys,has facilitated the discovery of radio galaxies over a large range in redshift.As a result,powerful radio galaxies are among the most extensively studied stellar systems at moderate and high redshifts.

By observing the emission-line properties of radio galaxies at di?erent redshifts,we can study the interplay between active galactic nuclei and their host galaxies as a function of cosmic epoch. The emission-line nebulae provide both a direct probe of the conditions in the interstellar medium of the host,and an indirect probe of the central source of ionizing radiation.The most spectacular emission-line nebulae in these systems can have luminosities above1037W,sizes of over100kpc, and a strong alignment with the radio axis(McCarthy et al.1987,Baum et al.1988,Chambers, Miley&van Breugel1987,1988,McCarthy&van Breugel1989,and McCarthy et al.1995).

The emission line nebulae of high redshift radio galaxies are usually studied via the redshifted Lyαline,which can,in principle,be strongly a?ected by extinction from even a small amount of dust due to resonant scattering.Although most powerful radio galaxies are bright in Lyα(McCarthy&Lawrence1993),dust and density strati?cation in the interstellar gas can have a strong e?ect on the?ux and spatial distribution of the line-emitting material seen in the rest frame ultraviolet.The distorted morphologies(Heckman et al.1986),large infrared luminosities (Golombek,Miley&Neugebauer1988),and molecular gas contents(Mirabel,Sanders&Kazes 1989;Mazzarella et al.1993;Evans1996)of radio galaxies at low-redshift imply a great deal

of dust in these systems.It might be expected therefore,that these conditions exist in some high-redshift radio galaxies as well.For example,4C+41.17and8C1435+63have been found to have sub–mm?ux densities consistent with thermal dust emission(Dunlop et al.1994,Ivison 1996).Also,recent observations of4C+05.41(Dey,Spinrad&Dickinson1995)and TX0211?122 (van Ojik et al.1994)have provided data in support of models of dusty systems at high redshift.

If high-redshift radio galaxies are dusty like many of their low redshift counterparts,the nebular properties(e.g.sizes,morphologies and luminosities)gleaned from observations of Lyαmight be misleading.Besides dust,associated HI absorption systems can apparently have a strong e?ect on the Lyαproperties of high redshift galaxies(van Ojik et al.1997)as well.However,by combining Lyαand Hαand/or[OIII]images of high redshift radio galaxies it might be possible to construct extinction and/or ionization“maps”of the emission-line nebulae.Since much work has been devoted to the study of the optical emission-line nebulae of low redshift galaxies,near infrared images of radio galaxies at z>2made through narrow-band?lters provide a means for direct comparison in the same rest-frame emission features.

Besides being valuable laboratories in which to study the interplay between relativistic radio plasmas and dense,galactic gas,the stellar properties of high-redshift radio galaxies can o?er signi?cant leverage on cosmological parameters.Powerful radio sources at z>2?3are likely to reside within massive galaxies which may still be growing via accretion at a time when the Universe was only10%?20%of its current age.Thus,the host galaxies of high redshift AGN may provide a glimpse of the formation and build up of the most massive stellar systems seen around us https://www.wendangku.net/doc/aa10860466.html,rge galaxies at high redshift may also mark the locations of young clusters whose cannibalised members form the stellar building blocks of the central host,and provide gas to stoke the engine of the radio source.

The combination of narrow-band?lters,sensitive,large format infrared arrays,and large aperture telescopes naturally lends itself to searches for high redshift active,or star forming galaxies in young clusters(see Bunker et al.1995,Mannucci&Beckwith1995,and Thompson, Mannucci&Beckwith1996).While clustering around powerful radio galaxies at z~0.5seems to be enhanced compared to lower redshifts(Hill&Lilly1991),the clustering properties of radio galaxies at z>1are largely unknown,with a pair of notable exceptions.Narrow-band Lyαimaging and follow-up spectroscopy of the?eld around the z=3.14radio galaxy MRC0316-257 con?rm the existence of two faint galaxies at the same redshift as the radio source(Le Fevre,et al.1997).In addition,Pascarelle et al.(1996)have discovered10?20objects in the?eld of the

z=2.39radio galaxy53W002via HST Lyαimaging and follow-up spectroscopy.Thus at least two groups or clusters are known around powerful radio galaxies at z>2.

As part of a program to study the continuum and emission-line properties of high redshift AGN in the near-infrared,we have imaged the z=3.594radio galaxy,4C+19.71(MG2141+19) using the W.M.Keck Telescope.4C+19.71is a double-lobed,steep-spectrum,FRII(Fanaro?and Riley class II-Fanaro?&Riley1974)radio source(Rottgering et al.1994).The two radio lobes are separated by8.1′′at a position angle on the sky of176?.The integrated?ux density in the lobes at1465MHz is S1465~3×10?27W m?2Hz?1,implying an emitted monochromatic power at a rest frequency of6.73GHz of approximately1028W Hz?1.The galaxy associated with4C +19.71has been imaged previously in the K-band by Eales&Rawlings(1996)and was shown to have two faint components separated by approximately4′′.The redshift of the galaxy is z=3.594 (Spinrad et al.1993).Eales&Rawlings present a K-band spectrum showing a weak feature at ~2.3μm,identi?ed as the[OIII]5007?A line,having a?ux of2.2±0.4×10?18W m?2.Here,we present both broad-band K and narrow-band2.3μm images of4C+19.71,which reveal a resolved central component at K,and a large,asymmetric emission line nebula aligned along the radio axis.The total[OIII]5007?A emission-line luminosity of the nebula is about3×1037W,and the linear extent along the major axis is74kpc.

In the following sections we describe the observations and present the imaging results.In section4we relate the nebular properties of4C+19.71to other powerful radio galaxies,estimate the rest frame blue continuum luminosity of4C+19.71,and discuss the limits on clustering around the radio galaxy probed by our narrow-band imaging.

Throughout this paper we adopt H o=75km s?1Mpc?1and q o=0,so that at the redshift of4C+19.71,9.3kpc projects to1′′on the sky.

2.Observations and Data Reduction

Observations of4C+19.71were made at the W.M.Keck Observatory on the night of5 September1996with the Near Infrared Camera(Matthews&Soifer1994).The radio galaxy was imaged through a standard K-band,2.0-2.45μm,?lter as well as through a narrow,2.284-2.311μm,?lter designed to sample the CO stellar absorption feature in zero redshift galaxies.At a redshift of z=3.594,the[OIII]5007?A emission-line feature,but not the4959?A feature,falls in the bandpass of the narrow-band?lter.The plate scale of the256x256InSb array is0.15′′per pixel. The total integration time through the K?lter was1080seconds while the total integration time through the narrow band?lter was2700seconds.In each case,individual images of60and150 seconds duration,respectively,were taken with the galaxy moved by about10′′on the array between successive exposures.An o?set guider employing a visual wavelength CCD was used to guide the telescope.A nearby star(referred to as star A in Fig.1)was visible in each exposure and used to register the individual frames.The conditions were photometric during the observations, and UKIRT faint standard stars(Casali&Hawarden1992)provided the?ux calibration.The seeing during the observations was0.4′′?0.5′′full width at half maximum(FWHM).To remove time-variable?uctuations in illumination,separate sky and normalized?at-?eld frames were created from the data for each three images,by taking the median of the nearest7-9frames.After being trimmed to a size of251×251pixels,the individual data frame are thus sky subtracted and?at?elded and are shifted to a common dc level after known bad pixels are?agged.These processed images are then aligned,using integer pixel shifts,and combined using a clipped mean algorithm.

3.Results

Near infrared K-band images of4C+19.71are presented in Figs.1and2.Fig.1is an image of the49′′×49′′?eld surrounding the radio galaxy.The FWHM of a stellar point source in Fig.1 is3.3pixels,or0.5′′.All con?dently detected sources,both resolved and unresolved,are labelled

in Fig. 1.The K-band magnitudes of these sources are given in Table1.We estimate a point source(3σ)detection limit of K~22.5mag in the?nal mosaic.

The radio galaxy4C+19.71has two obvious2.2μm components(see Fig2for an expanded picture)labelled as“a”and“b”after Eales&Rawlings(1996).Both components are resolved in our image,and component“a”has a faint extension to the south.The magnitudes of components “a”and“b”,as measured through2.0′′diameter circular apertures,are K=20.14±0.05mag and K=21.41±0.16mag,respectively.The total K magnitude of4C+19.71as measured from our data through a rectangular aperture of2′′×10′′,oriented north-south,centered on component“a”, and excluding G6,is K=18.92±0.04mag.In Table1,the K-band magnitudes of all the sources identi?ed in Figure1are given,as are the positions relative to component“a”of4C+19.71.

In Fig.2we show the central~20′′region of the4C+19.71?eld imaged through both the broad-band K and narrow-band2.3μm?lters.The di?erence in the appearance of4C+19.71in the broad and narrow-band images is striking.This di?erence is due to strong,redshifted[OIII] 5007?A emission.The[OIII]emission line nebula of4C+19.71is extended for over8′′,or about74 kpc,and is highly elongated in the north-south direction,with an axial ratio of about8:1.South of component“a”the nebula is nearly linear with a?aring at the end.Note that component “a”in Fig.2actually appears to be either double,or have a core plus“jet”morphology.The position angle between the primary and secondary(fainter)parts of component“a”(“a1”and “a2”respectively)is approximately160?,and it thus does not match the position angle of the nebula as a whole.North of component“a”the nebula is much more di?use,possibly curving to the west at the location of component“b”.The length of the nebula is comparable to the separation of the radio lobes mapped by Rottgering et al.(1994)and the overall position angle of the[OIII]emission is the same as the position angle of the radio lobes on the sky is176?±3?. Eales&Rawlings(1996)also note that the[OIII]emission appears extended to the north in their K-band spectrum.

From these data the4C+19.71nebula has an[OIII]5007?A emission-line?ux measured within a2′′×10′′rectangular aperture of1.54×10?18W m?2,corresponding to a5007?A line

luminosity of about3×1037W.Eales&Rawlings(1996)measure an[OIII]5007?A line?ux of ~10?18W m?2through a3.1′′×3.1′′slit,and~2×10?18W m?2through a3.1′′×12.4′′wide slit, oriented at a position angle of162?.By comparing the line?ux to the total emission measured through the K-band?lter,we estimate that the5007?A line alone comprises approximately34% of the broad K-band?ux.The rest frame equivalent width of the5007?A line,when averaged over the entire nebula,is560±58?A.Note that the total[OIII]line contribution to the measured K-band?ux is about45%.

The nature of the K~19.8mag source(G6)located approximately2.1′′northwest of component“b”is unknown.It’s proximity to the radio source argues that it may be associated, expecially since it lies along what appears to be a bend in the emission-line nebula beyond component“b”.However,G6has no discernible excess of?ux in the narrow band?lter,indicating that there is no strong[OIII]emission from this source.Given its lack of a narrow-band color excess and a K=19.78±0.05mag,we calculate a line?ux limit of about3×10?20W m?2from G6.If G6is at the redshift of4C+19.71,this corresponds to a limit on its[OIII]line luminosity of about7×1035W,or about2%of that measured for the radio galaxy nebula as a whole.

In Fig.3we present a plot of the di?erence between the broad and narrow-band magnitudes (the color“excess”)versus the broad-band magnitude for the objects in the4C+19.71?eld.All of the sources identi?ed in Fig.1and Table1are plotted here,except for G8and obj12,which have been excluded because the uncertainties in their narrow-band magnitudes are larger than50%. The large narrow-band color excess of4C+19.71is obvious in Fig. 3.We have indicated two points for4C+19.71in Fig.3-components“a”and“b”as de?ned above.The di?erent locations of the two points in Fig.3re?ect the variation of the[OIII]5007?A emission-line equivalent width along the extent of the4C+19.71nebula,in the sense that the equivalent width of the nebula is larger away from the galaxy nucleus.This simply re?ects the fact that there is very little continuum light in the galaxy(at least to the limits of this image)at large radii along the nebular axis.

Besides4C+19.71itself,there are three sources with an apparent narrow-band excess at

the3σlevel or above.These are the bright resolved sources labelled G1and G2,and the bright, apparently stellar source labelled star B.Sources G1and G2have narrow-band excesses at

the3?3.5σlevel,while star B has an excess at closer to the5σlevel.Object10has a large narrow-band excess(0.41mag),but the uncertainties are large(±0.23mag).The other nine sources plotted in Fig.3scatter about the K-NB=0.0mag line,with a total dispersion of about 0.2mag.An excess of0.2mag corresponds to an[OIII]5007?A rest frame equivalent width of about13?A at the redshift of4C+19.71.

4.Discussion

Since4C+19.71is one of only three galaxies at z>3to be imaged in an emission line,

it is instructive to place the4C+19.71nebula within the context of those seen around other powerful radio galaxies.The two largest sets of radio galaxy narrow-band imaging data are those compiled by Baum et al.(1988)and McCarthy,Spinrad&van Breugel(1995).The low redshift(0.003

2×1033?2×1037W(median L~6×1035W),corrected to[OIII]5007?A using emission-line ?ux ratios of[OIII]/Hα+[NII]=1,[OIII]/Lyα=0.3,and[OIII]/[OII]=3,and sizes of1?213kpc (median d~30kpc).Five systems in the McCarthy,Spinrad&van Breugel sample(about15%) at redshifts of z>0.5have nebulae larger than100kpc,as measured down to a surface brightness level of10?20W m?2arcsec?2.There are nebula found in both data sets with large,elongated features(e.g.3C227,3C458,3C435A),yet none are dominated by a single,high surface brightness,high axial ratio nebulosity as seen in4C+19.71.

Although the the Baum et al.(1988)and McCarthy et al.(1995)samples are the largest existing narrow-band imaging data sets,most of the objects are at redshifts well below z~2. From a sample of steep spectrum4C radio sources,Chambers et al.(1996a,b)image?ve galaxies

with2.348

At z>3there are two systems previously known to possess large,emission-line nebulae.

A large(~60kpc),apparently rotating Lyαhalo has been imaged around the z=3.57radio source4C03.24by van Ojik et al.(1997).The Lyαluminosity of the nebula in4C03.24is about 6×1037W.Graham et al.(1994)imaged the[OIII]+Hβnebula around the z=3.8radio galaxy 4C+41.17by taking the di?erence between images in a standard K-band and a K s(2.0?2.3μm)?lter.These authors show that the nebula has an extent of about15×40kpc,and that it is oriented along the radio axis.However,unlike4C+19.71,the4C+41.17nebula is wider,and appears to extend beyond the main radio components.Chambers,Miley&van Breugel(1990) showed that4C+41.17also has a large Lyαnebula which is similarly extended along the radio axis.

4.1.Relation of[OIII]and Radio Properties of4C+19.71

4C+19.71is a double-lobed,FRII radio source with a lobe separation of8.1′′at a position angle of176?.There is no obvious central radio core.The monochromatic power at a rest frequency of6.76GHz is approximately1028W Hz?1.The extent of the[OIII]nebula matches the separation of the radio lobes in4C+19.71,and the overall position angle of the nebula is similar to the position angle of the radio lobes.Although the absolute positioning of the infrared images is uncertain,and the radio map has no strong core,an R-band image of the?eld covering 10arcminutes and including six HST guide stars as well as“star A”indicates that to within an uncertainty of~2′′,the lobes are centered on component“a1”.If“a1”is the nucleus of the radio galaxy,and also the centroid of the two radio lobes,then the radio lobes are located on the[OIII] image as depicted in Fig.2.This positioning suggests that a narrow,slightly curving“corridor”of line-emitting gas?lls the regions between the galaxy nucleus and the radio lobes.This gas does not extend beyond the radio lobes to a limiting surface brightness of about2×10?20W m?2

arcsec?2.Note that the position angle between the primary and secondary parts of component “a”is not the same as that of the nebula as a whole or the radio lobe axis.Thus if“a2”represents a hotspot in the emission line gas caused by the interaction of an inner jet with the interstellar medium of4C+19.71,a precession of the jet,or a bend caused by the interaction of the jet with an overdense region,may explain the di?erence in the position angle between the inner and the outer nebula.

The emission-line?to?radio luminosity ratio in4C+19.71is entirely consistent with the relation found for other FRII radio sources over a large range in redshift.In Fig.2of McCarthy (1993)the luminosity in the[OII]3727?A line is plotted against the monochromatic radio power at an observed frequency of1400MHz.A clear correlation exists over?ve orders of magnitude in radio and emission-line power.If the[OIII]5007/[OII]3727line?ux ratio in4C+19.71is3.0,the [OII]luminosity is about1037W.Similarly the radio spectral index ofα=?1.24(Rottgering et al.1994),where fν∝να,implies a1400MHz?ux density of~3×10?27W m?2Hz?1,and thus a monochromatic power at an emitted frequency of6.43GHz of about1.4×1028W Hz?1.For this radio power,the predicted[OII]luminosity of4C+19.71is slightly larger than the average value, yet well within the scatter in the McCarthy plot.Ifα=?1.24out to an observed frequency of about300MHz,the monochromatic power at an emitted frequency of1400MHz is closer to1029 W Hz?1,thus placing4C+19.71near the center of the emission-line?radio power correlation. Thus both the morphology and the overall luminosity of the optical emission-line gas and radio plasma in4C+19.71suggest a causal link between the relativistic electrons and the104K ionized gas.

There are a number of possible explanations for the correlation of the optical emission-line gas and the radio plasma in4C+19.71.This gas could be photoionized by the active nucleus, or ionized by shocks associated with the radio jets responsible for supplying the radio lobes with energetic electrons.The emission-line gas may form a shell or channel around the radio jet as has been seen in the nearby Seyfert galaxy NGC1068(Gallimore et al.1996,Capetti,Macchetto& Lattanzi1997).Alternatively,the expanding jets could trigger star formation in the host galaxy,

and these hot stars could ionize the surrounding medium.This would indicate a more indirect connection between the radio emission and the optical line-emitting gas.The strong alignment in position angle between the radio lobes and the[OIII]gas,and the similarities in their overall dimensions could be used to argue for any or all of these possibilities.The560?A rest frame equivalent width of the[OIII]line is much larger than what is seen in starburst galaxies,yet is comparable to the largest values seen in powerful radio galaxies,being a factor of~2larger than the“typical”value tabulated by McCarthy(1993).Thus it is unlikely that the bulk of the [OIII]emission is produced by young stars.Longslit,near infrared spectroscopy of the2.3μm spectral region at high spectral resolution could provide a more concrete assessment of the source of ionizing photons through a measurement of the the linewidth and?ux ratio of the[OIII] 5007?A and Hβemission lines.In the spectrum presented by Eales&Rawlings(1996)the Hβemission-line is undetected,suggesting that the[OIII]5007/Hβline?ux ratio is greater than7-10, consistent with a hard source of ionizing photons(Veilleux&Osterbrock1987).

4.2.Dust and Gas in4C+19.71

4.2.1.Dust

By comparing the[OIII]5007?A emission-line?ux reported here with the?ux of another emission-line whose ratio to[OIII]is known,we can estimate the amount of reddening towards the ionized gas in4C+19.71.As pointed out by Dey,Spinrad&Dickinson(1995),the fact that there are only three known dusty,high redshift galaxies implies that these objects are either intrinsically very rare,or they have been systematically missed in surveys due to observational biases.However,since the presence of signi?cant amounts of dust in a high redshift galaxy requires the dust to have been formed at an even earlier epoch,dusty systems at z>2may imply very early star formation episodes.

Recombination lines are best suited to the task of determining reddening toward the emission-line gas,but pairs of these have not yet been measured in4C+19.71.However,a

total Lyα?ux has been measured in4C+19.71to be about1.1×10?18W m?2(H.Spinrad& M.Dickinson,private communication).Thus the measured[OIII]5007?A to Lyαline?ux ratio, averaged over the entire4C+19.71nebula,is~1.4.We can estimate the amount of reddening toward4C+19.71,by assuming the intrinsic spectrum is similar to the“average”radio galaxy spectrum compiled by McCarthy(1993).Since powerful radio galaxies typically have[OIII] 5007/Lyαline?ux ratios of about0.3(McCarthy1993),the increased ratio measured for4C

+19.71may indicate the presence of dust.To account for the increase in the[OIII]5007/Lyαline ?ux ratio seen here would require a visual extinction of only A V~1mag.With an l II=75?and a b II=?30?,the Galactic color excess towards4C+19.71is E(B-V)~0.04?0.05mag(Burstein &Heiles1982),implying an A V~0.12?0.16mag,negligible in comparison to the amount of extinction required if the intrinsic[OIII]5007/Lyαline?ux ratio is0.3.An A V~1mag is not very large,yet it implies a signi?cant mass of associated HI gas,if the dust is spread out over the entire nebula.If A V=5.3N H/1022mag(Bohlin,Savage&Drake1978),the HI column density toward the emission-line gas in4C+19.71is about2×1021cm?2.If this gas is spread out over a projected surface area of8′′×1′′or691kpc2,the implied HI gas mass is about1010M⊙.An atomic?to?molecular gas ratio similar to the Galaxy thus implies a similarly large mass of H2 (Young&Scoville1991).Of course,if Lyαis resonantly scattered throughout the nebula,a very small amount of dust could be responsible for the depleted line?ux,in turn implying much less HI,and by inference,much less molecular gas in4C+19.71.

Under dusty conditions we would expect the nebula as seen in Lyαto have a di?erent morphology and perhaps be much smaller in extent than the nebula as seen in[OIII].However, images of4C+19.71in the Lyαline(L.Max?eld&M.Dickinson,private communication)suggest the nebula has a projected size which is at least as large as the[OIII]nebula imaged here.Dust which is not uniformly distributed over the face of the galaxy,perhaps in a dusty disk or torus around the nucleus,could serve to remove much of the rest frame UV from our line of sight yet still allow ionizing photons to escape along the radio jet axis,as in the standard AGN uni?cation model(e.g.Antonucci1993).Alternatively the“average”extinction we have calculated may be dominated by small,dense clumps of dust and gas scattered throughout the nebula.

Although the average visual extinction required to bring the observed4C+19.71[OIII] 5007/Lyαline?ux ratio in line with most other powerful radio galaxies is small,it is predicated upon the assumption that the nebula is photoionized.In general this is a good assumption, since photoionization models can reproduce the emission-line?ux ratios measured in many radio galaxies(McCarthy1993).However,if there are signi?cant dynamical ionization processes such as shocks,the intrinsic[OIII]5007/Lyα?ux ratio can be as low as0.04(Dopita&Sutherland1995). In this case the required visual extinction to the4C+19.71nebula would be A V>2mag,using the measured[OIII]5007?A and Lyαline?uxes.

4.2.2.Ionized Gas

The size of the4C+19.71[OIII]nebula implies a distribution of potentially metal-rich gas on galactic scales.To determine the mass of ionized gas around4C+19.71we must know either the gas density,or the volume and volume?lling factor of the nebula.For a collisionally excited transition such as the[OIII]5007?A line,the cooling rate per unit volume in the low density limit is N2A21hν21=n e N1q12hν21(Osterbrock1974),where N2is the number density of O++ atoms in the excited state,N1is the number density of O++atoms in the lower state,A21is the radiative transition probability,and q12is the collisonal excitation rate.For the[OIII]5007?A line, A21hν21=7.5×10?14erg s?1and q12=4.1×10?9cm?3s,for T=104K(Osterbrock).The mass of O++can then be written as,

n?1e M⊙(1)

M OIII=8.7×107L5007

44

where L5007

is the luminosity in the5007?A line in units of1044erg s?1.Although we do not know 44

the ionization state of the gas from a single measurement of[OIII],if the ratio of O++/O in the nebula is about0.3(as it is in Cygnus A-Osterbrock),the total mass of ionized hydrogen is,

M=1.4×1011(n e Z)?1M⊙,(2)

where Z is the O/H ratio of the nebular gas in solar units.Thus if n e=102cm?3and Z=1, the total mass of ionized gas in the4C+19.71is about109M⊙.This is comparable to the most massive nebulae seen around low redshift radio galaxies in the light of visual emission-lines such as[OIII]5007?A and Hα+[NII](Baum&Heckman1989).

By combining these mass estimates with those made from a recombination line such as

Lyαor Hα,we can solve for the O/H ratio,given assumptions about the volume and the volume?lling factor of the gas.For pure case B recombination,the mass of ionized gas in the

)1/2M⊙or nebula as determined from the luminosity in the Lyαline is M=1.3×108(f5V68L Lyα

44

M=2.1×1010L Lyα

n?1e M⊙,where f5is the volume?lling factor in units of10?5,V68is the volume 44

is the Lyαline luminosity in units of1044erg s?1. of the nebula in units of1068cm3,and L Lyα

44

From measurements of the gas density in the optical line-emitting nebula of radio galaxies at low redshift,typical volume?lling factors of10?4?10?5have been derived(Heckman,van Breugel& Miley1984).Assuming cylindrical symmetry,we estimate the volume of the nebula in4C+19.71

=2.1(H.Spinrad,private

to be about1.5×1068cm3.Thus,if f5=1,V68=1.5,and L Lyα

44

communication),M=2.3×108M⊙,and n e=190cm?3.This assumes that the Lyαemitting nebula is of similar volume as the[OIII]nebula mapped here,and that A V=0mag.Note that if A V=1mag for the nebula and f5=1,the implied mass is closer to109M⊙and the electron density is about675cm?3.

Equating the masses derived from both the[OIII]and the Lyαlines suggests that Z~3for the nebular gas if A V=0mag.A galaxy at z=3.6might be expected to be in the early stages of its evolution and thus have low overall metallicity.Indeed,many damped Lyαabsorption line systems at high redshifts appear to have very low metallicities ranging from10?3?10?2of the solar value (e.g.Lu et al.1996),consistent with hierarchical models of structure formation(e.g.Rauch, Haehnelt&Steinmetz1997,Hellsten et al.1997).However,AGN emission lines typically imply solar or above metallicities over a large range in redshift.In high redshift quasars,this has been taken to imply early,and vigorous star formation pre-dating the AGN phase(Hamann&Ferland 1992,1993).If the[OIII]nebular gas in4C+19.71is metal-rich,it must have been processed

through stars before the observed epoch.At z=3.6,the maximum age of a stellar population is about2Gyr.This maximum age is closer to5×108yrs if the formation redshift is z~5.By comparison,values of Z~3are reproduced in the galactic chemical evolution models of Matteucci &Padovani(1993)for massive galaxies at ages of only about0.3Gyr for Salpeter(or?atter) IMF slopes.Flatter IMF spectral indices can generate O/H ratios of a few times solar after only 0.1Gyr.While O/H ratios above solar are easily explained in terms of an early starburst for4C +19.71,an A V=1mag for the nebula would imply an O/H ratio of about0.7solar,for values of the?lling factor and nebular volume given above.Since extinctions much larger than A V=1mag probably do not apply to the4C+19.71nebula overall,unless the intrinsic[OIII]5007?A?to?Lyαline?ux ratio is signi?cantly di?erent than what is seen in other powerful radio galaxies,it is unlikely that the emission-line gas has a metallicity far below solar.However,another uncertainty in this calculation is the temperature of the line-emitting gas.Lower metallicities imply hotter temperatures,stronger[OIII]emission,and lower oxygen masses.If T~2×104K,the oxygen mass is about36%of the value estimated above,and the O/H ratio closer to solar for an A V=0 mag.Thus,while early enrichment by a previous(z>3.6)episode of star formation is consistent with the data,the derivation of the metallicity of the gas is highly uncertain and values of the O/H ratio from a few tenths to a few times solar are possible.

4.2.3.Pressure Balance in the4C+19.71Radio Lobes

The radio properties of4C+19.71(under the assumption of pressure balance between the radio plasma and the gas),can also be used to estimate the mass of the nebula.Following Miley(1981), we can write the magnetic?eld strength,under energy equipartition,as B~1.5×10?3[F o/θ2S]2/7 G,where F o is the measured?ux density of the source in Jy,θ2is the surface area of the lobe in arcsec2,and S is the path length through the source in kpc.If the radio lobes have a spectral index ofα=?1.24,the volume?lling factor of the synchrotron emitting plasma is unity,and the radio lobes occupy spheres of1′′(about9kpc)diameter,each with a?ux density of0.15Jy at 1.465GHz,the minimum energy condition implies a magnetic?eld of~5×10?4G.If the radio

lobes are con?ned by static,thermal pressure from a gas at104K,a gas density of more than

6×10?21gm cm?3is implied.However,if the lobes are instead con?ned by ram pressure and we use the axial ratio of the emission line gas(approximately5:1from core to lobe)and a typical lobe advance speed of v~0.01c(Readhead et al.1996)we can estimate a lateral velocity of v~0.002c. The implied gas density is then greater than about2×10?25gm cm?3.If the volume?lling factor is10?5,the densities derived from the pressure equilibrium arguments imply gas masses of about 5×109M⊙and4×106M⊙,respectively,if the densities are representative of the entire nebula.

4.3.Continuum Luminosity of4C+19.71

In a2′′×10′′beam,the[OIII]5007?A line contributes approximately34%of the total?ux in the K band,and the observed2.2μm continuum of4C+19.71therefore has K~19.6mag,if the[OIII]4959?A line has1/3the?ux of the5007?A line.We have not subtracted a contribution from Hβto derive the continuum magnitude,but this is likely to be relatively small,perhaps 10%the?ux of the5007?A line(Eales&Rawlings1996).In order to compare the continuum magnitude to that expected from“normal”galaxies at z~3.6,we employ the luminosity function of Mobasher,et al.(1993),such that an E/S0galaxy has a characteristic absolute magnitude, M?B=?20.24mag,following Schecter(1976).Combining this characteristic absolute magnitude with the spectral energy distributions of Coleman,Wu&Weedman(1980),we estimate that the apparent K-band magnitude of an L?elliptical galaxy at z~3.6is K~23.5mag.Thus,if the rest frame visual continuum light from4C+19.71is produced by stars,it implies a40L?host galaxy. This is extremely luminous,yet the host galaxies of some radio loud quasars(Heckman et al.1991; Lehnert et al.1992;Armus et al.1997)as well as radio galaxies(Eales&Rawlings1993,1996; Evans et al.1996)at redshifts of z~2?3appear to have rest frame visual luminosities in this range.Although signi?cant fading(by2-4magnitudes in the rest frame visual)would be required for these systems to evolve into galaxies resembling even the brightest cluster ellipticals at the present epoch,this amount of fading can be accomodated by the postburst models of Chambers &Charlot(1990)and Charlot&Bruzual(1991).Of course,if much of the continuum emission

at these wavelengths in4C+19.71is scattered AGN light,the host galaxy may be signi?cantly fainter than K~19.6mag,and the amount of fading required would be correspondingly lower.

4.4.Clustering

Although the goal of this paper is not to search for young clusters or“protogalaxies”,it is worthwhile to explore the possibility of clustering around4C+19.71.At z=3.594an L?spiral galaxy has K~23.7mag.Since the3σpoint source detection limit our K-band image is K~22.5 mag,objects at the redshift of4C+19.71would be di?cult to detect in the continuum unless they were at least one magnitude brighter than an L?galaxy.However,strong sources of line emission at speci?c redshifts(z~3.6for[OIII]or z~2.5for Hα)could be detected through their excess light at2.3μm as measured with the narrow band?lter.

Besides4C+19.71itself,there are three sources with an apparent narrow-band excess at the 3σlevel or above.The bright sources G1(K~16.0mag)and G2(K~15.4mag)are well resolved, and it is likely they are foreground to4C+19.71.Infrared lines such as Brγat z~0.062,H2(1-0 S(1))at z~0.084,HeI at z~0.118,or Paαat z~0.227are possibilities for the source of the excess in G1and G2.The third source showing a signi?cant(~5σ)narrow-band?ux excess is star B.This source is intriguing since it is unresolved in the K-band image,but slightly elongated in the narrow-band2.3μm image.The true nature of this source is unknown,but its brightness (K~16.7mag)argues against it being at a redshift of z=3.6.The other ten sources plotted in Fig.3scatter about the K-NB=0.0mag line,with a total dispersion of about0.2mag,except for obj10which has a large uncertainty in its narrow-band excess.Since an excess of0.3mag corresponds to a rest frame equivalent width of about10?A at the redshift of4C+19.71,none of the remaining sources can be emission-line objects at the redshift of4C+19.71with[OIII] emission-line equivalent widths of more than about2%of that measured for4C+19.71.A source present at the3σlevel in both the K-band and the narrow-band2.3μm images would have an implied[OIII]line?ux of~2×10?20W m?2.The limiting line?ux for obj12(the faintest narrow-band source in the central part of Fig.1)is approximately10?20W m?2.If[OIII]/Hα~1,

then this?ux limit corresponds to an Hαluminosity of about2×1035W,and a star formation rate limit of about17M⊙yr?1(Kennicutt1983).The co-moving volume covered by our image is 212Mpc3at z=3.6(for H o=75km s?1Mpc?1and q o=0).Thus the space density of objects with K<22.5mag and[OIII]5007?A emission-line equivalent widths of more than about10?A in the vicinity of4C+19.71is less than5×10?3Mpc?3.Deeper images,at K and in the2.3μm?lter, would be required to place limits on L?or moderately star-forming galaxies around4C+19.71

5.Summary

Using a broad-band K and narrow-band2.3μm?lter,we have observed the z=3.594radio galaxy4C+19.71with the Near Infrared Camera on the W.M.Keck Telescope.These imaging data have allowed us to determine the following properties of the emission-line nebula in this high redshift,powerful radio galaxy:

(1)The[OIII]nebula has an extent of about74×9kpc,and is very narrow with some?aring and bending at the ends.The total luminosity of the nebula is L5007~3×1037W.The fraction of the light that is contributed by the5007?A line to the total K-band?ux is about34%.

(2)The length of the[OIII]nebula is nearly identical to the separation of the two radio lobes mapped at1465MHz by Rottgering,et al.(1994),and the position angle of the nebula is similar to that of the radio emission.4C+19.71falls on the emission-line vs.radio power correlation found for other powerful radio galaxies at low and high redshifts.Taken together,these facts suggest a direct link between the relativistic electrons and the104K gas in4C+19.71.

(3)The ratio of the[OIII]5007?A line luminosity to the total Lyαline luminosity(H.Spinrad and L.Max?eld,private communication)is~1.4,larger by a factor of about4?5than is typically seen in the spectra of powerful radio galaxies(McCarthy1993).This may indicate the presence of dust in4C+19.71,or an enhancement in the nebular O/H ratio.The nebula-averaged extinction required to produce the observed[OIII]?to?Lyαline?ux ratio is only A V~1mag if the intrinsic ratio is equal to the average value.If resonant scattering of the Lyαline is not severe,this

extinction implies an HI mass of109?1010M⊙,depending upon the distribution of the obscuring dust.

(4)We derive an ionized gas mass of2×108?109M⊙from the[OIII]and Lyαemission-line luminosities.The O/H ratio in the nebula is at least a few tenths solar,and may be as high as a factor of three above solar.The latter would argue for a starburst at z>3.6in4C+19.71.The gas masses derived by requiring that the radio lobes be either in thermal pressure or ram pressure equilibrium with the104K gas are5×109M⊙and4×106M⊙,respectively.

(5)The continuum magnitude of4C+19.71is K~19.6mag.At z=3.594,this corresponds to a 40L?E/S0galaxy.Although the fraction of the continuum light that is non-stellar in origin is not known,this value for the continuum?ux places4C+19.71along the K-z relation found for radio loud quasars and radio galaxies.

(6)There are three objects,besides4C+19.71,which have a?ux excess in the narrow band?lter. Two of these are bright(K~15.4?16.0mag)galaxies which are likely to be at low redshifts. The nature of the third object is unknown,but its brightness(K~16.7mag)also argues for a similarly low redshift.There are no candidate emission-line objects at the redshift of4C+19.71 having[OIII]rest frame equivalent width of more than about2%of the radio galaxy itself within a co-moving volume of212Mpc3.Thus the space density of objects with[OIII]emission-line luminosities of2?3×1035W and rest frame blue luminosities greater than3L?around4C+19.71 is less than about5×10?3Mpc?3.

The W.M.Keck Observatory is operated as a scienti?c partnership between the California Institute of Technology and the University of California.We thank the entire Keck sta?,especially Wendy Harrison,for making these observations possible.In addition,we thank David Hogg,Matt Lehnert,Pat McCarthy,and Tony Readhead for many helpful discussions.Hy Spinrad and Leslie Max?eld were kind enough to make their unpublished visual data available to us,and we thank them for that.The comments of an anonymous referee are also appreciated.Infrared astrophysics at Caltech is supported by grants from NASA.This research has made use of the NASA/IPAC

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