An unusual iron Lo-BAL quasar detected by ISOCAM

a r X i v :a s t r o -p h /0205022v 1 2 M a y 2002

Astronomy &Astrophysics manuscript no.(will be inserted by hand later)

An unusual iron Lo-BAL quasar detected by ISOCAM

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P.–A.Duc 1,2,P.B.Hall 3,D.Fadda 4,P.Chanial 2,D.Elbaz 2,P.Monaco 5,E.Pompei 6,B.M.Poggianti 7,

H.Flores 8,A.Franceschini 9,A.Biviano 10,A.Moorwood 11,and C.Cesarsky 11

1CNRS URA 2052

2CEA,DSM,DAPNIA,Service d’astrophysique,91191Gif–sur–Yvette Cedex,France 3

Ponti?cia Universidad Cat′o lica de Chile,Departamento de Astronom ′?a y Astrof ′?sica,Casilla 306,Santiago 22,Chile and Princeton University Observatory,Princeton,NJ 08544-1001,USA

4Instituto de Astro?sica de Canarias,Via Lactea s/n,E–38200La Laguna –Tenerife,Spain 5Dipartimento di Astronomia,via G.B.Tiepolo 11,34131Trieste,Italy 6European Southern Observatory,Santiago

7Osservatorio Astronomico di Padova,vicolo dell’Osservatorio 5,35122Padova,Italy

8DAEC/LUL,Observatoire de Paris–Meudon,5place Jules Janssen,92195Meudon,France 9Dipartimento di Astronomia,Universit`a di Padova,Vicolo dell’Osservatorio,5,I35122Padova,Italy 10INAF –Osservatorio Astronomico di Trieste,via G.B.Tiepolo,11,I–34131Trieste,Italy 11

European Southern Observatory,Karl–Schwarzschild–Strasse,2,D–85748Garching bei M¨u nchen,Germany

Accepted as a letter to A&A

Abstract.We report the discovery of an unusual low–ionization broad absorption line quasar at z =1.776which exhibits absorption lines from many excited states of Fe ii .This member of the rare class of ’FeLoBAL’quasars was serendipitously found in a mid–infrared (MIR)survey of distant clusters carried out with the ISOCAM camera.ISO J005645.1?273816has a high MIR to UV luminosity ratio,suggesting a strong dust obscuration plus emission from very hot dust.This characteristic makes MIR surveys particularly e?cient at detecting LoBAL quasars.Key words.quasars:absorption lines –quasars:individual:ISO J005645.1?273816–infrared:galaxies

1.Introduction

About 10%of optically selected quasars show absorption from gas with blueshifted out?ow velocities of typically <～0.1c (Weymann et al.,1991).These Broad Absorption Line (BAL)quasars may just be normal quasars seen along a particular line of sight,such that most quasars have BAL out?ows covering ～10–30%of the sky,with mass loss rates comparable to the quasar accretion rates (～1M ⊙yr ?1).It is also possible that BAL quasars represent a dust–enshrouded early phase in the lives of most,if not all,quasars (Becker et al.,2000).In either case,BAL out?ows must be understood to understand quasars as a whole.BAL quasars are divided into three observational sub-types depending on what type of absorption is seen.HiBAL quasars show absorption from high–ionization lines like C iv .LoBAL quasars (Voit et al.,1993)

also

2P.–A.Duc et al.:An unusual FeLoBAL quasar detected by ISOCAM

1991;Low et al.,1989;Boroson &Meyers,1992)vs.only ～1.5%in optical surveys.Dusty gas with a nearly 100%covering factor in LoBAL quasars helps explain many of their unusual properties,including ubiquitous signs of re-cent mergers in their host galaxies (Canalizo &Stockton,2001).

2.Observations

The quasar ISO J005645.1?273816(hereafter ISO 0056?2738)was serendipitously discovered as part of an on–going study of distant clusters of galaxies mapped in the mid–infrared by the ISOCAM camera on board the ISO satellite.This quasar was found in the ?eld of the z =0.56cluster J1888.16CL.

J1888.16CL was observed with ISOCAM at 6.75and 15μm in December 1997and with ISOPHOT at 200μm in December 1996.The ISOCAM data were processed fol-lowing the method presented by Fadda et al.(2000)which uses extensive simulations based of the addition of fake sources to the science images to estimate ?ux reconstruc-tion factors as well as error estimates.ISOPHOT data were reduced with the standard pipeline PIA (Gabriel et al.,1997).

B and R images of J1888.16CL were obtained in August and September 1999with the WFI camera in-stalled on the MPG/ESO 2.2m telescope at La Silla,as part of a program on supernova monitoring (PI:Patat).Near–infrared J,H and Ks broad–band images of the ?eld were taken in August 2001with SOFI on the NTT at La Silla.We retrieved a F814W image from the HST archives.The optical spectrum of ISO 0056?2738was obtained in September 2000with the FORS1instrument installed on the VLT UT1/Antu.The medium–resolution grism 300V (resolution 420)covering the wavelength range 4200?A –

8820?A was used associated with a slitlet of 1.2′′positioned

with the MOS unit.

3.Results

3.1.Source identi?cation

With a ?ux of 1.3mJy,ISO 0056?2738is one of the bright-est 15μm extragalactic sources in the ?eld of J1888.16CL.It is unambiguously associated with an object which ap-pears unresolved in the optical and near–infrared bands.On the HST image,its FWHM is lower than 0.22′′,and no clear extension is visible (see Fig.1).

The basic properties of the object,including position and photometry,are listed in Table 1.ISO 0056?2738was not detected by IRAS at 60and 100μm,nor at 200μm by ISOPHOT or at 20cm by the NRAO VLA Sky Survey (NVSS).

3.2.Spectroscopic Analysis

The VLT spectrum of ISO 0056?2738is shown in Figure 2.Extensive UV absorption plus narrow emis-

Fig.1.ISOCAM LW3(15μm)contours superimposed on a combined F606W+F814W HST image centered on the quasar.The levels are 2,2.5and 3μJy arcsec ?2.Table 1.Data

B (0.46μm)22.74±0.03mag R (0.65μm)20.95±0.02mag J (1.25μm)18.29±0.04mag H (1.65μm)17.68±0.04mag Ks (2.16μm)17.16±0.04mag L W2(6.75μm)0.51±0.15mJy L W3(15μm) 1.33±0.33mJy PHOT (200μm)<1Jy VLA NVSS (20cm)<2.3mJy

P.–A.Duc et al.:An unusual FeLoBAL quasar detected by ISOCAM

3 Fig.2.The VLT spectrum of ISO0056?2738.The horizontal dot–dashed line is the zero?ux level.Dotted lines above the spectrum show the wavelengths of the strong emission lines labeled across the top of the plot(including the expected wavelength of the C iii]line peak).Dashed lines below the spectrum show the wavelengths of the strong absorption lines from the z=1.7598system labeled across the bottom of the plot.

pear to reach zero?ux at1.7487±0.0008.The narrow absorption from the Fe iii UV34multiplet (λλλ1895.46,1914.06,1926.30)gives z=1.7598±0.0009.There are no obvious features in the C iv or Mg ii troughs at this redshift,but there is ab-sorption from Ni iiλλλ1709,1741,1751,Si iiλ1808,and Zn iiλλ2026,2062plus Cr iiλλλ2056,2062,2066.

Overall,however,the spectrum is dominated by absorption from ground and excited terms of Fe ii. Essentially every absorption trough not labelled in Figure2can be identi?ed with Fe ii.For example,the strong absorption just redward of C iv at1550–1600?A is from Fe ii UV multiplets44–46(Wampler et al.,1995;Hall et al.,2002).Fe ii multiplets from excited atomic terms are present up to at least UV191(～1787?A,excitation poten-tial～2.88eV).There may also be excited Cr ii UV5–UV8 absorption near2675and2835?A(de Kool et al.,2002). The3130?A trough could have contributions from even more highly excited Cr ii and possibly O iiiλ3133,as dis-cussed in section5.1.3of Hall et al.(2002).

Given the strength of the Fe ii absorption,the Mg ii absorption must be saturated.However,the?ux in the absorption troughs does not reach zero except for narrow regions in the C iv and Mg ii troughs.This means that the absorbing region does not cover the continuum source except at a narrow range of velocities,and that the Fe ii absorbing region probably has a smaller partial covering factor than the Mg ii and C iv regions.

4.Discussion and conclusions

The spectral energy distribution of ISO0056?2738in the rest–frame UV to infrared domain is compared in Figure3 to that of a sample of BAL quasars for which mid and far infrared?uxes are available in the literature.

The SED of ISO0056?2738appears to be similar,in shape and luminosity,to that of the nearby optically se-lected low ionization BAL quasar PG1700+518,at least within the wavelength range for which data are available for both objects.If this similarity extends to the far in-frared where IRAS?uxes are available for PG1700+518, one would infer for ISO0056?2738a total infrared lumi-nosity of L IR=4×1012L⊙.Instead one may use as tem-plate Mrk231,which is known as the nearest galaxy with a LoBAL signature,and has a strong IR bump at60μm. Taking the rest–frame5μm luminosity as the normaliz-ing factor,the total infrared luminosity of ISO0056?2738

4P.–A.Duc et al.:An unusual FeLoBAL quasar detected by

ISOCAM

Fig.3.Rest–frame spectral energy distribution of ISO J005645.1?273816(plain circles)and,for compari-son,of other BAL quasars with IR data:Mrk 231(LoBAL;z=0.04;data from NED);PG 1700+518(LoBAL;z=0.292;Andreani et al.,1999),ELAIS J164010+410502(BAL;z=1.099;Morel et al.,2001);FIRST J155633.7+351757(LoBAL;z=1.48;Clavel,1998),H1413+117(BAL;z=2.54;Barvainis et al.,1995).Note the lines joining the data points,indicated by plain squares,were drawn to guide the eye and by no means represent the real SEDs.We used H 0=75km s ?1Mpc ?1and q 0=0.5.

becomes 2×1013L ⊙.The PHOT measurement at 200μm ?ux indicates that this is most likely an upper limit.One may conclude for this analysis that unless its SED plunges at λ>10μm,ISO 0056?2738belongs to the class of ULIRGs and may even be a hyperluminous in-frared galaxy,like many quasars (e.g.Haas et al.,2000).ISO 0056?2738lies behind a rich cluster of galaxies;how-ever the distance from the cluster center,about 3.5′,is such that a gravitational lensing by the cluster is unlikely.The spectral energy distribution of the quasar in the rest–frame ultraviolet is much redder than that of the SDSS composite quasar of Vanden Berk et al.(2001).Using the SMC extinction curve of Prevot et al.(1984),we estimate a large but not unprecedented extinction of E (B ?V )=0.25±0.05.Besides,ISO 0056?2738seems to share with the few other LoBAL quasars with available mid–IR data (see Fig.3)a steep UV to MIR spectral index.In addition to the extinction in the UV,its very high MIR to UV luminosity ratio (rest–frame νL ν(5.40μm)/νL ν(0.16μm)=12)is likely due to the presence of unusually large quantities of very hot dust at 1000K,the emission of which peaks at about 3μm.Becker et al.(1997)claim that a relative excess of radio emission could be another property of LoBAL quasars.Note that ISO 0056?2738is not likely to be radio–loud (log(R ?)>1).It has log(R ?)<1.79using the Becker et al.(2000)de?-nition with αrad =?0.5,but corrected for reddening of E (B ?V )=0.25it has intrinsic log(R ?)<1.05.

To our knowledge,ISO 0056?2738is the ?rst FeLoBAL quasar so far directly found at mid–infrared wavelengths 1.At optical wavelengths,the SDSS Early Data Release Quasar Sample (Schneider et al.,2002)only contains four FeLoBAL quasars out of 200–300BAL quasars at 1.485

Acknowledgements.PBH acknowledges ?nancial support from Chilean grant FONDECYT/1010981.We are grateful to our referee,https://www.sodocs.net/doc/054678502.html,cy,for his comments which helped clarifying the paper.We thank G.Rodighiero for her help in reducing the PHOT data,E.Cappellaro and F.Patat for providing us the optical images of J1888.16CL.

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