The stellar content of low redshift BL Lac host galaxies from multicolour imaging
T. Hyvonen, J. K. Kotilainen, R. Falomo, E. Orndahl, T. Pursimo
aa r X i v : . [ a s t r o - ph ] S e p Astronomy&Astrophysicsmanuscript no. blcol˙final1 c (cid:13)
ESO 2018October 26, 2018
The stellar content of low redshift BL Lac host galaxies frommulticolour imaging
T. Hyv¨onen , J.K. Kotilainen , R. Falomo , E. ¨Orndahl and T. Pursimo Tuorla Observatory, University of Turku, V¨ais¨al¨antie 20, FIN–21500 Piikki¨o, Finlande-mail: [email protected], [email protected] INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italye-mail: [email protected] Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spaine-mail: [email protected]
Received; accepted
ABSTRACT
Context.
We present B -band imaging of 18 low redshift (z ≤ R -band and the near-infrared H -band. For a subset of the objects, also U - and V -band imaging is presented. Aims.
These multiwavelength data are used to investigate the blue–red–near-infrared colours and the colour gradients of the hostgalaxies of BL Lacs in comparison with other elliptical galaxies with and without nuclear activity.
Methods.
For all the BL Lacs observed in the B - and V -bands, and all objects at z < .
15 in the U -band, the host galaxy is clearlyresolved. In all cases galaxies are well represented by an elliptical model, with average absolute magnitude M B = − . ± . R e = ± Results.
The best-fit B -band Kormendy relation of ( µ e = e (kpc) + − ) is in reasonable agreement withthat obtained for normal ellipticals and radio galaxies. This structural and dynamical similarity indicates that all massive ellipticalgalaxies can experience nuclear activity without significant perturbation of their global structure. The distributions of the integratedblue / near–infrared colour (with average B – H = ± ∆ ( B – R ) /∆ (log r) = -0.14 ± / or steeper colour gradientsthan those in normal ellipticals. Conclusions.
The blue colours are likely caused by a young stellar population component, and indicates a link between star formationcaused by an interaction / merging event and the onset of the nuclear activity. This result is corroborated by stellar population modelling,indicating a presence of young / intermediate age populations in the majority of the sample, in agreement with low redshift quasar hosts.The lack of strong signs of interaction may require a significant time delay between the event with associated star formation episodesand the start of the nuclear activity. Key words. galaxies: active – galaxies: BL Lacertae objects: general – galaxies: interactions – galaxies: nuclei – galaxies: photometry– galaxies: structure
1. Introduction
BL Lac objects are an extreme class of active galactic nuclei(AGN) characterized by luminous, variable and polarized con-tinuum emission across the electromagnetic spectrum and strongcore-dominated radio emission with apparent superluminal mo-tion (e.g., Kollgaard et al. 1992). These properties suggest thatthey are strongly beamed objects dominated by synchrotronemission from a relativistic jet aligned close to the line of sight(Blandford & Rees 1978). BL Lac objects have many similar-ities to flat spectrum radio quasars (FSRQ) and they are oftengrouped together as blazars. According to the unified modelof radio-loud AGN (Urry & Padovani 1995), the parent popu-lation of BL Lac objects and FSRQs are low luminosity core-dominated FR I radio galaxies (RG), and high luminosity lobe-dominated FR II RGs, respectively. Consequently, their orienta-tion independent properties, such as host galaxies and environ-ments, should be identical to those of their parent populations.
Send o ff print requests to : T. Hyv¨onen A number of optical (e.g., Falomo & Kotilainen 1999;Urry et al. 2000; Nilsson et al. 2003; Heidt et al. 2004) and near-infrared (NIR) (e.g., Kotilainen et al. 1998; Scarpa et al. 2000;Cheung et al. 2003; Kotilainen & Falomo 2004; Kotilainen et al.2005) imaging studies have shown that virtually all nearby ( z < .
5) BL Lac objects are hosted in large and luminous ellipticalgalaxies, with average M R ∼ − . R e ∼
10 kpc, similarly to both FR I and FR II RGs (Govoni et al.2000). BL Lac hosts are much brighter than L ∗ galaxies (thecharacteristic luminosity of the Schechter luminosity function: M ∗ R = − .
2, Gardner et al. (1997); Nakamura et al. (2003)).While the morphology, such as jets and close companions, ofsome of the hosts indicates a recent interaction, the large major-ity of them are indistinguishable from inactive massive ellipti-cals at similar redshift (Scarpa et al. 2000).Until recently, however, imaging of BL Lac host galaxieswas obtained in one band only (usually R -band). Therefore,little colour information exist for them, especially in the bluedomain of the spectrum, where only a few objects havebeen studied in the B -band and even fewer in the U -band. Hyv¨onen et al.: The stellar content of low redshift BL Lac host galaxies
Kotilainen & Falomo (2004) observed a sample of 23 low red-shift ( z < .
3) BL Lac objects in the H -band and combined withprevious H -band imaging (Kotilainen et al. 1998; Scarpa et al.2000; Cheung et al. 2003) and optical R -band data from litera-ture (Falomo & Kotilainen 1999; Urry et al. 2000), investigatedthe integrated R – H colours and colour gradients of a sample of41 BL Lac host galaxies. They found that BL Lac host galax-ies appear to be systematically bluer than inactive ellipticals andhave a much wider distribution of host galaxy R – H colour (av-erage R – H = . ± .
4) and steeper colour gradient (average ∆ ( R – H ) / ∆ ( logr ) = − . ± .
60) than those for inactive ellipti-cal galaxies with dominant old stellar population (Peletier et al.1990). Similar behaviour has been found for the colours andcolour gradients in low redshift RGs (Govoni et al. 2000) andin low and intermediate redshift AGN and quasars with ellipti-cal hosts (Schade et al. 2000; ¨Orndahl et al. 2003; Jahnke et al.2004; Sanchez et al. 2004). The blue colours are most likelycaused by a young stellar population, indicating a recent star for-mation (SF) episode, possibly triggered by interaction or merg-ing between galaxies. The wide distribution of colours prob-ably reflects an object-to-object di ff erence in the age of themost recent SF episode. Note that there is also spectroscopicevidence for young / intermediate age populations in AGN hosts(e.g., Nolan et al. 2001; Raimann et al. 2005).In this study, we present multicolour optical imaging of asubsample of 18 BL Lac objects from the large, homogeneoussample of 41 sources for which high resolution R - and H -band imaging exists (Kotilainen et al. 1998; Scarpa et al. 2000;Cheung et al. 2003; Kotilainen & Falomo 2004). Most of the ob-served objects have bluer R – H host colour than inactive ellipti-cals and blue band observations are paramount to assess whethertheir blue colours are caused by a young stellar population. Allthe 18 BL Lac objects were observed in the B -band, while fora subsample, we obtained also U - and V -band imaging. The U -band observations were mainly restricted to the most nearby tar-gets in the sample. Combining the deep high spatial resolution U -, B - and V -band imaging with the existing R - and H -banddata, we are able to derive the colours and colour gradients overan extended wavelength range, where the H -band is sensitive toold stellar population that dominates the host galaxy mass, whilethe blue part of the spectrum traces the contribution from youngstellar populations to the excess blue light in the host galaxies.We also derive structural properties, such as morphology ande ff ective radius, as well as the absolute magnitude of the hostgalaxies in each observed band. The obtained U BVRH broadband colours are used in conjunction with stellar synthesis pop-ulation models to estimate the ages of the most recent SF episodein the host galaxies.In Section 2, we describe the sample, observations, datareduction, and methods of analysis. In Section 3, we presentthe results and discussion concerning the properties of the hostgalaxies. Summary and conclusions are given in Section 4.Throughout this paper, H =
70 kms − Mpc − , Ω m = . Ω Λ = .
2. Observations, data reduction, and analysis
The observations were carried out during several observing runswith di ff erent telescopes, with most of the observations done atthe 2.5m Nordic Optical Telescope (NOT) with the ALFOS C in-strument using Bessel U , B and V broad band filters. Nine targetswere observed only in the B -band, five in the U BV -bands andfour in the
U B -bands. One target (MRK 421) was additionallyobserved in the H -band, to complement the sample presented in Table 1.
Properties of the telescopes and instruments. a Band Telescope + instrument N obj Scalearcsec px − (1) (2) (3) (4) U , B , V NOT / ALFOSC 26 0.190 U , B NOT / MOSCA 3 0.217 H NOT / NOTCam 1 0.235 U , B ESO NTT / EMMI 3 0.273 a Column (1) gives the observed band; (2) the telescope and instru-ment used; (3) the number of observed targets, and (4) the spatialscale of the instrument.
Kotilainen & Falomo (2004). The B-band image of 3C 371, pub-lished in Nilsson et al. (1997), was kindly provided for us by K.Nilsson. A summary of the telescopes and instruments used isgiven in Table 1. Seeing during the observations varied between0.9 and 2.3 arcsec FWHM (average 1 . ± . . This consisted of bias subtraction, flat field division andcosmic ray rejection. For each night, bias and flat field imageswere made from several median combined bias frames and twi-light flat field exposures and cosmic rays were rejected usingIRAF procedures. Finally, individual images were aligned andcombined to form the final image of the target. The observa-tions were mostly done in photometric conditions (see Table 2)and several photometric standard stars from Landolt (1992) wereobserved during each night. Some targets were, however, ob-served during non-photometric nights and for these objects ad-ditional short exposures were subsequently obtained in photo-metric conditions to calibrate these frames using reference starsin the field. K -correction from Poggianti (1997) was applied tothe host galaxy magnitudes, but not to the nuclear magnitudes,since the nuclear component can be assumed to have a power-law spectrum ( f ν ∝ ν − α ) with α ∼ −
1. Absolute magnitudes werealso corrected for interstellar extinction calculated for each bandfrom the R -band extinction coe ffi cient from Urry et al. (2000).To derive the properties of the host galaxies, azimuthally av-eraged 1D radial luminosity profiles were extracted for each BLLac object and for a number of field stars. Any obvious extra fea-tures, such as nearby companions and / or foreground stars, weremasked out from the image to avoid contamination of the radialluminosity profile. To obtain an accurate model for the nuclearregion of the targets, it is important to have a well defined pointspread function (PSF). In most cases there were a number ofsuitable stars in the relatively large field of view surrounding theobjects to form a reliable PSF. The core and the wing of the PSFwere derived from a faint and a bright star in the frame, respec-tively, and they were combined to form the final PSF model. Thisfinal PSF was compared with the profiles of individual stars inthe frame to assure that this procedure resulted in a good and sta-ble representation of the true PSF. A representative case of thisPSF comparison is shown in Fig. 1, where the individual stellarprofiles are in good agreement with the adopted PSF model well IRAF is distributed by the National Optical AstronomyObservatories, which are operated by the Association of Universitiesfor Research in Astronomy, Inc., under cooperative agreement with theNational Science Foundation.yv¨onen et al.: The stellar content of low redshift BL Lac host galaxies 3
Table 2.
The sample and the journal of observations. a Name z V M B Filter T exp Date FWHM Photometric?(sec) (arcsec)(1) (2) (3) (4) (5) (6) (7) (8) (9)1ES 0229 +
200 0.139 18.0 -21.7 U / / B
900 12 / / V / / B
480 14 / / B
900 15 / / +
591 0.125 19.5 -21.0 U / / B / / V
600 27 / / U
900 08 / / B
300 08 / / H
150 25 / / U
900 06 / / B / / +
078 0.136 16.1 -23.6 B / / + B / / +
244 0.140 15.4 -24.3 B / / +
546 0.152 15.7 -23.7 U
900 07 / / B
700 07 / / +
428 0.129 16.5 -22.5 B / / +
122 0.162 17.0 -22.9 B / / U
700 07 / / B
900 15 / / + B / / U
600 07 / / B
240 08 / / V
180 08 / / U
500 07 / / B
900 25 / / V
300 08 / / U / / B / / V
480 08 / / B
720 01 / / a Column (1) gives the name of the BL Lac object; (2) the redshift; (3) the V -band apparent magnitude; (4) the B -band absolute magnitude; (5)the filter used; (6) the total integration time; (7) the date of the observation; (8) the seeing FWHM and (9) photometric (Y) or non-photometric(N) conditions. into the domain where the host galaxy becomes dominant overthe PSF. The only exception was the field of MRK 421 wherethere are no suitable stars available. In this case, the PSF wasestimated using standard stars observed during the same nightwith similar seeing conditions.The luminosity profiles were decomposed into a point source(represented by the PSF) and an elliptical galaxy components byan iterative least-squares fit to the observed profile. There arethree free parameters in the fit: the PSF normalization, the hostgalaxy normalization and the e ff ective radius of the host galaxy.The data were fit using the r / de Vaucouleurs law for ellipti-cal galaxies to represent the host galaxy. The host galaxy wasconsidered to be resolved if the PSF + host galaxy fit resulted ina considerably lower χ value than the PSF fit only. The uncer-tainty in the derived host galaxy magnitudes was estimated to be ∼ .
3. Results
From the images 1D azimuthally averaged radial luminosity pro-files of the BL Lac objects were extracted in the U -, B - and V - band. The profiles together with the best-fit model overlaid arepresented in Appendix in online version of the paper.We were able to clearly resolve the host galaxy in all objectsin the B - and V -bands, and in 8 / U -band. The onlytarget that remained unresolved in the U -band, PG 1418 + Fig. 2 shows the distribution of the B -band absolute magnitudesof the BL Lac host galaxies (this work), low redshift radio-loud(RLQ) and radio-quiet (RQQ) quasar hosts from Jahnke et al.(2004), low redshift RGs from Govoni et al. (2000) and inac-tive elliptical galaxies from Peletier et al. (1990), Colbert et al.(2001) and Bower et al. (1992). The average B -band absolutemagnitudes of the BL Lac objects, quasar and RG host galax-ies in these samples are presented in Table 4, where all absolutequantities were transformed into the cosmology adopted here.Since the selection criteria of the various samples are some-what non-homogeneous, there is a possibility of selection ef-fects. They are likely to a ff ect absolute magnitudes but since Hyv¨onen et al.: The stellar content of low redshift BL Lac host galaxies
Table 3.
Properties of the host galaxies. a Name Filter A z m nuc m host µ e r e R e M nuc M host N / H mag mag mag mag arcsec arcsec kpc mag mag(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)1ES 0229 + U B V B B + U B V U B H U B + B + B + B + U B + B + B U B + B U B V U B V U B V B a Column (1) gives the name of the object; (2) the filter; (3) the interstellar extinction in the U , B - and V -bands; (4) the redshift of the object; (5)the apparent nuclear magnitude; (6) the apparent host galaxy magnitude; (7) surface brightness at the e ff ective radius (8) and (9) the apparentand absolute e ff ective radius, respectively; (10) the absolute nuclear magnitude; (11) the absolute host galaxy magnitude and (12) the absolutenuclear / host luminosity ratio. the main focus of this study is on the colour properties of BLLac hosts as compared with other ellipticals, our main resultsare only marginally a ff ected by selection e ff ects. Furthermore,our BL Lac sample is well matched to RGs and quasars in red-shift (see Table 4) to minimize any bias introduced by possibleevolutionary e ff ects.The average B -band absolute magnitude of the 18 BL Lachosts is M B = − . ± .
7, i.e. ∼ M B = − . ± . M ∗ B + . M ∗ B + .
5, where M ∗ B = − . M B = − . ± .
7; (Colbert et al.2001)) but is ∼ M B = − . ± . ff erent orientation. The classification of BL Lacobjects with FR Is is based on their radio morphology, show-ing that BL Lac objects have similar extended radio propertiesto FR Is (e.g., Antonucci & Ulvestad 1985) but, however, thereare some indications that the radio properties of at least someBL Lac objects are similar to FR IIs rather than FR Is (e.g.,Kollgaard et al. 1992; Stanghellini et al. 1997). There is also ev-idence that some BL Lac objects share the luminosity propertiesof FR IIs (Cassaro et al. 1999). The average B -band luminos-ity of low redshift FR I and FR II RGs is M FRI = − . ± . M FRII = − . ± .
6, respectively, obtained by Govoni et al.(2000). BL Lac hosts appear, therefore, to be slightly fainter thanboth FR Is and FR IIs, but based on Kolmogorov-Smirnov statis-tics, the luminosity distributions for the BL Lac hosts and thecombined sample of RGs are indistinguishable. Our result sug-gests that both FR I and FR II RGs can be considered as a parentpopulation of BL Lac objects, consistent with the result obtainedby Falomo & Kotilainen (1999).The average e ff ective radius of the BL Lac host galaxies inthe B -band is R e = . ± . yv¨onen et al.: The stellar content of low redshift BL Lac host galaxies 5 Table 4.
Comparison of the average B -band host galaxy properties. a Name N < z > < M B ( host ) > < R e > mag kpc(1) (2) (3) (4) (5)BL Lacs (this work) 18 0 . ± . − . ± . . ± . . ± . − . ± . . ± . . ± . − . ± . . ± . . ± . − . ± . . ± . a Column (1) gives the sample; (2) the number of objects; (3) the average redshift; (4) the average host galaxy magnitude, and (5) the averagee ff ective radius ( < R e > for RGs are in R -band). Fig. 1.
Upper panel: Comparison of the PSF profile extractedfrom field stars (the solid line) with the profiles of individualstars (dotted lines) in the field of BL Lac object. Lower panel:The di ff erence between the profiles of individual stars and thePSF model.1999) that BL Lac objects are hosted in large elliptical galax-ies. BL Lac hosts are of similar size in the B -band to quasarhost galaxies ( R e = ± . R -band ( R e = . ± . ff ect.It is well known that the e ff ective radius of inactivelate-type galaxies increases toward shorter wavelengths (e.g., M¨ollenho ff & Heidt 2001). This dependence delineates the mor-phology of SF regions in the galaxies, in the sense that SF (bluebands) occurs in the spiral arms while the old stellar population(red bands) dominates the bulge component. On the other hand,de Grijs (1998) found no change in the e ff ective radius from B -to K -band for inactive early-type galaxies (up to Sa spirals). Theaverage e ff ective radii of our BL Lac host galaxy sample are R e ( U ) = . ± . R e ( B ) = . ± . R e ( V ) = . ± . H -band ( R e ( H ) = . ± . R -band ( R e ( R ) = ± ff ectiveradius of the early-type AGN host galaxies does not decreasewith wavelength. If anything, one can note a slight increasingtrend. However, note that this result should be taken with cau-tion because of the relatively large errors in determining the ef-fective radius depending on the well known degeneracy betweensurface brightness and e ff ective radius. According to the Kormendy relation, there is a tight relationbetween the e ff ective radius R e and the surface brightness µ e (Kormendy 1977; Kormendy & Djorgovski 1989). This rela-tion is a 2D projection of the 3D Fundamental Plane (e.g.,Dressler et al. 1987; Djorgovski & Davis 1987) that links R e and µ e with the stellar velocity dispersion σ . This relation, well es-tablished for inactive elliptical galaxies and the bulges of spiralgalaxies in nearby clusters (e.g., J¨orgensen et al. 1993, 1996),is related to the morphology and the dynamical structure of thegalaxies and gives important information about their formationprocesses, indicating underlying regularity within the galaxypopulations.Fig. 3 presents the B -band Kormendy relation for our sam-ple of 18 BL Lac objects, compared with that for nearbyRGs (Govoni et al. 2000), isolated inactive elliptical galaxies(Reda et al. 2005) and inactive ellipticals in the Coma cluster(J¨orgensen et al. 1993). The corresponding surface brightnesses µ e and e ff ective radii R e for the BL Lac hosts are given in Table 3.The surface brightnesses of the host galaxies were corrected forGalactic extinction and cosmological dimming (10 × log(1 + z)).Note that the surface brightnesses µ e and e ff ective radii R e referto the isophote that contains half of the total luminosity of thegalaxy. Another commonly used definition for µ e and r e is to de-rive these values directly from the de Vaucouleurs fitting of thegalaxy, but that definition is model dependent.The best-fit linear relation for the BL Lac hosts is µ e = e (kpc) + − . It is somewhat steeper thanthat for inactive elliptical galaxies and RGs. It is, however, con- Hyv¨onen et al.: The stellar content of low redshift BL Lac host galaxies
Fig. 2.
Histogram of the absolute B -band magnitudes of the BLLac hosts (top panel; this work), low redshift ( z < .
2) RLQand RQQ hosts (second panel; (Jahnke et al. 2004)), low red-shift RGs (third panel; (Govoni et al. 2000)) and inactive ellip-tical galaxies (bottom panel; Peletier et al. (1990); Colbert et al.(2001) and Bower et al. (1992)). The vertical short dashed linerepresent the luminosity of the L ∗ galaxies.sistent with the relation obtained by Falomo & Kotilainen (1999)and Kotilainen & Falomo (2004) for BL Lac hosts in the R - and H -bands, indicating that the dynamical structure of the hostsdoes not change with wavelength. Note that one BL Lac object(MS 1552.1 + µ e − R e -relation of the sample, towards large e ff ective radiusand high surface brightness. Note that MS 1552.1 + B – H = Fig. 3.
The B -band µ e − R e Kormendy relation for the BL Lachost galaxies (filled symbols), and for inactive elliptical galaxies(asterisks; J¨orgensen et al. (1993) and Reda et al. (2005)). Thesolid, dotted and long dashed lines represent the best linear fitfor BL Lac hosts, inactive ellipticals (J¨orgensen et al. (1993) andReda et al. (2005)), and low redshift RGs (Fasano et al. (1996)),respectively.rienced a phase of nuclear activity in the past with little influenceon the global structure of the galaxy.
The host galaxies of all the BL Lac objects in our sample werepreviously studied in the optical R -band and in the NIR H -band(Falomo & Kotilainen 1999; Kotilainen & Falomo 2004). Thenew observations presented here represents the first host galaxystudy performed in the blue part of the spectrum. The combina-tion of blue, optical and NIR data allows us to assess the issue ofthe optical-NIR colours of the BL Lac host galaxies. While the H -band is sensitive to old stellar populations, the blue domainof the spectrum is especially important in studying SF and thepresence of a young stellar population component in the galax-ies. The integrated rest-frame U – B , B – V and B – R colours ofour BL Lac sample are given in Table 5. The average coloursof the BL Lac hosts, compared to those observed in RLQand RQQ host galaxies (Jahnke et al. 2004), RGs (Govoni et al.2000), and inactive elliptical galaxies (Peletier et al. 1990;Bower et al. 1992; Colbert et al. 2001), and theoretically pre-dicted for inactive ellipticals (Fukugita & Ichikawa 1995;Fioc & Rocca-Volmerange 1999) are presented in Table 6.Because U -band data is available only for eight objects in oursample, we prefer to use the B – H colour as the longest baselinecolour.The average B – H colour of the BL Lac hosts is B – H = . ± . B – H = . ± .
3, Jahnke et al. (2004)) but slightly bluer than that ofinactive elliptical galaxies ( B – H = . ± .
3, Colbert et al. (2001)). yv¨onen et al.: The stellar content of low redshift BL Lac host galaxies 7
Fig. 4.
The distribution of the B – R colour for BL Lac hosts (toppanel; this work), low redshift ( z < .
2) RLQ and RQQ hosts(second panel; Jahnke et al. (2004)), low redshift RGs (thirdpanel; Govoni et al. (2000)) and inactive elliptical galaxies (bot-tom panel; Colbert et al. (2001) and Peletier et al. (1990)).The same situation applies when considering the B – V and B – R colours (Table 6). In fact, the optical colours of the BL Lachosts are very similar to those of intermediate / late-type (Sb -Sbc) inactive galaxies with significant ongoing SF (Table 6).Fig. 4 shows the distribution of the B – R colour for BL Lac hosts,quasar hosts (Jahnke et al. 2004), RGs (Govoni et al. 2000) andinactive ellipticals (Colbert et al. 2001). BL Lac hosts clearly ex-hibit a much wider colour distribution and have a significantlybluer host galaxy population that is not present for inactive el-lipticals.It has long been known that the integrated colours of ellip-tical galaxies become redder toward higher luminosity (mass).(e.g., Bower et al. 1992; Kodama & Arimoto 1997). This colour-magnitude relation links the properties of the stellar populationsof early-type galaxies with their structural properties and pro-vides important information about their formation and evolu- tion. It depends on the combined e ff ects of age and metallicityon the dominant stellar population (more massive galaxies areboth older and more metal-rich than less massive galaxies). It isidentical in di ff erent galaxy clusters, such as in Virgo and Comaclusters (Bower et al. 1992) and overall shows little dependenceon environment (Terlevich et al. 2001; Bernardi et al. 2003)and redshift (e.g., Aragon-Salamanca et al. 1993; Kodama et al.1998; Holden et al. 2004), indicating that massive ellipticalsformed in an intense starburst at high redshift followed by pas-sive evolution, with no major SF episodes since z ∼ B – R vs. R and B – H vs. H colour-magnitude diagramsfor the BL Lac hosts (this work), low redshift RLQ and RQQhosts (Jahnke et al. 2004), RGs (Govoni et al. 2000), inactive el-lipticals (Peletier et al. 1990; Colbert et al. 2001) and ellipticalsin Virgo and Coma clusters (Bower et al. 1992) are presentedin Fig. 5. It is evident that the BL Lac hosts do not followthe relatively tight colour-magnitude relation of inactive ellip-ticals. Instead, they have a significantly broader colour distribu-tion and the majority of them appear to be bluer than inactiveellipticals of similar luminosity. Indeed, such colours are moresimilar to those found in intermediate / late-type inactive galax-ies that have significant recent SF. This result is consistent withthe colours of low redshift RGs (Govoni et al. 2000) and quasarhosts (Jahnke et al. 2004). Although this colour di ff erence mayindicate that the colour-magnitude relation for elliptical galaxiesbreaks down at high luminosities as suggested by Govoni et al.(2000) and Kotilainen & Falomo (2004), note that the ellipticalgalaxies cover the same luminosity range as the BL Lac hosts.A more likely explanation for the blue colours of BL Lac hosts(and the other AGN hosts in the diagrams) is that they have expe-rienced recent SF. The wide colour distribution indicates a rangeof timescale since the latest SF episode, such that the bluest hostshave experienced the most recent SF whereas the reddest hostshave experienced little or no SF and are dominated by an old stel-lar population similar to those in inactive ellipticals. Especiallynote that there is one BL Lac host exhibiting red B – R colour ( B – R > .
0) and two hosts having red B – H colour ( B – H > . B – R = . ± .
1) in RLQ and RQQ hosts(Jahnke et al. 2004). Similarly blue host colours were also foundin low redshift quasars by Schade et al. (2000).
It is well known that nearby inactive elliptical galaxies donot have uniform colour but instead have negative colour gra-dients, i.e. they become bluer with increasing radius (e.g.,Peletier et al. 1990). These colour gradients have been widelyinterpreted as due to radial variations in the dust content and / orSF history of the galaxies (e.g., Kim 1989; Peletier et al. 1990;Goudfrooij et al. 1994; Michard 2000). As dust reddening is rel-atively small in ellipticals, while normal early-type galaxies canbe dusty (Tran et al. 2001), most of the colour gradients are usu-ally ascribed to the combined e ff ect of stellar metallicity andage gradients across the galaxies, with the outer regions beingyounger and / or having lower metallicity.For the BL Lac host galaxies, we derived the radial U – B , B – R and B – H colour gradients using the R - and H -bandhost galaxy luminosity profiles from Kotilainen et al. (1998);Falomo & Kotilainen (1999); Kotilainen & Falomo (2004);Scarpa et al. (2000). The B – R colour profiles are presented inFig. 6 and all colour gradients are presented in Table 5. Each Hyv¨onen et al.: The stellar content of low redshift BL Lac host galaxies
Table 5.
Colours and colour gradients of the BL Lac host galaxies. a Name U – B B – V B – R R – H ∆ ( U – B ) / ∆ ( logr ) ∆ ( B – R ) / ∆ ( logr ) ∆ ( B – H ) / ∆ ( logr )(1) (2) (3) (4) (5) (6) (7) (8)1ES 0229 +
200 0.4 0.3 2.0 2.5 0.92 –0.44 –0.17PKS 0521-365 0.9 0.9 2.3 –1.02 –1.66PKS 0548-322 1.1 2.1 1.10 –0.901H 0706 +
591 0.9 0.8 1.3 2.2 –0.01 0.17 –0.12MRK 421 –0.2 2.2 2.2 0.17 –0.91 –0.31MRK 180 0.4 0.5 2.7 0.75 –1.24 –0.911ES 1212 +
078 1.9 2.2 0.48 –0.85MS 1229.2 + +
244 0.9 2.1 –1.08 –0.91PG 1418 +
546 1.4 2.2 –0.21 0.861ES 1426 +
428 0.8 2.5 0.90 1.191ES 1440 +
122 1.6 2.4 0.16 –0.11AP LIBRAE –0.5 1.1 2.2 0.17 0.43 –0.31MS 1552.1 + a Column (1) gives the name of the object; (2) the U – B colour; (3) the B – V colour; (4) the B – R colour; (5) the R – H colour (fromKotilainen & Falomo (2004)); (6) the U – B colour gradient; (7) the B – R colour gradient and (8) the B – H colour gradient of the host galaxy. Table 6.
Average colours of the host galaxies. a The sample U – B B – V B – R R – H B – H (1) (2) (3) (4) (5) (6)BL Lacs (this work) 0 . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . a Column (1) gives the sample; and columns (2) – (6) the average U – B , B – V , B – R , R – H and B – H colour, respectively. gradient was cut at the radius < ∆ ( B – R ) / ∆ ( logr ) = − . ± .
75. The same neg-ative trend was also found for the R – H colour gradients of BLLacs ( ∆ ( R – H ) / ∆ ( logr ) = − . ± .
60 by Kotilainen & Falomo(2004)). The amplitude of the B – R colour gradients of the BLLac hosts is consistent with the average B – R gradient of RGs ∆ ( B – R ) / ∆ ( logr ) = − . ± .
17 (Govoni et al. 2000) and inac-tive ellipticals ∆ ( B – R ) / ∆ ( logr ) = − . ± .
07 (Peletier et al.1990), but the distribution is much wider than that of the RGsand inactive ellipticals. Similar distribution was also obtainedfor the R – H gradient of BL Lac hosts by Kotilainen & Falomo(2004).However, note that some BL Lac host galaxies have littleevidence for colour variation or even have an inverted (posi-tive) colour gradient, the steepest of them in PKS 0548-322, 1ES 1426 +
428 and MRK 501. Similarly steep positive R – H colour gradient for 1ES1426 +
428 was previously observed byKotilainen & Falomo (2004). Since the B – H colour is moresensitive to the dust content than the R – H colour, it indicatesthat the positive B – R and R – H gradients of this target aredue to radial variations in the dust content of its host galaxy.Indeed, recent HST observations of RGs (e.g., Martel et al.2000; Tremblay et al. 2007) have shown that there is a signif-icant number of RGs that contain dust in a variety of spatialdistributions, such as circumnuclear disks and dust lanes at kpcscales. On the other hand, dust is not a reasonable explanationfor e.g. MRK 501 which has a positive B – R but a negative R – H gradient. In this case, the inverted profiles indicates SF in theinner region of the host galaxy because the host galaxy is well re-solved. That indication is also supported by recent spectroscopicobservations of ongoing SF in the nuclear regions of a BL Lacobject (PKS 2005-489; Bressan et al. (2006)).Fig. 7 shows the U – B colour profiles for the eight BL Lachost galaxies for which we have the available data (Table 5). yv¨onen et al.: The stellar content of low redshift BL Lac host galaxies 9 Fig. 6.
The B - R colour profiles for the BL Lac host galaxies, derived using the R -band data from Falomo & Kotilainen (1999);Scarpa et al. (2000) and Kotilainen & Falomo (2004).All the host galaxies, except one (3C 371), have a positivecolour gradient. 3C 371 has some signature of recent interaction(Nilsson et al. 1997) which might be the reason for the negative U – B colour gradient. Average colour gradient for all eight ob-jects is ∆ ( U – B ) / ∆ ( logr ) = . ± .
46, although for four objectsthe gradient is very flat.
The
U BV colours combined with the previously observed R - and H -band data can be used in conjunction with stellar synthesispopulation models to estimate the approximate ages of the recentSF episodes in the BL Lac host galaxies. In this context, the U -and B -bands are especially important as they provide photometry shortward of the 4000 Å break, without which only approximateaverage ages can be determined. Note also that dust reddeninghas little e ff ect, since any reddening only would imply even bluerintrinsic colours. The SED modelling utilizes all the colour in-formation simultaneously by fitting the SED of evolution syn-thesis model to calibrated fluxes of the host galaxies at di ff erentwavelengths. Each wavelength band represents one data point inthe SED of the object. Comparing fits made for di ff erent modelspectra gives an estimate of the ages of the dominant stellar pop-ulation components in the host galaxy. Such SED modelling hasbeen performed by Jahnke et al. (2004) for low redshift RLQ andRQQ hosts. For the analysis we used the PEGASE2 evolution-ary model (Fioc & Rocca-Volmerange 1997) and a single stellarpopulation (SSP) model of single metallicity because the age- Fig. 7.
The U – B colour profiles for BL Lac hosts.metallicity degeneracy in models cannot be resolved with mul-ticolour data only. We used instantanous burst models which as-sume that the young stellar population is formed in a short burstwith an IMF and evolves passively thereafter.Synthesized galaxy spectra composed from libraries of stel-lar spectra can be used to derive the colours of the galaxies overa large wavelength range from UV to NIR for di ff erent ages andmetallicities. For generating the synthetic spectra we used theScalo (1990) initial mass function, solar metallicity and five dif-ferent model ages (0.1, 0.7, 2, 6 and 14 Gyr), to allow a directcomparison with the results of Jahnke et al. (2004).From the synthesized spectra we have derived the U – B , B – V , B – R , R – H and B – H colours for each model ages and comparedthose with the observed colours of each BL Lac using an iter-ative least squares fitting, with age as the only free parameter.The results of the fitting are presented in Table 7, where the bestfitting model is marked as 1 and the poorest fitting model as 3.The fitting results indicate that the best fitting models aregenerally consistent with a young / intermediate age stellar pop-ulation. Only for two BL Lacs (1ES 0229 +
200 and MRK 421),the best fitting model is older than 2 Gyr and is best consistentwith the 6 Gyr model. Note that these objects also have the red-dest host galaxies in the sample ( B – H ∼ .
4, see Table 5). Themodel fitting is thus consistent with the host galaxy colours, sug-gesting that the host galaxies of these objects are dominated byan old stellar population, with no evidence for a young popula-tion. Eight objects are best fit with the 2 Gyr model, while theremaining eight objects in the sample require an even youngermodel, 0.7 Gyr, and in the case of MS 1552.1 + + Table 7.
Colours of the single SSP fit compared to the coloursof the BL Lac sample. 1 represents the best and 3 the worst fit.
Name 0.1 0.7 2.0 6.0 14.0Gyr Gyr Gyr Gyr Gyr1ES 0229 +
200 3 1 2PKS 0521-365 3 1 2PKS 0548-322 2 1 31H 0706 +
591 2 1 3MRK 421 2 1 3MRK 180 3 1 21ES 1212 +
078 1 2 3MS 1229.2 + +
244 2 1 3PG 1418 +
546 2 1 31ES 1426 +
428 3 2 11ES 1440 +
122 1 2 3AP LIBRAE 3 1 2MS 1552.1 +
12 out of their 19 objects, and an even younger population (0.7Gyr) in five objects, while an older population (6 Gyr) was pre-ferred in only two objects. Note that, as is the case for the quasarhosts (Jahnke et al. 2004), none of the elliptical BL Lac hostsare best modeled with the 14 Gyr model, i.e. a very old, evolvedpopulation as would be expected for early-type host galaxies.On the other hand, for both types of AGN hosts, there is alsolittle evidence for massive ongoing starbursts with a signifi-cant very young population (age << yv¨onen et al.: The stellar content of low redshift BL Lac host galaxies 11 -18 -20 -22 -24 -2600.511.522.5 M(R)-20 -22 -24 -26 -282345 M(H) Fig. 5.
Upper: The B – R vs. R colour-magnitude diagram for BLLac host galaxies (filled circles), RLQ and RQQ hosts (opencircles; Jahnke et al. (2004)), RGs (open squares; Govoni et al.(2000)) and inactive early-type galaxies (asterisks; Peletier et al.(1990); Colbert et al. (2001)). Lower: The B – H vs. H colour-magnitude diagram for BL Lac host galaxies and comparisonsamples. Open circles are from Jahnke et al. (2004) and aster-isks from Bower et al. (1992).of the early-type host galaxies of moderately luminous AGNare caused by them having experienced a relatively recent SFepisode. Further support for this conclusion is provided by spec-troscopic evidence for young / intermediate age populations inAGN hosts (e.g., Nolan et al. 2001; Raimann et al. 2005). Onthe other hand, both the predominantly late-type hosts of lowluminosity AGN (e.g., Kotilainen & Ward 1994; Schade et al.2000; Jahnke et al. 2004) and the elliptical hosts of very lumi- nous AGN (e.g., Dunlop et al. 2003) exhibit similar colours totheir inactive counterparts.The blue colours and steep colour gradients of early-typeAGN hosts found in an increasing number of studies are mostlikely caused by a young stellar population, and indicate a linkbetween SF and the onset of the nuclear activity, both likely trig-gered by a tidal interaction or a minor or major merging event.However, the lack of obvious signs of interaction (companiongalaxies, tidal tails, disturbed morphology) in the close environ-ment of the large majority of the host galaxies may require asignificant time delay (at least hundreds of Myr) between theevent with associated SF episodes and the start of the nuclearactivity. Such a time delay is indeed predicted by simulations ofgas reaching the galaxy center after a merger / interaction (e.g.,Lin et al. 1988).
4. Conclusions
We have presented B -band imaging of a sample of 18 low red-shift ( z < .
3) BL Lac objects for which their host galaxieswere previously resolved in the optical R - and NIR H -bands.Subsamples consisting of nine and five BL Lacs were also im-aged in the U - and V -bands, respectively. All the BL Lac ob-jects were clearly resolved in the B - and V -bands while 8 / U -band. All the host galaxies arewell described by an elliptical de Vaucouleurs model. These dataare combined with previous optical and NIR data to study theoptical-NIR colours and colour gradients of the host galaxies.BL Lac host galaxies are luminous (massive) and large ellip-tical galaxies having average B -band absolute magnitude M B = − . ± . ff ective radius R e = . ± . B -band Kormendyrelation for BL Lac hosts is slightly steeper than that of RGsbut does not deviate significantly from the Fundamental Planeof inactive elliptical galaxies. This indicates that BL Lac hostsare dynamically similar to normal ellipticals and that the activenuclear phase does not have any significant influence on the dy-namical structure of the galaxy. Thus it is possible that all mas-sive galaxies can experience an AGN phase.The distributions of the integrated blue / NIR colours ( B – H = . ± . B – R = . ± .
5) and colour gradients ( ∆ ( B – R ) / ∆ ( logr ) = − . ± .
75) of the BL Lac hosts are much widerthan those for normal ellipticals with old stellar populations, andmany BL Lac objects have bluer hosts and / or steeper colourgradients than those in normal ellipticals. The blue colours andsteep colour gradients are most likely caused by a young stel-lar population, and indicate a link between SF caused by aninteraction / merging event and the onset of the nuclear activity.Many targets have inverted (positive) colour gradients which insome cases, e.g. 1ES 1426 + / intermediate age populations in themajority of the sample, in agreement with low redshift quasarhosts. However, the lack of obvious signs of interaction may re-quire a significant time delay between the event with associatedSF episodes and the start of the nuclear activity.Future work in this area should address the correlation be-tween colour and statistics of companion galaxies and morpho-logical disturbances as an indicator for the interaction, for welldefined samples of AGN hosts and inactive galaxies. In a forth-coming paper, we shall present NIR spectroscopy of BL Lac hosts and RGs with blue colours, to analyze their stellar con-tent and SF properties, based on emission and absorption linediagnostics, in more detail than is a ff ordable with imaging. Acknowledgements.
Based on observations made with the Nordic OpticalTelescope, operated on the island of La Palma jointly by Denmark, Finland,Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias. This work was sup-ported by the Italian Ministry for University and Research (MIUR) under COFIN2002 / / IPAC Extragalactic Database (NED) which is operated by the JetPropulsion Laboratory, California Institute of Technology, under contract withthe National Aeronautics and Space Administration. We thank Valentin Ivanovfor obtaining for us the observations with NTT / EMMI.
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The observed U -band radial luminosity profiles (solid points with error bars) for each BL Lac, overlaid with the PSF model(dotted line), the de Vaucouleurs r / model (long-dashed line) and the fitted PSF + host galaxy model profile (solid line). The X-axisis in arcsec and the Y-axis in mag arcsec − Fig. 9.
The observed B -band radial luminosity profiles and model profiles. For explanation, see the caption of Fig. 8. yv¨onen et al.: The stellar content of low redshift BL Lac host galaxies 15 Fig. 9. (continued)
Fig. 10.