Table Of ContentESO-VLT optical spectroscopy of BL Lac objects: II. New
redshifts, featureless objects and classification assessments.
B. Sbarufatti1, A. Treves
Universita` dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
6
0
R. Falomo
0
2 INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
n
a
J. Heidt
J
3 Landenssterwarte Heidelberg, Ko¨nigstuhl, D-69117 Heidelberg, Germany
2
1 J. Kotilainen
v
Tuorla Observatory, University of Turku, Va¨isa¨la¨ntie 20, FIN-21500 Piikkio¨, Finland
6
0
5
R. Scarpa
1
0 European Southern Observatory, 3107 Alonso de Cordova, Santiago, Chile
6
0
/
h
p
- ABSTRACT
o
r We report on ESO Very Large Telescope optical spectroscopy of 42 BL Lacertae objects of
t
s unknown redshift. Nuclear emission lines were observed in 12 objects, while for another six we
a detected absorptionfeatures due to their host galaxy. The new high S/N spectra therefore allow
:
v us to measure the redshift of 18 sources. Five of the observed objects were reclassified either as
i starsorquasars,andone is ofuncertainnature. Forthe remaining18the opticalspectraappear
X
without intrinsic features in spite of our ability to measure rather faint (EW ∼0.1 ˚A) spectral
r
a lines. For the latter sources a lowerlimit to the redshift was setexploiting the veryfact that the
absorption lines of the host galaxy are undetected on the observed spectra.
Subject headings: BL Lacertae objects: general
1. Introduction grouped together in the blazar class. From the
spectroscopical point of view BL Lacs are char-
BL Lac objects (hereinafter BL Lacs or BLL)
acterized by quasi featureless optical spectra. In
are active galactic nuclei (AGN) characterized
facttheirspectraareoftendominatedbythenon–
by luminous, rapidly variable UV–to–NIR non–
thermal continuum that arises from the nucleus.
thermal continuum emission and polarization,
Tothisemissionitissuperimposedathermalcon-
strong compact flat spectrum radio emission and
tribution due to the stellar componentofthe host
superluminal motion. Similar properties are ob-
galaxy. Like in other AGN, emission lines could
servedalsoinflatspectrumradioquasars(FSRQ)
be generated by fluorescence in clouds surround-
and the two types of active nuclei are often
ingthecentralblackhole. Moreover,asithappens
for high z quasars in some cases absorption lines
1alsoatUniversita`diMilano-Bicocca
due to intervening gas in the halo of foreground
1
galaxiescanbeobservedinthespectraofBLLacs perwe adoptedthe followingcosmologicalparam-
and one can derive a lower limit to the redshift of eters: H = 70 km s−1 Mpc−1, Ω =0.7, Ω =0.3.
0 Λ m
the object. The detectability of spectral features
dependsonthebrightnessofthenuclearsource: in 2. The sample
factduringlowbrightnessstates,intrinsicabsorp-
ThesampleofBLLacobjects(andcandidates)
tion features can be more easily revealed, while
observed with the VLT telescopes was selected
during high states one can better discover inter-
from two extended lists of BL Lacs: the Padovani
vening absorption systems. Because of the strong
& Giommi (1995a) collection of BL Lacs and the
contribution from the continuum the equivalent
Sedentary Survey (Giommi et al. 1999, 2005, in
width (EW) of all these spectral features is of-
the following addressed as SS). The Padovani &
ten very small and their detection represents a
Giommi(1995a)listcontainsallobjects identified
challenging task.
asBL Lacsbelonging to the complete samples ex-
In the past decade a number of projects were
isting at the time of its compilation, selected in
carried out to derive the redshift of BL Lac ob-
the radio, optical and X-ray bands (e.g.: 1 Jan-
jects. Most of these works were based on op-
sky survey – 1-Jy, Stickel et al. (1991), Palomar-
tical spectra collected with 4 m class telescopes,
Greensurvey–PG,Greenetal.(1986),Extended
and are therefore limited by relatively low signal-
Medium Sensitivity Survey – EMSS, Gioia et al.
to-noise ratio (S/N), low spectral resolution and
(1990),Slewsurvey,Perlmanetal.(1996),White-
limited wavelengthrange (e.g.Falomo et al.1993;
Giommi-Angelini catalogue – WGA White et al.
Stickel & Ku¨hr 1993; V´eron-Cetty & V´eron 1993;
(1994)). ItincludesalsosourcesfromtheHewitt&
Bade, Fink, & Engels 1994; Falomo, Scarpa, &
Burbidge(1993)andV´eron-Cetty&V´eron(1993)
Bersanelli1994;Falomo 1996;Marcha˜et al. 1996;
catalogues (in the latter case we checked that the
Drinkwater et al. 1997; Laurent-Muehleisen et al.
source was still included in the 2001 version), for
1998; Landt et al. 2001; Rector & Stocke 2001;
a total of 233 objects. The criteria used to define
Londish et al. 2002; Carangelo et al. 2003; Hook
a BL Lac object in Padovani & Giommi (1995a)
etal.2003). Recently,however,someobservations
dependonthesampleoforigin. Inmostcases,the
with 8 m class telescopes were carried out (Heidt
EWofthelinesisrequiredtobe≤5˚A,butalsoUV
et al. 2004; Sowards-Emmerd et al. 2005). De-
excess, optical polarization and variability, radio-
spite these efforts, a significant fraction of known
to-optical spectral index are used as selecting cri-
BL Lacs (e. g. 50 % in V´eron-Cetty & V´eron
teria. The SS was obtained cross-correlating the
(2003) catalogue) have still unknown redshift.
National Radio Astronomy Observatory (NRAO)
In order to improve the knowledge of the red-
VeryLargeArray(VLA)SkySurvey(NVSS)data
shift of BL Lacs we carried out a project to ob-
(Condon et al. 1998) with the ROSAT All Sky
tain optical spectra of sources with still unknown
Survey–Bright Source Catalogue (RBSC) list of
or uncertain redshift using the European South-
sources (Voges et al. 1999). SS selected a com-
ernObservatory(ESO)8-mVeryLargeTelescopes
plete sample of 150 High energy peaked BL Lacs
(VLT). This allows one to improve significantly
(HBL, see Padovani & Giommi 1995b, for defini-
the S/N of the spectra and therefore the capabil-
tion) down to a 3.5 mJy radio flux limit. BL Lac
ity to detect faint spectral features. A first report
classification in the SS is based on the position of
onthis work,givingthe redshiftof12objects,has
the sources on the α −α plane.
OX RO
been presented by Sbarufatti et al. (2005a, Paper
The Padovani & Giommi (1995a) and SS
I), and here we refer on the results for the full
datasets lead to a combined list containing 348
sample of 42 observed sources.
objects. The distribution of the V magnitude for
The outline of this paper is the following. In
these objects is reported in Fig. 1. The bulk of
section 2 we give some characterization of the 42
themhaveVbetween15and20,andthe the frac-
observed objects. The observation and analysis
tion of objects with unknown redshift increases
procedures are described in section 3. In sections
with the apparent magnitude and reaches ∼ 50%
4 and 5 we report the results of our spectroscopic
at the faintest magnitudes. Note, however, that
study. Finallyinsection6asummaryandconclu-
also at V ∼ 15-17 about 20% of the sources have
sions ofthis study aregiven. Throughoutthis pa-
2
not known redshift. The total number of objects Although this programdid not require optimal
with unknown redshifts is 105. photometric conditions, most of the observations
Fromthecombinedlistweselectedsourceswith were obtained with clear sky. This enables us to
δ <+15◦, for observability from the VLT site. perform a spectrophotometric calibration of the
Moreover to grant a sufficiently high S/N level of acquireddatausingstandardstars(Oke1990)ob-
the optical spectra we required V<22. Thus we served in the same nights. From the database of
gathered a list of 59 objects. During three obser- sky conditionsatParanalweestimate thata pho-
vationalcampaigns,performedinservicemode,we tometric accuracy of 10% was reached during our
completed this optical spectroscopy program, ob- observing nights. The spectra were also corrected
taining data for ∼70%ofthe sample (42 sources). for Galactic extinction, using the law by Cardelli,
Our sample is similar to the parent sample of 348 Clayton, & Mathis (1989) and assuming values of
objectsintermsofmeanapparentmagnitudeand EB−V from Schlegel, Finkbeiner, & Davis (1998).
subdivisioninLow(LBL)andHighenergypeaked
BL Lacs. 4. Results
In Fig. 2 we give the optical spectrum of each
3. Observations and data analysis
source. In order to show more clearly the contin-
Optical spectra were collected in service mode uum shape and the faint features we report both
with the FOcal Reducer and low dispersion Spec- the flux calibrated and the normalized spectrum
trograph(FORS1,Appenzeller et al.1998)onthe for each object. The main emission and absorp-
VLT. The observations were obtained from April tion features are identified. Those due to the
2003 to March 2004 with UT1 (Antu) and from galactic interstellar gas are indicated as “ISM”
April to October 2004 with UT2 (Kueyen). We and“DIB”(Diffuse InterstellarBands,seesection
used the 300V+I grism combined with a 2” slit, 4.2.3),while telluric absorptionsaremarkedas⊕.
yieldingadispersionof110˚A/mm(corresponding
to 2.64 ˚A/pixel) and a spectral resolution of 15– 4.1. The continuum emission
20 ˚A covering the 3800−8000 ˚A range. The see-
Inafirstapproximation,the opticalcontinuum
ingduringobservationswasintherange0.5−2.5”,
of a BL Lac object is due to the superposition of
withanaverageof∼1”. Relevantinformationson
two components: the non-thermal emission of the
the sample objects are given in Table 1.
active nucleus, Doppler-enhanced because of the
DatareductionwasperformedusingIRAF1(Tody alignment of the jet with the line of sight, and
1986, 1993) following standard procedures for theemissionofthehostgalaxy. Dependingonthe
spectral analysis. This includes bias subtraction, relativestrengthofthenucleuswithrespecttothe
flat fielding and cleaning for bad pixels. For each galaxy light, the spectral signature of the latter
target we obtained three spectra in order to get can be either easily detected or diluted beyond
a good correction of cosmic rays and to check the the point of recognition. Taking into account the
reality of weak features. The individual frames robust evidence that the host galaxies are giant
were then combined into a single average image. ellipticals (e.g. Urry et al. 2000), to describe the
Wavelength calibration was performed using the continuumandderivetheopticalspectralindexof
spectra of a Helium/Neon/Argon lamp obtained thenon-thermalcomponent,wefittedapowerlaw
during the same observing night, reaching an ac- (F ∝λ−α,the spectralindicesaregiveninTable
λ
curacy of ∼ 3 ˚A(rms). From these images we 1) plus the spectrum of a typical elliptical galaxy
extracted one-dimensional spectra adopting an asdescribedbythe Kinneyetal.(1996)template.
optimal extraction algorithm (Horne 1986) to While in most cases the contribution of the host
improve the S/N. galaxywas negligible, in 6 sources it was not, and
theluminosityofthehostcanthusbederived. For
1IRAF (Image Reduction and Analysis Facility) is dis- these six sources (three of them were presented
tributedbytheNationalOpticalAstronomyObservatories,
in Paper I) we give the best fit decomposition in
which are operated by the Association of Universities for
ResearchinAstronomy,Inc.,undercooperativeagreement Fig.3 and report the parameters in Table 2. The
withtheNationalScienceFoundation. derived absolute magnitudes of the host galaxies
3
are consistent with the distribution of M of BL 1319+019),while object0841+129remains ofun-
R
Lac hosts given by Sbarufatti et al. (2005b). certain nature.
4.2. Spectral features and redshifts 4.2.3. Lineless BL Lacs.
The detection and the measurement of very InspiteofthehighS/N18objectsexhibitspec-
weakspectralfeaturesisdifficulttoassessbecause tra lacking any intrinsic feature. In several spec-
itdependsonthechoiceoftheparametersusedto tra we clearly see absorptionfeatures fromthe in-
define the spectral line and the continuum. In or- terstellar medium (ISM) of our Galaxy. In par-
der to apply an objective method for any given ticular, we are able to detect CaII λλ3934,3968,
spectrum we evaluate the minimum measurable NaI λ5892 atomic lines, and a number of DIBs
equivalent width (EW ) defined as twice the λλ 4428,4726,4882,5772, generated by complex
min
rms ofthe distributionofallEWvalues measured molecules in the ISM (e.g. Galazutdinov et al.
dividing the normalized spectrum into 30 ˚A wide 2000, and references therein). In Fig. 4 we re-
bins (details for this automatic routine are given port the average spectrum of the interstellar ab-
inPaperI).WecheckedthattheS/Nratiodepen- sorptions. In three cases absorptionlines from in-
denceinsidetheconsideredspectralrangevariesat tervening gas are detected, leading to lower limits
mostby20%,remaining<10%overalargewave- on the redshift of the objects (0841+129,z>2.48;
lengthrange. This reflects into a similarvariation 2133–449,z>0.52; 2233–148,z>0.49).
of EWmin. The procedure for calculating EWmin For these 18 sources we have estimated a red-
was applied to all featureless or quasi-featureless shiftlowerlimitbasedontheEW oftheirspec-
min
spectra to find faint spectral lines. All features traandtheapparentmagnitudesofthenuclei. We
above the EWmin threshold, ranging from ∼ 1 ˚A reportthese in Table 1. The procedure to obtain
to 0.1 ˚A in our data, were considered as line can- these limits is described in section 4.2.4.
didates and were carefully visually inspected and
measured. The results are summarized in Tab. 1. 4.2.4. Redshift lower limits procedure.
Based on the detected lines and the shape of the
Inthissectionwedescribetheproceduretoob-
continuumweconfirmtheBLLacclassificationfor
tainredshiftlowerlimits forBL Lacswithlineless
36 objects, while 6 sources were reclassified. De-
spectra(seeTable1)fromtheEW ofthespec-
pending on the observed spectral properties the min
trum and the observed magnitude of the object.
objects can be assembled in three groups.
Under the assumption that the host galaxy lumi-
nosityisconfinedinanarrowrange(Sbarufattiet
4.2.1. Confirmed BL Lacs with measured z.
al. 2005b) it is in fact possible to constrain the
Twelve objects belonging to this group were position of the source on the nucleus-to-host flux
reported in paper I. Six more are presented here ratio (ρ) vs redshift plane.
(Table 1). Three have redshift derived from emis-
We assume that the observed spectrum of a
sionlines(0723–008,z=0.128;2131–021,z=1.284;
BL Lac object is given by the contribution of two
2223–114, z=0.997) and three from absorption
components: 1- a non-thermal emission from the
lines (1212+078, z=0.137; 1248–296, z=0.382;
nucleus that can be described by a power law
2214–313, z=0.460). Details on each source are (F(λ)=Cλ−α,whereC isthe normalizationcon-
given in section 5.
stant); 2 - a thermal component due to the host
galaxy. Depending on the relative contribution of
4.2.2. Misclassified and uncertain nature ob-
the two components the optical spectrum will be
jects.
dominated by the non-thermal (featureless) emis-
Despite their classification as BL Lac objects sionorbythespectralsignatureofthehostgalaxy.
in one or more input catalogues, six sources Theobservedequivalentwidth(EWobs)ofagiven
havespectraincompatiblewiththisidentification. spectral absorption line is diluted depending on
Five of them were reclassified either as quasars theratioofthetwocomponents. Detectionofthis
(0420+022,1320+084)orstars(1210+121,1222+102, spectral feature requires a spectrum with a suffi-
cientlyhighS/N.ThisisillustratedinFig5,where
4
a simulated spectrum (ρ=5, z=0.5) is reproduced Ontheotherhandthequantityρ dependsalso
0
withtwodifferentS/Nratios. TheS/N=300spec- on the observed magnitudes of the object, since
trum grants a secure detection of the CaII fea-
tures,whilewithS/N=30thelinesareundetected. log(ρ0)=−0.4[Mn(z)−Mh(z)] (4)
In order to estimate the redshift of an object
where M is the nucleus absolute magnitude and
fromtheEW weneedtoknowthe relationbe- n
min M is the host absolute magnitude, and
tween EW and the nucleus-to-hostflux ratio ρ. h
obs
For a spectral absorption line of intrinsic equiva- M (z)=m +5−5logd (z)−k (z) (5)
n n l n
lent width EW the observed equivalent width is
0
given by the relation (see also Sbarufatti 2005): where m is the nucleus apparent magnitude,
n
d (z) is the luminosity distance and k (z) is the
L n
(1+z)×EW
0 nucleusk-correction,computedfollowingWisotzki
EW = (1)
obs 1+ρ×A(z) (2000). The absolute magnitude of the host is
The nucleus-to host ratio ρ can be represented M (z)=M∗ −E(z) (6)
by h h
∗
whereM =–22.9istheaverageRbandmagnitude
F(λ) h
ρ(λ)= (2) of BL Lac hosts at z=0 and E(z) is the evolution
G(λ) correction, as given by Bressan et al. (1998).
where G(λ) is the giant elliptical spectral tem- An example of the procedure described above
plate by Kinney et al. (1996,see also section 4.1), isgiveninFig. 8,wheretherelationshipsbetween
and A(z) is a correction term that takes into ac- log(ρ0)andtheredshiftforagivenvalueofEWmin
countthelossoflightinsidetheobservedaperture. and mn are shown. The intersection of the two
Inthis workthe apertureis a2”×6”slitthatcap- curves yields a lower limit to the redshift of the
tures&90%ofthenuclearlight,butnotthewhole target. Whenitgoesbeyondtheobservedspectral
surroundinggalaxythatismoreextendedthanthe range, we set the redshift limit to the value cor-
aperture(inparticularforlowztargets). Inorder responding to the considered feature reaching the
to estimate this effect we evaluated the amount upperlimitoftheobservedwavelengthrange(z∼1
of light lost from the galaxythrough the aperture in the case of CaII λ3934 line). The uncertainty
in use from simulated images of BL Lacs (point of this procedure depends mainly on the spread
source plus the host galaxy). The main parame- of the distribution of the host galaxy luminosity.
tersinvolvedaretheshapeandthesizeofthehost This issueisdiscussedinUrryetal.(2000)andin
galaxy. According to the most extensive imaging Sbarufatti et al. (2005b), where it is shown that
studies of BLL (Falomo 1996; Wurtz et al. 1996; the 64 BL Lacs hosts of known redshift resolved
Falomo&Kotilainen 1999;Heidtetal.1999;Nils- with HST are well represented by an elliptical of
sonet al.2003;Urry et al.2000)we assumedthat MR=-22.9,with 68% of them in the interval -23.4
the host galaxy is a giant elliptical of effective ra- – -22.4.
dius R = 10 kpc. The fraction of starlight lost This procedure can be used for any absorption
e
then depends on the redshift of the object and is line belonging to the host galaxy and for which
particularly significant at z< 0.2, producing the an estimate of the un-diluted EW is available. In
bending of the curves in Fig. 6. thisworkweconsideredtheCaIIabsorptionlineat
Since we want to refer the observed equivalent λ=3934 ˚A(EW0=16 ˚A), we assumed a power law
width to the nucleus-to-hostratio ρ =ρ(λ ) at a spectral index α=0.7 (Falomo et al. 1993), and
0 0
fixedwavelengthλ ,equation(1)canberewritten we referred to the effective wavelength of the R
0
as: band (λ0=6750 ˚A) to compute ρ0 (which implies
(1+z)×EW0 ∆=4.3).
EW = (3)
obs 1+ρ0×∆×A(z) In order to test this procedure we considered
eight BL Lacs for which the CaII line of the host
where∆(λ)isthenucleus-to-hostrationormalized
galaxy has been measured. Five of these objects
to that at λ (∆(λ)=ρ(λ)/ρ(λ ); see Fig. 7).
0 0
derive from the observations discussed here and
5
in paper I, three others are from observations ob- (1993) catalogue. Falomo (1996) detected the
tainedattheESO3.6(Carangeloetal.2003;Sbar- host galaxy with ground-based imaging, propos-
ufatti 2005). ThesespectraarereportedinFig. 9 ing z∼0.2–0.3. The optical spectrum taken by
and the relevant parameters are given in Table 4. Falomo,Scarpa,&Bersanelli(1994)isfeatureless.
The comparison between the redshifts estimated Our spectrum (S/N=230)does not showevidence
by our procedure with the spectroscopic ones in- for intrinsic spectral features from the host, sug-
dicatesareasonablegoodagreement(seeFig. 10). gesting a very high N/H ratio. Interstellar ab-
sorptions from NaI λ5892 and DIBs at 5772 and
5. Notes to individual objects. 4726 ˚A are well detected. Based on EW , we
min
estimate z>0.31.
0047+023 This compact and flat spectrum ra-
dio source was classified as a BL Lac by Hewitt
0627–199 Hooketal.(2003)obtainedalineless
& Burbidge (1993) on the basis of UV color and
spectrum for this radio selected BL Lac object.
featureless spectra. Further featureless optical
Our VLT spectrum, of moderate S/N (50), shows
spectraobtainedbyAllington-Smithetal.(1991);
no spectral features. From EW we set z>0.63.
V´eron-Cetty & V´eron (1993) confirmed the BL min
Lac. Even in our S/N∼ 80 spectrum no spectral
features were found. Based on the minimum de- 0723–008 Wills & Wills (1976) classified this
tectable EW the sourceis most likely at z > 0.82. source as a Narrow Emission Line Radio Galaxy
based on an optical spectrum, giving z=0.127.
Rusk & Seaquist (1985) report an optical polar-
0048–097 Previous optical observations of this
ization of 1.5 %, classifying the source as a Low
well known BL Lac object belonging to the 1-Jy
Polarization QSO. V´eron-Cetty & V´eron (2001)
sample,reportedafeaturelessspectrum(Stickelet
reportthissourceasaBLLacobject. Henriksenet
al.1991;Falomo,Scarpa,&Bersanelli1994). Rec-
al. (1984) gives broad band indices α =0.7 and
tor & Stocke (2001),however,suggested the pres- RO
enceofanemissionlineat6092˚A(possiblyidenti- αOX=1.0, which are compatible with a BL Lac
or a FSRQ classification. Our optical spectrum
fiedwith[OII]λ3727atz=0.634or[OII]λ5007at
is clearly dominated by a non thermal emission
z=0.216). Falomo (1996) proposed z>0.5, based
with spectral index α=0.7. Superposed to this,
on the non detection of the host galaxyin the op-
strong narrow emission lines and absorption lines
tical images of the source. Our S/N=250 opti-
from the underlying host galaxy at z=0.127 are
cal spectrum does not confirmthe presence of the
emissionlineat6092˚A,andapartofsometelluric detected, confirming the redshift. From the val-
ues of the spectral indices and the measured EW
lines and a number of Galactic absorptions it is
for the spectral lines, we suggest that this object
found featureless. From our EW estimate, we
min
is of intermediate nature between a BL Lac and a
infer that this source is at z>0.3.
quasar.
0420+022 Fricke et al. (1983) classified this
0841+129 This source, first identified by C.
source as a BL Lac candidate on the basis of a
Hazard (see Jaunsen et al. 1995, and references
featureless (although noisy) optical spectrum. El-
therein),isaDampedLymanαAbsorption(DLA)
lison et al. (2001) through an unpublished optical
QSOatz>2.48asderivedfromthetwoverystrong
spectrum propose a redshift z=2.277 and classify
DLAsat∼4100and∼4225˚A(seeforexamplePet-
the source as a radio loud QSO. In our optical
tinietal.1997;Prochaskaetal.2001;Warrenetal.
spectrum we are able to clearly detect emission
2001, and references therein). The classification
linesLy λ1419OVI]λ1034,CIVλ1549andCIII]
α
as a BL Lac object was motivated by the absence
λ1909, at z = 2.278. A recent spectrum obtained
of prominent emission lines (Hewitt & Burbidge
byHooketal.(2003)alsoconfirmourfindingsand
1993).
the classification of the object as a QSO.
Ourspectrum,inadditiontoseveralabsorption
lines, exhibit three possible broad emission struc-
0422+004 Thisobjectisawellknownradiose-
tures at∼4310, ∼4850 and ∼5370 ˚A. These could
lectedBLLac,includedinthe Hewitt&Burbidge
6
be interpreted as NV λ1240, SiIV λ1397 and CIV vation of a featureless spectrum. Burbidge (1996)
λ1549 at z∼2.47. This is consistent with z∼2.5, estimatesthisobjectapossiblecandidateofexpul-
deduced from the observed position of the onset sionfromagalacticnucleus. Thesharpabsorption
of the absorption of the Ly forest Warren et al. lines detected in our spectrum clearly indicate a
α
(2001). An alternative explanation is, however, stellar origin. The measured colors lead to a tem-
that these structures are pseudo-emissions result- perature of ∼10000 K. If the object were a main
ing from the depression of the continuum caused sequence or a supergiant star, the corresponding
by the envelope of many unresolved narrow ab- distance willput it outside the Galaxy,but notat
sorptionfeatures. Higherresolutionspectraofthe the distance of NGC 4380. We are therefore led
object in the spectral range 4200 to 5800 ˚A are toconsiderawhitedwarf,whichwouldbeat100–
needed to distinguish between the two possibili- 200 pc. The absence of H lines indicates a DQ
ties. or DXP white dwarf (Schmidt et al. 2001, 2003).
SomeofthelinesarereferabletoHeIandCItran-
sitions. The object clearly deserves further study;
1210+121 Hazard&Murdoch(1977)proposed
in particular polarizationmeasurements would be
that this object was the optical counterpart of
interesting.
a radio source in the Molongo Catalogue (MC2
Sutton et al. 1974); the separation was however
16”. Zotov & Tapia (1979) reported large optical 1248–296 Perlman et al. (1996) obtained a low
variability and polarization, apparently reinforc- S/N spectrum of this source, and proposed a BL
ing the identification. Baldwinetal.(1973)found Lac at z=0.487based on the possible detection of
afeaturelessopticalspectrum. OurVLTspectrum the host galaxy features. In our VLT spectrum
clearly shows that the source is a type B star in CaII,Gband,H areclearlydetectedatz=0.382,
β
our Galaxy. confirmingthefindingsofWooetal.(2005),while
inthe blue partthe contributionofanon-thermal
component is clearly visible. The best fit decom-
1212+078 OurVLTspectrumclearlyshowsthe
positiongivesα=0.92forthe non-thermalcompo-
presence of a strong thermal component due to
nent visible below 5000 ˚A, and M =-22.7 for the
the host. We detected CaII λλ3934, 3968, G R
host, in good agreement with result from the di-
band λ4305, MgI λ5175 and H λ6563 in emis-
α
rect detection of the host in HST imaging (Urry
sionatz=0.137,confirmingtheredshiftestimated
et al. 2000; Sbarufatti et al. 2005b).
by Perlman et al. (1996). The contribution of
thenon-thermalcomponentisvisibleinthebluest
part of the spectrum. The best fit decomposition 1319+019 This object was initially selected as
of the spectrum gives α=1.17 for the non-thermal a BL Lac candidate onthe basis of the University
component and M =-22.0 for the host. Though of Michigan objective prism survey (MacAlphine
R
this is somewhat fainter than expected for a BL & Williams 1981) designed to find AGN and it is
Lac host galaxy, we can not exclude that given included as BL Lac in the V´eron-Cetty & V´eron
the low redshift and the consequent large appar- (2001) catalogue. No radio counterpart for this
ent size of the host, partofthe light did not enter sourcehasbeenfoundinliterature. LaterThomp-
in the slit. son&Djorgovski(1990)proposeditsclassification
asaBLLac,basedonalowS/Nopticalspectrum
that was found featureless. In our much better
1222+102 This is a blue stellar object in the
quality spectrum we clearly see many absorption
direction of the Virgo-Coma cluster. Its apparent
features that characterize the object as a galactic
position in the sky is very close to the center of
star of spectral type ∼A. Our findings are also in
the galaxy NGC 4380, still well inside the galaxy
agreement with the spectral classification of the
boundaries. The projected separation to the nu-
cleusattheredshiftofthegalaxyis∼10kpc. The 2dF QSO Redshift survey (2QZ, see Croom et al.
2004).
object is considered a candidate BL Lac in the
Burbidge & Hewitt (1987) list, selected because
of its UV excess. Arp (1977) reports the obser-
7
1320+084 This source is part of the BL Lac 1722+119 Griffithsetal.(1989)reportedaten-
sampleextractedfromtheEINSTEINSlewSurvey tative redshift z=0.018 for this X-ray selected,
and a radio counterpart was reported by Perlman highly polarized BL Lac. This estimate was not
et al. (1996). Our VLT data show the source has confirmed by more recent observations (V´eron-
a QSO like spectrum at z=1.5, in contrast with Cetty & V´eron 1993; Falomo et al. 1993; Falomo,
a featureless spectrum observedby Perlmanet al. Scarpa, & Bersanelli 1994). Our VLT spectrum
(1996). Several intervening absorption lines, in (S/N=350)shows only absorptionfeatures due to
particular MgII at z=1.347 were also detected. ourgalaxyISM:CaIIλλ3934,3968,NaIλ5892and
DIBs at 4428˚A, 4726˚A, 4882˚A, 5772˚A, with no
evidence of intrinsic features. From the minimum
1349–439 The spectrum of this X-ray selected
EW we derive z>0.17.
BL Lac (della Ceca et al. 1990), shows a number min
of absorption lines from the interstellar medium:
CaIIλλ3934,3968,the5772˚ADIB,NaIλ5892. No 2012–017 Consistently with previous observa-
intrinsic features were detected, and the deduced tions of this radio selected BL Lac (White et
redshift lower limit is z>0.39. As already pointed al. 1988; V´eron et al. 1990; Falomo, Scarpa, &
out by V´eron (1996), the value z=0.05 sometime Bersanelli 1994), also our S/N=130 VLT spec-
reportedfor this object is consequence of a confu- trum is featureless. The optical spectral index is
sionwith the nearbySeyfert1galaxyQ 1349-439. α=0.49,inmarginalagreementwithα=0.33±0.12
reported by Falomo, Scarpa, & Bersanelli (1994).
From EW we derive z>0.94.
1442–032 ThisX-raysource,the radiocounter- min
part of which was found in the NVSS survey, was
first classified as a BL Lac in the RBSC-NVSS 2128–254 The spectrum of this X-ray selected
samplebyBaueretal.(2000),andthenconfirmed BLLaccandidateisreportedasfeaturelessbySS.
by the SS. There are no published optical spec- We confirm this result and set a lower limit of
tra for this source. Our optical spectrum is fea- z>0.86 for the redshift.
tureless, with the exception of the NaI λ5892 ab-
sorptionfeaturefromourgalaxyISM.TheEW
min 2131–021 Rector&Stocke(2001)andDrinkwa-
value for this objects leads to z>0.51.
ter et al. (1997) proposed a redshift of 1.285
for this source, based on the detection of CIII]
1500-154 This X-ray selected BL Lac is part of λ1909, MgII λ2798 and [OII] λ3727, opposed to
the RSBC-NVSS sample (Bauer et al. 2000) and the z=0.557 suggested by Wills & Wills (1976).
entersinSS.Nopreviousopticalspectroscopyhas While [OII] falls outside our spectral range, we
beenfoundintheliterature. Ourspectrumiscom- confirm the presence of CIII] and MgII emission
pletelyfeatureless,leadingtoz>0.38fromtheob- lines at z=1.283, also detecting the fainter CII]
tained EW . λ2326 feature at the same redshift.
min
1553+113 This source is an optically selected 2133–449 This source was discovered because
BL Lac from the Palomar-Green survey. The of its optical variability by Hawkins et al. (1991).
redshift estimate z=0.360 given in the Hewitt & OpticalspectroscopybyHawkinsetal.(1991)and
Burbidge (1993) catalogue was disproved by later Heidt et al. (2004) led to completely featureless
spectroscopy (Falomo & Treves 1990; Falomo, spectra. Our VLT observations clearly show the
Scarpa, & Bersanelli 1994). While no intrinsic presence of an intervening absorption feature at
featuresweredetectedinourS/N=250VLTspec- 4250˚A,atentativeidentificationofwhichisinter-
trum, a number of absorption lines due to our vening MgII at z=0.52(see Churchill et al. 2005).
galaxyISMwererevealed: CaIIλλ3934,3968,NaI ThelowerlimitonzderivedfromEW isz>0.98
min
λ5892 and DIBs at 4428,4726,4882,5772 ˚A. The .
EW estimate for this object gives a limit
min
z>0.09.
2136–428 The spectrum obtained by Hawkins
et al. (1991), who discovered this source study-
8
ing its optical variability, is completely feature- z=0.492, while, using the EW estimate from
min
less. Our VLT observations shows severalabsorp- the spectrum, z>0.65 is found.
tionfeaturesduetotheISMofourgalaxy: DIBat
4428˚A,4726˚A,4882˚Aand5772˚A,CaIIλλ3934,
2254–204 Previousopticalspectroscopy(V´eron-
3968 and NaIλ 5892 atomic lines. The feature at
Cetty & V´eron1993;Hook et al. 2003)of this BL
5942 ˚A could be CaII λ3968 at z=0.497. Since
Lacobjectfromthe1Jysampleshowedcompletely
at this redshift the CaII λ3934should fall at 5890
featureless spectra. With VLT we are able to de-
˚A, where it will be strongly contaminated by the
tect faint interstellar absorptions of CaII λλ3934,
interstellar NaI absorption, the redshift estimate
3968 and NaIλ 5892, but no intrinsic or interven-
is only tentative. The lower limit deduced by the
ing spectral lines are found. The inferred redshift
minimum measurable EW is z>0.24.
limit is z>0.47.
2214–313 Our VLT spectrum of this object
2307–375 This source was first classified as a
clearly shows the typical spectral signature of the
BL Lac in the RSBC-NVSS sample (Bauer et al.
hostgalaxy(CaIIλλ3934,3968andGbandλ4305)
2000). The classification was then confirmed by
atz=0.46. Thebestfitdecompositiongivesα=0.9
the SS.Nopreviousopticalspectroscopyhasbeen
for the non-thermal component and M =–22.3
R published. Our VLT spectrum is featureless, al-
for the host. Previous optical spectroscopy per-
lowing us to set only a lower limit to the redshift
formed by Bade, Fink, & Engels (1994) with the
of z>1.
ESO 3.6m telescope failed to detect any spectral
feature.
2342–153 ThissourceispartoftheEMSSsam-
ple of BL Lac objects. Our VLT data, as well
2223–114 Optical observations of this radio
as previous optical spectroscopy with the 6.5 m
source obtained by V´eron-Cetty & V´eron (1993)
telescope of Multi Mirror Telescope Observatory
did not show any intrinsic spectral feature. In
(Rectoretal.2000)showedafeaturelessspectrum.
our spectrum, that extends further in the red,
From EW we derive z>1.
min
we detect a single narrow emission line at λ7367
(EW = 5 ˚A). This is a realfeature since it clearly
2354–021 This object was discussedin paper I.
appears on each of the 3 individual spectra (see
Here we report only the spectrum, in Fig. 2.
section 3). A possible identification of this line is
[OII]λ 3727 at z=0.977, while MgII λ2798 gives
z=1.633. We discarded this second classification 2354–175 This X-ray source from ROSAT All
because the line FWHM (1200 km s−1), is typi- SkySurvey,isclassifiedasBLLaccandidateinthe
cal for a narrow line such [OII], while for MgII a RBSC-NVSSsample(Baueretal.2000)andinthe
larger value would be expected. Moreover, with SS.Nopreviousspectroscopywaspublishedinlit-
a MgII identification both CIV λ1549 and CIII] erature. Our S/N=150 VLT spectrum is feature-
λ1909 broad lines would be expected inside the less, allowing only to set a lower limit of z>0.85
observedspectral range,but no other features are to the redshift.
detected.
6. Summary and conclusions
2233–148 The redshift z=0.325 reported by
Out of 42 objects observed we confirm the BL
Johnston et al. (1995) is due to confusion with
Lac classification for 36 sources and for 18 of
the source HB89 2233+134 in Schmidt & Green
themweareabletomeasure/confirmtheredshift.
(1983). Drinkwater et al. (1997) report an in-
This information allows us to derive the luminos-
tervening system at z=0.609, but without giving
ity of the objects. The distribution in the V band
an identification of the corresponding absorption
luminosity-distance plane is indeed fully consis-
feature. We detect several absorption features on
tent with what observed for BL Lacs of known
the spectrum. In particular we propose to iden-
redshift in the combination of the Padovani &
tify the features at 4165 and 4183 ˚A as MgII at
Giommi (1995a) and the SS sample (see Fig. 11).
9
We note that the sources in this combined list REFERENCES
are affected by the typical selection effect of in-
Abraham, R. G., Crawford, C. S., & McHardy,
complete, flux limited samples: the envelope of
I. M. 1991, MNRAS, 252, 482
the objects follows in fact the expected behavior
for sources with constant V magnitude, centered Allington-Smith, J.R., Peacock,J.A., & Dunlop,
aroundV=18,with a spreadof∼ 3 mag. The ob- J. S. 1991, MNRAS, 253, 287
jectsdiscussedhere(filledcirclesandlowerlimits)
follow the same distribution, with absolute mag- Appenzeller et al., 1998, Messenger 94, 1
nitudes ranging between -21.5 and -27.5, slightly
Arp, H. 1977, Coll. Intl. No. 263, Paris-Centre
increasing with the redshift.
Nat. Recherche Sci, p. 377
In 18 cases the optical spectra remain lineless
in spite of the high S/N of the obtained optical Bade, N., Fink, H. H., & Engels, D. 1994, A&A,
spectra. This indicates that if they are hosted by 286, 381
galaxies of standard luminosity they have likely
Bauer,F.E.,Condon,J.J.,Thuan,T.X.,&Brod-
very luminous or extremely beamed nuclei (see
erick, J. J. 2000, ApJS, 129, 547
also Sbarufatti et al. 2005b). In the latter case
one may expect to see the most extreme cases of
Baldwin,J.A.,Burbidge,E.M.,Hazard,C.,Mur-
relativistic beaming, making these sources ideal
doch,H.S., Robinson,L.B.,& Wampler,E.J.
targets for milli-arcsec resolution radio observa-
1973,ApJ, 185, 739
tions. Alternatively, if the host galaxies were
under-luminous,theseobjectscouldberareexam- Bressan, A., Granato, G. L., & Silva, L. 1998,
ples of dwarf galaxies hosting an AGN (see Sbar- A&A, 332, 135
ufatti et al. 2005b).
Burbidge, G., & Hewitt, A. 1987,AJ, 93, 1
The high S/N of most of the optical spectra
obtained at VLT represents a frontier for the de- Burbidge, G. 1996, A&A, 309, 9
termination of the redshift of BL Lacs with cur-
rent instrumentation and further improvement of Carangelo,N., Falomo, R., Kotilainen, J., Treves,
the issue will not be easy to get. In particu- A., & Ulrich, M.-H. 2003, A&A, 412, 651
lar the four brightest objects (R<16: 0048–099,
Cardelli, J. A., Clayton, G. C., & Mathis, J. S.
1553+113, 1722+119, 2136-428) we observed at
1989,ApJ, 345, 245
the VLT have values of EW smaller than 0.25
min
˚A. These objects belong to an interesting sub- Churchill, C. W., Kacprzac, G. G. & Steidel,
population of BL Lac objects with extreme nu- C. C., 2005, proceedings for IAU colloquium
clear (and/or host) properties for which it is ac- 199: ProbingGalaxiesthroughQuasarAbsorp-
tually not possible to derive the intrinsic physical tion Lines, Cambridge University Press, 2005
parameters. Onepossibilityistoimagethesource
when it is particularly faint in order to improve Condon,J.J.,Cotton,W.D.,Greisen,E.W.,Yin,
the detection of the host galaxy and to derive an Q.F.,Perley,R.A.,Taylor,G.B.,&Broderick,
imaging redshift (Sbarufatti et al. 2005b). Deep J. J. 1998, AJ, 115, 1693
spectroscopicobservationsinthenear-IRmayalso
Croom, S. M., Smith, R. J., Boyle, B. J., Shanks,
prove to be effective in the determination of the
T., Miller, L., Outram, P. J., & Loaring, N. S.
redshift considering this region of the spectrum is
2004,MNRAS, 349, 1397
poorly known.
della Ceca, R., Palumbo, G. G. C., Persic, M.,
We would like to thank the referee, Dr. John Boldt, E. A., Marshall, E. E., & de Zotti, G.
Stocke, for his accurate and helpful comments, 1990,ApJS, 72, 471
which allowed us to improve the quality of this
Drinkwater, M. J., et al. 1997, MNRAS, 284, 85
work.
Ellison, S. L., Yan, L., Hook, I. M., Pettini, M.,
Wall, J. V., & Shaver, P. 2001,A&A, 379, 393
10
