ESO VLT Optical Spectroscopy of BL Lac Objects IV. New spectra and properties of the full sample

25 Pages·2013·0.94 MB·English

ESO VLT Optical Spectroscopy of BL Lac Objects IV - New
spectra and properties of the full sample.
3 M. Landoni
1
Universita` degli Studi dell’ Insubria. Via Valleggio 11, I-22100 Como, Italy.
0
2 INAF - Osservatorio Astronomico di Brera, Via Emilio Bianchi 46, I-23807 Merate, Italy
INFN - Istituto Nazionale di Fisica Nucleare
b
e [email protected]
F

1
R. Falomo
1
INAF - Osservatorio Astronomico di Padova, Vicolo dell’ Osservatorio 5, I-35122 Padova, Italy

]
O
C A. Treves
h. Universita` degli Studi dell’ Insubria. Via Valleggio 11, I-22100 Como, Italy.
p INAF - Istituto Nazionale di Astrofisica and INFN - Istituto Nazionale di Fisica Nucleare
-
o
r B. Sbarufatti
t
s
a INAF - Osservatorio Astronomico di Brera, Via Emilio Bianchi 46, I-23807 Merate, Italy
[

1 M. Barattini
v
1 Universita` degli Studi dell’ Insubria. Via Valleggio 11, I-22100 Como, Italy.
9
5
2 R. Decarli
.
2 Max-Planck-Institut fur Astronomie, Konigstuhl 17, 69117 Heidelberg, Germany
0
3
1 J. Kotilainen
:
v Finnish Centre for Astronomy with ESO (FINCA) - University of Turku, Va¨isa¨l¨antie 20, FI-21500 Piikkio¨,
i Finland
X
r
a
ABSTRACT
Wepresentthelastchapterofaspectroscopyprogramaimedatderivingtheredshiftoralower
limittotheredshiftofBLLacobjectsusingmediumresolutionspectroscopy. Herewereportnew
spectrafor33BLLacobjectcandidatesobtainedin2008-2009confirmingtheBLLacnatureof25
sources and for 5 objects we obtained new redshifts. These new observations are combined with
ourpreviousdatainordertoconstructahomogeneoussampleof∼70BLLacswithhighquality
spectroscopy. All these spectra can be accessed at the website http://www.oapd.inaf.it/zbllac/.
The average spectrum, beaming properties of the full sample, discussion on intervening systems
and future perspectives are addressed.
Subject headings: BL Lacertae objects: general, Galaxies: quasars: absorption lines. Instrumentation:
spectrographs
1
1. Introduction 2. Observations and data reduction
BL Lac objects are extragalactic sources for The spectra were obtained in Service Mode
which the redshift z cannot be easily derived. using the FOcal Reducer and a low dispersion
This is due to the combination of intrinsically Spectrograph (FORS2, Seifert et al. (2000)). The
weak emission lines and strongly beamed nuclear observations took place between April 2008 and
non thermal emission. The determination of z June 2008at VLT-UT1 (Antu) and from Septem-
is obviously important in order to properly char- ber2008toFebruary2009atVLT-UT2(Kueyen).
acterize their physical properties. While other The spectrograph was configured with a 300V+I
distance indicators were introduced like the pho- grismwithaslitof2′′ achievingthespectralrange
tometry of the host galaxy (imaging redshift, see of3800-8300˚Awithadispersionof∼3.2˚A/pixel.
e.g. Sbarufatti et al. (2005a)), or TeV γ-ray ab- Detailed informations on the observations of each
sorptionby the EBL(e.g. Prandini et al.(2010)), sourcearegiveninTable1. Theseeingduringthe
′′ ′′
the direct spectroscopic line observation remains observation was in the range 0.65 -2.50 with an
the onlywaytoproducearobustmeasureoftheir average of 1.20′′ (rms ∼0.4′′).
distance. The determination of z is becoming
even more compelling by the recent development Data reduction was executed using IRAF1 adopt-
of γ-ray astronomy, which demonstrated that the ing standard procedures. In particular, for each
extragalactic γ-ray sky is dominated by this class target we performed bias subtraction, flat field-
of objects. ing and bad pixels or cosmic rays rejection. For
each object we secured 2-6 individual exposures
In order to improve the quality (in terms of that were then combined into a single average
throughput, spectral resolution and S/N ratio) image. The wavelength calibration was achieved
of the spectrum, we have carried out systematic using the spectra of a Helium Neon Argon lamp
campaigns at European Southern Observatory reaching an accuracy of ∼ 3 ˚A rms. Even though
Very Large Telescope (VLT), aimed to observe this program did not require photometric condi-
∼90BL Laccandidatesofunknownredshift. Re- tions during observations, checking the weather
sults on the first three runs were presented in database at Paranal, the sky was clear during
Sbarufatti et al. (2005b, 2006a, 2009), Paper I, II mostofthe time. Thisenablesustofluxcalibrate
and III respectively , where the criteria of selec- our spectra using standard stars (Oke 1990). The
tion of the sample (see also Section 5.1), details flux calibration was assessed through the relative
on the used spectrometer, and the reduction and photometric calibration of the acquisition image
analysis procedure are given. and the overall uncertainty is ∆m ∼ 0.1 mag
(rms). Finally, each spectrum has been dered-
In this paper we report on observations of 33 dened applying the extinction law described in
new BL Lac candidates that complete our obser- Cardelli et al. (1989) assuming the EB−V values
vational campaign and for 25 of them we confirm computed by Schlegel et al. (1998).
their BL Lac nature. In Section 2 we summarize
the observations and data reduction process and 3. Results
we show the results in Section 3. The average
From the spectroscopic point of view we con-
spectrum of the class is presented in Section 4,
sider as bona-fide BL Lac the sources that either
while a discussions of the properties for the full
have a featureless spectrum or in which their in-
sample is reported in Section 5.
trinsic features have equivalent width (EW) . 5
˚A. Based on this classification we confirm the
Throughout the paper we adopt the following
concordant cosmology: H = 70 km s−1 Mpc−1, BL Lac nature for 25 objects. Their spectra
0
Ω =0.27 and Ω =0.73.
m Λ
11 IRAF (Image Reduction and Analysis Facility) is dis-
tributedbytheNationalOpticalAstronomyObservatories,
which are operated by the Association of Universities for
ResearchinAstronomy,Inc.,undercooperativeagreement
withtheNationalScienceFoundation.
2
Table 1
Journal of the observations
Object R.A.(J2000) Decl.(J2000) R Date Exposuretime N S/N Seeing Ref
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
GC0109+224 011209.103 +224435.052 15.60 2008-09-20 2040 6 200 2.00 V01
PKS0140-059 014240.810 −054326.472 18.20 2008-10-26 1362 2 65 1.00 V06
RBS0337 023829.549 −311633.312 16.40 2008-09-30 1362 2 130 0.65 V01
PKS0302-623 030356.678 −621140.812 17.90 2008-09-27 1362 2 150 1.40 S94
RBS0499 035855.903 −305402.664 17.60 2008-09-20 1362 2 40 1.00 V01
PKS0410-519 041141.858 −514931.548 16.50 2008-10-01 1362 2 70 1.60 G04
PKS0439-299 044122.987 −295231.296 18.00 2008-10-25 1362 2 30 1.20 V01
PKS0443-387 044514.474 −383834.296 17.00 2008-10-26 1362 2 140 1.70 G04
RBS0589 044840.733 −163251.684 16.60 2008-10-28 1362 2 130 0.80 V01
ZS0506+056 050925.900 +054221.000 15.10 2008-11-02 800 2 200 2.00 V01
PMNJ0529-3555 052940.800 −355541.160 18.90 2008-10-29 1362 2 30 1.30 G04
PKS0539-543 054050.200 −541758.700 17.30 2008-10-29 1362 2 70 1.60 G04
PKS0607-605 060800.334 −603127.732 17.60 2008-10-29 1362 2 50 1.90 G04
MS-0622.5-5256 062342.667 −525801.704 19.20 2008-10-26 1362 2 45 2.50 V01
PKS0735+17 073808.100 +174303.600 15.80 2009-01-13 2720 4 160 1.30 V01
OJ-131 082055.562 −125935.340 16.50 2008-04-09 1362 2 100 1.60 V01
RXJ1022.7-0112 102246.704 −011311.280 16.90 2008-04-09 1361 2 100 1.50 V01
RXJ1023.9-4336 102356.446 −433646.980 16.70 2008-04-10 1361 2 200 1.40 V01
PKSB1056-113 105909.478 −113436.516 16.20 2008-04-18 1361 2 100 1.50 V06
1ES1106+244 110918.245 +241045.156 18.70 2008-04-22 2720 4 40 1.30 V01
RXJ1117.0+2014 111707.522 +201449.632 15.10 2009-02-16 2612 4 130 1.00 V01
RXJ1241.8-1455 124149.862 −145642.756 17.70 2008-04-22 2720 4 140 0.95 V01
1ES1239+069 124151.314 +063606.840 18.50 2008-04-17/22 2720 4 50 1.00 V01
4C12.46 130931.862 +115350.676 18.20 2008-04-18 1361 2 70 1.00 V01
OQ-240 142703.307 +234739.840 14.50 2008-04-07/17 800 4 170 1.00 V01
OR103 150427.670 +102917.520 18.90 2009-02-16 1361 2 150 1.70 V01
RBS1478 151616.718 −152418.000 18.50 2008-04-02 1361 2 50 0.90 V01
PKS1815-553 181944.678 −552206.204 21.50 2008-04-22 1362 2 10 1.30 S94
OW-080 205008.688 +040815.792 19.00 2008-04-22 1361 2 90 0.70 V01
OX-183 215227.972 +173430.396 18.00 2008-06-02 2722 4 50 0.90 V01
RBS1899 224913.829 −130005.616 18.20 2008-06-08 1361 2 60 1.00 V01
RXJ2319.6+1611 231944.371 +161232.688 17.30 2008-06-08 1361 2 100 1.70 V01
1ES2322-409 232446.238 −404131.128 15.70 2008-05-28 1361 2 60 0.90 V01
Note.—Descriptionofcolumns: (1)Objectname;(2) Rightascension(J2000); (3) Declination(J2000); (4) Rapparentmagnitude
fromcatalog;(5)Dateofobservation;(6)Exposuretime(s);(7)Numberofspectracollected;(8)Meansignal-to-noiseratio;(9)Mean
seeingduringobservation(10)References: G04: Giommietal.(2004);S94: Stickeletal.(1994);V01: V´eron-Cetty&V´eron(2001);
3
are reported in Figure 1 while confirmed non- or rejection. Spectral parameters, line properties
BL Lac spectra are shown in Figure 2. All and relative identification and EW for the BL
min
BL Lac spectra can be accessed at the website Lacobjectsinvestigatedinthispaperarereported
http://www.oapd.inaf.it/zbllac/ In particular, 20 in Table 2.
sources have no intrinsic lines, 3 of them show
weak absorption features arising from their host 3.3. Redshift lower limits
galaxy and other 3 show weak narrow emission
Despite high quality of the available instru-
lines. For only one object (OX 183)a weak broad
mentation in terms of throughput and the high
emission line is apparent. We identify spectral
signal-to-noise ratio reached, 20 objects still show
features arising from the source or from interven-
a featureless continuum. For these sources, we
ingsystemswiththeatomicspecies(ifapplicable)
applied the procedure fully illustrated in Paper
while the Diffused Interstellar Bands are marked
II and III in order to measure a lower limit on
with the label DIB. Telluric absorption are in-
the redshift of eachsource. Briefly,assuming that
dicated by the symbol ⊕. The identification of
the BL Lac host galaxy is a giant elliptical with
the lines,theirequivalentwidth, fullwidthathalf
Mhost =−22.90±0.50ifonemeasurestheEW
maximumandredshiftestimationaregiveninTa- R min
andthenucleusapparentmagnitudeitispossibile
ble 2 for BL Lac objects and in Table 3 for the
to infer a lower limit on the z of the source using
othersources. WealsoreportinAppendixAshort
Equation (Paper III)
comments on interesting individual sources .
(1+z)×EW
0
3.1. The continuum emission EWobs = (1)
1+ρ/A(z)
In the optical region the emission of BL Lac
where EW is the observedminimum equivalent
objects is characterised by a superposition of a obs
width, EW is the equivalent width of the fea-
non-thermal component arising by the accreting 0
ture in the host galaxy template of Kinney et al.
nucleusandathermalonethatoriginatesfromits
(1996), ρ is the nucleus-to-hostflux ratio, z is the
host galaxy. In particular, the emission from the
redshift and A(z) is the aperture correction. The
nucleus can be described by a power law model
(F ∝ λ−α) while the galaxy contribution can be lower limits are reported in Table 2.
λ
modeledbyanellipticalgalaxytemplatespectrum
with Mhost = −22.90 ± 0.50 (Sbarufatti et al. 4. The mean spectrum of BL Lac objects
R
2005a). For the objects that do not present host
Inordertoinvestigatetheoverallspectralchar-
galaxy absorption features we fitted the contin-
acteristics of the BL Lac objects, we constructed
uum adopting a single power law, while for the
a mean optical spectrum combining the normal-
BL Lacwhere galacticabsorptionaredetectedwe
ized spectra of the 23 sources with z measured
performed the fit by adding the host galaxy tem-
through intrinsic emission or absorption lines. In
plate contribution, following procedure described
details, for each spectrum we first carefully re-
in Paper II and III. Resulting power law indexes
moved spectral region contaminated by telluric,
are reported in Table 2. The mean value is
interstellarmediumorinterveningsystemabsorp-
α =0.87±0.35whichisconsistentwithresult
mean tion and then we corrected the spectrum to rest
for power law indexes obtained in Paper I, II and
frame reference by adopting the measured z. We
III (α =0.90±0.40).
mean finallyaveragedthespectraaccordingtotheirS/N
ratio. The obtained mean spectrum is reported
3.2. Line detection and EW
min in Figure 3 and it covers the spectral range 1800
We measured the minimum detectable equiva- ˚A∼ 6700 ˚A, rest frame. The first panel shows
lent width (EW ) for each spectrum adopting the mean spectrum assuming for the continuum
min
the recipe described in Paper I. We considered as a power-law with index α = 0.90 while the nor-
line canditates all the spectral features revealed malisedspectrumisreportedinthebottompanel.
by an automatic procedure selecting EW above Wemeasurethespectralfeaturesdetectableonthe
EW . Each candidate has been carefully in- averagespectrumreportingthelineidentification,
min
spected for the consequent feature identification equivalent width and full width at half maximum
4
Table 2
Spectral line parameters for the BL Lac objects
Object z α R EWmin LineID λ zline Type FWHM EW(observed)
˚A ˚A kms−1 ˚A
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
GC0109+224 >0.10 0.85 14.20 0.11
PKS0140-059 >0.46 0.45 18.50 0.45
RBS0337 0.232 1.30 16.00 0.35
CaII 4850 0.233 g +0.60±0.15
CaII 4889 0.232 g +0.55±0.15
GBand 5304 0.232 g +0.80±0.13
MgI 6375 0.233 g +1.00±0.18
RBS0499 >0.45 1.00 18.60 0.65
PKS0439-299 >0.68 0.30 20.10 1.00
MgII(?) 4708 [0.68] i +2.15±0.50
RBS0589 >0.45 1.20 17.00 0.20
ZS0506+056 >0.15 0.92 15.30 0.18
MS0622.5-5256 0.513 1.00 19.30 0.65
CaII 5953 0.513 g +1.50±0.20
CaII 6002 0.513 g +1.45±0.20
GBand 6501 0.512 g +1.40±0.40
PKS0735+17 >0.18 0.84 15.70 0.20
OJ-131 0.539 0.44 17.20 0.25
[OII] 5736 0.539 e 1200±150 −1.15±0.30
Hβ 7481 0.539 e 1100±100 −0.70±0.15
[OIII] 7703 0.538 e 1250±100 −2.50±0.20
RXJ1022.7-0112 >0.30 1.34 16.60 0.20
RXJ1023.9-4336 >0.24 1.28 15.80 0.12
PKSB1056-113 >0.27 0.66 16.50 0.20
1ES1106+244 >0.37 0.78 18.20 0.65
RXJ1117.0+2014 0.140 1.40 16.10 0.40
[OII] 4245 0.139 e 1200±120 −1.20±0.20
CaII 4489 0.140 g +0.70±0.15
CaII 4522 0.140 g +0.60±0.10
GBand 4908 0.140 g +0.70±0.20
NaIBlend 6710 0.139 g +1.10±0.40
RXJ1241.8-1455 >0.32 1.05 16.80 0.20
1ES1239+069 >0.47 1.00 18.10 0.35
4C-12.46 >0.37 0.43 17.60 0.30
OQ-240 >0.10 1.00 14.20 0.15
RBS1478 >0.50 0.95 18.60 0.50
OW-080 >0.47 0.42 18.10 0.25
OX-183 0.870 0.16 18.70 0.50
MgII 5236 0.870 e 4400±300 −5.80±1.60
[OII] 6970 0.870 e 1100±200 −3.50±0.70
RBS1899 >0.53 0.93 18.60 0.45
RXJ2319.6+1611 >0.97 0.96 17.20 0.25
MgII(?) 5527 [0.97] i +1.50±0.30
1ES2322-409 >0.16 0.84 15.90 0.32
Note.—Descriptionofcolumns: (1)Objectname;(2)averageredshift(orlowerlimit);(3)power-lawspectralindex;(4)Rmagnitude
measuredonthespectrum;(5)Minimumequivalentwidthdetectable;(6)Lineidentification(7)Barycenteroftheline(8)Redshiftof
theline;(9)Linetypee: emissionlinefromBLLac;g: absorptionlinefromBLLachostgalaxy;i: absorptionlinefromintervening
system;(10)FullWidthathalfmaximum;(11)Measuredequivalentwidth
5
Table 3
Spectral line parameters for the objects confirmed non-BL Lacs
Object z R LineID λ zline Type FWHM EW(observed)
˚A kms−1 ˚A
(1) (2) (3) (4) (5) (6) (7) (8) (9)
PKS0302-623 1.350 17.70
CIII] 4485 1.349 e 5500±300 −20.00±2.00
MgII(?) 5756 [1.056] i +2.50±0.30
MgII 6585 1.350 e 7000±300 −35.00±8.00
PKS0410-519 1.254 17.10
CIII] 4300 1.252 e 5200±300 −63.50±5.00
MgII 6310 1.253 e 3400±300 −48.00±6.00
PKS0443-387 0.537 16.80
MgII 4298 0.535 e 7400±500 −53.50±3.00
[NeV] 5143 0.537 e 1100±100 −0.60±0.20
[NeV] 5268 0.537 e 1500±200 −2.60±0.30
[OII] 5730 0.537 e 1200±100 −2.50±0.20
[NeIII] 5946 0.537 e 2000±300 −5.30±0.60
Hǫ 6098 0.537 e 1200±200 −1.70±0.30
Hδ 6304 0.537 e 1050±150 −0.90±0.20
Hγ 6670 0.537 e 5300±400 −30.00±3.00
Hβ 7455 0.534 e 5300±400 −30.00±3.00
[OIII] 7697 0.537 e 1100±100 −35.00±8.00
PMNJ0529-3555 0.323 19.20
MgII 3705 0.323 e 6400±400 −90.30±10
CaII 5206 0.323 g +4.60±0.70
CaII 5250 0.323 g +4.50±0.50
GBand 5689 0.323 g +4.50±0.50
[OIII] 6560 0.323 e 1300±100 −4.70±0.80
[OIII] 6623 0.323 e 1100±200 −9.80±1.20
PKS0539-543 1.191 17.60
CII] 5097 1.191 e 3200±300 −1.40±0.30
MgII 6135 1.191 e 4000±400 −50.00±8.00
PKS0607-605 1.100 17.30
CIII] 4000 1.095 e 3400±300 −30.90±3.00
MgII 5880 1.100 e 4000±300 −45.50±4.00
[OII] 7821 1.100 e 1100±100 −13.50±2.00
OR103 1.833 15.70
MgII(?) 3778 [0.349] i +2.80±0.5
CIV 4393 1.834 e 5200±300 −13.50±2.00
CIII] 5410 1.833 e 4800±400 −5.00±0.70
PKS1815-553 1.633 20.80
CIII] 5030 1.634 e 6800±600 −92.00±13.00
MgII 7370 1.632 e 6100±400 −115.00±15.00
Note.—Descriptionofcolumns: (1)Objectname;(2)averageredshift;(3)Rmagnitudemeasuredonthespectrum;(4)Lineidenti-
fication(5)Barycenteroftheline(6)Redshiftoftheline;(7)Linetypee: emissionlinefromBLLac;g: absorptionlinefromBLLac
hostgalaxy;i: absorptionlinefrominterveningsystem;(8)FullWidthathalfmaximum;(9)Measuredequivalentwidth
6
Fig. 3.— Mean spectrum of BL Lac objects obtained combining the 23 objects of our campaign in which
intrinsic spectral features are detected. The first panel reports the mean spectrum assuming for the contin-
uum a power-law with index α = 0.90 (which corresponds to the mean spectral index of the whole BL Lac
sample). In the second panel normalised spectrum is shown.
7
Table 4
Spectral line parameters from the BL Lac mean optical spectrum.
Lineidentification λ EW FWHM EW[min;max] FWHM[min;max] Nobs /Ntot Linetype
˚A ˚A kms−1 ˚A kms−1
(1) (2) (3) (4) (5) (6) (7) (8)
CIII] 1909 1.20±0.10 4000±350 [1.30;1.90] [2000;4000] 3/4 e
MgII 2800 1.00±0.20 3500±200 [0.30;3.20] [1900;6100] 9/13 e
[OII] 3727 0.30±0.10 1300±200 [0.10;2.50] [1000;4700] 9/18 e
CaII 3934 0.80±0.20 [0.30;6.10] 12/17 g
CaII 3968 0.65±0.10 [0.40;4.20] 12/17 g
GBand 4305 0.75±0.10 [0.42;4.00] 12/17 g
Hβ 4862 0.25±0.10 [0.50;3.00] 2/14 g
[OIII] 5007 0.50±0.10 1100±200 [0.80;1.47] [500;2400] 3/14 e
MgI 5162 0.90±0.20 [0.26;14.00] 3/12 g
NaI 5892 0.40±0.10 [1.00;3.00] 2/6 g
Hα 6563 0.90±0.15 1300±200 * * 1/3 e
Note.—Descriptionofcolumns: (1)Ionidentification;(2)Restframewavelength; (3)Measuredequivalentwidth(4)Fullwidthat
halfmaximum(5)Equivalentwidthmeasurementrange(6)Fullwidthathalfmaximummeasurementrange(7)Ratiobetweennumber
ofdetectionsandnumberofspectrainwhichthelinecouldbemeasures(8)Typeofline: e: emissionlinefromBLLac;g: absorption
linefromBLLachostgalaxy;
inTable4. Wenotethatduetothelargedifference select ∼ 155 observable sources from the Paranal
in BL Lacs of the ratio between the non-thermal with VLT+FORS. We obtained time for the ob-
emission and the thermal one associated to stel- servationof88sourcesselectedcombiningthe155
lar component (their host galaxy) the mean BL objects from the 11th Veron catalog with fur-
Lac spectrum cannot be considered as represen- ther44sourcesfromPadovaniandGiommi1995a.
tative as that of quasars. On the other hand, it Thankstothehighqualityopticalspectra,wecon-
illustrates emission or absorption lines that could firmed as bona-fide BL Lacs 69 sources while 15
be measured in individual objects. In particular, have been classified as QSO or FSRQ. The re-
absorption lines from the host galaxy are always maining 4 objects are stars. For the sources that
presentinthespectrumbuttheirdetectabilityde- have been classified as bona-fide BL Lac we were
pends both on S/N ratio and on the nucleus-to- able to measured a spectroscopic redshift for 23
host ratio of the source (see also 5.4). Conversely, of them. In particular, in 12 sources we clearly
emission lines could be missing. detected the absorption features from their host
galaxy and in 15 objects emission lines are ob-
5. Discussion and conclusions servable. Broad emission lines (such as Mg II or
C III]) are detected in only 10 BL Lacs. We mea-
5.1. Summary on the campaign
sured the luminosity of Mg II emission lines in
all the sources in which the line is detected. The
According to the 11th Veron-Cetty catalog of
mean value is logL(Mg II) ∼ 42.40±0.30, where
AGN objects (Veron 2003) at the time of the
L (Mg II) is in erg s−1 . This value is lower of
scheduling of our campaign in 2003, 797 sources
an order of magnitude of that of quasars which is
were known as candidate or confirmed BL Lacs
logL(Mg II) ∼ 43.60± 0.45 (Shen et al. (2011))
and 313 objects of the list were left without a
but the dispersions are rather larger. For the re-
reliable measure of their redshift. Considering
only the sources observable from the southern maining 46 sources we inferred a redshift lower
limit (see Section 3.3) . The full dataset, with
sky (δ < 15) a total of 169 BL Lacs objects re-
spectra and line identification, is available at the
mains of unknown redshift. From this set, we
website http://www.oapd.inaf.it/zbllac/.
selected objects with V < 21 in order to se-
cure an acceptable S/N ratio spectrum for each
source. This additional constraint allowed us to
8
5.2. Optical beaming factor
In the radio emission, the relativistic beaming
is manifest in BL Lac objects because of the fre-
quent appearanceofjets with superluminalveloc-
ities. This isrequiredinhighenergyemissions(in
γ-rays)bytheshorttimescaleofvariability,which
can be accommodated in a standard SSC model
(e.g. (Maraschi & Tavecchio 2003)) only invoking
largeDopplerbeaming factors. Followingthe uni-
fiedmodelofActiveGalaticNucleiweassumethat
the opticalemissionarisingfromthe nuclearcom-
ponent is basically the superposition of a thermal
emission of luminosity L associated with a disk
th
structureandanonthermaloneL ,whichdom-
nth
inates the output power in BL Lac objects. Since
the former component is isotropic and the latter
supposedly is beamed, we can define as beaming
factor(seee.g. Farina et al.(2012),Landoni et al.
(2012))
L
tot
δ = (2)
L
th
where L = L + L .
tot th nth
We further assume that the thermal compo-
nents of BL Lac objects can be described as that
of quasars. The large collection of Type I AGN
spectra available in the SDSS allows to find rela-
tions between the broad line luminosity L and
line
the contiguous (thermal) luminosity for Type I
quasars. They aregenerallyexpressedinthe form
Fig. 4.— Optical beaming factor for the sources
observed in the campaign. Red dashed line in-
λL (λ )
λ ref
log =K (3) dicates the lower limit value calculated through
L
line Equation 4 assuming z = 0.70. Filled blue circle
Forsimplicity,wereferonlytoMgIIrelationsince are the δ of bona-fide BL Lac objects with Mg
itisthetightestone(K =1.91±0.26,λref =3000, II emission line while the open magenta circle are
seeDecarli et al.(2011);Shaw et al.(2012)). Tak- the beaming factor for other intermediate sources
ingintoaccountthe contributionofthecosmolog- (classifiedasconfirmednon-BLLacs). Dottedline
ical redshift and combining equation (2) and (3) delimit the area of the EW −δ plane where the
with the usual defintion of the equivalent width , source are of intermediate nature between pure
one has QSO (δ ∼1) and BL Lacs (δ & 4).
λ
δ =(1+z)× ref (4)
10K ×EW
δ is obviously equal to ∼1, within the dispersion,
in the case Type I quasar. When a broad Mg II
line is observed in a BL Lac object Eq. (4) gives
an estimate of the beaming. If no emission line is
detectedonecanstilluseEq. (4)andfromtheup-
per limit on the equivalent width deduce a lower
limit on δ. We suppose that the redshift is such
9
that the relation can be applied because the Mg n = 69 (23 with redshift measured and 46 lower
t
II emission line is within band of observation and limits on z) and those which have z < z are
min
wealsoconsiderthepositive1-σ dispersiononthe 13. Therefore, excluding sources with z < z
min
relation,whichyieldsK =2.17. Inparticular,for the number of BL Lacs which can contribute to
featureless BL Lacs we assume z ∼ 0.7 while for the observed absorptions is at most 56. On the
the BL Lac in which the redshift is known we use other hand the minimum number which for sure
the actualvalue for the redshift. The wide-spread contributes to the statistics of the absorbers is 8,
criterionofdefining aBL Lac a Type I AGN with which are the observed systems associated to BL
line EW < 5, which was adopted also in this pa- Lacs in our sample, plus the number of BL Lacs
per,thereforetranslatesthroughEq. (4)inδ &4. without intervening absorptions with z > z
min
In Figure 4 we show the lower limit relation ex- thatare20. FollowingZhu & M´enard(2012),who
pressed in Eq. (4) for the beaming factor of BL considerQSOsintheSDSS,foranarbitrarypoint-
Lacs with no broad emission lines (dashed line) ingdirectionthenormalisedmeannumberofMgII
and the actual value of δ for the sources in which absorberswithEW&1˚Aisn ∼0.40inthered-
Q
the broad Mg II emission line is detected (filled shiftrangegivenabove. Withintheseassumptions
blue circle). We note that ∼ 7 BL Lac candidates we expect that the number of Mg II absorption
considered in our sample have spectral properties systems is 11< N <22. Since in our sample we
exp
thatdonotcompletelysatisfythecriterionforBL observe N = 8 intervening systems we do not
obs
Lac classification. On the other hand, these ob- confirm the suggestion of Stocke & Rector (1997)
jectsdifferalsofromthetypicalspectrumofType andweconcludethatabsorbersrelatedtoBLLacs
I AGN.Interesting enoughthey havetheir optical are not more numerous than those associated to
beaming factor (open magenta circle of Figure 4) QSOs. Moreover, if we assume that the distribu-
muchlargerofthatofquasar,whereδshouldbe∼ tionofMgII interveningsystemforBLLacisnot
1, suggesting a substantial non-thermal emission different from those of quasars,the number of ob-
from the nucleus which indicates that the objects served absorption system could suggest that the
areindeedofintermediatenaturebetweenBLLac BL Lacs without z in our sample should have low
and quasars. value for their redshift, in order to comply with
the discussed statistical properties for intervening
5.3. Mg II intervening absorption systems system.
Since BL Lac objects are extragalactic sources
5.4. Future perspective: BL Lac spec-
it is reasonable to expect that their spectra could
troscopy in the ELT era
present absorption features from intervening sys-
tems. In particular, Stocke & Rector (1997) InthespectrumofaBLLacobjectthe absorp-
claimedthatastrongexcessofafactor∼4onthe tion lines associated to the stellar components of
number of Mg II intervening system is apparent the host galaxy are always present. On the other
in the spectra of BL Lac objects, with respect to hand,theirdetectabilitydependsbothontheS/N
those of quasars. ratio and on the nucleus to host ratio (N/H) of
In our sample there are 7 confirmed BL Lacs ob- the source (see e. g. Sbarufatti et al. (2005a)).
jects in which absorption systems are detected. With the state of the art 8-mt class telescopes
These absorptions are interpreted as Mg II inter- and modern instrumentation it is possible to ob-
vening systems on ourline of sighttowardthe BL tainspectrawithS/Nabout100andR∼2000for
Lac. In particular, the total number of absorb- object of V ∼ 17 (assuming exposure time of ∼ 1
ing systems is N = 8, partly associated with hour). For sources with V & 20 the situation is
obs
BL Lacs with unknown redshift. The most natu- evenworsesincetheS/Ntypicallydropsbelow50.
ral interpretation is that the features are due to Thislimitsthecapabilitytodetectthehostgalaxy
interveningMgIIatλ =2800. Thefeaturesasso- features either for the sources with high intrinsic
0
ciatedtointerveningMgIIsystemsareobservable N/H ratio and for faint objects. For this rea-
in our sample in the redshift interval z ∼ 0.5 son, in order to significantly improve these limits,
min
and z ∼1.5 . a future 40-mt class telescope like the European
max
The total number of BL Lacs we consider is Extreme Large Telescope (E-ELT) is mandatory
10