The Astrophysical Journal, 747:10 (21pp), 2012 March 1 doi:10.1088/0004-637X/747/1/10
C
2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
X-RAY AND MULTIWAVELENGTH INSIGHTS INTO THE NATURE OF WEAK
EMISSION-LINE QUASARS AT LOW REDSHIFT
Jianfeng Wu
1,2
, W. N. Brandt
1,2
, Scott F. Anderson
3
, Aleksandar M. Diamond-Stanic
4,8
, Patrick B. Hall
5
,
Richard M. Plotkin
6
, Donald P. Schneider
1
, and Ohad Shemmer
7
1
Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA; jfwu@astro.psu.edu
2
Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
3
Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
4
Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92903, USA
5
Department of Physics & Astronomy, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada
6
Astronomical Institute “Anton Pannekoek,” University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
7
Department of Physics, University of North Texas, Denton, TX 76203, USA
Received 2011 October 24; accepted 2011 December 9; published 2012 February 8
ABSTRACT
We report on the X-ray and multiwavelength properties of 11 radio-quiet quasars with weak or no emission lines
identified by the Sloan Digital Sky Survey (SDSS) with redshift z = 0.4–2.5. Our sample was selected from the
Plotkin et al. catalog of radio-quiet, weak-featured active galactic nuclei (AGNs). The distribution of relative X-ray
brightness for our low-redshift weak-line quasar (WLQ) candidates is significantly different from that of typical
radio-quiet quasars, having an excess of X-ray weak sources, but it is consistent with that of high-redshift WLQs.
Over half of the low-redshift WLQ candidates are X-ray weak by a factor of 5, compared to a typical SDSS quasar
with similar UV/optical luminosity. These X-ray weak sources generally show similar UV emission-line properties
to those of the X-ray weak quasar PHL 1811 (weak and blueshifted high-ionization lines, weak semiforbidden
lines, and strong UV Fe emission); they may belong to the notable class of PHL 1811 analogs. The average X-ray
spectrum of these sources is somewhat harder than that of typical radio-quiet quasars. Several other low-redshift
WLQ candidates have normal ratios of X-ray-to-optical/UV flux, and their average X-ray spectral properties are
also similar to those of typical radio-quiet quasars. The X-ray weak and X-ray normal WLQ candidates may belong
to the same subset of quasars having high-ionization “shielding gas” covering most of the wind-dominated broad
emission-line region, but be viewed at different inclinations. The mid-infrared-to-X-ray spectral energy distributions
(SEDs) of these sources are generally consistent with those of typical SDSS quasars, showing that they are not likely
to be BL Lac objects with relativistically boosted continua and diluted emission lines. The mid-infrared-to-UV
SEDs of most radio-quiet weak-featured AGNs without sensitive X-ray coverage (34 objects) are also consistent
with those of typical SDSS quasars. However, one source in our X-ray-observed sample is remarkably strong in
X-rays, indicating that a small fraction of low-redshift WLQ candidates may actually be BL Lac objects residing in
the radio-faint tail of the BL Lac population. We also investigate universal selection criteria for WLQs over a wide
range of redshift, finding that it is not possible to select WLQ candidates in a fully consistent way using different
prominent emission lines (e.g., Lyα,Civ,Mgii, and Hβ) as a function of redshift.
Key words: BL Lacertae objects: general – galaxies: active – galaxies: nuclei – quasars: emission lines – X-rays:
galaxies
Online-only material: color figures, extended figure, machine-readable table
1. INTRODUCTION
Strong and broad-line emission is a common feature of quasar
spectra in the optical and UV bands. However, since multi-color
quasar selection at high redshift in the Sloan Digital Sky Survey
(SDSS; York et al. 2000) is mostly based on the presence of
the Lyα forest and Lyman break (e.g., Richards et al. 2002), the
SDSS can also effectively select high-redshift quasars with weak
or no emission lines. About 90 such weak-line quasars (WLQs)
at high redshift have been found with Lyα +Nv rest-frame
equivalent widths of REW < 15 Å (e.g., Fan et al. 1999, 2006;
Anderson et al. 2001; Collinge et al. 2005; Diamond-Stanic
et al. 2009, hereafter DS09). Some of these objects show a hint
of weak Lyα emission but no other lines; others are completely
bereft of detectable emission lines even in high-quality spectra.
High-redshift SDSS quasars show an approximately lognormal
distribution of Lyα +Nv REW with a mean of ≈62 Å (DS09).
8
Center for Galaxy Evolution Fellow.
The WLQs constitute 3σ negative deviations from the mean,
and there is no corresponding population with 3σ positive
deviations. The majority of these high-redshift WLQs are radio
quiet (α
ro
> −0.21; α
ro
is the slope of a nominal power law
between 5 GHz and 2500 Å in the rest frame; see Section 4 for
a full definition).
WLQs have mainly been studied at high redshifts due to
the fact that the Lyα forest enters into the SDSS spectroscopic
coverage for quasars at z>2.2. However, there is no appar-
ent reason to believe that these objects should also not exist
at lower redshifts. Indeed, a few apparent analogs of WLQs
at lower redshifts have been found serendipitously over the
past ≈15 years; e.g., PG 1407+265 (z = 0.94; McDowell
et al. 1995), 2QZJ2154−3056 (z = 0.49; Londish et al. 2004),
and PHL 1811 (z = 0.19; Leighly et al. 2007a, 2007b). As
a byproduct of a systematic survey for optically selected BL
Lacertae objects (hereafter BL Lacs) in SDSS Data Release 7
(DR7; Abazajian et al. 2009), Plotkin et al. (2010a) discovered
about 60 additional radio-quiet WLQ candidates at z<2.2
1
The Astrophysical Journal, 747:10 (21pp), 2012 March 1 Wu et al.
for which all emission features have REW < 5 Å. These ob-
jects are perhaps the first low-redshift SDSS counterparts of
the previously identified high-redshift SDSS WLQs. Following
the nomenclature that has been established by previous work
on WLQs (e.g., Shemmer et al. 2009), we define “high red-
shift” as z>2.2 and “low-redshift” as z 2.2 because WLQs
are selected with different approaches for these redshift ranges
(see above). Although WLQs are rare, their exceptional char-
acteristics constitute a challenge to our overall understanding
of quasar geometry and physics, especially the quasar broad
emission-line region (BELR). Analogously, physical insights
have been gained by investigating other minority populations
with exceptional emission-line or absorption-line properties,
such as narrow-line Seyfert 1 galaxies and broad absorption line
(BAL) quasars. Therefore, extensive studies of the multi-band
properties of WLQs should have scientific value.
There are several candidate explanations for the physical
nature of WLQs. Their UV emission lines may be weak due
to an “anemic” BELR with a significant deficit of line-emitting
gas (e.g., Shemmer et al. 2010). It has also been speculated that
WLQs may represent an early stage of quasar evolution in which
an accretion disk has formed and emits a typical continuum, but
BELR formation is still in progress (e.g., Hryniewicz et al. 2010;
Liu & Zhang 2011).
The weak UV emission lines may also be a consequence
of a spectral energy distribution (SED) that lacks high-energy
ionizing photons. This soft SED may be a result of unusual
accretion rate. For example, an extremely high accretion rate
might produce a UV-peaked SED (e.g., Leighly et al. 2007b).
In this scenario, high-ionization lines, like C iv, should be
suppressed relative to low-ionization lines like Hβ. However,
Shemmer et al. (2010) estimated the normalized accretion
rates, L/L
Edd
, of two high-redshift WLQs via near-infrared
spectroscopy and found their accretion rates were within the
range for typical quasars with similar luminosities and red-
shifts. Alternatively, a combination of low accretion rate and
large black hole mass may lead to a relatively cold accre-
tion disk that emits few ionizing photons. Laor & Davis
(2011) predicted a steeply falling SED at λ<1000 Å for
quasars with cold accretion disks, and such an SED was ob-
served in the WLQ SDSS J0945+1009 by Hryniewicz et al.
(2010).
High-energy ionizing photons (including X-rays) may be
heavily absorbed before they reach the BELR. Wu et al. (2011)
studied a population of X-ray weak quasars with unusual UV
emission-line properties like those of PHL 1811 (weak and
highly blueshifted high-ionization lines, weak semiforbidden
lines, and strong UV Fe emission). All of their radio-quiet
PHL 1811 analogs were found to be X-ray weak by a factor
of ≈13 on average. These objects also show a harder average
X-ray spectrum than those for typical quasars which suggests
the presence of X-ray absorption. PHL 1811 analogs appear
observationally to be a significant subset (≈30%) of WLQs. The
existence of a class of quasars with high-ionization “shielding
gas” covering most of the BELR, but little more than the BELR,
could potentially unify the PHL 1811 analogs and WLQs via
orientation effects (see Section 4.6 of Wu et al. 2011). The
shielding gas would absorb high-energy ionizing photons before
theyreachtheBELR,resultinginweakhigh-ionizationemission
lines. When such a quasar is observed through the BELR and
the shielding gas, a PHL 1811 analog would be seen; when it is
observed along other directions, an X-ray normal WLQ would
be observed.
Another possibility is that instead of being intrinsically weak,
the UV emission lines of WLQs could in principle be diluted by
a relativistically boosted UV/optical continuum as for BL Lac
objects. However, this scenario is not likely for most WLQs.
Shemmer et al. (2009) found that the X-ray properties of high-
redshift WLQs are inconsistent with those of BL Lac objects.
Furthermore, there is no evidence of strong optical variability
or polarization for these WLQs (see DS09; Meusinger et al.
2011). The UV-to-infrared SEDs of high-redshift WLQs are
also similar to those of typical quasars, while the SEDs of
BL Lac objects are much different (DS09; Lane et al. 2011).
Nevertheless, it is possible that the population of BL Lac objects
has a small radio-quiet tail (e.g., Plotkin et al. 2010b) and that a
small fraction (5%; see Lane et al. 2011) of the general WLQ
population may be BL Lac objects.
Most previous studies of WLQs were based on high-redshift
objects. To investigate the nature of the overall WLQ population,
we obtained new X-ray observations of low-redshift WLQs se-
lected mainly from the catalog of radio-quiet BL Lac candidates
in Plotkin et al. (2010a). We also utilized sensitive archival X-ray
coverage of the sources in their catalog. Our closely related
science goals are the following: (1) enable comparison of the
broadband SEDs of low-redshift WLQs to those of high-redshift
WLQs, typical radio-quiet quasars, and BL Lac objects; (2) pro-
vide basic constraints on X-ray spectral properties via band-ratio
analysis and joint spectral fitting; (3) clarify if there is broadband
SED diversity among low-redshift WLQs; and (4) allow reliable
planning of future long, spectroscopic X-ray observations.
In Section 2, we describe the selection of our sample of low-
redshift, radio-quiet WLQ candidates. In Section 3, we detail
their UV/optical observations and the measurement of their
rest-frame UV spectral properties. In Section 4, we describe
the relevant X-ray data analyses. Overall results and associated
discussion are presented in Section 5. Throughout this paper,
we adopt a cosmology with H
0
= 70.5kms
−1
Mpc
−1
,
Ω
M
= 0.274, and Ω
Λ
= 0.726 (e.g., Komatsu et al. 2009).
2. SELECTION OF THE LOW-REDSHIFT
WLQ CANDIDATES
We obtained Chandra snapshot observations (3.0–4.1 ks)
of six low-redshift (z = 0.40–1.67) WLQ candidates. Five
of the six targets were identified by Plotkin et al. (2010a)
as radio-quiet, weak-featured SDSS quasars with all emis-
sion features having REW 5 Å. An additional source,
SDSS J0945+1009, was similarly identified as a weak-featured
quasar by Hryniewicz et al. (2010). All the objects are suffi-
ciently bright in the optical band (m
i
18) for short Chan-
dra observations to provide tight constraints on their X-ray-to-
optical SEDs.
We further utilized the weak-featured quasar catalogs in
Plotkin et al. (2010a) to search for low-redshift, radio-quiet
sources having sensitive archival X-ray coverage. To ensure
our sample has the high X-ray detection fraction necessary to
provide physically meaningful constraints, we only selected
sources covered by Chandra or XMM-Newton observations.
9
An additional five sources were thereby added into our sample.
Three of them (J1013+4927, J1139−0201, and J1604+4326)
appear in the radio-quiet, weak-featured quasar catalog (Table 6
9
We also checked for pointed ROSAT PSPC observations with an exposure
time greater than 5 ks and an off-axis angle less than 19
(within the inner ring
of the PSPC detector). However, none of the radio-quiet, low-redshift sources
in the catalogs of Plotkin et al. (2010a) are covered by ROSAT observations
meeting these criteria.
2
The Astrophysical Journal, 747:10 (21pp), 2012 March 1 Wu et al.
Tab le 1
X-Ray Observation Log
Object Name z
a
Δ
Opt-X
b
Detector Observation Observation Exposure Time Off-axis Angle References
(SDSS J) (arcsec) Date ID (ks) (arcmin)
Chandra Cycle 12 Objects
081250.79 + 522530.81.153 0.8 ACIS-S 2010 Dec 28 12710 4.10.31
094533.98 + 100950.11.671 ... ACIS-S 2011 Jan 12 12706 3.00.32
110938.50 + 373611.70.397 0.3 ACIS-S 2011 Feb 27 12711 3.10.31
125219.47 + 264053.91.289 0.3 ACIS-S 2011 Mar 12 12709 3.40.31
153044.08 + 231013.41.406 0.4 ACIS-S 2011 Apr 15 12707 3.00.31
161245.68 + 511816.91.595 0.4 ACIS-S 2011 Feb 1 12708 3.20.31
Archival X-ray Data Objects
101353.46 + 492758.11.640 ... MOS
c
2004 Apr 23 0206340201 22.76.41
113900.55 −020140.01.903 0.2 ACIS-S 2004 Jul 21 4871 14.90.61
160410.22 + 432614.61.538 0.2 ACIS-I 2006 Jun 25 6933 26.73.71
0.2 ACIS-I 2006 Jun 23 7343 19.43.7
211552.88 + 000115.52.500 ... ACIS-S 2008 Dec 24 10388 9.50.3 1,3,4
232428.43 + 144324.31.417 0.7 ACIS-S 2009 May 31 10386 5.00.3 1,3,4
Notes.
a
Redshift for each source. See Section 3.1 for details about redshift measurements.
b
Angular distance between the optical and X-ray positions; no entry indicates no X-ray detection.
c
This object was observed by both the MOS and pn detectors. We list MOS detector parameters here.
References. (1) Plotkin et al. 2010a; (2) Hryniewicz et al. 2010; (3) Collinge et al. 2005; (4) Plotkin et al. 2010b.
in Plotkin et al. 2010a). J1139−0201 was targeted by Chandra
as an optically selected BL Lac candidate in Cycle 5, while
J1013+4927 and J1604+4326 were serendipitously covered by
Chandra or XMM-Newton observations. The other two objects
(J2115+0001 and J2324+1443) were initially identified as weak-
featured quasars by Collinge et al. (2005). They were also listed
in the catalog of Plotkin et al. (2010a). These two sources did not
have constraints on their radio fluxes in Collinge et al. (2005)
or Plotkin et al. (2010a) but were later confirmed as radio-quiet
sources by the Very Large Array (VLA) observations of Plotkin
et al. (2010b). They were targeted by Chandra as radio-quiet
BL Lac candidates in Cycle 10; their observations were briefly
reported in Plotkin et al. (2010b). Table 1 presents the X-ray
observation log for our sample.
Our sample includes 11 WLQs in total. All of the sources in
our sample have redshifts of z<2.2, except J2115+0001 which
has a slightly higher redshift of z = 2.4995 (see Section 3.1
for redshift measurements). For comparison, all the radio-quiet
WLQs studied in X-rays by Shemmer et al. (2006, 2009)have
z>2.7 (see Figure 1). Figure 2 shows the SDSS spectra of
the sources in our sample. The spectra show no evidence for
dust reddening or intrinsic BALs; i.e., there is no indication
that their UV/optical continua or BELRs are obscured. We will
compare the multiwavelength properties of our sample to those
of the high-redshift WLQs in Shemmer et al. (2006, 2009)in
Section 5.
3. UV/OPTICAL OBSERVATIONS
3.1. UV Emission-line Measurements
The redshift values (see Table 1) for our low-redshift WLQs
are generally those from Hewett & Wild (2010) which are the
best available measurements for large SDSS quasar samples.
There are three sources lacking Hewett & Wild measurements.
For two quasars (J1109 + 3736 and J1139−0201), the redshift
values are taken from the catalog of Plotkin et al. (2010a). The
redshift of the other source (J2115+0001; z = 2.4995 ±0
.0052)
is measured based on a Lyα +Civ absorption system.
10
To obtain accurate measurements of the weak emission lines,
we manually measured rest-frame emission-line properties for
C iv,Siiv,theλ1900 complex,
11
and Fe iii UV48 (see Table 2)
following the method in Section 2.2 of Wu et al. (2011), which
is summarized below. We first smoothed the SDSS spectra
with a 5 pixel sliding-box filter, and manually interpolated over
strong narrow absorption regions. We then fitted a power-law
local continuum for each line between their lower and upper
wavelength limits λ
lo
and λ
hi
(see Table 2 of Vanden Berk et al.
2001). After subtracting the local continuum, we measured the
REW value for each line. The C iv blueshifts were calculated
between the lab wavelength in the quasar rest frame (1549.06 Å;
see Table 2 of Vanden Berk et al. 2001) and the observed
mode of all pixels with heights greater than 50% of the peak
height, where mode = 3×median − 2×mean. For comparison,
we also include in Table 2 the corresponding measurements
of the spectrum of PHL 1811 (Leighly et al. 2007a) and of
the composite spectrum of typical SDSS quasars in Vanden
Berk et al. (2001). The spectral measurements of PHL 1811 are
included here because some of our low-redshift WLQ candidates
show similar unusual UV/optical spectral properties to those of
PHL 1811 (see Section 5.2). The Mg ii measurements from Shen
et al. (2011) are also listed in Table 2. These measurements
are reliable because the Fe ii component, which could affect
the Mg ii strength measurement, was well modeled. These
REW(Mg ii) values somewhat exceed the selection criterion of
REW 5 Å for BL Lac candidates in Plotkin et al. (2010a). This
10
Plotkin et al. (2010b) did not report the redshift for this source. In this
work, we adopt the redshift of the Lyα +Civ narrow absorption system as the
systemic redshift. Nestor et al. (2008) fit a Gaussian distribution centered at
v = 0kms
−1
with σ = 450 km s
−1
to the distribution of narrow C iv systems
around quasar systemic redshifts. We measured the redshift using that
Gaussian dispersion as the redshift uncertainty to obtain z = 2.4995 ±0.0052.
11
Mainly C iii] λ1909, but also including other features; see note “b” of
Table 2.
3
The Astrophysical Journal, 747:10 (21pp), 2012 March 1 Wu et al.
Figure 1. SDSS absolute i-band magnitude, M
i
, plotted vs. redshift, z. The red filled circles and triangles show our sample of low-redshift WLQ candidates; the blue
filled squares show high-redshift WLQs from Shemmer et al. (2006, 2009); the gray dots represent the 105,783 objects in the SDSS DR7 quasar catalog (Schneider
et al. 2010).
(A color version of this figure is available in the online journal.)
Tab le 2
Quasar UV Emission-line Measurements
Object Name MJD C iv Blueshift REW REW REW REW REW
(SDSS J) (C iv)(Siiv)
a
(λ1900 Å)
b
(Fe iii)(Mgii)
Chandra Cycle 12 Objects
081250.79 + 522530.8 53297 ... ... ... 3.8 ± 2.1 <4.58.4± 0.7
094533.98 + 100950.1 52757 −7300 ± 1700 3.0 ± 1.2 ... 4.9 ± 1.5 1.7 ± 1.5 17.1 ± 0.8
110938.50 + 373611.7 53499 ... ... ... ... ... ...
125219.47 + 264053.9 53823 ... ... ... 8.8 ± 1.2 2.9 ± 0.9 8.7 ± 0.4
153044.08 + 231013.4 53878 ... ... ... 4.9 ± 1.5 2.6 ± 1.2 12.95 ± 0.4
161245.68 + 511816.9 52051 −4700 ± 1300 3.4 ± 1.8 ... 5.1 ± 1.5 3.0 ± 1.5 9.5 ± 0.6
Archival X-ray Data Objects
101353.46 + 492758.1 52076 ... ... ... 3.4 ± 1.8 <6.96.2± 0.8
113900.55 −020140.0 52294 ... <9.0 <9.9 <10.8 <9.0 11.1 ± 1.1
113900.55 −020140.0 (HET) 55702 −2950 ± 1550 3.2 ± 2.7 ... 11.7 ± 1.8 5.5 ± 1.5 ...
160410.22 + 432614.6 52756 ... ... ... <1.9 <1.85.8± 1.0
211552.88 + 000115.5 52443 ... ... ... ... ... ...
232428.43 + 144324.3 52258 ... ... 7.6 ± 2.1 <5.4 ... 8.6 ± 0.8
PHL 1811
c
... −1400 ± 250 4.7 ± 0.9 4.8 ± 0.9 8.3 ± 0.6 4.7 ± 0.6 ...
V01 composite
c,d
... −570 ± 30 30.0 ± 0.3 8.7 ± 0.3 21.7 ± 0.2 2.9 ± 0.1 ...
Notes. The blueshift values are in units of km s
−1
. All REW values are in units of Å.
a
This line is a blend of Si iv and O iv]; we refer to it as Si iv simply for convenience.
b
Mainly C iii] λ1909, but also including [Ne iii] λ1814, Si ii λ1816, Al iii λ1857, Si iii] λ1892, and several Fe iii multiplets (see Table 2 of Vanden
Berk et al. 2001).
c
These measurements are taken from Wu et al. (2011).
d
The composite spectrum from Vanden Berk et al. (2001).
discrepancy mainly originates from differences in measurement
methods. For Plotkin et al. (2010a), it was impractical to define
reference wavelengths to model the continuum in a uniform
way for the entire large sample since many objects lack redshift
measurements. The REW values in Plotkin et al. (2010a)were
measured manually after defining the continuum by eye for
most sources. While this method generally performed well for
BL Lac objects, it did not properly model blended Fe emission
for unbeamed objects.
Only two sources (J0945+1009 and J1612+5118) have high-
quality C iv coverage in their SDSS spectra so that we are able
to measure their C iv REW and blueshift values. Both sources
have weak and highly blueshifted C iv lines. J1139−0021 has
no clearly detectable C iv line in its SDSS spectrum; we could
4
The Astrophysical Journal, 747:10 (21pp), 2012 March 1 Wu et al.
Figure 2. SDSS spectra for the 11 sources in our sample of low-redshift WLQ candidates ordered by Δα
ox
(see Section 4 for definition). The Δα
ox
values and their
error bars (if the source is detected in X-rays) are shown for each source. The name of each source is labeled in the format of “Jhhmm+ddmm.” The y-coordinates
are the flux density (F
λ
) in arbitrary linear units. The tick marks on the y-axis show the zero flux-density level for each normalized spectrum. The spectra have been
smoothed using a 5 pixel sliding-box filter. The spectrum of the radio-quiet BL Lac candidate J1109+3736 is shown separately in the lower right panel for convenience
of presentation (since its redshift is much lower than those of the other sources in our sample). Emission lines, including C iv λ1549, C iii] λ1909, and Mg ii λ2799,
are labeled in the left and upper right panels. The Mgii λ2799, Hβλ4862, [O iii] λ5007, and Hαλ6564 lines are labeled in the lower right panel; the Ca ii H/K break
is also marked by the dotted lines. All the quoted values here are vacuum wavelengths. The spectral resolution is R ≈ 2000. Also included are the composite spectrum
of SDSS quasars by Vanden Berk et al. (2001) and the mean spectrum of the high-redshift WLQs of Shemmer et al. (2009).
only obtain an upper limit on its REW. Therefore, we obtained
follow-up UV spectroscopy for this source with the Low-
Resolution Spectrograph (Hill et al. 1998) on the Hobby–Eberly
Telescope (HET; Ramsey et al. 1998). The UV emission-line
measurements based on the HET spectroscopy are also listed in
Table 2. J1139−0021 has a weak and strongly blueshifted C iv
line in its HET spectrum.
All of the sources having C iii] coverage show weaker
C iii] semiforbidden lines than those of typical quasars. The
Fe iii UV48 strength of our low-redshift WLQ candidates
is generally similar to those of typical quasars. The SDSS
spectrum of J1109+3736 does not have coverage of these rest-
frame UV emission lines because of its much lower redshift,
while the signal-to-noise ratio (S/N) of the SDSS spectrum
5
