Issue |
A&A
Volume 513, April 2010
|
|
---|---|---|
Article Number | A18 | |
Number of page(s) | 6 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200913864 | |
Published online | 15 April 2010 |
The radio-loud active nucleus in the "dark lens'' galaxy J1218+2953
S. Frey1,2 - Z. Paragi3,2 - R. M. Campbell3 - A. Moór4
1 - FÖMI Satellite Geodetic Observatory, PO Box 585,
1592 Budapest, Hungary
2 -
MTA Research Group for Physical Geodesy and Geodynamics, PO Box 91,
1521 Budapest, Hungary
3 -
Joint Institute for VLBI in Europe, Postbus 2,
7990 AA Dwingeloo, The Netherlands
4 -
MTA Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary
Received 14 December 2009 / Accepted 22 January 2010
Abstract
Context. There is a possibility that the optically
unidentified radio source J1218+2953 may act as a gravitational lens,
producing an optical arc
away from the radio position. Until now, the nature of the lensing
object has been uncertain since it is not detected in any waveband
other than the radio. The estimated high mass-to-light ratio could even
allow the total mass of this galaxy to be primarily in the form of dark
matter. In this case, J1218+2953 could be the first known example of a
"dark lens''.
Aims. We investigate the nature of J1218+2953 by means of
high-resolution radio imaging observations to determine whether there
is a radio-loud active galactic nucleus (AGN) in the position of the
lensing object.
Methods. We report on Very Long Baseline Interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.6 and 5 GHz.
Results. Our images, having angular resolutions of 1 to
10 milli-arcseconds (mas), reveal a rich and complex radio structure extending to almost
.
Based on its radio spectrum and structure, J1218+2953 can be classified
as a compact steep-spectrum (CSS) source, and as a medium-size
symmetric object (MSO). The object harbours an AGN. It is also found as
an X-ray source in the XMM-Newton EPIC (European Photon Imaging
Cameras) instrument serendipitous source catalogue.
Conclusions. Rather than being a dark lens, J1218+2953 is most
likely a massive, heavily obscured galaxy in which the nuclear activity
is currently in an early evolutionary stage.
Key words: radio continuum: galaxies - galaxies: active - galaxies: individual: FIRST J121839.7+295325 - techniques: interferometric - gravitational lensing: strong
1 Introduction
It is now widely accepted that 95%
of the mass of galaxies and clusters is composed of some unknown form
of dark matter. The observational evidence comes from the flat galactic
rotation curves, the mass required for gravitational lensing, and the
presence of hot X-ray-emitting gas in galaxy clusters (see e.g. Freese 2009,
for a recent review). It is possible that empty dark matter halos may
also exist. Strong gravitational lensing may in principle be a useful
tool to detect massive dark matter halos that do not contain
significant amounts of ordinary matter (Rusin 2002). Hawkins (1997),
based on a sample of supposedly lensed double quasars without visible
lensing galaxies, envisaged a large number of dark galaxies, three
times more than galaxies with normal mass-to-light ratios. However,
those quasars later proved to be physical binaries rather than lensed
images. In fact, the extensive systematic Cosmic Lens All-Sky Survey
(CLASS) that identified gravitationally lensed objects in the radio,
did not find any convincing evidence for dark lens among 22 lens
systems (Jackson et al. 1998; Browne et al. 2003).
Another suspected "dark galaxy'', VIRGOHI 21 has earlier been
found in the Virgo Cluster, based on the broadening of its 21-cm H I emission line
by the presumed rotation of the gravitationally bound neutral
hydrogen gas (Davies et al. 2004; Minchin et al. 2005). In this model, the total mass of the object
could reach 1011
,
with a baryonic fraction at least ten
times smaller than observed in visible disk galaxies in the Universe
(Minchin et al. 2007). On both observational and theoretical grounds, others claim
that VIRGOHI 21 is in fact tidal debris left over from an interaction that
involved the nearby bright spiral galaxy NGC 4254 (Haynes et al. 2007; Duc & Bournaud 2008). The
existence of dark galaxies is therefore far from being established.
Recently Ryan et al. (2008)
investigated a hypothesis that the optically unidentified radio source
FIRST J121839.7+295325 (J1218+2953 hereafter) is strongly lensing a
background galaxy. The arc-like lensed image is
south-west of the radio source. Its estimated photometric redshift is
.
However, the suspected foreground object is so far detected only in the radio. Its integrated flux density at
GHz frequency is S=33.9 mJy in the Faint Images of the Radio Sky at Twenty-centimeters
(FIRST) survey (White et al. 1997). The source is unresolved in FIRST, with a deconvolved size smaller than
.
Other total radio flux density measurements found in the literature for
this object at low frequencies are: 284 mJy at 74 MHz (Cohen
et al. 2004), 151 mJy at 151 MHz (Hales et al. 2007), and 129 mJy at 330 MHz (Westerbork Northern Sky Survey Catalogue, WENSS
; de Bruyn et al. 1998). A power-law fit to the total flux density data at these four frequencies gives a radio spectral index
(
).
Deep optical and infrared imaging (Russell et al. 2008; Ryan et al. 2008) did not reveal any
counterpart to the radio source. The limiting magnitudes are 25.5, 22.0 and
20.7 in the V, J and H bands, respectively. Based on the 1.4-GHz radio
flux density and the optical non-detection (Russell et al. 2008), Ryan et al. (2008) place
the source in an approximate redshift range of
.
The lower limit derives from the optical non-detection, so if the object is an unusually
obscured galaxy, it could possibly be even closer.
The best-fit lens model assuming an isothermal ellipsoid
(Ryan et al. 2008) provides a lensing galaxy with an Einstein radius of
and a dynamical mass
.
The apparently very
large mass-to-light ratio led Ryan et al. (2008)
to raise the possibility that
J1218+2953 may be in fact a galaxy dominated by dark matter. Suspected
dark-matter galaxies, of which this could be a very rare (or the only)
example,
would of course be impossible to image directly in the optical.
Alternatively, the lensing radio source J1218+2953 may be a massive
obscured galaxy with a central active galactic nucleus (AGN) visible in
the radio. In this case, according to the optical non-detection and the
lens model, the stellar mass in this galaxy would at best be only about
1% of its total dynamical mass (Ryan et al. 2008).
Firm evidence for a radio-loud AGN can be obtained with high-resolution radio
interferometric observations. Here we report on our dual-frequency Very Long
Baseline Interferometry (VLBI) experiments with the European VLBI Network (EVN), which have revealed a complex
structure on angular scales from 1 to several hundred
milli-arcseconds (mas) within J1218+2953. Our VLBI observations also provide an accurate
astrometric position of the source. We give the details of the experiments and the data
analysis, as well as our search for additional data at other wavebands in Sect. 2. We describe the observed radio structure of the
source in Sect. 3 and discuss the possible implications of our
results in Sect. 4.
2 Observations
2.1 Radio interferometric imaging
We observed J1218+2953 with the EVN at 1.6 GHz and
5 GHz frequencies, using the e-VLBI technique (Szomoru 2008). Unlike
traditional VLBI, in which the remote radio telescopes of the
network record their signals onto physical media, which are shipped
to the central processor for subsequent correlation,
e-VLBI streams the signals over optical fibre networks directly to the central processor for
real-time correlation. This process vastly compresses the time-scale between
observations and the availability of the correlated data for
further analysis.
On 2009 January 23 we conducted an exploratory 1.6-GHz e-VLBI experiment.
This lasted only for 2 h, but allowed us to verify that the source is sufficiently compact for VLBI detection.
Its radio emission was clearly detectable at 25 mas angular resolution,
and the resulting image revealed two major components separated by
500 mas. Based on these exploratory observations, we conducted
deeper dual-frequency observations.
To make a more sensitive 1.6-GHz VLBI image, we used
nine antennas of the EVN during an 8 h observation on 2009 April 21.
Participating stations included Effelsberg (Germany),
Jodrell Bank Lovell Telescope, Cambridge, Darnhall (UK), Medicina (Italy),
Onsala (Sweden), Torun (Poland), Arecibo (Puerto Rico) and the phased array
of the 14-element Westerbork Synthesis Radio Telescope (WSRT, the Netherlands).
At most of the antennas, a data transmission rate of 512 Mbit s-1 was
achieved in real time, which resulted in a total bandwidth of 64 MHz in both
left and right circular polarizations, using 2-bit sampling.
We also carried out an 8-h e-VLBI observation with EVN at 5 GHz on
2009 March 24. The maximum data rate was 1024 Mbit s-1, corresponding to
128 MHz bandwidth per polarization. The successfully participating radio
telescopes were Effelsberg, Jodrell Bank Mk2, Knockin (UK), Medicina, Onsala,
Torun, and the WSRT.
The correlation of the VLBI data from both observations took
place at the EVN MkIV Data Processor at the Joint Institute for VLBI in Europe
(JIVE) in Dwingeloo, the Netherlands.
We also analysed the synthesis array data recorded at the WSRT during the e-VLBI observations. Here, the source J1218+2953 appeared unresolved on arcsecond scales at both 1.6 and 5 GHz, allowing us to obtain simultaneous total flux density measurements.
Because our target radio source is relatively weak, we observed in
phase-reference mode. This method is usually applied to increase the total
coherent integration time spent on the source and consequently to improve the
sensitivity of the observations. Phase-referencing is done by regularly
changing between the target source and a bright, compact reference
source lying nearby on the plane of the sky (e.g. Beasley & Conway 1995).
We chose J1217+3007, a BL Lac object with compact VLBI structure
(Bondi et al. 2004) having an angular separation of
from J1218+2953, as
the phase-reference calibrator. We took the
position of J1217+3007 from
the US National Radio Astronomy Observatory (NRAO) Very Long Baseline Array
(VLBA) Calibrator
Survey
,
which quoted right ascension and declination uncertainties of 0.21 mas and
0.41 mas, respectively.
The delay, delay rate and phase solutions
derived for the phase-reference calibrator were interpolated and applied to
J1218+2953 within the target-reference cycle time of
5 min. The
target source was observed for 3.5-min intervals in each cycle. The total
observing time on J1218+2953 was nearly 5 h and 4 h at 1.6 GHz and 5 GHz,
respectively.
We used the NRAO Astronomical Image Processing System (AIPS; e.g. Diamond 1995)
for the data calibration. The visibility amplitudes were calibrated
using system temperatures and antenna gains measured at the antennas.
Fringe-fitting was performed for the calibrator (J1217+3007) and
fringe-finder sources (J0927+3902, J1159+2914, J1407+2827) using 3-min
solution intervals.
We exported the data to the Caltech Difmap package (Shepherd
et al. 1994)
for imaging. The conventional hybrid mapping procedure involving
several iterations of CLEANing and phase (then amplitude)
self-calibration resulted in the images and brightness distribution
models for the calibrators. Overall antenna gain correction factors (10% or less) were determined and applied to the visibility amplitudes in AIPS.
We then repeated fringe-fitting for the phase-reference calibrator in AIPS, now
taking its CLEAN component model into account in order to compensate for
residual phase corrections resulting from its non-pointlike structure. The
solutions obtained were interpolated and applied to the target source data. The calibrated and phase-referenced
visibility data of J1218+2953, unaveraged in time, were also exported to Difmap
for imaging.
The total intensity
images at 1.6 GHz (Figs. 1, 2)
and 5 GHz (Fig. 3) were made after several cycles of CLEANing
in Difmap. The lowest contours are drawn at
image noise levels.
The coordinates in all images are relative to the a priori position taken from the FIRST catalogue (
,
). This position was used as the correlation phase center for both e-EVN experiments.
![]() |
Figure 1:
The naturally weighted 1.6-GHz VLBI image of J1218+2953. The positive
contour levels increase by a factor of 2. A Gaussian taper with the
value of 0.5 at the projected baseline length of 10 million wavelengths
(M |
Open with DEXTER |
![]() |
Figure 2:
The uniformly weighted 1.6-GHz VLBI image of J1218+2953. The positive
contour levels increase by a factor of 2. The first contours are drawn
at -60 and 60 |
Open with DEXTER |
![]() |
Figure 3:
The naturally weighted 5-GHz VLBI image of J1218+2953. The positive
contour levels increase by a factor of 2. The first contours are drawn
at -50 and 50 |
Open with DEXTER |
2.2 Infrared and X-ray data
At the position of J1218+2953, the archive of the Submillimetre
Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell
Telescope (JCMT) does not contain data. To look for archival
observations at shorter wavelengths, we searched the Spitzer Space
Telescope archive for possible measurements at our target position.
This resulted in two Infrared Array Camera (IRAC) images at 3.6 and
5.8 m,
which belong to the same observation (AOR key 12451840) targeting the
nearby Seyfert galaxy Markarian 766 (NGC 4253). We started
the data processing of these images with the BCD files (Basic
Calibrated Data) and then performed additional corrections as described
by Hora et al. (2008). Since
J1218+2953 was not detected in the final images, we derived upper
limits at the position of our radio source of 0.030 mJy (
)
and 0.28 mJy (
)
at 3.6 and 5.8
m, respectively. However, these limits are not very deep.
The object 2XMM J121839.6+295327 found in the XMM-Newton EPIC
(European Photon Imaging Cameras) instrument serendipitous source
catalogue (Watson et al. 2009) is
away from the center of the radio source, to the north. This difference is comparable to the formal X-ray positional error (
).
Therefore we identify our radio source with the X-ray object. It is
detected in the three highest-energy bands out of the five bands, with
flux values of
W m-2 (1-2 keV band),
W m-2 (2-4.5 keV band), and
W m-2 (4.5-12 keV band). The total-band (0.2-12 keV) X-ray flux is
W m-2.
Although the uncertainties are large, we can estimate the absorbing
neutral hydrogen column density based on the detections in the 3
hardest EPIC bands, assuming a typical power-law spectral slope
of 1.8. The value of
cm-2 is about two orders of magnitude higher than the Galactic one.
3 The radio structure of J1218+2953
Our tapered 1.6-GHz VLBI image (Fig. 1) reveals a rich and complex structure in an "inverted S'' shape, spanning almost
.
This corresponds to a projected linear size of 5-6 kpc, if the source is indeed in the
redshift range. (We assume a flat cosmological model with
km s-1 Mpc-1,
and
.)
The major components of the brightness distribution are labeled in the
images. The two brightest ones (SE1 and NW2) seen also in the
highest-resolution image (Fig. 2)
are on opposite sides of the approximate center of symmetry (component
C). The 1.6-GHz data measured on the longest baselines to Arecibo
indicate that there isn't any compact unresolved component visible with
the sensitivity of those baselines (Fig. 2). Therefore SE1 itself is in fact not a "core-jet'' structure: the linear
100-mas
long jet-like feature in the south-east ends in a "hot spot''. Here
the shape of the radio structure abruptly changes at right angles,
ending up in component SE2. On the northwestern side of C, a similar
but somewhat smoother directional change occurs between NW1 and NW2.
The sum of flux densities in the CLEAN components is 20.8 mJy. Our WSRT data
give 27 mJy total flux density, closer to the FIRST value (33.9 mJy). The
correlated flux density measured on only the shortest e-VLBI baseline in our
experiment (Jodrell Bank Lovell Telescope-Darnhall, 17.6 km) is also
consistent with this higher value. Therefore we believe that there is still
some extended emission in the
angular scale which our e-VLBI
observations resolved out.
The 5-GHz image (Fig. 3) shows only those components that have somewhat flatter spectra and compact structures not resolved out at this higher observing frequency. The sum of CLEANed flux densities is 1.6 mJy, while the WSRT data give 9 mJy total flux density. The difference is attributed to extended radio emission. The total flux density is consistent with the steep overall radio spectrum of the source.
The astrometric position of component C (
,
)
was determined with the AIPS task MAXFIT using the 5-GHz image, and is
accurate to 1 mas. We suggest that this coincides with the center
of the galaxy.
4 Discussion
Here we follow the assumption of Ryan et al. (2008) that the optical arc seen
away
from the
radio position of J1218+2953 is a result of gravitational lensing.
However, we cannot provide any further proof for this scenario based on
our own observations. We stress nevertheless that our interpretation of
the compact radio source itself is independent of the lensing
hypothesis.
4.1 The nature of the radio source: lensed or lens?
Are J1218+2953 and the optical arc all gravitationally lensed images of the same background object, or does the compact radio source reside in the lens forming the optical arc?
A static gravitational lens would be achromatic and
conserve surface brightness in its images; these characteristics form
the
basis for the principal objection to the former interpretation.
The radio flux density of the
images should scale with their size, and since the arc is elongated,
its radio
flux density should be much higher than we observe in the compact
J1218+2953 components. However, the optical arc does not show
sufficient
radio emission. At 1.6 GHz, both our VLBI radio source and the
optical arc are located within the same WSRT restoring beam. Yet the
measured WSRT flux density is only 25%
greater. In principle it might be possible that we see a gravitational
lens system in which one of the ray paths suffers high obscuration in
the optical, while the other one does in the radio, for example due to
free-free absorption (e.g. Mittal et al. 2007).
However, we find this configuration very unlikely, because of the
extended area of the arc that would require free-free absorption in
order to mask a radio detection.
![]() |
Figure 4:
The critical surface mass density as a function of the lens
redshift for the case of a background source at z=2.5.
The constant factor (
|
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The structure seen in Fig. 1 may suggest the appearance of two close images of a single bent structure in a background source. While the configuration of an extended arc plus two more compact images is quite feasible (e.g. B0128+437; Biggs et al. 2004, Biggs 2004), conservation of surface brightness would imply that the relative flux densities of the two components in each "image'' should be similar. However, it is clear that NW1 is rather fainter than NW2, and the opposite case holds for SE1 and SE2.
It thus seems very unlikely that the radio structure of J1218+2953 and the optical arc arise from graviational lensing of the same background source. Therefore we interpret the host galaxy of J1218+2953 as (part of) the gravitational lens forming the optical arc.
Assuming a background object at z=2.5, even if the lensing galaxy is much closer than estimated by Ryan et al. (2008), i.e.
at
0.15<z<0.8, the total lensing
mass required to produce the same image-plane geometry would not change
dramatically, only within a factor
of 2.
Figure 4 plots the critical surface mass density,
![]() |
(1) |
supressing the explicit constants and the net factor of H0/c arising from the ratio of the usual lensing angular diameter distances. Therefore the interpretation of the VLBI radio source as belonging to the lensing object is not terribly sensitive to the redshift of the lens, even if the lensing galaxy moves significantly closer, reflecting higher obscuration in the optical.
Note that the center of the lens model (Fig. 2 of Ryan et al. 2008)
is offset from our radio position by
,
beyond the
positional uncertainty
of the HST-WFPC2 observations (
;
Russell et al. 2008).
In their model, Ryan et al. (2008)
treated the position of the lensing galaxy, an isothermal ellipsoid, as
a free parameter. The accurate coordinates derived from our VLBI
observations could provide further constraints to refine the lens
modeling, leading to somewhat different values for other parameters. On
the other hand, similar radio sources are often associated with merging
or disturbed-morphology hosts. Thus it is also possible that the VLBI
structure does not coincide with the mass center of the galaxy.
4.2 The classification of the radio source
Based on its radio spectrum and structure, J1218+2953 can be classified as a compact steep-spectrum (CSS; e.g. O'Dea 1998) source, and a medium-size symmetric object (MSO; Fanti et al. 1995). These objects are believed to be young radio sources, recently (about 105-106 years ago) triggered AGNs. Their host galaxies are the most easily identified in the nearby Universe. Beyond redshift 1, the hosts have typical R-band magnitudes around 24. These galaxies often have very red colors (R-K>5). (See e.g. de Vries 2003; Holt 2009, for reviews.)
The radio source J1218+2953 is probably entirely contained within its host galaxy. A plausible assumption is that the mass center of the host coincides with component C. The interaction of the outflowing plasma in the jets with the dense interstellar medium could result in the observed two-sided bent radio structure. This qualitative picture is consistent with the supposed presence of a large mass of gas and dust in this "dark'' galaxy. Notably, the position angle of the practically linear inner part of the radio source (SE1-NW1) is coincident with the minor axis of the gravitational lens model (Fig. 2 of Ryan et al. 2008). This is to be expected if we assume that the inner jets mark the spin axis of the central black hole, which itself coincides with the rotation axis of the entire galaxy.
4.3 Is J1218+2953 a highly obscured galaxy?
According to our VLBI imaging results, J1218+2953 can most naturally
be identified with the lensing galaxy, if the optical arc is indeed due
to gravitational lensing as we assume.
The lens is therefore not completely dark matter, but rather not (yet)
detected in the optical and near-infrared wavebands. According to Ryan
et al. (2008), this galaxy has an extremely high (over )
dynamical-to-stellar mass ratio. However,
due to the suspected high obscuration, the stellar mass could take more
of the total mass in this galaxy.
It is feasible to expect mid- and far-infrared emission from the dust,
and (hard) X-rays from the high-energy photons originating from the
accretion disk around the central black hole that can penetrate the
dense material in the galaxy.
![]() |
Figure 5: The SED of J1218+2953. Measured values are indicated with open circles, upper limits with arrows. The values for RX J1011.2+5545 (Barcons et al. 1998) divided by 5 are plotted with triangles. The data are taken form the NASA/IPAC Extragalactic Database (NED). Dashed lines characterise the radio (left) and optical (middle) part of the comparison SED. The dashed line at the X-rays (right) is the approximation of the spectrum of RX J1011.2+5545 from Fig. 6 of Barcons et al. (1998). |
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The early evolutionary stages of the growth of massive black holes are highly obscured. The model of Fabian (1999)
accounts for the observed hard X-ray background and states that the
growth of the central black hole continues until the obscuring gas is
ejected from the galaxy. Before that, the absorbed radiation from the
obscured AGN is re-emitted mostly in the far-infrared and
sub-millimeter bands. Direct X-ray radiation from around the accreting
black hole may be observed above 30 keV. According to theoretical
models, CSS sources should generally be strong X-ray emitters. However,
at
and beyond, the angular resolution of current X-ray telescopes is not
sufficient to resolve the expected kpc-scale structure spatially
(Siemiginowska 2009, and references therein).
The flux density upper limit in the 3.6-m
Spitzer band and the measured XMM-Newton EPIC flux allow us to compare
our source with a sample of obscured AGNs selected by Eckart
et al. (2010) from moderately
deep fields observed by both the Spitzer Space Telescope and the
Chandra X-ray Observatory. While our source fits in the general
X-ray-infrared dependence, our upper limit indicates that J1218+2953
could be relatively faint in the mid-infrared compared to other
obscured AGNs.
We constructed the spectral energy distribution (SED) of
J1218+2953 from the measured data in the radio and X-rays, and from the
upper limits available in the optical and near-infrared (Fig. 5). For comparison, we plotted the SED of an X-ray-selected moderately obscured radio-loud AGN at z=1.25, RX J1011.2+5545 (Barcons et al. 1998).
The flux densities of the comparison object were scaled down by a
factor of 5 throughout the whole electromagnetic spectrum, such that
the 1.4-GHz flux densities of the two sources were aligned. While the
two resulting SEDs are remarkably similar in the radio and X-ray bands,
J1218+2953 appears at least an order of magnitude fainter in the
optical, suggesting an additional extinction
.
Without dust obscuration, Ryan et al. (2008) place the dynamical mass-to-light ratio in the range of 30 to 150
for J1218+2953. A strong extinction of
would imply higher luminosity and consequently a mass-to-light ratio consistent with the typical values for other galaxies.
The
ratio in our case (
10-22 mag cm2) is less than the standard Galactic value of
mag cm2, but there is observational evidence that this ratio is nearly always lower in other AGNs, typically by factors of
3 to 100 (Maiolino et al. 2001).
Thus for J1218+2953, there is a rough agreement between the likely
amount of extinction and the column density of the absorbing neutral
hydrogen inferred from the X-ray data.
5 Conclusion
Our EVN observations have revealed that the optically unidentified
radio source J1218+2953 has a radio-loud AGN in its center. The tapered
1.6-GHz e-VLBI image (Fig. 1) shows a complex, two-sided radio structure. The angular size of the radio source is less than
.
Although the redshift of the object is unknown, we used the range
estimated by Ryan et al. (2008)
to conclude that the source is confined to a sub-galactic projected
linear size, 5-6 kpc. Our higher-resolution images at 1.6 GHz
(Fig. 2) and 5 GHz (Fig. 3)
indicate two "hot spots'' and a possible weak central component. The
latter may mark the location of the massive black hole in the center of
this galaxy. We determined the position of this radio component to an
accuracy of 1 mas.
The suspected lensing object thus coincides with a compact
radio source associated with an AGN. According to well-established
models, its radio jet activity is driven by accretion onto a
supermassive (typically 108
)
black hole (see e.g. Urry & Padovani 1995
for a review). Moreover, we found that the object also emits X-rays. It
is therefore detected in two different electromagnetic wavebands, which
indicates that its total mass could not be exclusively in the form of
dark matter.
Based on its radio spectrum and morphology, J1218+2953 can be classified as a compact steep-spectrum (CSS) source, and as a medium-size symmetric object (MSO). Our object is unique in a sense that the CSS samples studied to date contain radio sources at least twice as bright. The faintest CSS sample of Tschager et al. (2003) was compiled from unresolved WENSS sources with at least 250 mJy flux density at 330 MHz. However, even those sources usually lack high-resolution VLBI data, therefore their pc-scale properties are unknown.
A few relatively weak MSOs have been observed by Kunert-Bajraszewska et al. (2005) with the NRAO Very Large Array (VLA) and the UK Multi-element Radio Linked Interferometer Network (MERLIN), with an eventual goal of establishing a more complete evolutionary scheme for radio sources. In our case, J1218+2953 has attracted much attention as an potential "dark'' gravitational lens, not because it was a faint CSS. The complex structure we observe draws attention to the importance of in-depth high-resolution studies of faint CSS sources, and obscured radio-loud X-ray-selected AGNs. These would help provide a better understanding of the early evolution of radio AGNs, the interaction between the host galaxies and the expanding radio sources, and AGN feedback processes.
Our results render the "dark lens'' interpretation of Ryan et al. (2008) unlikely. Supported by the X-ray detection of J1218+2953, we suspect that it is a heavily obscured galaxy in which the nuclear activity is in an early evolutionary stage. Deeper observations in the future in the mid-infrared, far-infrared, and sub-millimeter bands might be able to detect the source and put more constraints on its spectral energy distribution.
AcknowledgementsWe are grateful to the chair of the EVN Program Committee, Tiziana Venturi, for granting us short exploratory e-VLBI observing time in January 2009. The EVN is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. The Westerbork Synthesis Radio Telescope is operated by the ASTRON (Netherlands Institute for Radio Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO). This effort is supported by the European Community Framework Programme 7, Advanced Radio Astronomy in Europe, grant agreement no. 227290, and the Hungarian Scientific Research Fund (OTKA, grant no. K72515). The e-VLBI developments in the EVN are supported by the EC DG-INFSO funded Communication Network Developments project "EXPReS'', Contract No. 02662 (http://www.expres-eu.org/). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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Footnotes
- ... Twenty-centimeters
- http://sundog.stsci.edu
- ... WENSS
- http://cdsarc.u-strasbg.fr/viz-bin/Cat?VIII/62
- ...
Survey
- http://www.vlba.nrao.edu/astro/calib/index.shtml
All Figures
![]() |
Figure 1:
The naturally weighted 1.6-GHz VLBI image of J1218+2953. The positive
contour levels increase by a factor of 2. A Gaussian taper with the
value of 0.5 at the projected baseline length of 10 million wavelengths
(M |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
The uniformly weighted 1.6-GHz VLBI image of J1218+2953. The positive
contour levels increase by a factor of 2. The first contours are drawn
at -60 and 60 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
The naturally weighted 5-GHz VLBI image of J1218+2953. The positive
contour levels increase by a factor of 2. The first contours are drawn
at -50 and 50 |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
The critical surface mass density as a function of the lens
redshift for the case of a background source at z=2.5.
The constant factor (
|
Open with DEXTER | |
In the text |
![]() |
Figure 5: The SED of J1218+2953. Measured values are indicated with open circles, upper limits with arrows. The values for RX J1011.2+5545 (Barcons et al. 1998) divided by 5 are plotted with triangles. The data are taken form the NASA/IPAC Extragalactic Database (NED). Dashed lines characterise the radio (left) and optical (middle) part of the comparison SED. The dashed line at the X-rays (right) is the approximation of the spectrum of RX J1011.2+5545 from Fig. 6 of Barcons et al. (1998). |
Open with DEXTER | |
In the text |
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