E. Örndahl - J. Rönnback - E. van Groningen
Department of Astronomy and Space Physics, Uppsala University, Box 515, 751 20 Uppsala, Sweden
Received 24 February 2003 / Accepted 1 April 2003
Abstract
We have conducted an optical imaging study aimed at resolving the host
galaxies of 79 radio-loud and radio-quiet quasars at
z=0.4-0.8,
extending the number of investigated objects in this redshift range
by 45%.
Observations were performed mainly in the R band but also in V and I
band using the Nordic Optical Telescope on La Palma.
In this paper we discuss the sample
composition and observations and the reduction techniques used.
The quasars were selected in pairs of radio-loud and radio-quiet objects
matched in the z-V plane in order to facilitate a
statistical comparison. The radio-loud part of the sample
contains comparable numbers of flat and steep radio spectrum sources which
also are matched in redshift and V magnitude.
Point spread function subtraction was performed using one-dimensional
luminosity profiles both on the quasar image and on a field star, and
subtracted images and luminosity profiles are shown for each quasar field.
The detection rate
is 60% for the radio-quiet host galaxies and 80% for radio-loud hosts.
The host galaxies have magnitudes which make them brighter than an L* galaxy by a factor of 1.5-4
at the low end of the redshift range, which increases by
2-3 times towards the higher end of the redshift range.
Both radio-quiet and radio-loud hosts
follow the radio galaxy R-z Hubble relation well.
Analysis and discussion of colours and morphology is presented
in Örndahl & Rönnback (2003).
Key words: galaxies: active - quasars: general - galaxies: fundamental parameters - galaxies: photometry
The properties of quasar host galaxies are of fundamental importance in that they further our understanding of the AGN phenomenon. By studying the hosts, clues to the origin and fuelling of the AGN may be provided and the subset of the galaxy population which is capable of producing and sustaining high nuclear activity defined. Such studies can also help constrain physical models of quasar evolution and provide insight on the links between the growth of supermassive black holes and the formation of galaxies.
Using both ground-based instruments as well as the Hubble Space Telescope (HST), investigations of quasar host galaxies at low redshifts have in recent years yielded a wealth of information, both in the optical and near-infrared. The hosts of luminous quasars all seem to be bright galaxies having a luminosity >L* (Bahcall et al. 1997; McLeod & McLeod 2001; Taylor et al. 1996), but the early indications of a host galaxy morphology determined by radio-loudness in the sense that radio-loud quasars (RLQ) reside in early-type galaxies and radio-quiet quasars (RQQ) in spiral hosts (Smith et al. 1986; Hutchings et al. 1989) have been superseded by a picture in which the nuclear luminosity determines the morphology so that only massive spheroidals can host AGNs above a certain luminosity limit (McLure et al. 1999; Dunlop et al. 2003). In addition, a correlation has been found between the luminosities of nearby quasars and those of their host galaxies (McLeod & Rieke 1995; Hooper et al. 1997). Studies at low redshift have furthermore revealed that the relation between bulge luminosity and black hole mass found for nearby galaxies holds also for host galaxies (Laor 1998; Magorrian et al. 1998) but with an apparent offset between the masses of the black holes powering RLQ and RQQ which grows with increasing redshift (Kukula et al. 2001; McLure et al. 1999; Laor 2000).
Host galaxies at higher redshifts are increasingly difficult to resolve and
also suffer from rapid cosmological surface brightness dimming of the host
in contrast to the
nucleus. The similarity between the strong evolution of the
quasar population with redshift and the rate of galaxy formation has driven
a wide interest in host galaxies at z > 1
in an effort to uncover the connections between nuclear activity and host
properties (e.g. Heckman et al. 1991; Aretxaga et al. 1998; Lehnert et al. 1999). The hosts
of RLQ have brightened considerably more than RQQ hosts by
(Kukula et al. 2001; Falomo et al. 2001; Ridgway et al. 2001), perhaps indicating pronounced differences
between the two categories of host galaxy.
In contrast, host galaxies in the intermediate redshift range
(corresponding to a look-back time of around 3.5-5.5 Gyr)
have not been as extensively investigated. Apart from
the occasional few objects entering the tail end of a sample targetting
low ()
or high (
)
redshift
or being sources of special interest (as e.g. Brotherton et al. 1999; Véron-Cetty & Woltjer 1990),
not many dedicated surveys have been carried out.
The earliest such studies were performed in the optical (Hutchings et al. 1989; Romanishin & Hintzen 1989)
but later authors have concentrated on near-infrared imaging so as to
achieve larger host flux ratios relative to the nucleus
(Kotilainen & Falomo 2000; Carballo et al. 1998; Márquez et al. 1999; Kotilainen et al. 1998; Márquez et al. 2001),
with the exception of the HST R band study of 16 RLQ and RQQ in the interval
0.4 < z < 0.5 by Hooper et al. (1997). The results are consistent with the
findings at low redshift: the hosts are similar to or brighter than an L*
galaxy and obey the correlation between host and quasar luminosity in that
more powerful quasars reside in more luminous hosts.
The sample presented in this paper constitutes the main part of our
investigation
of RLQ and RQQ in the redshift interval
,
the first
part of which was published in Rönnback et al. (1996) (hereafter R96).
We aim to compare the properties of the host galaxies of RLQ and RQQ
found at the less well explored intermediate redshifts, in order to discern
possible differences between the two host classes and to link the results
obtained at low redshift to those at high redshift. Since previous
studies in this interval mainly have concentrated on RLQ while the by far
most common quasar type is radio-quiet, we have constructed a sample
composed of equal numbers of RLQ and RQQ matched in magnitude
and redshift.
The (R96) sample is comprised of 23 objects which to date is the
largest uniform survey available in the intermediate redshift range.
These hosts are luminous, with a difference between the magnitudes
of RLQ hosts and RQQ hosts of only 0.3 mag (corresponding to
). Furthermore, the colours obtained from R, V and
Gunn i indicate host galaxies as blue as late-type spirals
or irregular galaxies. In this paper an additional
79 objects are presented, increasing the collected total
number of investigated sources
at intermediate redshifts by
45%. Imaging in R, V and I band
was carried out using the Nordic Optical Telescope under favourable seeing
conditions.
The layout of this paper is as follows. In Sect. 2 the sample properties are discussed, with special attention given to the possible influence the several emission lines entering the optical broadband filters may have (Sect. 2.3). Section 3 details the observations and in Sect. 4 the reduction process and the process of point spread function (PSF) subtraction are addressed. The derived host galaxy properties are presented in Sect. 5 and discussed in Sect. 6, and in Appendix A we show images and luminosity profiles of the sample quasars and their host galaxies. The full analysis of the complete sample including the (R96) data will be published in Örndahl & Rönnback (2003) (Paper II), where the colours of the host galaxies and morphological considerations will be discussed.
Throughout this paper we adopt an Einstein-de Sitter universe (q0 = 0.5)
and a Hubble constant of H0 = 75 km s-1 Mpc-1.
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Figure 1: Redshifts and apparent V magnitudes of the sample quasars. Radio-quiet quasars are marked by crosses, flat spectrum radio-loud quasars and steep spectrum radio-loud quasars by open squares and filled squares respectively. Non-classified radio-loud quasars are marked with a triangle. |
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The sample was
selected from the catalogues of Véron-Cetty & Véron (1993)
and Hewitt & Burbidge (1993), where the selection criteria
used were the redshift
range (
)
and the constraints imposed by the sky
coordinates at the observation site.
To ensure that the radio-loud and radio-quiet subsamples were drawn from the
same distribution in the z-V plane, matched
pairs of RLQ and RQQ with approximately the same
redshift and apparent V magnitude were selected
in order to make a direct statistical comparison between the two classes
easier (as in e.g. Hutchings et al. 1989 and Dunlop et al. 1993).
A two-dimensional Kolmogorov-Smirnov test confirms that no
statistically significant difference exists between the two subsamples in
this respect.
The sample of 79 quasars is presented in Table 1 and
Fig. 1.
The values of
,
the V magnitude and the redshift have been updated with current data
from the NASA/IPAC Extragalactic Database (NED). We use the criterion for
radio-loudness from Kellermann et al. (1989), namely a luminosity of
,
which for our assumed cosmology translates into
.
Observations of the quasar radio luminosity function have shown it to be
of a bimodal nature (Miller et al. 1993; Kellermann et al. 1994), where a wide gap separates
the radio-loud from the radio-quiet objects. There is however a class of
objects termed radio-intermediate quasars (Falcke et al. 1996; Miller et al. 1993) which have
radio luminosities falling between these two principal groups.
The radio-intermediate quasars are possibly intrinsically radio-quiet
objects with a jet-producing central engine where the radio emission
has been relativistically boosted along the line of sight (Falcke et al. 1996)
or, depending on the
possible existence and size of
associated extended lobe emission, they may be RLQ of
very low luminosity (Kukula et al. 1998).
Using the VLA FIRST survey (Becker et al. 1995), recent work by White et al. (2000)
on the other hand shows it to be likely that the radio emissivity is
not a discontinuous property so that radio-selected quasars instead
occur at all levels of radio-loudness.
The quasars in our sample, however, neatly fall into a bimodal
distribution and published radio luminosities for our objects do not indicate
that any radio-intermediate quasars have been included.
![]() |
Figure 2: Position of the filter profiles at z = 0.5 and z=0.8. We use template galaxy spectra from Kinney et al. (1996) and the composite quasar spectrum from Vanden Berk et al. (2001). The intensity scales are arbitrary. |
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A more
thorough investigation has been performed by Wold et al. (2001) for
their sample of RQQ at
.
They examine
the 1.4 GHz NVSS (Condon et al. 1998) and the VLA FIRST survey
for detections of their objects, calculate
the flux density at 5 GHz and find that out of 20 RQQ, only three have
luminosities that surpass the limit for radio-quietness. Thus, these
objects are better classified as radio-intermediate.
Likewise, Kukula et al. (1998) detect two
such sources when determining the radio properties of their sample of
27 RQQ at low redshift (z < 0.3).
We therefore conclude that even though a few of the radio-quiet sources in
our sample have no available radio flux measurements, the likelihood that
their emission in radio is strong enough to make them radio-intermediate is
small.
Radio-loud
quasars can be subdivided into flat spectrum (FS) and steep spectrum (SS)
sources, with different properties. Flat spectrum quasars exhibit rapid
variability and have high and variable polarization. Their radio emission is
core-dominated and they more often display superluminal motion than the
lobe-dominated SS quasars, which in the unified scheme is
explained by SS objects having a larger viewing angle between the radio axis
and our line of sight (e.g. Urry & Padovani 1995).
During the sample selection process the radio-loud objects
were not monitored for radio spectral index .
Subsequent calculation of the spectral indexes
(setting the division between FS and SS
objects at
)
shows that this resulted in a blind draw of
16 FS and 20 SS sources. Three objects could not be classified.
Investigating the two subsets of RLQ
with a two-dimensional Kolmogorov-Smirnov test gives the result that
their distributions in the z-V plane do not differ significantly and
thus that
the radio-loud subsample in itself is matched in terms of spectral index.
When selecting the sample, no adjustment was made for the effect of emission lines entering the filters at different redshifts. In Fig. 2 we plot the position of our filter profiles at different redshifts, together with a composite quasar spectrum obtained from the SDSS (Vanden Berk et al. 2001) and template spectra for an elliptical galaxy and a starburst galaxy (Kinney et al. 1996).
As can be seen, both the [OIII]5007 and H
emission lines
are present in the wing of the R filter (crossing below the half power point
of the R band at
), and
affect the I filter throughout the observed redshift range.
Likewise, the [OII]
3727 emission line passes out of the V band
wing at
and
lies within the R filter profile for all of the sample.
Flux contributions from these strong emission lines could therefore
influence the derived host galaxy magnitudes and colours. The lines
can be produced in star formation regions in
the host galaxy itself and in extended emission line envelopes,
and can also be present due to scattered light from the quasar.
Still, the host galaxies of lower redshift
quasars seem to be if not almost exclusively of elliptical type
(Dunlop et al. 2003), then at least predominantly so (Schade et al. 2000).
Spectroscopical studies of
low-redshift samples ()
also show a very small contribution from
younger stellar populations to the host galaxy light, typically less than 1%
by mass from a 0.1 Gyr old component (Nolan et al. 2001; Jahnke et al. 2001).
At higher redshifts, the host galaxies still have properties consistent
with them being drawn from a population of inactive field galaxies,
preferentially massive ellipticals. The luminosity profiles of the hosts
of both RLQ and RQQ also follow a Kormendy relation similar to that of
giant massive ellipticals (Kukula et al. 2001; Kotilainen & Falomo 2000; Kotilainen et al. 1998).
There are a few exceptions to the rule, though.
Blue colours were found for the host galaxies in (R96),
and also in the
three radio-loud host galaxies with
studied
by Kirhakos et al. (1999). Furthermore,
Brotherton et al. (1999) have
discovered a quasar at z=0.634 with spectral features
suggesting a starburst event only 400 Myr old, which may have an even younger
component (Brotherton et al. 2002), but which in any case is too old to display any
[OII]
3727 emission.
While the additional uncertainties which may be introduced should be kept
in mind, we assume that in general the contribution of light to the
emission lines in our filters caused by young stars in the host galaxy
is small.
Stockton & MacKenty (1987) found extended nebulosities in [OIII]5007 for
somewhat less than half of their sample of quasars at z< 0.5, mainly
around steep spectrum RLQ. Of their objects with z> 0.35, six sources
(35% of the sample), all steep spectrum, had envelopes.
Hes et al. (1996) have conducted an emission-line imaging study of steep spectrum RLQ in [OII]
3727 at intermediate redshifts, and have also
compiled literature data on additional objects. Since the [OII] line has
much lower critical density for de-excitation
it is expected to be radiated at larger radii than [OIII].
In the work by Crawford & Vanderriest (1997,2000)
on steep spectrum RLQ in the same redshift interval as our sample, the [OIII] and [OII] envelopes have similar sizes within the detection limits.
In either case the
diameters of these envelopes cover the same range as those determined by
Hes et al. (15-25 kpc), and
are consistent with more recent findings for host galaxies at both
intermediate and high redshifts where the line emission is either compact
and well inside the body of the host galaxy (Hutchings et al. 2001; Márquez et al. 1999; Kirhakos et al. 1999)
or not detected at all (Aretxaga et al. 1998; Lehnert & Becker 1998), though
Stockton & Ridgway (2001) find [OII]3727 emission around a quasar at z=1.2which clearly is compact but distributed quite differently than the light in
the K band. It should also be noted that a few extreme cases exist where the
emission envelopes are of more than double the size quoted here.
About half of the objects imaged by Hes et al. show extended
nebulosities, with a mean total luminosity in [OII]3727 for the
quasars of log
W. This coincides with the
value obtained by Crawford & Vanderriest
who also estimate the mean contribution made by only the emission envelope to
the line luminosity to be log
W. The
[OIII]
5007
luminosities are higher, with a mean value for the envelope part of
log
W.
Computing the mean magnitude of the [OII]
3727 line and comparing it
to the mean
R magnitude of our sample reveals a contribution from the line of less than
0.05 mag to the host galaxy magnitude, even when the line
is centered in the filter.
For the V band images the line contribution is of similar size.
The [OIII]
5007 line
exerts a larger influence and supplies in the worst-case
scenario an additional
0.3 mag to the mean I magnitude.
For individual objects the line can of course play a much less significant
role, depending on the redshift of the object and on whether an envelope
even exists. Corrections to the host magnitudes to compensate for possible
emission line envelopes are further addressed in Sect. 5.2.
The mean value of the quasar H
luminosity has been found to be
log
W from comparison of relative fluxes in
Vanden Berk et al. (2001). Therefore, scattered light from the quasar H
line can
contribute more than 5% of the host galaxy flux when the
line is centered in the R or I band filter. In addition, the H
line is passing out of the I band at the low end of the redshift range
investigated here, but being the brightest Balmer line it still can contribute
around a tenth of a magnitude. The Mg II emission line is however not strong
enough to affect the V band, since the line only begins to enter the filter
for the highest redshifts in our range. Corrections for scattered line emission
will be further addressed in Sect. 5.2.
The main part of the observations were performed during two runs of
ten nights each in June respectively October 1994 at the 2.56 m Nordic
Optical Telescope (NOT) on La Palma. For the June 1994 run we used
a Thomson 10241024 CCD, kindly on loan from the Instituto de
Astrofísica de
Canarias, with a pixel scale of 0
14/pixel. During the
October 1994 run the detector used was the Brorfelde TEK
CCD,
with a pixel scale of 0
18/pixel. A few additional fields were obtained
at the NOT in December 1998, using the ALFOSC 2k Loral CCD with pixel scale
0
188/pixel.
All quasars were observed in the R band. To keep the core unsaturated we obtained for each object a series of three to five dithered exposures of 300 to 500 s, giving a total integration time of in most cases 2000 s. For 24 objects where an indication of a host galaxy was seen (either by eye in the raw frames or in coadded images combined on site) we also performed observations in V and/or I, with integration times of typically two to four times 500 to 700 s. Standard star fields from Landolt (1992) were taken throughout the nights during all runs. The data obtained is almost entirely of photometric quality - only a few objects observed in October could not be calibrated, and most of these were subsequently calibrated with frames taken during the December run.
A summary of the observations is presented in Table 1, where
the FWHM is calculated from the coadded frames.
The median seeing value for the coadded frames
is less than 0
85, with 15% of the frames having a seeing better than 0
55.
There is no significant difference in seeing between radio-loud and
radio-quiet frames if the December data (which suffered from much poorer
seeing conditions and mainly targeted radio-quiet objects) is excluded.
This also holds for the SS and FS radio-loud subsamples.
For a handful of quasars we encountered various problems. The objects US 3150 and 4C 09.72 were saturated (thus rendering PSF subtraction impossible), and the object OU 401 was accidentally placed on a bad column in the CCD. Furthermore, ZC 2354+002 has a very uncertain redshift determination, and the position of 1951+4950 coincides very closely with that of a field star so that no PSF subtraction was possible. These five objects were thus excluded from the sample.
Table 1: General properties and observation summary of the sample quasars.
We first subtracted bias and corrected for bad columns,
then flattened each science frame using either twilight skyflats or
superflats obtained from stacks of typically ten dithered science images
normalized to the sky.
The dithered images of each
object were then aligned to an accuracy of 0.05 pixels and
combined to a master frame, in order to
increase the signal to noise and permit efficient removal
of artifacts, bad pixels and cosmic hits. Finally,
the combined frames were photometrically calibrated.
When an object or object field is referred to henceforth in this paper,
it is the coadded frame which is implied.
All reduced frames were flat to less than 0.2%. The photometric calibration of the object frames was accomplished via aperture photometry on each standard star, with an uncertainty in the photometric zero points measured to be 0.03 mag. The photometric error due to sky variations is less than 0.04 mag for a source brighter than 21 mag, except for the more sensitive I band where the error is less than 0.04 mag for a source of less than 20 mag and below 0.08 mag for a source brighter than 21 mag. For a few objects from the October run the images obtained were of non-photometric quality; however, additional photometric calibration observations were carried out in December 1998.
Some of the sources in the sample were observed on multiple occasions. When such was the case, we chose to use the observation with superior image quality, either due to lower seeing or being the deeper exposure. For some objects the double observations are of very similar quality, in which case we analysed both fields and computed the mean magnitude, thus obtaining a single magnitude value for each object.
The data has been analysed using two different methods for determining the PSF in each image: a purely empirical one and a PSF constructed from a combination of empirical and model data.
The advantage of using a noise-free model PSF is that no extra noise is added in the PSF subtraction thus making it possible to reach fainter levels of host galaxy flux, but in frames where the core part of the PSF is slightly non-circular the empirical PSF is difficult to model. When constructing the combined PSF, we therefore retained the high-signal empirical core but replaced the low-signal wings by a noise-free model. The model used is the profile from Saglia et al. (1993), which provides a better fit to stellar profiles than either a Gaussian or a Moffat function. The composite PSF was used to good advantage in (R96), and for further details of the construction method we refer to this paper.
In each object frame we chose a field star to use for constructing the empirical PSF. An optimal star is one whose intensity is comparable to or higher than that of the quasar, so that this source of noise contribution to the residual image from the subtraction process is minimised. When possible, we chose a star close to the object to eliminate effects on the residual from variations of the PSF over the field: however, the FWHM of the PSF is stable to within 1-2% over the field.
We first rebinned the star to pixel center and next performed background subtraction, where the sky level was determined from star-free regions well away from the objects. The centering was checked again and azimuthally averaged luminosity profiles and growth curves were computed. The growth curves provide an extra control that the background has been correctly subtracted: when these curves did not show proper asymptotic behaviour the process was repeated. When necessary we performed masking of close companions, stars or other features which would give an unwelcome contribution to the luminosity profile.
In order to determine the magnitudes of the host galaxies, the nuclear and stellar light contributions must be disentangled. We have achieved this by subtracting a scaled PSF from the quasar images. Since the amount of flux originating from the point source itself is unknown, care is required in the choice of scaling factor. A subtraction to zero flux in the central pixels will remove all quasar light to a certainty, but also an unknown amount of host galaxy light. To minimise the oversubtraction of the host galaxy, we have chosen a scaling factor which results in a residual having a flat-top luminosity profile and positive flux at all radii. This monotonically decreasing residual is a more realistic representation of a real galaxy, but is still likely to be an oversubtraction for elliptical galaxies and spirals with a bulge since these have a peaked profile. The magnitudes we obtain will as a result be upper limits. As a first rough estimate of the scaling factor the PSF was normalized to the quasar and then subtracted. By utilising the luminosity profile of the resulting residual we iteratively arrived at a scaling factor which rendered the luminosity profile monotonous while at the same time ensuring that no negative pixels were present in the central parts of the host galaxy image.
There are cases when the two requirements of flat-top residual and non-negativity are incompatible. This can happen for a host galaxy which is off-centered from the quasar source but also when the central part of the PSF profile for some reason is broader than that of the quasar, either because the quasar is close to saturation or because of variability of the PSF shape over the CCD. In such cases we have relaxed the flat-top criterion from demanding that the flux in the two first luminosity profile points be equal, to accepting a slightly lower value for the second point as long as the third point equals the first. This procedure as well as the slight resulting oversubtraction is acceptable as long as no large errors arise in the growth curves of these objects. The magnitudes obtained in this way have a larger uncertainty and have been marked in Tables 3 and 4. For eight fields, however, the second point in the profile deviated so much that a proper scaling factor (and consequently, host galaxy flux) could not be reliably determined, even after testing different PSF stars when such were available in the field. These fields were deselected from further analysis, leading to four objects falling out of the sample.
To independently check the PSF it was also subtracted from one or more field stars, since the residual in these cases should become negligible. This exercise was possible for all but seven fields (where there was no star other than the PSF star) and resulted in barely detectable stellar residues in most cases, proving the overall validity of the PSF.
For all objects we performed subtraction of both the empirical
and the combined PSF. The difference between the two methods is only
discernible in the very outermost parts of the host galaxy, where the
composite PSF method detects very faint structures.
The resulting total magnitudes are therefore very similar.
![]() |
Figure 3: Contour plots of the quasar KUV 0200-0858. To the left the residual after subtraction of the empirical PSF and to the right after subtraction of the combined PSF. North is up, east to the left. The contours mark the 21, 22, 23, 24 and 25 mag arcsec-2 levels and the image scale is in arcseconds. |
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In Fig. 3 we show as an example the two different residuals for the quasar KUV 0200-0858. The model subtraction is less influenced by sky noise, and also uncovers a slight extension to the northwest. The extension towards the east is a low-level charge transfer streak which can be seen in the unsubtracted image. The presence of the streak is much less visible in the empirical subtraction due to the inclusion of this feature in the wings of the empirical PSF.
Unfortunately, the Saglia et al. models have circular luminosity profiles and do not allow for a proper treatment of ellipticity. In combination with the charge transfer problem noted above this has a serious impact on our attempts to determine the morphology of the host galaxies. These questions will be further discussed in Paper II, since only detections and magnitudes are addressed here, none of which depend on the method of subtraction. We will therefore present values from empirical subtractions alone. The images and luminosity profiles of the sample quasars and their host galaxies are presented in Fig. A.1.
There are a number of different errors that can contribute to the host galaxy
magnitude estimation, divisible into two categories: the Poisson error in
the subtraction method and the systematic error due to
oversubtraction of the core component.
The sizes of the photometric zeropoint errors have been measured
to be quite small, as have also
the uncertainties arising from determining the proper growth curve limit.
Typically, the growth curve flattens at a surface brightness of
26 mag arcsec-2 for the R and V band
images, and at
25 mag arcsec-2 in the I band, whereas the
variation in sky level always was found to correspond to a surface brightness
more than a magnitude fainter. Thus, uncertainties in the sky
background are not a dominant source of error.
By comparing the magnitude determinations made for stars in the field the
photometric error was measured to be
0.1 mag.
The influence of the PSF shape variation on the derived magnitude was
investigated by choosing objects having several suitable PSF stars
distributed over the field and performing the subtraction using all of these
stars. The error obtained was found to be
0.1 mag.
Scaling and subtracting the PSF from stars in the field provides a check of
how well the subtraction method performs.
This control yielded a mean flux of the stellar residuals which is
7% 10% of the host galaxy flux derived for that field,
placing them below
of
the Poissonian noise expected from the subtraction
technique by combining the error sources dealt with above.
The small size of this error is attributable to the overall high quality of
the data and the instrumental setup.
To investigate the systematical oversubtraction error simulated
host galaxies were created. A field star including background was added to an
elliptical galaxy model which previously had been convolved by the model PSF
which best fitted the stellar image used. Then PSF subtraction was carried
out in the same way as for real data. This was done for a small set
of point source and galaxy brightnesses and resulted in values
very similar to those derived in (R96),
leading us to adopt that error
determination as valid for the data set in this paper.
For a galaxy magnitude of R = 19, the extracted host
flux is less than the original one by an amount corresponding to
mag, while the difference for R = 20 and 21 mag is
and
respectively.
A further oversubtraction ensues in the case when a relaxed flat-top has to be adopted when determining the scaling factor (as discussed in Sect. 4.3). To control the size of this oversubtraction as well as the stability of the procedure itself, two of the authors have independently computed scaling factors for a subset of the object frames. As long as the requirements of flat-top residual and non-negativity were met the scaling factors differed by a negligible amount. However, in those instances when the flat-top criterion had to be relaxed the oversubtraction could increase by up to 0.1 mag, also increasing the spread in this error by about 0.03 mag. Thus, a more problematic luminosity profile has a larger error than indicated in the simulations.
By comparing the magnitudes of the host galaxy residuals
extracted from those objects for which we have multiple observations the
accuracy of the PSF subtraction can be tested. The difference between residuals
was measured to be 0.2 mag but is the mean of
a range of host galaxy magnitudes, whereas the size of the intrinsic error is
expected to be larger for fainter hosts. The number of sources observed at
more than one occasion are however too few to allow for a more finely tuned
estimation of the differences. Brighter hosts will have magnitudes
recoverable by subtraction with an error within
of the total
(Poisson plus intrinsic) error, whereas faint hosts or those where the
flat-top criterion had to be modified have a less well-defined magnitude.
Summing up, the error in the host galaxy magnitudes amounts to
0.25-0.3 mag, which is in the same range as error estimates compiled
by other authors comparing objects observed on more than one occasion
(Kotilainen & Falomo 2000; Véron-Cetty & Woltjer 1990; McLeod & McLeod 2001; Maraziti & Stockton 1994).
Of the initially 79 objects, 70 remain for analysis after the deselection of saturated or otherwise observationally flawed objects and those for which a scaling factor could not be reliably determined (as in Sect. 4.3). Kolmogorov-Smirnoff tests however show that both the radio-loud and radio-quiet subsamples as well as the SS and FS radio-loud subsamples still are matched with respect to redshift and quasar V magnitude.
Two additional tests were performed on parts of the sample to help discriminate between detections and artifacts caused by the subtraction process. Scaling of the PSF to remove stars in the field was used to evaluate the quality of the PSF shape, and was carried out on all objects but the seven which had no other star than the PSF star in the field. When the test star remainder was substantial the host galaxy residual was compared to it, and in case the shapes were very similar the residual was classified as a non-detection. We found this to be the case for ten fields.
It is possible that the flat-top criterion may have created a spurious host galaxy detection, in particular when the residual is small and the morphology quite circular. To look into this possibility the test of subtracting the quasar to zero in the center was performed for most objects, since even after subtraction to zero an undisputed host galaxy should have sizeable flux. Thus, based on comparison of the obtained zero-residual both to the result from flat-top quasar subtraction and to the stellar residue, eleven circular cases were indeed concluded to be non-detections.
In this way we detected host galaxies in 28 RLQ (14 of which
have steep spectra)
and 21 RQQ, comprising 80% and 60%
of the total number of respective sources.
Of these, one of the radio-loud
(TEX 1423+1438) and two of the radio-quiet (0256+0140
and 2141+0402) detections must be
deemed marginal. Eight other objects have been marginally resolved
in one wavelength band but are clearly visible in the other band(s) employed
so that they without a doubt constitute a detection.
On the whole, more than
half of the objects imaged in more than one band (13 out of 23) are
detected in all bands.
The difference between the mean seeing of the radio-loud and the
radio-quiet subsamples is very slight, as already pointed out in
Sect. 3. Comparison of the mean seeing for the frames where we
detect a host galaxy to the mean computed for frames where no detection
is made shows that the detection rate
stays at a mean of 70% regardless of whether
the seeing is below 1
,
below 0
8 or below 0
55.
Since the sample was investigated primarily in the R band regardless of source redshift, some early-type hosts may have escaped
detection due to the shift of the 4000 Å break from R into I for
redshifts above 0.7. As most of the energy of an early-type galaxy
is emitted longwards of this break, such a host will be much better visible in
the I band than in R. Indeed, the detection rate for RLQ falls
off by
20% and for RQQ by all of
35%
above this redshift limit. The radio-loud quasar TEX 1423+1438
at z=0.78is an excellent example of the principle: while imaged in all three bands,
only the I band image shows a host galaxy though very faint
structures may be guessed at in the other bands.
Almost all non-detections have been classified as such by
comparison with the results from stellar subtraction: only for a handful of
cases is the quasar residue unresolved and so faint as to
be virtually non-existent.
From the simulations performed in (R96), the lower limiting
host magnitude was found to be
.
In this sample, four hosts
have magnitudes fainter than R = 21 and two are fainter than R = 21.5.
Inspection of the residual images make it clear that the hosts
with
are resolved, whereas the two
even fainter objects only are marginally resolved. The upper limit to the
host magnitude determined from the unresolved faint sources in this sample is
.
The claim made by Bahcall et al. (1995) that a large fraction of quasar hosts have luminosities substantially less than L* has been shown to be erroneous by subsequent reanalysis of the original data (Bahcall et al. 1997; McLeod & Rieke 1995) and new imaging (McLure et al. 1999). Given that all unresolved objects have redshifts where the 4000 Å break has moved out of the filter used for observation, the reason for our failure to resolve these hosts is most likely not that they are so faint as to be completely outshone by the quasar, but rather because we are sampling an unsuitable region in the host galaxy spectrum.
In the object 3C 380 we detect no host galaxy in the R band
when investigating the centre of the image. However, there is an
off-centered structure, coinciding with the optical synchrotron
hotspots detected by de Vries et al. (1997) with HST.
Observations in [OII]3727 suggest that the two knots are
dominated by optical continuum light instead of emission lines (O'Dea et al. 1999).
In spite of the non-detection in the R band
the host galaxy is clearly discernible in the V and I band,
as is the hotspot feature.
The magnitudes for the detected hosts were extracted at growth curve limit
and measured with the help of the luminosity profiles.
For a few objects with double
observations of very similar quality, the differences between the two
magnitude measurements were below 0.1 mag.
Thus, for the objects B2 1512+3701 in V band,
PKS 2209+0804 in R band, and 2217+0845
in I band, both fields were
analysed and the mean of the magnitude used. Values for the Galactic
extinction were taken from Schlegel et al. (1998) for all objects in the sample.
Due to the lack of constraints on the internal extinction in the host galaxies
no such correction was performed.
It is of interest to compare the quasar V magnitudes obtained by us to the
V magnitudes found in catalogues and listed in Table 1.
For the 18 quasars imaged in V, the difference between observed and
previously measured values amounts to 0.4 mag with a mean of
+0.2 mag,
but it is to be remembered that these values refer to different epochs
and thus could be influenced by the intrinsic variability of the objects.
To compensate for the influence from possible emission line envelopes in
[OIII]5007
as discussed in Sect. 2.3.2, we apply a generic magnitude
correction using the mean envelope
luminosity of log
W (Crawford & Vanderriest 2000).
The correction is
detailed in the upper part of Table 2 and is
only made for steep spectrum radio-loud objects, since these are most
prone to have extended emission line regions.
The sample is divided into
three redshift intervals chosen to center around the approximate main and
half power points of the R and I filters. However, only the two
lowest redshift intervals are used since for the highest bin the
[OIII] line is
in practice outside the R filter and in I the only object afflicted
is 3C380, for which individual data could be found.
For this quasar the
luminosity in [OIII] is log
W
(Lawrence et al. 1996), leading to
a correction of 0.08 mag in I when assuming an envelope
contribution of
30% to the total luminosity (Crawford & Vanderriest 2000).
It was possible to find a value for the quasar luminosity also for the
object 3C 275.1. Here, log
W
(Jackson & Rawlings 1997) giving an envelope
contribution in I of 0.08 mag, but negligible in R. Finally, the object
B2 1512+3701 is known to have a very bright envelope in [OIII].
Using the value
for its [OIII] luminosity derived by Crawford & Vanderriest (2000), we find that a correction
of 0.41 mag is necessary in R.
Table 2:
Mean magnitude corrections to compensate for extended
[OIII]5007 emission line envelopes (upper part of table) and
scattered quasar Balmer emission (lower part of table).
Also shown are the number of sources affected in each interval,
excepting the objects for which individual corrections for [OIII]
emission were made.
Table 3:
Apparent magnitudes of the radio-loud quasars and
host galaxies. The hosts of the
objects PKS 0130+2412
and PKS 2351-0036 were also resolved but have
not been included in the table due to not being calibrated. Colon signs
mark a marginal detection and an asterisk a steep spectrum source.
Table 4: Apparent magnitudes of the radio-quiet quasars and host galaxies. The hosts of the objects 0010+0146 and 0020-0300 were also resolved but have not been included in the table due to not being calibrated. Colon signs mark a marginal detection.
The host magnitudes must also be corrected for scattered quasar light in the Balmer lines (Sect. 2.3.3). The brightness of a typical quasar in these lines has been calculated from the composite in Vanden Berk et al. (2001), and the assumption made that 10% of the light is scattered. The generic magnitude corrections are listed in the lower part of Table 2 and are applied to all sources imaged in the shown redshift and filter combinations.
The resulting magnitudes are presented in Tables 3 and 4. The mean R magnitude of the radio-loud subsample
shifts by only 0.03 mag when applying the modifications from
Table 2, whereas the mean I magnitude shifts to 0.16
mag fainter. The mean I magnitude of the radio-quiet subsample shifts
to 0.13 mag fainter.
![]() |
Figure 4:
The distribution of host galaxy R magnitudes with redshift.
No marginally resolved objects have been included in the plot.
Open squares mark flat spectrum radio-loud
objects, filled squares steep spectrum radio-loud objects and triangles
non-classified radio-loud objects. Radio-quiet objects are marked with
crosses. The solid line is the Hubble R-z
relation for radio galaxies (O'Dea et al. 1996).
The long-dashed line shows the apparent magnitude of an L*
galaxy calculated using the K-correction from Fukugita et al. (1995) for an E
galaxy. For the short-dashed line the Sbc galaxy type K-correction was
used, while for the dash-dotted line the Im type K-correction was applied.
The typical ![]() |
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![]() |
Figure 5: The distribution with redshift of host galaxy R magnitudes from the literature. The lines are the same as in Fig. 4. Shown here is the (R96) NTT data (symbols for radio-quiet, FS and SS radio-loud objects as in Fig. 4) and samples from Hooper et al. (1997), Márquez et al. (1999), Kotilainen et al. (1998) and Kotilainen & Falomo (2000), marked as indicated in the figure. Only resolved objects have been plotted. |
Open with DEXTER |
The detected residuals have luminosities which typically are 15-25% of that of the quasar nucleus. Inspection of the residual images rarely reveal any unambiguous morphological markers at these redshifts: possible explanations for the nature of the diffuse light thus include not only stellar light but also nebular continuum produced by ionized gas and scattered quasar light.
As has been shown earlier, the emission line contributions from
envelopes ionized by the quasar are not much more than a tenth of a
magnitude. The continuum emission from undiscovered extended envelopes
is therefore probably too weak to contribute all of the detected host flux.
Indeed, no prominent narrow emission lines are seen in any of the off-nuclear
host galaxy spectra obtained by Hughes et al. (2000) at
,
indicating the relatively unimportant fraction of light coming
from an envelope.
Quasar light which is scattered from dust and/or electrons can make a substantial contribution to the continuum emission in the ultraviolet, as is well known from spectropolarimetric measurements of radio galaxies (Cimatti et al. 1993; Tadhunter et al. 1992; Fabian 1989). If the unification scheme for radio-loud AGN is correct then the same effects which operate in radio galaxies can also influence the observed distribution of extended light around RLQ.
The fraction of scattered nuclear continuum light was estimated
by Fosbury (1997) to be 10% of the total quasar light, which would
make the measured hosts spuriously brighter by around a magnitude.
This effect will be more prominent at
shorter wavelengths where the light from an older stellar population is
dominated by that of the AGN, and thus becomes more important
at z>0.5 where the ultraviolet emission has moved into the
V band. For the R band this happens at redshift >0.8. Furthermore,
the polarimetric properties of a complete
sample of radio galaxies at redshifts up to
have been investigated by Tadhunter et al. (2002), who find that
only in 30% of the objects at z>0.4 does the scattered
light make a significant contribution to the UV excess.
The applicability of this result to quasar host galaxies is however far from
certain.
Both spectroscopic and polarimetric observations are needed to disentangle
how important the contribution of scattered quasar continuum light is
in general as well as in the individual case.
Given the difficulties involved no attempts were made to correct for scattered quasar light or nebular continuum, but the additional possible uncertainties in the host galaxy magnitudes should be kept in mind. In general, though, the colours of the host galaxies as determined from annular apertures are redder than those of the quasars, suggesting a minor influence from light sources other than the host galaxy itself. We will in the following assume that the dominant source of extended light is stellar emission originating in the host galaxy.
The host galaxies apparent R magnitudes have been plotted versus redshift
in Fig. 4, together with the R-z Hubble
relation for radio galaxies (O'Dea et al. 1996). Objects which were only marginally
resolved have not been included in the plot.
For comparison, the brightness of a field galaxy having a characteristic
(Schechter) magnitude of *=-20.9
(Lin et al. 1996) has been plotted
as well. To compute the corresponding apparent magnitude of the field
galaxy, K-corrections appropriate for Hubble type E and Sc have been
applied following the calculations of Fukugita et al. (1995).
The hosts follow the radio galaxy R-z relation,
concurrent with the result for hosts at low as well as intermediate redshifts
(Márquez et al. 1999; Dunlop et al. 2003; Hooper et al. 1997). Their luminosities are
brighter than that of an L* galaxy, and they also do not fade with redshift
quite as rapidly as the field galaxy.
At lower redshifts a simple least-squares fit to the data corresponds to
1.5-4 L* depending on galaxy type, which increases to
2.5-12 L* at the
higher end of the redshift range.
These values are consistent with the findings at redshifts of
0.1-0.3, where the
hosts of RLQ and RQQ are found to have luminosities
ranging between 1-4 L* (Dunlop et al. 2003, R band; Jahnke & Wisotzki 2000, B
band; Boyce et al. 1998, V band;
McLeod & Rieke 1994, H band), and also with the results from
the intermediate redshift surveys by Kotilainen & Falomo (2000) and Kotilainen et al. (1998)
who find a typical host luminosity of
6 L* in the H band.
At redshifts above unity the luminosities of radio-loud hosts have increased by an amount well accounted for by passive evolution whereas the luminosities of radio-quiet hosts seem to be more or less unchanged, perhaps indicating less massive radio-quiet hosts at these redshifts (Kukula et al. 2001; Falomo et al. 2001; Ridgway et al. 2001). It is not possible to discern any such trend towards the high end of the redshift range in the radio-quiet sample investigated here, as only three radio-quiet host galaxies were detected in R at a redshift larger than z=0.64 (and of these, one source could not be calibrated).
In Fig. 5 we plot the (R96) NTT data and samples from the literature (Kotilainen & Falomo 2000; Márquez et al. 1999; Kotilainen et al. 1998; Hooper et al. 1997). Only resolved objects having redshifts in the range 0.4-0.8 are shown (thus excluding the marginally resolved hosts from Kotilainen et al. and Kotilainen & Falomo, as well as from (R96)).
In the work of Hooper et al. magnitudes were determined both for an elliptical and a disk fit to the data. Here we use the magnitudes obtained for the elliptical fit (typically 0.5-1 mag brighter than those of the disk fit), thus assuming elliptical morphology for the host galaxies. The HST sample of Hooper et al. has been recalculated to R magnitude by the authors, whereas Márquez et al. have observed in J band and Kotilainen et al. in H band. Colour transformations were performed using R-K = 2.5, calculated by using V-K = 3.9 (Poggianti 1997) and V-R = 1.44 (Fukugita et al. 1995) for elliptical galaxy types. This R-K value coincides with that found for both radio-loud and radio-quiet host galaxies at low redshift (Dunlop et al. 2003). Using values of J-K and J-H from Poggianti we find R-J = 1.4 and R-H = 2.2.
From Fig. 5 it is clear that the literature samples follow the radio galaxy R-z relation well, as already mentioned. The very bright hosts (mainly belonging to the Kotilainen et al. and Kotilainen & Falomo samples) are associated with brighter than average quasars, in accordance with the results from e.g. McLeod & Rieke (1995) that the minimum host galaxy luminosity increases with quasar luminosity. Comparing the host magnitudes shown in Fig. 4 to those in Fig. 5 reveals that the latter are somewhat brighter, at least toward the high end of the redshift range, which is not surprising given that the mean total magnitude of the sample investigated here is fainter at these redshifts than the objects collected from the literature.
To evaluate the possible difference between the R magnitude distributions with redshift of the radio-loud and radio-quiet host galaxies in our sample, a two-dimensional Kolmogorov-Smirnoff test was performed. The result shows that the two distributions are statistically indistinguishable (with a significance level of p = 0.36). There would thus seem to be no difference between the apparent magnitudes of radio-loud and radio-quiet hosts at the intermediate redshifts under study here, confirming and expanding the results of Hooper et al. (1997). A full analysis incorporating K-corrections is deferred to Paper II.
PSF subtraction was performed with the help of one-dimensional luminosity profiles both on the quasar image and on a field star, to better control the quality of the subtraction. Host galaxies were detected in a total of 49 objects, corresponding to a 60% detection rate for radio-quiet hosts and an 80% detection rate for radio-loud hosts.
Apparent magnitudes are presented for the host galaxies and compared to a field galaxy having a Schechter luminosity as well as to the R-z Hubble relation for radio galaxies. The hosts are brighter than an L* galaxy by a factor of 1.5-4 at the low end of the redshift range (depending on galaxy type), which increases to 2.5-12 L* towards the higher end of the redshift range. They are found to follow the radio galaxy relation, regardless of whether they harbour a radio-loud or a radio-quiet quasar.
In Paper II we will include the data from (R96) and analyse and discuss absolute magnitudes, colours and morphological considerations of the full sample of host galaxies.
Acknowledgements
The Instituto de Astrofísica de Canarias is gratefully acknowledged for lending us the CCD used during the June 1994 run. This research has made use of NASA's Astrophysics Data System Abstract Service, and also 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.
In Fig. A.1
we show images and luminosity profiles of selected quasars and
their host galaxies, presented in order of right ascension.
All fields shown in the printed version were taken in Rband. The full set of images and profiles for the sample can be
found in the electronic version of the article at
EDP Sciences.
For each object field we present five plots. In the
leftmost graph the luminosity profile is plotted (in mag arcsec-2)
versus radius in arcseconds. The points mark the quasar profile, the
full-drawn line is the PSF, and the dotted line the residual after PSF
subtraction. In the
fourfold greyscale plot the image sizes are always
(except
for the case of EX 0240+0044, where it is
), with
the objects always centred in the plots.
Top left shows the host galaxy residual, top right is the unsubtracted
quasar frame and in the bottom right graph we show the
residual left from the scaling of the PSF to a field star.
The bottom left graph is a contour plot of the host galaxy
residual, where the number inside the plot denotes the value of the lowest
contour in mag arcsec-2 and the spacing between contours is 1 mag arcsec-2. The contours have been smoothed by a
box for
better clarity of low-intensity features.
North is up and east is to the left.
In the electronic version we in addition
show fields observed in V or I band,
where the luminosity profile has been labelled also with the filter name.
Non-calibrated objects have been indicated by a #-symbol, and plots where we
present a quasar residual subtracted to zero in the center in order to
highlight the non-detection of a host galaxy have been marked with "zero''.