Integrated Modeling of Telescopes: 377 (Astrophysics and Space Science Library)

The results from the modelling of the synthetic quasars are listed in Tables 1—3. Two features of these results are worthy of individual comment. First, it can be seen that for both host morphologies the errors associated with the determination of all the parameters steadily increase with redshift, as is expected because of the inevitable drop in signal-to-noise ratio. Secondly, with regards to the host scalelength it can be seen to be significantly easier to accurately determine this parameter for disc hosts than for ellipticals.

Results of the two-dimensional modelling tests using synthetic quasars with elliptical host galaxies. Column 1 gives the redshift of the quasar.

1 Introduction

Column 2 gives the actual scalelength of the simulated host in kpc. Columns 3—8 give the mean percentage error in the reclaimed value of the relevant parameter.

Results of the two-dimensional modelling tests using synthetic quasars with disc host galaxies. Columns as Table 1. The range of percentage errors in the reclaimed values of the host-galaxy parameters from the synthetic quasar modelling tests. Columns 1 and 2 detail the actual host-galaxy morphology and redshift of the synthetic quasars. Although the high degree of accuracy achieved in these simulations is impressive, it should be noted that these error estimates are only valid for the high-resolution data provided by HST. As will be seen in Section 6 , the errors associated with ground-based data can be significantly larger.

As a result of the success of the programme of tests outlined above, it was felt that the level of information present in the HST data justified an extension of the modelling code to cover more than just fixed elliptical and disc host galaxies. The modelling code as described so far is able to determine host-galaxy morphology only to the extent that it is elliptical-like or disc-like. This question has particular relevance because of recent studies of the cores of inactive elliptical galaxies using both HST Lauer et al.

During the latter stages of the analysis of the HST host galaxies presented by Dunlop et al. To investigate whether an improved fit could be achieved with a two-component model the central model-building algorithm was extended to produce combined disc and bulge models. During the fitting of these combined models the eight parameters controlling the form of the galaxy surface-brightness distributions were left free.

In combination with the normalization of the nuclear component this required the fitting of a total of nine free parameters.

With regards to host-galaxy morphology, the clear conclusion from both sets of modelling tests presented here is that if the host galaxies of the AGN in the HST imaging study are consistent with standard de Vaucouleurs or Freeman models, then the two-dimensional modelling code will successfully discriminate between the two. With the arrival of active and adaptive-optics systems the prospect of near diffraction-limited ground-based imaging has become a reality. Within the context of the study of quasar host galaxies this development perhaps has most relevance in the near-infrared. The inherent advantages of observing host galaxies in the near-infrared were discussed by Dunlop et al.

In this section the results are presented from a short observing run that was designed to explore whether the subarcsec imaging quality now readily attainable with tip-tilt active optics at UKIRT could rectify this problem. The observed sample consists of nine objects Table 4 which were chosen to fully quantify the improvements that could be gained over the previous K -band imaging study. Six of the quasars were taken from the original object sample described in Taylor et al.

The remaining three objects were taken from the sample of Bahcall et al. These objects presented an ideal opportunity to investigate the advantages to be gained by confining the nuclear flux to within the central 0. The remaining two objects, in contrast, were specifically chosen because the model fits from the previous analysis were regarded as secure, with a strong preference being shown for one particular host morphology. The first six objects listed are taken from the original K -band imaging sample Taylor et al. The final three objects have been taken from the sample imaged by Bahcall et al.

Column five lists the on-source integration time for each object. Column six details any problems experienced with the images: EN, electronic noise; PD, pointing drift. Redshifts and V magnitudes have been taken from Taylor et al. The following observational procedure was used. Each object was observed using a four-point jitter pattern with each point consisting of 3 min of integration broken into 18 coadditions of s duration.

This combination was chosen specifically in order to provide unsaturated but background-limited images. This min jitter pattern was repeated six times for each object, providing a total of 72 min of on-source integration. Considering the desirability in host-galaxy observations of confining the quasar nuclear flux to as small an angular extent as possible, the quasar nucleus itself was used as the tip-tilt guide-star in an attempt to obtain the best possible resolution. In order to provide high signal-to-noise ratio measurements of the IRCAM3 PSF, and to calibrate our photometry, observations of standard stars were taken before and after the completion of each jitter pattern.

Following dark-frame subtraction each object was flat-fielded using concurrent sky flat-fields produced by a process of median filtering of the 24 individual 3-min integrations. These frames could be median filtered without fear of contamination from host-galaxy light. The four flat-field quadrants produced in this fashion were then added together and normalized to unit median to produce the final flat-field. After flat-fielding the individual 3-min frames were corrected for the known non-linearity of the IRCAM3 detector before being re-registered and stacked to produce the final deep images for analysis.

Several problems with the data obtained during this observing run limited the number of objects which could be successfully analysed with the modelling code. The consequent wandering of the telescope pointing during the observations of these two objects resulted in final mosaicked images from which it was impossible to accurately separate the host and nuclear light. It proved impossible to construct a reliable flat-field for this object, and it is was therefore dropped from the modelling process.

The data for two more of the nine objects listed in Table 4 were of insufficient quality to successfully model the underlying host galaxies. The result of these technical difficulties was that only four objects yielded data of the high quality required by the two-dimensional modelling code. However, the remaining four objects still allowed the main objectives of the run to be achieved. Three of the objects were allocated unreliable fits from the previous K -band imaging, and therefore presented a good test of the improvements to be gained from the increased resolution of tip-tilt imaging.

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