HI Absorption in the Luminous QSO PDS 456
C. Young, C. Brogan, C. Cheung, B. Jones, M.
Lystrup, E. McGrath, N. Mohan, D. Osgood, A. Petric, C. Schwartz, H. Smith
NRAO Summer Intern/REU Program 1999
M. Yun
National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801
Abstract
We present results from the Summer Student Project at the Very Large Array (VLA) in
Socorro, NM for 1999. We chose to observe a quasar which was recently discovered in
the Pico dos Dias survey (Torres et al. 1997). This object, also an IRAS source, has
a relatively high inferred dust content and is also the most luminous quasar in our
nearby universe with a redshift of ~0.18 (Simpson et al. 1998; Torres et al.
1997). We chose to observe this object at ~1200 MHz in the hopes that HI gas
might be present around this quasi-stellar object (QSO) and, thus, exhibit HI
absorption.
Introduction
PDS 456 (RA = 17 28 19.796, DEC = -14 15 55.87) is of
interest in the study of QSO formation because it is relatively bright in both the
infrared and optical regimes. We hope to reveal the nature of the gas surrounding PDS
456 by searching for HI absorption towards this object. The detection of HI in
absorption could show a number of possible features. For example, if the HI were
part of a cold quiescent halo around PDS 456, we would expect to see absorption at the
systemic velocity of the galaxy. If the HI were being expelled as a result of the
QSO shedding its outer layers, the absorption should appear at blueshifted velocities.
A third possibility is that HI is accreting onto the central engine, in which
case we should detect redshifted HI absorption (see, for example, van Gorkom et al.
1989).
Observations
We observed PDS 456 on 1999 July 16 with the VLA in A-configuration. The source
3C48 has a well-known flux density and was observed once for flux calibration (Perley
et al. 1998). Also, 1733-130 (J2000) was observed intermittently to bracket PDS 456
and provide adequate phase calibration. This phase calibrator had a high flux density
(5.2 Jy) and was used, along with 3C48, for bandpass calibration.
We observed in the spectral line mode of the VLA and opted to use online Hanning
smoothing to overcome the effects of an interference source at 1200 MHz. Basing our
choice of observing mode on optimal bandwidth and velocity resolution, we observed in
4IF mode with 32 channels in each band. This mode provided a total bandwidth of 6.25
MHz (1563 km/s), and a velocity resolution of 24.4 km/s.
The rest frequency of HI absorption is 1420.406 MHz. However, the expected frequency
for HI absorption toward PDS 456 is 1199.667 MHz due to its estimated redshift of ~
0.18 (Simpson et al. 1998). Unfortunately, several factors degrade VLA observations
at this frequency: (1) a strong source of radio frequency interference (RFI) is
present at 1200 MHz, (2) the system temperature is increased at this frequency
compared to the standard 1.4 GHz, and (3) the ground becomes a significant source of
noise at these low frequencies. The latter two of these factors must be accepted, but
we took measures to decrease the interference caused by the RFI at 1200 MHz.
Ideally, all recorded channels should be reliable, but the outer channels are often
degraded due to imperfect electronics. For this reason, these outermost channels are
usually discarded. We took advantage of this fact and chose our two IFs such that the
two 32-channel bands overlapped at exactly 1200 MHz, the frequency of the RFI. By this
arrangement the data is least affected by the interference.
Data Reduction
We used the standard procedure for reducing spectral line data (see the VLA Spectral
Line Users Guide). The initial appearance of a weak detection prompted us to take
extra measures to ensure its reliability. Due to the 1200 MHz continuum strength of
PDS 456, we were able to self-calibrate the phases of the continuum map and apply these
corrections to the line data. Also, the maps were CLEANed such that all sources within
30 arcmin of the phase center and with a flux density greater than 5 mJy (as detected by the
NVSS) were cleaned separately to improve map quality (Condon et al. 1998). Even after
many different combinations of self-calibration and the aforementioned CLEANing
process, the weak detection remained constant. We also imaged the RCP and LCP line
data separately and determined that the apparent detection was evident in both
polarizations.
Results
PDS 456 has been observed before by the VLA. The NVSS found this source to have a flux
density of 22.7 mJy at 1.46 GHz, and other observations at 8.4 GHz and 4.6 GHz verify
that this source is an unresolved steep spectrum source (Simpson et al. 1998; Yun
1998, private communication). However, no one has ever attempted to search for HI
absorption toward PDS 456, probably because of the difficult nature of the
observations. Fortunately, our project was intended to be a tool of instruction rather
than discovery, so we were allowed to proceed with this observation despite reasonable
doubt of its success.
As alluded to before, we did detect what appears to be an HI absorption line. However,
preliminary data reduction confirms our detection at only about the 2 1/2 sigma level,
where one sigma is equal to the root mean square (RMS) noise. The measured RMS noise
in our maps is ~1.4 mJy which agrees with the theoretical value calculated by
considering time on source, declination of the source, and system imperfections at the
observing frequency.
The smoothed spectrum (see
Figure 1) which seems to depict a ~5 sigma detection is
actually somewhat misleading. The unsmoothed spectrum (see
Figure 2), however,
exhibits the ~2.5 sigma detection previously mentioned. We are confident that
this one-channel detection is not an artifact created during data reduction for two
reasons: (1) the detection has remained constant despite numerous attempts to remove
it through methods of calibration and CLEANing, (2) the second IF does not exhibit the
same artifact (see Figure 3), and (3) the spectrum of a field source of ~50 mJy
does not exhibit the same structure in IF1 (see Figure 4).

We have submitted a proposal for an additional 3-4 hours of VLA time to verify this
weak detection. Publication of the results from these tentative observations will
follow in a timely manner.
Discussion
Assuming that the HI absorption is real and associated with PDS 456, we can
speculate on the origin and nature of this gas. Our initial hypothesis, which prompted
these observations, was that PDS 456 is in a transition period from a dust-enshrouded
(i.e. ultraluminous infrared galaxy) to naked QSO. This claim was made because PDS
456 is an IR-bright source, but it is also very bright optically with a magnitude of B
= 14.69 (Torres et al. 1998). However, in this scenario we would expect to see HI
at velocities ranging from 0 km/s to ~-300 km/s. Since we observe infalling HI
at a velocity of about 200 km/s istead, our view of this source has changed. Now we
postulate that we have detected the infall of HI gas onto PDS 456. Considerable work
has been conducted in this field of HI astronomy (see, for example, van Gorkom et
al. 1989 for a good review), and the infall of cold gas into galaxies has been
suggested by observations. In these studies, no blue-shifted HI has been detected;
likewise, we have detected no blueshifted HI absorption in PDS 456. This lack of
detection could be a result of the source being unresolved by the VLA. The detection
of emission lines from this source might provide further insight about the nature of
the surrounding gas.
References
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Greisen, E. W., Yin, Q. F., Perley, R. A., Taylor, G. B., Broderick, J.
J. 1998, AJ, 115, 1693
Perley, R. A. and Taylor,
G. B. 1998, VLA Calibration Manual
Simpson, C., Ward, M., O'Brien, P.,
Reeves, J. 1998, MNRAS, 303, L23
Torres, C. A. O., Quast, G. R.,
Coziol, R., Jablonski, F., de la Reza, R., Lepine, and Gregorio-Hetem,
J. 1998, ApJ, 488, L19
van Gorkom, J. H.,
Knapp, G. R., Ekers, R. D., Ekers, D. D., Laing, R. A., and Polk, K.S.
1989, AJ, 97, 708
Acknowledgements
We thank NRAO for granting observing time to this group of undergraduate
and graduate students. Also, our gratitude is extended to Greg
Taylor, Liese van Zee, and Chris Fassnacht, directors of the
1999 REU summer program at NRAO, Socorro.
NRAO is operated by Associated Universities, Inc., under a cooperative
agreement with the National Science Foundation.
Questions? E-mail Chad Young.