The Tautenburg - Calar Alto VPM Survey

A Quasar Search via Variability and Zero Proper Motion



Helmut Meusinger, TLS Tautenburg
Jens Brunzendorf, TLS Tautenburg
Ralf Scholz, AI Potsdam
Mike Irwin, IoA Cambridge
Michel Laget, Lab. d'Astrophysique, Marseille
Jan Kohnert, TLS Tautenburg




Motivation

A subfield from the VPM QSO search in the vicinity of the globular cluster M3. Quasar hunting is like seeking a needle in a haystack. One of the starlike objects in the image above is a quasar. (Click here to see which one.) Quasars -- the spectacular superlative of active galactic nuclei -- are the most energetic and most luminous (quasi-stationary) objects in the Universe. As such, they are ideal indicators of the large-scale structure and the long-term evolution of matter. Quasars have to be found, of course, before they can be studied. For many tasks using quasars as tracers it is important to understand the selection effects of quasar searches. Most criteria for the conventional selection of quasar candidates are based on the differences of their spectral energy distribution compared to stars. In order to estimate which fraction of quasars are eventually ignored by such surveys, one has to perform alternative search strategies that do not directly rely on the spectral energy distribution. Brightness variations and very large distances are such properties. As a consequence of their large distances, quasars do not show proper motions, contrary to a large fraction of stars. The search for objects which are both variable in brightness and stationary in position is supposed to be a powerful technique for efficiently finding quasars. The selection effects of such a search are expected to be quite different from those of the more conventional optical search techniques.

Search Strategy

For an efficient VPM (= variability and proper motion) survey, a large number of homogeneous observations of a large number of faint galaxies are needed, with high astrometric and photometric accuracy and covering a long time baseline of several years up to some decades. These requirements can be matched by archival plates from large Schmidt telescopes.

The basic observational material of our VPM survey is provided by about 200 plates from the Tautenburg Schmidt telescope in two fields with a time baseline of more than three decades. The plates were digitised by means of the APM facility, Cambridge, UK, or the Tautenburg Plate Scanner, TPS.

More than 60 000 star-like objects were detected on the plates, but only a small fraction show both overall-variability, long-term variability, and zero absolute proper motions. These are considered prime candidates for quasars. Spectroscopic follow-up observations are performed at the German-Spanish Astronomical Centre on Calar Alto, Spain.

Variability - proper motion diagram for the M3 field.
Figure: Illustration of the basic idea of the quasar candidate selection. The diagram shows indices for variability and proper motion, respectively, of about 12 000 star-like objects in the high-galactic latitude field around the globular cluster M3 (the cluster region was excluded because of strong image crowding). Quasars detected by a search based on slitless spectroscopy are marked by red squares, all other star-like objects by white dots. No significant variability is measured for the majority of the stars. Further, for a substantial fraction of the stars (especially the intrinsically faint stars in our neighbourhood) significant absolute proper motions can be measured. Quasars, on the other hand, have immeasurably low proper motions, but most of them show detectable brightness fluctuations.


Preliminary Results

The VPM method provides a powerful, yet time-consuming, technique for efficiently finding quasars. Up to now, spectroscopic follow-up observations revealed about 350 quasars/Seyfert1s in the two VPM search fields with an total effective area of 16 square degrees. The success rate of the survey is higher than 40 per cent with a completeness limit at B ~ 19.5 (the limits of the deepest plates are two mags fainter).

For B < 18.5, the VPM quasar surface density is higher than that derived from other surveys. At fainter magnitudes, the completeness decreases due to the strong dependence of the success rate on the photometric accuracy.

The most interesting result is that, up to the limit of the survey, the properties of the VPM-quasar population are very similar to the properties of quasar samples from more conventional optical surveys. There are no significant differences found for the spectral energy distributions. Although there was a substantial fraction of the VPM quasar candidates with exceptionelly red colours, all spectroscopically confirmed quasars have normal (i.e., ``blue'') colours. All unusually red candidates turned out to be galactic stars. Up to the limit of the survey, the fraction of ``red quasars'' seems to be negligible.

The quasar sample from the VPM survey will be suitable for the statistical investigation of quasar variability .


composite spectrum Figure: Composite spectrum of VPM QSOs in the rest frame. (Click on the image for a larger version.)



2 dim composite Figure:
This is a 2-dimensional composite from ~220 QSO spectra in the observer's frame. The wavelength range is 4000 A (left) to 8000 A (right). The redshifts cover the interval from 0.3 (bottom) to 3.3 (top).



colour-redshift diagrams Figure: The colour-redshift diagrams. The quasars from the VPM survey are shown as red squares, catalogued quasars from other surveys as blue dots. (Click on the image for a larger version.)


Publications

Meusinger, H., Froebrich, D., Haas, M., Irwin, M., Laget, M., Scholz, R.-D.:
VPMS J1342+2840 -- an unusual quasar from the variability and proper motion survey
Astronomy & Astrophysics, 433 (2005), L25
Paper with figures (PDF file, 428 KB)

Meusinger, H., Froebrich, D., Haas, M., Irwin, M., Kohnert, J., Laget, M., Scholz, R.-D.:
Unconventional quasars from the variability and proper motion survey
Astron. Nachrichten, 326 (2005), 553
Abstract with figures (PDF file, 105 KB)

Meusinger, H., Scholz, R.-D., Irwin, M., Laget, M.:
``Quasars from the Tautenburg-Calar Alto Variability and Proper Motion Survey'',
Astron. Nachrichten, Suppl. Issue 3, 324 (2003), 50
Abstract (gzipped postscript file, 21 KB)

Meusinger, H., Laget, M., Brunzendorf, J.:
``A QSO survey via optical variability and zero proper motion in the M92 field. V.Completion of the QSO sample'',
Astron. Nachrichten, 324 (2003), 474
Paper with figures (gzipped pdf file, 457 KB)

Meusinger, H., Scholz, R.-D., Irwin, M., Lehmann, H. :
``QSOs from the variability and proper motion survey in the M3 field'',
Astronomy & Astrophysics, 392, (2002) 851
Paper with figures (gzipped postscript file, 446 KB)

Meusinger, H., Scholz, R.-D., Irwin, M., Lehmann, H. :
``QSOs in the M3 field´´,
Catalogue at CDS

Meusinger, H., Brunzendorf, J., Scholz, R.-D., Irwin, M.:
``QSOs from a Variability-and-Proper Motion Survey'',
In: R. Green et al. (eds.): AGN surveys, IAU Colloq. 184, ASP Conference Series, in press
Paper with figures (gzipped postscript file, 260 KB)

Meusinger, H., Brunzendorf, J.:
``Properties of the Low-z NELGs from the VPM Survey'',
In: R. Green et al. (eds.): AGN surveys, IAU Colloq. 184, ASP Conference Series in press
Paper with figures (gzipped postscript file, 90 KB)

Brunzendorf, J., Meusinger, H.:
``A QSO survey via optical variability and zero proper motion in the M92 field. IV. More QSOs due to improved photometry'',
Astronomy & Astropysics, 390, (2002) 879
Paper with figures (gzipped postscript file, 1038 KB)

Meusinger, H., Brunzendorf, J.:
``A QSO survey via optical variability and zero proper motion in the M92 field. III. Narrow emission line galaxies'',
Astronomy & Astropysics, 390, (2002) 439
Paper with figures (gzipped postscript file, 348 KB)

Meusinger, H., Brunzendorf, J.:
``A QSO survey via optical variability and zero proper motion in the M92 field. II.Follow-up spectroscopy and properties of the QSO sample'',
Astronomy & Astropysics 374 (2001), 878
Paper with figures (gzipped pdf file, 367 KB)

Brunzendorf, J., Meusinger, H.:
``A QSO survey via optical variability and zero proper motion in the M92 field. I.QSO candidates and selection effects'',
Astronomy & Astropysics 373 (2001), 38
Paper with figures (gzipped pdf file, 446 KB)

Meusinger, H., Scholz, R.-D., Irwin, M.:
``Variable BSS Candidates in M3 Proved to be Quasars''
Information Bulletin on Variable Stars 5037 (2001)
Paper with figures (gzipped postscript file, 47 KB)

Meusinger, H., Brunzendorf, J.:
``J1714.9+4210: A Variable Faint High-Latitude Carbon Star''
Information Bulletin on Variable Stars 5035 (2001)
Paper with figures (gzipped postscript file, 47 KB)

Meusinger, H., Brunzendorf, J.:
``Variable Narrow Emission Line Galaxies'',
In: R. E. Schielicke (ed.): Astron. Ges. Abstract Ser. 17 (2000), 86

Brunzendorf, J., Meusinger, H.:
``Variability and Proper Motion Selected QSOs in the M92 Field'',
In: R. E. Schielicke (ed.): Astron. Ges. Abstract Ser. 17 (2000), 87

Meusinger, H., Brunzendorf, J., Scholz, R.-D., Irwin, M.:
``A variability/proper motion search for QSOs on Schmidt plates'',
In: P. Kroll (ed.): Treasure Hunting in Astronomical Plate Archives, Harri Deutsch Verlag, (1999), 122
Paper with figures (gzizzed postscript file, 107 KB)

Meusinger, H., Scholz, R.-D., Irwin, M., Brunzendorf, J., Lehmann, H., Ball, M.:
``A status report on the Tautenburg VPM QSO survey'',
In: R. E. Schielicke (ed.): Astron. Ges. Abstract Ser. 14 (1998), 109

Scholz, R.-D., Meusinger, H., Irwin, M.:
``A UBV/variability/proper motion QSO survey from Schmidt plates'',
Astronomy & Astrophysics 325 (1997), 457
Paper with figures (gzizzed postscript file, 1330 KB)

Meusinger, H., Ball, M., Scholz, R.-D., Irwin, M.:
``TAUMOK Multi-Object Spectroscopy of QSO candidates from a Variability/Proper Motion Survey'',
In: R. E. Schielicke (ed.): Astron. Ges. Abstract Ser. 13 (1997), 233

Meusinger, H., Scholz, R.-D., Boller, T., Brunner, H., Lamer, G., Irwin, M.:
``Optical and X-Ray variability of AGNs'',
In: H. U. Zimmermann et al. (eds.): Röntgenstrahlung from the Universe, MPE Report 263, 483, (1996), 737
Paper with figures (gzipped postscript file, 305 KB)

Scholz, R.-D., Meusinger, H., Irwin, M.:
``Absolute proper motions with Hipparcos-like accuracies for a complete sample of stars down to B = 18.5 in a field of 10 square degrees'',
In: R. E. Schielicke (ed.): Astron. Ges. Abstract Ser. 12 (1996), 82

Meusinger, H., Klose, S., Ziener, R., Scholz, R.-D.:
``A Study of the Variability and Proper Motions of Quasars based on Tautenburg Schmidt Plates'',
In: J. Chapman et al. (eds.): The future utilisation of Schmidt telescopes, IAU Coll. 148, ASP Conference Series 84 (1995), 486
Paper with figures(gzipped postscript file, 354 KB)

Meusinger, H., Klose, S., Ziener, R., Scholz, R.-D.:
``Optical long-term variability of a sample of quasars in the field around M3'',
Astronomy & Astropysics 289 (1994), 67
Paper with figures (gzipped pdf file, 446 KB)

Meusinger, H., Klose, S., Ziener, R.:
``A Study of the Optical Long-Term Behaviour of Quasars'',
In: H. T. MacGillivray et al. (eds.): Astronomy from wide-field imaging, IAU Symp. 161, Kluwer Academic Publishers, Dordrecht, (1994), 737
Paper(gzipped postscript file, 216 KB)

Meusinger, H., Klose, S., Ziener, R.:
``A three-color study of optical variability of quasars'',
In: R. E. Schielicke (ed.): Astron. Ges. Abstract Ser. 10 (1994), 223

Meusinger, H., Klose, S., Ziener, R.:
``The optical long-term behaviour of a sample of quasars'',
In: R. E. Schielicke (ed.): Astron. Ges. Abstract Ser. 9 (1993), 187