A&A 382, L26-L28 (2002)
D. Reimers1 - H.-J. Hagen1 - R. Baade1 - S. Lopez2 - D. Tytler3
1 - Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
2 - Departemento Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
3 - Department of Physics and Center for Astrophysics and Space Sciences, University of California, San Diego, C-0424, La Jolla, CA 92093-0424
Received 3 December 2001 / Accepted 16 December 2001
We announce the probable discovery of a new very bright gravitationally lensed QSO, HS 0810+2554 (z=1.50, V=15.3). The gravitational lens character has been discovered serendipitously by means of short (12 times 1 s) HST/STIS target acquisition images. The coadded images show a close bright double A (V=16.0) and B (16.7) separated by 0 25 plus two fainter images C (17.4) and D (18.8). There is also evidence for a lens galaxy in the center of the images which is fainter than A by a factor of 30 at 7150 Å. The image configuration resembles very much that of PG 1115+080 except that in HS 0810+2554 image splittings (1'') are smaller by a factor of 2.5 and HS 0810+2554 is brighter by 1.
Key words: galaxies: quasars: individual HS 0810+2554 - galaxies: quasars: general - cosmology: gravitational lensing
Gravitational lenses have become an important tool of observational cosmology. Multiply imaged QSOs can be used to determine the Hubble constant H0 from observed time delays between the images (Refsdal 1964), to study the distribution of matter in a mass-selected sample of galaxies, to investigate the QSO geometry by means of wavelength dependent image amplification due to microlensing (Wisotzki et al. 1993, 1998), and last not least to get information on the spatial distribution of intergalactic matter in the distant universe (Lopez et al. 1999).
The Hamburg-All Sky Bright QSO Surveys (cf. Hagen et al. 1995; Reimers & Wisotzki 1997) which are based on digitized objective prism plates over 22500 deg2 from the Calar Alto (HQS) and ESO Schmidt telescopes (HES) have produced roughly half of the known bright (B<17) QSOs and have as such been a rich source of gravitationally lensed QSOs; in fact 6 multiple QSOs have been discovered so far (for reference cf. Hagen & Reimers 2000).
Due to seeing limitations optical imaging surveys from the ground can detect only multiply images with fairly large image separations which favours giant E galaxies as lenses.
In this letter we announce the serendipitous discovery of a multiply
imaged QSO with four images within 1'' during high resolution
spectroscopy of a bright QSO with HST/STIS.
|Figure 1: Johnson V image of HS 0810+2554 (Calar Alto 3.5 m telescope, 120 s, March 21, 1996). The small panels show logarithmic plots of the coadded acquisition STIS/HST images (effective wavelength 7150 Å, exposure time 12 s). The displayed brightnesses were computed by dividing the total brightness of V=15.3 according to the counts ratios. Notice the faint probable lensing galaxy in the center of the 4 images.|
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HS 0810+2554 was discovered on a digitized objective prism plate of the HQS (Hagen et al. 1995) and published in a list of bright QSOs (Hagen et al. 1999) with V=15.3 and z=1.5. Due to its extreme brightness we proposed it as a target for high-resolution UV spectroscopy with HST/STIS with the aim to detect the warm-hot intergalactic medium at z >1 via intergalactic OVI absorption lines (cf. Reimers et al. 2001) and got 19 orbits HST time.
The HST observations were conducted between November 5-11 2001, with a surprising result: the target acquisition images taken with STIS show four images, in a configuration similar to that of PG 1115+080 (cf. Fig. 1).
There are several arguments which leave no doubt that we have discovered a new four-image lensed QSO:
PG 1115+080 was the second lensed QSO discovered (Weymann et al. 1980). In addition to the four images, the QSO host galaxy forms an Einstein ring visible in Nicmos H band (Impey et al. 1998). PG 1115+080 is one of the few lenses where there is sufficient information to infer the Hubble constant from measured time delays (Schechter et al. 1997; Impey et al. 1998).
While the morphological similarity of HS 0810+2554 with PG 1115+080
(CASTLE survey (http://cfa-www.harvard.edu/castles) is striking, the
image separations in the former are smaller by roughly a factor of 2.5 suggesting a smaller mass of the lensing galaxy if the geometry (lens
redshift) should turn out to be similar. Since the time delays vary
with the square of the angular sizes of lens systems, the observed
time delays in PG 1115+080 of the order of 20 days suggest a rather
short time delay for HS 0810+2554.
|Figure 2: The STIS spectrum taken with both A and B in the aperture shows blacked out cores (zero flux) in the strongest Ly forest lines which is only possible if the absorption spectra of both components are identical.|
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Without redshift information the lensing galaxy G of HS 0810+2554 is difficult to compare with that in PG 1115+080. The latter is at z=0.29 and fainter in V by 3 84 than image A (CASTLE survey (http://cfa-www.harvard.edu/castles). HS 0810+2554 has a similar ratio between G and A at 7150 Å. This would imply a lower M/L of the lensing galaxy at a comparable redshift. Further speculations on the lens redshift are not useful at the present time. The next observational steps to be taken are obvious, and this rapid letter intends to motivate observers to conduct the observations:
This work has been supported by the Verbundforschung of the BMBF/DLR under grant No. 50 OR 9911 1. SL acknowledges financial support by a FONDECYT grant No. 3000001.