The field surrounding NGC 7603: Cosmological or non-cosmological redshifts?

A&A 421, 407-423 (2004)
DOI: 10.1051/0004-6361:20034260

The field surrounding NGC 7603: Cosmological or non-cosmological redshifts?

M. López-Corredoira¹ and C. M. Gutiérrez²

¹ Astronomisches Institut der Universität Basel. Venusstrasse 7. 4102 Binningen, Switzerland
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain

(Received 2 September 2003 / Accepted 31 December 2003 )

Abstract
We present new observations of the field surrounding the Seyfert galaxy NGC 7603, where four galaxies with different redshifts - NGC 7603 ( z=0.029), NGC 7603B ( z=0.057) and two fainter emission line galaxies ( z=0.245 and z=0.394) - are apparently connected by a narrow filament, leading to a possible case of anomalous redshift. The observations comprise broad and narrow band imaging and intermediate resolution spectroscopy of some of the objects in the field. The new data confirm the redshift of the two emission-line objects found within the filament connecting NGC 7603 and NGC 7603B, and settles their type with better accuracy. Although both objects are point-like in ground based images, using HST archive images we show that the objects have structure with a FWHM = 0.3-0.4 arcsec. The photometry in the R-band obtained during three different campaigns spread over two years does not show any signs of variability in these objects above 0.3-0.4 mag. All the above information and the relative strength and width of the main spectral lines allow us to classify these as HII galaxies with very vigorous star formation, while the rest of the filament and NGC 7603B lack star formation. We delineate the halo of NGC 7603 out to 26.2 mag/arcsec ² in the Sloan r band filter and find evidence for strong internal distortions. New narrow emission line galaxies at z=0.246, 0.117 and 0.401 are also found at respectively 0.8, 1.5 and 1.7 arcmin to the West of the filament within the fainter contour of this halo. We have studied the spatial distribution of objects in the field within 1.5 arcmin of NGC 7603. We conclude that the density of QSOs is roughly within the expected value of the limiting magnitude of our observations. However, the configuration of the four galaxies apparently connected by the filament appears highly unusual. The probability of three background galaxies of any type with apparent B-magnitudes up to 16.6, 21.1 and 22.1 (the observed magnitudes, extinction correction included) being randomly projected on the filament of the fourth galaxy (NGC 7603) is $\approx$ $3\times 10^{-9}$ . Furthermore, the possible detection of very vigorous star formation observed in the HII galaxies of the filament would have a low probability if they were background normal-giant galaxies; instead, the intensity of the lines is typical of dwarf HII galaxies. Hence, a set of coincidences with a very low probability would be necessary to explain this as a fortuitous projection of background sources. Several explanations in terms of cosmological or non-cosmological redshifts are discussed.

Key words: galaxies: individual: NGC 7603 -- galaxies: statistics -- galaxies: peculiar -- galaxies: starburst -- cosmology: distance scale

Offprint request: M. López-Corredoira, cgc@ll.iac.es

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