Astronomy Reports, Vol. 49, No. 5, 2005, pp. 343–353. Translated from Astronomicheski˘ı Zhurnal, Vol. 82, No. 5, 2005, pp. 387–397. Original Russian Text Copyright c 2005 by Sazhin, Khovanskaya. The Object CSL-1 as an Effect of Projection M. V. Sazhin and O. S. Khovanskaya Sternberg Astronomical Institute, Universitetski ˘ı pr. 13, Moscow, 119992 Russia Received April 23, 2004; in final form, December 3, 2004 Abstract—We discuss possible interpretations of the extragalactic double galaxy CSL-1. CSL-1 can be explained either as the projection of two morphologically identical galaxies or as the effect of gravitational lensing by a cosmic string. We discuss the first of these possibilities in detail. More accurate observations will enable unambiguous conclusions about the nature of CSL-1. c 2005Pleiades Publishing, Inc. 1. THE OBSERVED FEATURES OF CSL-1 to each other along the vertical axis); the right panel shows the same for the NTT. The spikes in the spectra The object CSL-1 (Capodimonte–Sternberg are due to bad pixels on the CCD chip. Lens candidate No. 1) was discovered in the course of a deep survey by the Capodimonte Observa- The TNG spectra were taken under non- tory (further OACDF, Osservatorio Astronomico di photometric conditions, and the correlation coeffi- Capodimonte Deep Field). cient is about 0.6. The NTT observations taken under The OACDF is a deep image of a 0.5 × 1 square- photometric conditions demonstrate that the two degree area of sky at a high Galactic latitude taken spectra of the CSL-1 components are strongly corre- in various filters [1] with the 3.5 m NTT telescope of lated. The correlation coefficient for the raw spectra is the European Southern Observatory (ESO, La Silla, 0.9771 for a sample of one thousand data points. After Chile). The object CSL-1 was imaged in ten filters the reduction of the spectra, the correlation coefficient using the NTT: the three standard broadband filters— was 0.8452 (Fig. 3). The spectra were background- B (blue), V (visual), and R (red)—and seven narrow- reduced: since the two objects are both elliptical band filters, at 753, 770, 791, 815, 837, 884, and galaxies their spectra should have essentially identical 914 nm. Later, the 3.5 m National Galileo Telescope slopes, and so a linear fit of one spectrum to the (TNG, Canaries) was used to obtain additional im- other was undertaken in order to remove errors due to ages of CSL-1 in the infrared H filter (1.6 µm). The errors in the CCD detector. The differences between characteristics of the OACDF are presented in [1, 2]. the reduced spectra represent random noise without CSL-1 is discussed by Sazhin et al. [3]. a determinate component, with the autocorrelation CSL-1 consists of two giant, slightly elliptical function being close to unity (Fig. 4). This difference corresponds to a non-Gaussian random distribution. galaxies with absolute magnitudes MR = −22.3 ± 0.1 and the same redshift, z =0.46 ± 0.008;itcould The width of the correlation peak corresponds to in principle correspond to a single galaxy with these chaotic velocities of about 400 km/s in the galaxies. properties, together with an image created by a grav- The radial-velocity difference is 27 ± 25 km/s. itational lens. The distance from the solar system’s baricenter to CSL-1 is about 1900 Mpc for a Hubble One hypothesis that has been suggested to explain − − the existence of these two close images with very constant of H =65 km s 1 Mpc 1. The apparent angular separation of the two components is small, similar spectra is that a dust lane with a special hour- glass shape crosses the minor diameter of a single about 20 kpc (1.9 ). Though the object belongs to a giant, very elliptical galaxy. This dust lane mimics scattered cluster of galaxies, the field is quite sparse: the appearance of two separate objects with virtually other objects are located from tens of arcseconds to several arcminutes away on the sky (Fig. 1). undistorted round isophotes. The appearance (the presence of metallic absorp- The two CSL-1 component galaxies have low el- tion lines) and the slope of the spectra clearly indi- lipticity, so that their brightness-distribution isopho- cate that both components correspond to elliptical tes can be approximated well by a two-dimensional galaxies with redshift z =0.46 ± 0.008 (Fig. 2). The de Vaucouleurs law [4]: left panel of Fig. 2 shows the spectra of the CSL-1 1/4 r components taken with the TNG telescope (for ease IS(r)=I0 exp −b , of presentation, the spectra have been shifted relative rc 1063-7729/05/4905-0343$26.00 c 2005 Pleiades Publishing, Inc. 344 SAZHIN, KHOVANSKAYA N 40′′ AB E 20 6′ ~10′′ 0 20 40 60 80′′ Fig. 1. The object CSL-1 in the R filter (to the left and in the middle). The distance between the brightness centers of the two components is 1.9. One pixel of the image corresponds to 0.238, and the resolution is about 1. The photometric accuracy is 10%. Each of the components is a slightly elliptical giant galaxy with redshift z =0.46 ± 0.008 and absolute magnitude MR = −22.3 ± 0.1. The right panel shows the isophotes of CSL-1 in the 914 nm filter, i.e., sections of the 3D brightness distribution with reference surfaces separated by a given brightness difference. The brightness profile of CFL-1 agrees with a de Vaucouleurs law. I, rel. units 80 0.6 z = 0.46 ± 0.008 60 0.4 40 Mg 0.2 G band 20 CaI (H, K) 0 0 Mg –0.2 5000 6000 7000 8000 3000 4000 5000 6000 7000 8000 9000 λ, Å λ, Å Fig. 2. Left: the TNG spectra of the components of CSL-1. The spectra have been arbitrarily shifted relative to one another for ease of presentation. Absorption lines of calcium, the G band due to metals in galaxies, night-sky lines (telluric absorption lines), and a magnesium line are evident. The lines and the slope of the spectra unambiguously show that both components are elliptical galaxies with redshift z =0.46 ± 0.008. The spikes in the spectra are due to bad pixels of the CCD chip. Right: the NTT spectra of the components. where gave results that disagreed with the observations. 1+e 1 − e The extinction law was assumed to have the form r = (x2 + y2)+ (x2 + y2)cos2ψ exp(−τ(x)),wherex is the coordinate along the pro- 2 2 file and the extinction coefficient is 1 − e 1/2 n + xy sin 2ψ , τ(x)=f(x)/λ , 2 where f(x) is a geometric factor describing the distri- x and y are curvilinear coordinates with their origin at bution of dust at a given point x and n is the dust in- the central peak of the brightness distribution, e is the dex [5–9], which assumes values in the interval [1, 2]. eccentricity of the corresponding isophote, and ψ in- If an extended galaxy were crossed by a dust lane dicates the position angle of the isophote. This model having this extinction law, a brightness peak in the in- represents the best approximation to the brightness frared would have been observed between the centers distribution of CSL-1 [3]. of the two components of CSL-1, since the dust ex- It was demonstrated in [3] that modeling the dust tinction coefficient, which is inversely proportional to with the extinction law characteristic of our Galaxy the wavelength to the nth power (n ∈ [1, 2]), is small ASTRONOMY REPORTS Vol. 49 No. 5 2005 THE OBJECT CSL-1 AS AN EFFECT OF PROJECTION 345 Parameters of the CSL-1 components in various filters FWHM, arcsec A A B Filter MA MB re re /re A B PSF B 1.59 1.67 1.14 22.73 ± 0.15 22.57 ± 0.15 V 1.59 1.67 1.01 20.95 ± 0.13 21.05 ± 0.13 6.3 1.4 R 1.98 1.98 0.98 19.67 ± 0.20 19.66 ± 0.20 3.0 2.5 H 1.19 1.11 0.85 753 nm 1.11 1.19 0.87 770 nm 1.27 1.27 0.86 7.4 0.6 791 nm 1.67 1.59 0.97 914 nm 1.27 1.27 0.79 8.8 1.4 in the infrared, so that the dust is transparent at these If we normalize each profile so that the highest wavelengths. In fact, a gap is observed between the intensity in each filter is equal to unity (Fig. 6), two components of CSL-1, indicating the presence the minima have the same depth, and hence are of two separate objects rather than a single, strongly frequency-independent. This enables us to rule out elliptical galaxy. the dust-lane hypothesis. Therearevariousmodelsforthedust,however. Let us consider the observed properties of CSL-1. For example, a possible dust lane in the galaxy The table presents the observed parameters for the NGC2841 was considered in [10]; this lane is bright components of CSL-1. It is important that CSL-1 is in the optical due to the reflection of light from resolved. Since the uncertainty of a Gaussian func- the galaxy’s central region, whereas it is dark in tion is 1%,thedifference between the telescope’s the infrared. Nevertheless, the extinction coefficient resolving power (the point-spread function, PSF) and depends on the wavelength in all models.