Hubble imaging excludes cosmic string lens

The galaxy image pair Capodimonte-Sternberg-Lens Candidate no. 1 (CSL-1) has been a leading candidate for a cosmic string lens. High quality imaging data from the Hubble Space Telescope presented here show that it is not a lens but a pair of galaxies. The galaxies show different orientations of their principal axes, not consistent with any lens model. We present a new direct test of the straight-string lens model, using a displaced difference of the image from itself to exclude CSL-1 at high confidence.

Figure 1

Color composite of HST imaging data, generated using tools described by R. Lupton et al., Publ. Astron. Soc. Pac. 116, 133 (2004). Inset shows a logarithmic contour plot of surface brightness in F775W for the CSL-1 galaxy pair; contours are separated by a factor of 1.88 in surface brightness, and the brightest corresponds to 19.405 mag per square arcsec. The orientations of the isophotes are clearly different, strong evidence against a string. The position angles of the principal axes, measured eastward from north, are $51.7\pm 1.1\mathrm{deg}$ for the eastern component and $-2.6\pm 1.5\mathrm{deg}$ for the western component.

Figure 2

Left and right panels show HST F625W images of the two galaxies, with angular scale in units of 0.05 arcsec pixels. The center panel shows the difference between the two for the best-fit cosmic string lens model. The figure shows the galaxies rotated so that the $x$-axis aligns with the translation axis (perpendicular to the “string” angle). The color bar refers to the center panel and indicates the flux difference in units of standard deviations per pixel. The best-fit straight-string model gives a translation of 37.9 pixels at an angle of $-7.1\mathrm{deg}$. The reduced ${\chi }^2$ for the F625W difference image is 5.8 and for the F775W image is 5.0 within a strip 37 pixels wide by 100 pixels long. The string model predicts a null difference image, with reduced ${\chi }^2=1$ to high accuracy.