Spectroscopical Confirmation and Lens Modeling of a Complex Strong Lensing System Produced by a Close Galaxy Pair at $z_d=0.79$

Abstract

We report the spectroscopic confirmation and lens modeling of the complex strong-lens system J0233-0205, in which the deflector consists of a pair of galaxies at zd = 0.790 +/- 0.022, the background source lies at zs = 2.160 +/- 0.002, and the circularized Einstein radius is thetaE = 1.680 arcsec +/- 0.003 arcsec. Our lens modeling requires two lens components, namely two elliptical galaxies with Einstein radii of 0.669 arcsec +/- 0.002 arcsec and 0.735 arcsec +/- 0.002 arcsec, respectively, and a projected separation of 0.513 arcsec (about 3.832 kpc), as well as three source components: two disk galaxies separated by 0.4965 arcsec (about 3.712 kpc), plus a point-like component closely aligned with one of the disks. From a joint lensing and stellar-population analysis, we infer a total stellar mass within the critical curve of the lens pair of (1.956 +/- 0.418) x 10^11 solar masses and a total enclosed mass of (1.107 +/- 0.008) x 10^12 solar masses, corresponding to a projected dark-matter fraction of 82 +/- 4%. The stellar masses of the two lens galaxies are (8.548 +/- 2.128) x 10^10 solar masses and (1.525 +/- 0.295) x 10^11 solar masses, implying dark-matter fractions within the z-band effective radius of 57 +/- 11% and 70 +/- 6%, respectively. The small separation of the lens pair, together with its relatively high deflector redshift, makes J0233-0205 a potentially ideal laboratory for probing the mass distribution and dark-matter content of close galaxy pairs. In addition, the two disk galaxies and the associated point-like source make this system valuable for investigating the merger process in the source plane.

Figures (6)

• Figure 1: Spectroscopic follow-up of HSC J0233$-$0205 with P200/DBSP. Top row: RGB cutouts showing the DBSP slit placements for the two exposures (Oct. 15: left; Oct. 16: right). North is up and east is to the left. The lensed compact galaxies A (blue) and B (red) are marked, and the scale bar (1) and image separation (320) are indicated; yellow lines outline the slit. Second row: 1D spectrum extracted from image B in the Oct. 15 exposure. Third row: 1D spectrum extracted from image A in the Oct. 16 exposure. Fourth row: 1D spectrum extracted from image B in the Oct. 16 exposure. Vertical dashed lines mark prominent emission features (Ly$\alpha$, Si iv, C iv, and Mg ii) at the derived source redshift, and the $\oplus$ symbol denotes a telluric absorption band. From these spectra, we measure $z_s = 2.160 \pm 0.001$.
• Figure 2: Optical spectrum of the lens galaxies in HSC J0233$-$0205, showing a secure redshift of $z_d=0.790\pm0.022$. The blue curve shows the observed spectrum, while the red curve shows the best-fitting template spectrum. The dotted vertical lines mark the expected observed wavelengths of prominent stellar absorption features at this redshift (e.g., Ca H&K, G band, Mg, and Balmer lines), and the gray shaded regions indicate wavelength intervals strongly affected by atmospheric/telluric absorption (and thus masked in the fit).
• Figure 3: Best-fitting results. North is up and east is to the left. From left to right, the six columns show the data, the lens-light model, the lens-light-subtracted data, the lensed-image model, the residual, and the source-light model. The first five rows correspond to the $g$, $r$, $i$, $z$, and $y$ bands, respectively, and the last row shows a color-composite image generated from all five bands. The first five columns cover $6\farcs55 \times 6\farcs55$ with a pixel scale of $0\farcs168$, while the last column covers $3\arcsec \times 3\arcsec$ with a pixel scale of $0\farcs0168$. In the last row, the residual panel shows the unnormalized residual (data $-$ model), whereas the other rows show normalized residuals.
• Figure 4: Visualization of the best-fitting lens model. North is up and east is to the left. Left: RGB composite of the modeled lensed image in the image plane, overlaid with the model-predicted critical curves (red) and caustics (green). Middle: dimensionless convergence map, $\kappa$, for the total mass model; the labeled markers indicate the positions of the two lens components (lens1 and lens2). Right: lensing magnification map shown as the absolute magnification, $|\mu|$. All three panels cover the same field of view, $6\farcs55 \times 6\farcs55$, and share the same coordinate system, with axes labeled in arcseconds. The middle and right panels are displayed with a pixel scale of $0\farcs0168$ per pixel. Color bars indicate the corresponding values of $\kappa$ and $|\mu|$.
• Figure 5: CIGALE SED fitting result. Open and filled circles correspond to the observed and model-predicted photometry.