Confirming lensed-quasar candidates with DESI and P200 spectroscopy I. 14 lensed quasars and 8 lensed galaxies

Abstract

Lensed quasars are powerful probes of cosmology, the co-evolution of supermassive black holes and their host galaxies, and the distribution of dark matter. We cross-match 1,724 previously identified candidates from KiDS, HSC, and the DESI Legacy Imaging Surveys (DESI-LS) with DESI DR1, obtaining 937 DESI spectra for 677 unique systems. Combining DESI spectroscopy with observations from the Palomar 200-inch Double Spectrograph (P200/DBSP), we confirm two lensed quasars with source redshifts $z_s={1.93,3.23}$ and Einstein radii $θ_{\rm E}={0.39,1.07}$ arcsec, respectively. We further identify 12 likely lensed quasars that are well reproduced by a simple SIE model, exhibit lens galaxies in the image modeling, and have at least one available spectrum; across these, $θ_{\rm E}$ spans $045$-$2.34$ arcsec and $z_s$ spans $1.13$-$2.88$. In 9 of the 12 cases, the systems already satisfy our lensing criteria except that only one quasar image currently has a spectrum; obtaining a second spectrum for the other image would enable immediate confirmation. Moreover, we report eight new static strong lenses spanning galaxy- to group- scale lenses. These results provide valuable targets for follow-up studies and underscore the efficiency of wide-field spectroscopic surveys such as DESI in confirming gravitationally lensed quasars and galaxies.

Figures (12)

• Figure 1: Postage-stamp cutouts of confirmed (red boxes) and likely (blue boxes) lensed quasars. North is up and east is left. Colour composites are constructed from the $g$, $r$, $i$-band imaging of HSC Public Data Release 3 (PDR3) for J0954$-$0022, J0954$+$0323, J1215$-$0058, J1220$+$0112, J1633$+$4235, J2212$-$0103, and J2225$+$0409, and from the $g$, $r$, $z$-band imaging of DESI-LS DR10 for the remaining systems. Note that all the color images were generated by us using HumVIMarshall2016. For J1215$-$0058, the cutout size is $73\times73$ pix$^2$, corresponding to a field of view of $\sim12\arcsec\times12\arcsec$. For the other systems, HSC cutouts are $33\times33$ pix$^2$ and DESI-LS cutouts are $21\times21$ pix$^2$, both corresponding to $\sim5.5\arcsec\times5.5\arcsec$ fields of view. A white short line in each panel indicates 1.
• Figure 2: P200/DBSP spectra. Top row: the P200 spectra of J1800+5305 are plotted as blue (image A) and red (image B) curves, each smoothed with a 5-pixel Gaussian kernel (effective smoothing $\sim$7.5-12.5Å). Middle and bottom rows: gray curves show the unsmoothed P200 flux densities; black curves show the same spectra smoothed with a 5-pixel Gaussian kernel (effective smoothing $\sim$7.5-12.5Å). In all panels, where available, DESI spectra are overplotted in green and smoothed with a 10Å kernel. Additional observing details are provided in Table \ref{['tab:p200']}. The $y$-axis is flux density in units of $10^{-17}\ \mathrm{erg\ s^{-1}\ cm^{-2}\ \AA^{-1}}$, and the $x$-axis is wavelength (Å).
• Figure 3: DESI-DR1 spectra of the confirmed and likely lensed quasars. Flux densities are smoothed with a Gaussian kernel of width $1.5,\text{\AA}$. The $y$-axis unit is $10^{-17}\ \mathrm{erg\ cm^{-2}\ s^{-1}\ \AA^{-1}}$; the $x$-axis unit is Å. The DESI spectra of J1800$+$5305 and J1809$+$5451 are shown in Fig. \ref{['fig:specs']} (green curves) and are therefore omitted here. Panels marked with a red "C" indicate confirmed lenses; all other systems are classified as likely lenses.
• Figure 4: Image decomposition results for confirmed and likely lensed quasars. Rows 1-6 correspond to J0047$+$0322, J0422$-$0447, J0642$+$5617, J0954$-$0022, J0954$+$0323, and J1215$-$0058. Settings of cutouts follow Fig.\ref{['fig:cutouts']}. In the Data panels, dashed circles mark the DESI fiber (diameter $1\farcs5$). Labels “A” and “B” in the PSF panels follow the convention used in Table \ref{['tab:psfs']} and throughout the text.
• Figure 5: (continued) Row 7-12: J1220+0112, J1531+3724, J1633+4235, J1800+5305, J1809+5451, J2212-0103.