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Register a new userA Redshift for the First Einstein Ring, MG 1131+0456
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MG 1131+0456 is a radio-selected gravitational lens, and is the first known Einstein ring. Discovered in 1988, the system consists of a bright radio source imaged into a ring and two compact, flat-spectrum components separated by 2.1 arcsec. The ring is optically faint (R = 23.3), rising steeply into the near- and mid-infrared (K = 17.8; W2 = 13.4). The system has been intensively studied in the intervening years, including high-resolution radio imaging, radio monitoring, and near-infrared imaging with Hubble and Keck. The lensing galaxy is at z(lens) = 0.844. However, to date, no spectroscopic redshift had been reported for the lensed source. Using archival Keck data from 1997, we report the robust detection of a single narrow emission line at 5438 Angstroms, which we associate with CIII] 1909 from a type-2 quasar at z(source) = 1.849. Support for this redshift identification comes from weaker emission associated with CIV 1549 and HeII 1640, typical of type-2 quasars, as well as the lack of emission lines in archival near-infrared Keck spectroscopy. We also present, for the first time, Cycle 1 Chandra observations of MG 1131+0456, which clearly resolves into two point sources with a combined flux of ~1e-13 erg/cm2/s and a best-fit column density of ~3e22 /cm2. We suggest a new method to identify candidate lensed active galactic nuclei from low-resolution X-ray surveys such as eROSITA by targeting sources that have anomalously high X-ray luminosity given their mid-infrared luminosity.
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We report the discovery of a partial Einstein ring of radius 1.48arcsec produced by a massive (and seemingly isolated) elliptical galaxy. The spectroscopic follow-up at the VLT reveals a 2L* galaxy at z=0.986, which is lensing a post-starburst galaxy at z=3.773. This unique configuration yields a very precise measure of the mass of the lens within the Einstein radius, (8.3e11 +- 0.4)/h70 Msolar. The fundamental plane relation indicates an evolution rate of d [log (M/L)B] / dz = -0.57+-0.04, similar to other massive ellipticals at this redshift. The source galaxy shows strong interstellar absorption lines indicative of large gas-phase metallicities, with fading stellar populations after a burst. Higher resolution spectra and imaging will allow the detailed study of an unbiased representative of the galaxy population when the universe was just 12% of its current age.
We have obtained and modeled new NICMOS images of the lens system MG1131+0456, which show that its lens galaxy is an H=18.6 mag, transparent, early-type galaxy at a redshift of about z_l = 0.85; it has a major axis effective radius R_e=0.68+/-0.05 arcsec, projected axis ratio b/a=0.77+/-0.02, and major axis PA=60+/-2 degrees. The lens is the brightest member of a group of seven galaxies with similar R-I and I-H colors, and the two closest group members produce sufficient tidal perturbations to explain the ring morphology. The host galaxy of the MG1131+0456 source is a z_s > 2 ERO (``extremely red object) which is lensed into optical and infrared rings of dramatically different morphologies. These differences imply a strongly wavelength-dependent source morphology that could be explained by embedding the host in a larger, dusty disk. At 1.6 micron (H), the ring is spectacularly luminous, with a total observed flux of H=17.4 mag and a de-magnified flux of 19.3 mag, corresponding to a 1-2L_* galaxy at the probable source redshift of z_s > 2. Thus, it is primarily the stellar emission of the radio source host galaxy that produces the overall colors of two of the reddest radio lenses, MG1131+0456 and B~1938+666, aided by the suppression of optical AGN emission by dust in the source galaxy. The dusty lens hypothesis -- that many massive early-type galaxies with 0.2 < z_l < 1.0 have large, uniform dust opacities -- is ruled out.
A deep spectrum taken with the Echelle Spectrograph and Imager (ESI) at the Keck II Telescope as part of the Lenses Structure and Dynamics (LSD) Survey reveals the redshifts of the extremely red source of the radio Einstein Ring in the gravitational lens system MG1549+305 ($z_{rm s}=1.170pm 0.001$) and an intermediate redshift lensed spiral galaxy ($z_{rm G2}=0.604pm 0.001$). The source redshift allows us to determine the mass of the SB0 lens galaxy enclosed by the Einstein Radius ($R_{rm E}=1farcs15pm0farcs05$) $M_{rm E}$$equiv$$M(<R_{rm E}) = 8.4pm0.7times 10^{10} h_{65}^{-1}$ M$_odot$. This corresponds to a Singular Isothermal Ellipsoid (SIE) velocity dispersion $sigma_{rm SIE}=214pm5$ kms, in good agreement with the measured stellar velocity dispersion $sigma=227pm18$ kms (Lehar et al. 1996). The mass-to-light ratio within the Einstein Radius ($sim$1.4 effective radii) is $10pm1 h_{65}$ mlu. This is only marginally larger than typical stellar mass-to-light ratios of local early-type galaxies, indicating that dark matter is not likely to be dominant inside the Einstein Radius.
We report an ultra-bright lensed submillimeter galaxy at $z_{spec}=1.4796$, identified as a result of a full-sky cross-correlation of the AllWISE and Planck compact source catalogs aimed to search for bright submillimeter galaxies at $z sim 1.5-2.8$. APEX/LABOCA observations of the candidate galaxy reveal a source with flux (S$_{870 mu m}= 54pm 8$ mJy). The position of the APEX source coincides with the position of the AllWISE mid-IR source, and with the Einstein ring GAL-CLUS-022058s, observed with the HST. Archival VLT/FORS observations reveal the redshift of this Einstein ring, $z_{spec}=1.4796$, and detection of the CO(5-4) line at $z_{spec} = 1.4802$ with APEX/nFLASH230 confirms the redshift of the submillimeter emission. The lensed source appears to be gravitationally magnified by a massive foreground galaxy cluster lens at $z = 0.36$. We use Lenstool to model the gravitational lensing, which is near to a fold arc configuration for an elliptical mass distribution of the central halo, where four images of the lensed galaxy are seen; the mean magnification is $mu_{rm L} =18pm 4$. We have determined an intrinsic rest-frame infrared luminosity of $L_{IR} approx 10^{12} L_odot $ and a likely star formation rate of $sim 70-170$ $M_odot yr^{-1}$. The molecular gas mass is $M_{mol} sim 2.6 times 10^{10} M_odot$ and the gas fraction is $f = 0.34pm 0.07$. We also obtain a stellar mass log$(M_ast/M_odot) = 10.7 pm 0.1$ and a specific star formation rate log$(sSFR/Gyr^{-1})=0.15 pm 0.03$. This galaxy lies on the so-called main sequence of star-forming galaxies at this redshift.
We report on three redshift $z>2$ quasars with dramatic changes in their C IV emission lines, the first sample of changing-look quasars (CLQs) at high redshift. This is also the first time the changing-look behaviour has been seen in a high-ionisation emission line. SDSS J1205+3422, J1638+2827, and J2228+2201 show interesting behaviour in their observed optical light curves, and subsequent spectroscopy shows significant changes in the C IV broad emission line, with both line collapse and emergence being displayed on rest-frame timescales of $sim$240-1640 days. These are rapid changes, especially when considering virial black hole mass estimates of $M_{rm BH} > 10^{9} M_{odot}$ for all three quasars. Continuum and emission line measurements from the three quasars show changes in the continuum-equivalent width plane with the CLQs seen to be on the edge of the full population distribution, and showing indications of an intrinsic Baldwin effect. We put these observations in context with recent state-change models, and note that even in their observed low-state, the C IV CLQs are generally above $sim$5% in Eddington luminosity.
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