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Register a new userTwo Peculiar Fast Transients in a Strongly Lensed Host Galaxy
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A massive galaxy cluster can serve as a magnifying glass for distant stellar populations, with strong gravitational lensing exposing details in the lensed background galaxies that would otherwise be undetectable. The MACS J0416.1-2403 cluster (hereafter MACS0416) is one of the most efficient lenses in the sky, and in 2014 it was observed with high-cadence imaging from the Hubble Space Telescope (HST). Here we describe two unusual transient events that appeared behind MACS0416 in a strongly lensed galaxy at redshift $z = 1.0054 pm 0.0002$. These transients---designated HFF14Spo-NW and HFF14Spo-SE and collectively nicknamed Spock---were faster and fainter than any supernova (SN), but significantly more luminous than a classical nova. They reached peak luminosities of $sim10^{41}$ erg s$^{-1}$ ($M_{rm AB} < -14$ mag) in 5 rest-frame days, then faded below detectability in roughly the same time span. Models of the cluster lens suggest that these events may be spatially coincident at the source plane, but are most likely not temporally coincident. We find that HFF14Spo can be explained as a luminous blue variable (LBV), a recurrent nova (RN), or a pair of stellar microlensing events. To distinguish between these hypotheses will require a clarification of the positions of nearby critical curves, along with high-cadence monitoring of the field that could detect new transient episodes in the host galaxy.
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We report the detection of extended Lyman-$alpha$ emission from the host galaxy of SDSS~J2222+2745, a strongly lensed quasar at $z = 2.8$. Spectroscopic follow-up clearly reveals extended Lyman-$alpha$ in emission between two images of the central active galactic nucleus (AGN). We reconstruct the lensed quasar host galaxy in the source plane by applying a strong lens model to HST imaging, and resolve spatial scales as small as $sim$200 parsecs. In the source plane we recover the host galaxy morphology to within a few hundred parsecs of the central AGN, and map the extended Lyman-$alpha$ emission to its physical origin on one side of the host galaxy at radii $sim$0.5-2 kpc from the central AGN. There are clear morphological differences between the Lyman-$alpha$ and rest-frame ultraviolet stellar continuum emission from the quasar host galaxy. Furthermore, the relative velocity profiles of quasar Lyman-$alpha$, host galaxy Lyman-$alpha$, and metal lines in outflowing gas reveal differences in the absorbing material affecting the AGN and host galaxy. These data indicate the presence of patchy local intervening gas in front of the central quasar and its host galaxy. This interpretation is consistent with the central luminous quasar being obscured across a substantial fraction of its surrounding solid angle, resulting in strong anisotropy in the exposure of the host galaxy to ionizing radiation from the AGN. This work demonstrates the power of strong lensing-assisted studies to probe spatial scales that are currently inaccessible by other means.
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