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| NASA’s James Webb Space Telescope’s NIRCam image of the galaxy cluster PLCK G165.7+67.0 reveals the magnifying effect of the foreground cluster, located 3.6 billion light-years away. The zoomed-in section shows the triply imaged supernova H0pe, highlighted with white dashed circles, due to gravitational lensing. Color coding indicates different wavelengths: blue (0.9, 1.15, 1.5 microns), green (2.0, 2.77 microns), and red (3.56, 4.1, 4.44 microns). Full-resolution images are available for download from the Space Telescope Science Institute. |
Measuring the Hubble constant, which quantifies the rate of the universe’s expansion, is a crucial focus for astronomers globally. Using data from both ground and space observatories, including NASA’s James Webb Space Telescope, researchers are advancing this effort. Recently, astronomers analyzed Webb data featuring Cepheid variables and Type Ia supernovae—key indicators of distance—to validate previous measurements from the Hubble Space Telescope.
Now, a team led by Brenda Frye from the University of Arizona is exploring a novel approach: gravitationally lensed supernovae. After Webb captured three unexpected points of light in a densely populated galaxy cluster, the team, intrigued by their potential as supernovae, launched the initiative dubbed Supernova H0pe. The unexpected visibility of these points in Webb’s images, absent in earlier Hubble observations, prompted their investigation. The team had selected this particular field for its high star formation rate—over 300 solar masses per year—which suggests an increased likelihood of supernova occurrences. Dr. Frye is set to share more about how this gravitational lensing is shedding light on the Hubble constant.
“Initial analyses confirmed that these points corresponded to an exploding star with unique characteristics. First, it’s identified as a Type Ia supernova, the explosion of a white dwarf, known as a ‘standard candle’ due to its predictable intrinsic brightness. Additionally, this supernova is gravitationally lensed.
“Gravitational lensing plays a crucial role in this study. A cluster of galaxies situated between the supernova and Earth bends the light, creating multiple images of the supernova, akin to how a trifold vanity mirror reflects different angles of a person. In the Webb image, this effect was strikingly evident, as the middle image appeared flipped relative to the other two—a phenomenon consistent with theoretical predictions.
“To generate these three images, light from the supernova traveled along three distinct paths. Each path, having different lengths, resulted in the supernova being observed at three separate times during its explosion. Using the mirror analogy, this time-delay means that one image shows a person lifting a comb, another depicts hair being combed, and the middle image reveals the person setting the comb down.”
“Trifold supernova images are significant because the time delays, supernova distance, and gravitational lensing properties can be used to calculate the Hubble constant, or H0 (pronounced H-naught). The supernova was named SN H0pe, reflecting the optimism it brings to astronomers striving to better understand the universe’s expanding rate.
“To investigate SN H0pe further, the PEARLS-Clusters team submitted a Webb Director’s Discretionary Time (DDT) proposal, which underwent a dual-anonymous review by scientific experts and was subsequently recommended for observations by the Webb Science Policies Group. Simultaneously, data were collected from the MMT, a 6.5-meter telescope on Mt. Hopkins, and the Large Binocular Telescope on Mt. Graham, both located in Arizona. Through the analysis of these observations, the team confirmed that SN H0pe is linked to a background galaxy that existed 3.5 billion years after the Big Bang, situated well behind the cluster.
SN H0pe stands as one of the most distant Type Ia supernovae observed to date. Another team member conducted a time delay measurement by examining the evolution of its light spectrum from Webb, further confirming its classification as a Type Ia supernova.
Seven subgroups contributed lens models that describe the 2D matter distribution of the galaxy cluster. Given that the Type Ia supernova is a standard candle, each lens model was evaluated based on its accuracy in predicting the time delays and brightness of the supernova relative to the actual measured values.
To eliminate biases, the results from these independent groups were kept blinded until a designated ‘live unblinding’ event, where findings were revealed simultaneously. The team reported the Hubble constant value as 75.4 kilometers per second per megaparsec, with an uncertainty of plus 8.1 or minus 5.5. (One parsec equals approximately 3.26 light-years.) This marks only the second measurement of the Hubble constant using this method, and the first involving a standard candle. The lead investigator of the PEARLS program stated, ‘This is one of the great Webb discoveries, leading to a better understanding of this fundamental parameter of our universe.’
Our results are significant: the measured Hubble constant aligns with other measurements in the local universe but shows some tension with values derived from the early universe. Upcoming Webb observations in Cycle 3 aim to refine these uncertainties, providing more precise constraints on H0.