We present recent JWST NIRCam imaging observations of SPT0615-JD (also known as the Cosmic Gems Arc), lensed by the galaxy cluster SPT-CL J0615-5746. The 5-arcsec-long arc is the most highly ...magnified \(z>10\) galaxy known, straddling the lensing critical curve and revealing five star clusters with radii \(\sim 1\) pc or less. We measure the full arc to have F200W 24.5 AB mag, consisting of two mirror images, each 25.3 AB mag with a magnification \(\mu \sim 60\) (delensed 29.7 AB mag, \(M_{UV} = -17.8\)). The galaxy has an extremely strong Lyman break F115W\(-\)F200W \(>3.2\) mag (\(2\sigma\) lower limit), is undetected in all bluer filters (\(< 2\sigma\)), and has a very blue continuum slope redward of the break (\(\beta = -2.7 \pm 0.1\)), resulting in a photometric redshift \(z_{phot} = 10.2 \pm 0.2\) (95% confidence) with no significant likelihood below \(z < 9.8\). Based on SED fitting to the total photometry, we estimate an intrinsic stellar mass of \(M_{*} \sim 2.4 - 5.6 \times 10^{7} M_{\odot}\), young mass-weighted age of \(\sim 21 - 79\) Myr, low dust content (\(A_V < 0.15\)), and a low metallicity of \(\lesssim 1\%~Z_{\odot}\). We identify a fainter third counterimage candidate within 2.2 arcsec of the predicted position, lensed to AB mag 28.4 and magnified by \(\mu \sim 2\), suggesting the fold arc may only show \(\sim60\)% of the galaxy. SPT0615-JD is a unique laboratory to study star clusters observed within a galaxy just 460 Myr after the Big Bang.
We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of \(z_{\mathrm{phot}}\simeq4.8\), previously detected in JWST/NIRCam imaging of the strong lensing ...(SL) cluster MACS J0647+7015 (\(z=0.591\)). The spectroscopic observation allows us to precisely measure the redshift of the host arc at \(z_{\mathrm{spec}}=4.758\pm0.004\), and the star's spectrum displays clear Lyman- and Balmer-breaks commensurate with this redshift. A fit to the spectrum suggests a B-type super-giant star of surface temperature \(T_{\mathrm{eff,B}}\simeq15000\) K with either a redder F-type companion (\(T_{\mathrm{eff,F}}\simeq6250\)K) or significant dust attenuation (\(A_V\simeq0.82\)) along the line of sight. We also investigate the possibility that this object is a magnified young globular cluster rather than a single star. We show that the spectrum is in principle consistent with a star cluster, which could also accommodate the lack of flux variability between the two epochs. However, the lack of a counter image and the strong upper limit on the size of the object from lensing symmetry, \(r\lesssim0.5\) pc, could indicate that this scenario is somewhat less likely -- albeit not completely ruled out by the current data. The presented spectrum seen at a time when the Universe was only \(\sim1.2\) Gyr old showcases the ability of JWST to study early stars through extreme lensing.
ABSTRACT
We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of zphot ≃ 4.8, previously detected in JWST/NIRCam imaging of the strong lensing (SL) ...cluster MACS J0647+7015 (z = 0.591). The spectroscopic observation allows us to precisely measure the redshift of the host arc at zspec = 4.758 ± 0.004, and the star’s spectrum displays clear Lyman- and Balmer-breaks commensurate with this redshift. A fit to the spectrum suggests a B-type super-giant star of surface temperature $T_{\mathrm{eff,B}}\simeq 15\, 000$ K with either a redder F-type companion ($T_{\mathrm{eff,F}}\simeq 6\, 250$ K) or significant dust attenuation (AV ≃ 0.82) along the line of sight. We also investigate the possibility that this object is a magnified young globular cluster rather than a single star. We show that the spectrum is in principle consistent with a star cluster, which could also accommodate the lack of flux variability between the two epochs. However, the lack of a counter image and the strong upper limit on the size of the object from lensing symmetry, r ≲ 0.5 pc, could indicate that this scenario is somewhat less likely – albeit not completely ruled out by the current data. The presented spectrum seen at a time when the Universe was only ∼1.2 Gyr old showcases the ability of JWST to study early stars through extreme lensing.
The gravitationally lensed star WHL0137-LS, nicknamed Earendel, was
identified with a photometric redshift $z_{phot} = 6.2 \pm 0.1$ based on images
taken with the Hubble Space Telescope. Here we ...present James Webb Space
Telescope (JWST) Near Infrared Camera (NIRCam) images of Earendel in 8 filters
spanning 0.8--5.0$\mu$m. In these higher resolution images, Earendel remains a
single unresolved point source on the lensing critical curve, increasing the
lower limit on the lensing magnification to $\mu > 4000$ and restricting the
source plane radius further to $r < 0.02$ pc, or $\sim 4000$ AU. These new
observations strengthen the conclusion that Earendel is best explained by an
individual star or multiple star system, and support the previous photometric
redshift estimate. Fitting grids of stellar spectra to our photometry yields a
stellar temperature of $T_{\mathrm{eff}} \simeq 13000$--16000 K assuming the
light is dominated by a single star. The delensed bolometric luminosity in this
case ranges from $\log(L) = 5.8$--6.6 $L_{\odot}$, which is in the range where
one expects luminous blue variable stars. Follow-up observations, including
JWST NIRSpec scheduled for late 2022, are needed to further unravel the nature
of this object, which presents a unique opportunity to study massive stars in
the first billion years of the universe.
The gravitationally lensed star WHL0137-LS, nicknamed Earendel, was identified with a photometric redshift \(z_{phot} = 6.2 \pm 0.1\) based on images taken with the Hubble Space Telescope. Here we ...present James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) images of Earendel in 8 filters spanning 0.8--5.0\(\mu\)m. In these higher resolution images, Earendel remains a single unresolved point source on the lensing critical curve, increasing the lower limit on the lensing magnification to \(\mu > 4000\) and restricting the source plane radius further to \(r < 0.02\) pc, or \(\sim 4000\) AU. These new observations strengthen the conclusion that Earendel is best explained by an individual star or multiple star system, and support the previous photometric redshift estimate. Fitting grids of stellar spectra to our photometry yields a stellar temperature of \(T_{\mathrm{eff}} \simeq 13000\)--16000 K assuming the light is dominated by a single star. The delensed bolometric luminosity in this case ranges from \(\log(L) = 5.8\)--6.6 \(L_{\odot}\), which is in the range where one expects luminous blue variable stars. Follow-up observations, including JWST NIRSpec scheduled for late 2022, are needed to further unravel the nature of this object, which presents a unique opportunity to study massive stars in the first billion years of the universe.
