The Largest Gravitational Lens: MACS J0717.5+3745 (z=0.546)

The Astrophysical Journal (Impact Factor: 6.28). 07/2009; DOI: 10.1088/0004-637X/707/1/L102
Source: arXiv

ABSTRACT We identify 13 sets of multiply-lensed galaxies around MACS J0717.5+3745
($z=0.546$), outlining a very large tangential critical curve of major axis
$\sim2.8\arcmin$, filling the field of HST/ACS. The equivalent circular
Einstein radius is $\theta_{e}= 55 \pm 3\arcsec$ (at an estimated source
redshift of $z_{s}\sim2.5$), corresponding to $r_e\simeq 350\pm 20 kpc$ at the
cluster redshift, nearly three times greater than that of A1689 ($r_e\simeq 140
kpc$ for $z_{s}=2.5$). The mass enclosed by this critical curve is very large,
$7.4\pm 0.5 \times 10^{14}M_{\odot}$ and only weakly model dependent, with a
relatively shallow mass profile within $r<250 kpc$, reflecting the unrelaxed
appearance of this cluster. This shallow profile generates a much higher level
of magnification than the well known relaxed lensing clusters of higher
concentration, so that the area of sky exceeding a magnification of
$>10\times$, is $\simeq 3.5\sq\arcmin$ for sources with $z\simeq 8$, making
MACS J0717.5+3745 a compelling target for accessing faint objects at high
redshift. We calculate that only one such cluster, with $\theta_{e}\ge
55\arcsec$, is predicted within $\sim 10^7$ Universes with $z\ge 0.55$,
corresponding to a virial mass $\ge 3\times 10^{15} M_{\odot}$, for the
standard $\Lambda CDM$ (WMAP5 parameters with $2\sigma$ uncertainties).

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    ABSTRACT: The \emph{Grism Lens-Amplified Survey from Space} (GLASS) is a Hubble Space Telescope (\HST) Large Program, which will obtain 140 orbits of grism spectroscopy of the core and infall regions of 10 galaxy clusters, selected to be among the very best cosmic telescopes. Extensive \HST\ imaging is available from many sources including the CLASH and Frontier Field programs. We introduce the survey by analyzing spectra of faint multiply-imaged galaxies and $z\gtrsim6$ galaxy candidates obtained from the first seven orbits targeting the core of the Frontier Field cluster MACS0717.5+3745. Using the G102 and G141 grisms to cover the wavelength range 0.8--1.7$\mu$m, we confirm 4 strongly lensed systems by detecting emission lines in each of the images. For the 9 $z\gtrsim6$ galaxy candidates clear from contamination, we do not detect any emission line down to a 1-$\sigma$ noise level of $\sim$5$\times$10$^{-18}$\cgs. Taking lensing magnification into account, our flux sensitivity reaches $\sim$0.2--5$\times$10$^{-18}$\cgs. These limits over an uninterrupted wavelength range rule out the possibility that the high-$z$ galaxy candidates are instead strong line emitters at lower redshift. These results show that by means of careful modeling of the background--- and with the assistance of lensing magnification---interesting flux limits can be reached for large numbers of objects, avoiding pre-selection and the wavelength restrictions inherent to ground-based multi-slit spectroscopy. These observations confirm the power of slitless \HST\ spectroscopy even in fields as crowded as a cluster core.
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    ABSTRACT: We report the LBT/MODS1 spectroscopic confirmation of two images of faint Lyman alpha emitters at z=6.4 behind the Frontier Fields galaxy cluster MACSJ0717.5+3745. A wide range of lens models suggests that the two images are highly magnified, with a strong lower limit of mu>5. These are the faintest z>6 candidates spectroscopically confirmed to date. These may be also multiple images of the same z=6.4 source as supported by their similar intrinsic properties, but the lens models are inconclusive regarding this interpretation. To be cautious, we derive the physical properties of each image individually. Thanks to the high magnification, the observed near-infrared (restframe ultraviolet) part of the spectral energy distributions and Lya lines are well detected with S/N(m_1500)>~10 and S/N(Lya)~10-15. Adopting mu > 5, the absolute magnitudes, M_1500, and Lya fluxes, are fainter than -18.7 and 2.8x10^-18 erg/s/cm2, respectively. We find a very steep ultraviolet spectral slope beta=-3.0+/-0.5 (F_lambda=lambda^beta), implying that these are very young, dust-free and low metallicity objects, made of standard stellar populations or even extremely metal poor stars (age <~ 30 Myr, E(B-V)=0 and metallicity 0.0 - 0.2 Z/Z_solar). The objects are compact (< 1 kpc^2), and with a stellar mass M* < 10^8 M_solar. The presence of the Lya line and the intrinsic FWHM (<300 km/s) of these newborn objects do not exclude a possible leakage of ionizing radiation. We discuss the possibility that such faint galaxies may resemble those responsible for cosmic reionization.


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