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Abstract:

One of the most common consequences of extensions to the standard models of particle physics or cosmology is the emergence of a fifth force. While generic fifth forces are tightly constrained at Solar System scales and below, they may escape detection by means of a screening mechanism which effectively removes them in dense environments. We constrain the strength ΔG/GN and range λC of a fifth force with Yukawa coupling arising from a chameleon- or symmetron-screened scalar field—as well as an unscreened fifth force with differential coupling to galactic mass components—by searching for the displacement it predicts between galaxies’ stellar and gas mass centroids. Taking data from the Alfalfa survey of neutral atomic hydrogen (HI), identifying galaxies’ gravitational environments with the maps of [H. Desmond, P. G. Ferreira, G. Lavaux, and J. Jasche, Mon. Not. R. Astron. Soc. 474, 3152 (2018)] and forward modeling with a Bayesian likelihood framework, we find, with screening included, 6.6σ evidence for ΔG>0 at λC≃2Mpc. The maximum-likelihood ΔG/GN is 0.025. A similar fifth force model without screening gives no increase in likelihood over the case ΔG=0 for any λC. Although we validate this result by several methods, we do not claim screened modified gravity to provide the only possible explanation for the data: this conclusion would require knowing that the signal could not be produced by “galaxy formation” physics. We show also the results of a more conservative—though less well-motivated—noise model which yields only upper limits on ΔG/GN, ranging from ∼10−1 for λC ≃ 0.5 Mpc to ∼ few ×10−4 at λC ≃ 50 Mpc. Corresponding models without screening receive the somewhat stronger bounds ∼ few ×10−3 and ∼ few ×104 respectively. We show how these constraints may be improved by future galaxy surveys and identify the key features of an observational program for directly constraining fifth forces on scales beyond the Solar System. This paper provides a complete description of the analysis summarized in [H. Desmond, P. G. Ferreira, G. Lavaux, and J. Jasche, arXiv:1802.07206].

Abstract:

We present Atacama Large Millimeter/Submillimeter Array observations of the rest-frame far-infrared (FIR) dust continuum emission of six bright Lyman-break galaxies (LBGs) at z ≃ 7. One LBG is detected (5.2σ at peak emission), whilst the others remain individually undetected at the 3σ level. The average FIR luminosity of the sample is found to be LFIR≃2×1011L⊙⁠, corresponding to an obscured star formation rate (SFR) that is comparable to that inferred from the unobscured UV emission. In comparison to the infrared excess (IRX=LFIR/LUV⁠)–β relation, our results are consistent with a Calzetti-like attenuation law (assuming a dust temperature of T = 40–50 K). We find a physical offset of 3kpc between the dust continuum emission and the rest-frame UV light probed by Hubble Space Telescope imaging for galaxy ID65666 at z=7.17+0.09−0.06⁠. The offset is suggestive of an inhomogeneous dust distribution, where 75 per cent of the total star formation activity (SFR≃70M⊙/yr⁠) of the galaxy is completely obscured. Our results provide direct evidence that dust obscuration plays a key role in shaping the bright end of the observed rest-frame UV luminosity function at z ≃ 7, in agreement with cosmological galaxy formation simulations. The existence of a heavily obscured component of galaxy ID65666 indicates that dusty star-forming regions, or even entire galaxies, that are ‘UV dark’ are significant even in the z ≃ 7 galaxy population.