Title: UV Luminosity Functions from HST and JWST: A Possible Resolution to the High-Redshift Galaxy Abundance Puzzle and Implications for Cosmic Strings
Authors: Mattéo Blamart, Adrian Liu, Robert Brandenberger, Julian B. Muñoz, and Bryce Cyr
First Writer’s Institute: Division of Physics, McGill College, and Trottier Area Institute, Montreal, Canada
Standing: Revealed in Bodily Assessment D [closed access, preprint available on arXiv]
Not lengthy after the James Webb Space Telescope (JWST) opened its eyes, it began discovering galaxies that shouldn’t be there. These galaxies weren’t inconceivable in response to our fashions. However at redshifts above 10, when the universe was lower than half a billion years previous, JWST saved discovering vivid galaxies in far higher numbers than astronomers had predicted. Three years later, that surplus continues to be unexplained, and the record of proposed fixes has grown lengthy.
- Did early galaxies convert gasoline into stars much more effectively?
- Did their brightness fluctuate wildly?
- Did they kind unusually huge stars?
- Was there much less mud in the best way?
At this time’s paper proposes one thing very completely different. What if the additional galaxies had been seeded by cosmic strings, defects in spacetime left over from a section transition within the universe’s first fraction of a second?
The observable on the coronary heart of that is the UV luminosity function, or UVLF, which is just a rely: what number of galaxies are there at every brightness, at every time frame? It’s probably the most direct measurements we’ve of when and the way galaxies assembled. The Hubble Space Telescope (HST) mapped the UVLF out to about redshift 10. JWST has pushed that boundary to redshift 17 and on the vivid finish, on the highest redshifts, it finds extra galaxies than ΛCDM plus commonplace galaxy-formation physics comfortably accommodates.
Right here’s the half that makes this puzzle genuinely exhausting, and it’s the center of as we speak’s paper. Any cosmological rationalization, something that modifications the universe’s substances quite than the astrophysics of star formation, has to string a really slim needle. Increase the expansion of construction sufficient to supply JWST’s galaxies at redshift 17, and you’ll usually additionally increase it at redshift 6, the place Hubble already measured the UVLF and located nothing uncommon. Earlier work has proven precisely this: improve the matter power spectrum sufficient to match JWST, and also you break settlement with HST. The repair has to work early after which get out of the best way.
Strings that fade
That is the place cosmic strings change into fascinating. Cosmic strings are one-dimensional topological defects. You may consider them as cracks frozen into the material of spacetime when the early universe cooled by means of a symmetry-breaking phase transition. They’re a generic prediction of many Grand Unified Theories. A community of them would consist principally of closed loops, every characterised by its vitality per unit size. That vitality is often quoted because the dimensionless string pressure, Gμ, and it units how strongly the strings gravitate. Up to now, the tightest limits have come from the cosmic microwave background, which constrains Gμ to under roughly 10⁻⁷.
What makes strings a pure candidate right here is the form of their affect over cosmic time. A string loop can pull matter onto itself and seed a dark matter halo at primarily any epoch, together with absurdly early ones. In commonplace ΛCDM, in contrast, huge halos at very excessive redshift are exponentially suppressed, the universe merely hasn’t had time to construct them. So strings make their greatest relative distinction precisely the place JWST sees the excess. And as cosmic time goes on and unusual construction formation catches up, the string contribution turns into a smaller and smaller fraction of the whole, fading into irrelevance by the redshifts that Hubble probes. Early assist, late silence. That’s exactly the form the information calls for.
To truly take a look at this, the authors wanted to foretell UVLFs throughout a variety of each astrophysical and cosmological assumptions, which is generally the province of hydrodynamical simulations that take far too lengthy to run 1000’s of instances. As a substitute they constructed cosmic strings into Zeus21, a semi-analytic code that may generate a predicted UVLF in milliseconds. That velocity is what lets them do the important factor: discover the degeneracies and map out how a lot of the noticed surplus may very well be defined by strings versus by extra environment friendly star formation, and whether or not the 2 will be instructed aside.
Their reply is that cosmic strings can account for the measured UVLFs from redshift 4 all the best way to 17 without having an abrupt bounce in star-formation effectivity or excessive stochasticity the substances most astrophysical explanations lean on. On the redshifts Hubble probes, including strings modifications nearly nothing, and the information are match completely effectively with out them. On the redshifts that JWST probes, the 2 circumstances pull aside, and you may watch it occur throughout the panels of Determine 1. At redshift 9, the fashions with and with out strings are practically indistinguishable. By redshift 12 and past, the string-free mannequin falls in need of the noticed abundance of vivid galaxies by orders of magnitude, whereas the mannequin together with strings continues to trace the information. The impact grows with redshift precisely because the argument requires.
And there’s a bonus. As a result of UVLFs grow to be so delicate to strings, the truth that no string signature is required at decrease redshift locations a brand new higher restrict on the string pressure: Gμ ≲ 10⁻⁸, roughly an element of ten higher than the earlier cosmic microwave background constraints.
The authors are cautious in regards to the end result. The restrict carries some dependence on the mannequin and priors they selected, and, as they level out, the selection of the best way to parameterize star formation is itself a form of prior. The dominant uncertainty isn’t the strings in any respect; it’s how poorly we presently know the star-formation effectivity of early galaxies, notably within the JWST samples. There’s additionally a wrinkle they deal with in an appendix, the place accounting for the velocities of string loops weakens the certain considerably, although it nonetheless improves on Planck satellite tv for pc’s observations of the cosmic microwave background.
That final level can also be the place the trail ahead lies. The degeneracy on the coronary heart of this. A lift in huge halos seems to be so much like extra environment friendly star formation. This is perhaps damaged by taking a look at how galaxies cluster. Galaxies born in probably the most huge halos cluster extra strongly than these in smaller ones. So if cosmic strings actually are seeding additional huge halos at early instances, they need to depart a particular imprint on galaxy clustering. Measurements of that clustering have gotten doable out to redshift 10 and past.
What’s fascinating about this result’s that it doesn’t require the additional galaxies to be unusual. In most astrophysical explanations, early galaxies need to behave in another way from later ones, burning by means of gasoline quicker, or flickering extra, or making completely different stars. On this image, galaxies behave the identical method all through; there are merely just a few extra locations for them to kind when the universe was very younger. Whether or not that’s what really occurred is a query for higher measurements of early star formation and galaxy clustering, and people are coming.
Astrobite edited by: Nicki Bond
Featured Picture Credit score: JADES/ESA/Webb. ESA/Webb, NASA, ESA, CSA, B. Robertson (UC Santa Cruz), B. Johnson (Middle for Astrophysics, Harvard & Smithsonian), S. Tacchella (College of Cambridge), M. Rieke (Univ. of Arizona), D. Eisenstein (Middle for Astrophysics, Harvard & Smithsonian), A. Pagan (STScI)
