16 research outputs found
Nebular dominated galaxies: insights into the stellar initial mass function at high redshift
We identify a low-metallicity (12 + log(O/H) = 7.59) Ly-emitting galaxy at = 5.943 with evidence of a strong Balmer
jump, arising from nebular continuum. While Balmer jumps are sometimes observed in low-redshift star-forming galaxies, this
galaxy also exhibits a steep turnover in the UV continuum. Such turnovers are typically attributed to absorption by a damped Ly
system (DLA); however, the shape of the turnover and the high observed Ly escape fraction ( esc,Ly ⌠27%) is also consistent
with strong nebular two-photon continuum emission. Modelling the UV turnover with a DLA requires extreme column densities
(HI > 1023 cmâ2
), and simultaneously explaining the high esc,Ly requires a fine-tuned geometry. In contrast, modelling the
spectrum as primarily nebular provides a good fit to both the continuum and emission lines, motivating scenarios in which (a)
we are observing only nebular emission or (b) the ionizing source is powering extreme nebular emission that outshines the stellar
emission. The nebular-only scenario could arise if the ionising source has âturned offâ more recently than the recombination
timescale (âŒ1,000 yr), hence we may be catching the object at a very specific time. Alternatively, hot stars with eff âł 105 K
(e.g. Wolf-Rayet or low-metallicity massive stars) produce enough ionizing photons such that the two-photon emission becomes
visible. While several stellar SEDs from the literature fit the observed spectrum well, the hot-star scenario requires that the
number of âł 50 Mâ stars relative to ⌠5 â 50 Mâ stars is significantly higher than predicted by typical stellar initial mass
functions (IMFs). The identification of more galaxies with similar spectra may provide evidence for a top-heavy IMF at high
redshift
3D intrinsic shapes of quiescent galaxies in observations and simulations
We study the intrinsic 3D shapes of quiescent galaxies o v er the last half of cosmic history based on their axial ratio distribution. To this end, we construct a sample of unprecedented size, e xploiting multiwav elength u -to- K s photometry from the deep wide- area surv e ys KiDS + VIKING paired with high-quality i -band imaging from HSC-SSP. The dependences of the shapes on mass, redshift, photometric bulge prominence and environment are considered. For comparison, the intrinsic shapes of quenched galaxies in the IllustrisTNG simulations are analysed and contrasted with their formation history. We find that o v er the full 0 < z < 0.9 range, and in both simulations and observations, spheroidal 3D shapes become more abundant at M*< 10 11 M â, with the effect being most pronounced at lower redshifts. In TNG, the most massive galaxies feature the highest ex situ stellar mass fractions, pointing to violent relaxation via mergers as the mechanism responsible for their 3D shape transformation. Larger differences between observed and simulated shapes are found at low to intermediate masses. At any mass, the most spheroidal quiescent galaxies in TNG feature the highest bulge mass fractions, and, conversely, observed quiescent galaxies with the highest bulge-to-total ratios are found to be intrinsically the roundest. Finally, we detect an environmental influence on galaxy shape, at least at the highest masses, such that at fixed mass and redshift, quiescent galaxies tend to be rounder in denser environments.</p
Novel Branches of (0,2) Theories
We show that recently proposed linear sigma models with torsion can be
obtained from unconventional branches of conventional gauge theories. This
observation puts models with log interactions on firm footing. If non-anomalous
multiplets are integrated out, the resulting low-energy theory involves log
interactions of neutral fields. For these cases, we find a sigma model geometry
which is both non-toric and includes brane sources. These are heterotic sigma
models with branes. Surprisingly, there are massive models with compact complex
non-Kahler target spaces, which include brane/anti-brane sources. The simplest
conformal models describe wrapped heterotic NS5-branes. We present examples of
both types.Comment: 36 pages, LaTeX, 2 figures; typo in Appendix fixed; references added
and additional minor change
Linear Sigma Models with Torsion
Gauged linear sigma models with (0,2) supersymmetry allow a larger choice of
couplings than models with (2,2) supersymmetry. We use this freedom to find a
fully linear construction of torsional heterotic compactifications, including
models with branes. As a non-compact example, we describe a family of metrics
which correspond to deformations of the heterotic conifold by turning on
H-flux. We then describe compact models which are gauge-invariant only at the
quantum level. Our construction gives a generalization of symplectic reduction.
The resulting spaces are non-Kahler analogues of familiar toric spaces like
complex projective space. Perturbatively conformal models can be constructed by
considering intersections.Comment: 40 pages, LaTeX, 1 figure; references added; a new section on
supersymmetry added; quantization condition revisite
Deciphering Lyman- Emission Deep into the Epoch of Reionisation
A major event in cosmic history is the genesis of the first starlight in our
Universe, ending the ''Dark Ages''. During this epoch, the earliest luminous
sources were enshrouded in neutral and pristine gas, which was gradually
ionised in a process called ''reionisation''. Hence, one of the brightest
emission lines in star-forming galaxies, Lyman- (Ly-), was
predicted to emerge only towards the end of the epoch of reionisation, about
one billion years after the Big Bang. However, this picture has been challenged
over the past decade by the surprising detection of Ly- in galaxies
less than 500 million years old. Here we show, by taking advantage of both
high-resolution and high-sensitivity images from the James Webb Space Telescope
programs PRIMER, CEERS and FRESCO, that all galaxies in our sample of
Ly- emitters deep in the epoch of reionisation have close companions.
To understand the physical processes that lead to the observed Ly-
emission in our sample, we take advantage of novel on-the-fly radiative
transfer magnetohydrodynamical simulations with cosmic ray feedback. We find
that in the early Universe, the rapid build up of mass through frequent
galactic mergers leads to very bursty star formation which in turn drives
episodes of high intrinsic Ly- emission and facilitates the escape of
Ly- photons along channels cleared of neutral gas. These merging
galaxies reside in clustered environments thus creating sufficiently large
ionised bubbles. This presents a solution to the long-standing puzzle of the
detection of Ly- emission deep into the epoch of reionisation.Comment: Submitted to Nature. 38 pages, 9 figures, 2 table
The ionising photon production efficiency at z~6 for a sample of bright Lyman-alpha emitters using JEMS and MUSE
We study the ionising photon production efficiency at the end of the Epoch of
Reionisation () for a sample of 35 bright Lyman-
emitters, this quantity is crucial to infer the ionising photon budget of the
Universe. These objects were selected to have reliable spectroscopic redshifts,
assigned based on the profile of their Lyman- emission line, detected
in the MUSE deep fields. We exploit medium-band observations from the JWST
extragalactic medium band survey (JEMS) to find the flux excess corresponding
to the redshifted \ha\ emission line. We estimate the UV luminosity by fitting
the full JEMS photometry, along with several HST photometric points, with
\texttt{Prospector}. We find a median ultra-violet continuum slope of for the sample, indicating young stellar populations
with little-to-no dust attenuation. Supported by this, we derive
with no dust attenuation and find a median value of
log. If we
perform dust attenuation corrections and assume a Calzetti attenuation law, our
values are lowered by dex. Our results suggest Lyman-
emitters at the Epoch of Reionisation have enhanced compared to
previous estimations from literature, in particular, when compared to the
non-Lyman- emitting population. This initial study provides a promising
outlook on the characterisation of ionising photon production in the early
Universe. In the future, a more extensive study will be performed on the entire
dataset provided by the JWST Advanced Deep Extragalactic Survey (JADES). Thus,
for the first time, allowing us toComment: 11 pages, 5 figures in main paper. 10 pages, 30 figures in appendix.
Submitted to MNRA
JWST-JADES. Possible Population III signatures at z=10.6 in the halo of GN-z11
Finding the first generation of stars formed out of pristine gas in the early
Universe, known as Population III (PopIII) stars, is one of the most important
goals of modern astrophysics. Recent models suggest that PopIII stars may form
in pockets of pristine gas in the halo of more evolved galaxies. Here we
present NIRSpec-IFU and NIRSpec-MSA observations of the region around GN-z11,
an exceptionally luminous galaxy at , which reveal a 5
detection of a feature consistent with being HeII1640 emission at the
redshift of GN-z11. The very high equivalent width of the putative HeII
emission in this clump (170 A), and the lack of metal lines, can be explained
in terms of photoionisation by PopIII stars, while photoionisation by PopII
stars is inconsistent with the data. It would also indicate that the putative
PopIII stars likely have a top-heavy initial mass function (IMF), with an upper
cutoff reaching at least 500 M. The PopIII bolometric luminosity
inferred from the HeII line would be , which
(with a top-heavy IMF) would imply a total stellar mass formed in the burst of
. We find that photoionisation by the Active
Galactic Nucleus (AGN) in GN-z11 cannot account for the HeII luminosity
observed in the clump, but can potentially be responsible for additional HeII
emission observed closer to GN-z11. We also consider the possibility of in-situ
photoionisation by an accreting Direct Collapse Black Hole (DCBH) hosted by the
HeII clump; we find that this scenario is less favoured, but it remains a
possible alternative interpretation. We also report the detection of a
Ly halo stemming out of GN-z11 and extending out to 2 kpc, as
well as resolved, funnel-shaped CIII] emission, likely tracing the ionisation
cone of the AGN.Comment: Submitted to A&A, 13 pages, 8 figures; some typos corrected and some
minor additional information added to match submitted versio
Evidence for a Low Lyman Continuum Escape Fraction in Three Massive, Ultraviolet-bright Galaxies at z > 7
Although low-mass star-forming galaxies are the leading candidates of the reionization process, we cannot conclusively rule out high-mass star-forming galaxies as candidates. While most simulations indicate the former is the best candidate, some models suggest that at z â„ 6 massive, UV-bright galaxies â âoligarchsâ â account for at least 80% of the ionizing budget. To test this hypothesis, we target massive ( ), UV-bright ( M _UV ⌠â22) Ly α emitters at z > 7 in archival data, observed with similar resolution spectrographs (Very Large Telescope/X-shooter and Keck/MOSFIRE). To increase the reliability of our conclusions, we stack all spectra and obtain a deep-stacked spectrum of 24.75 hr. The stacked Ly α profile displays a clear asymmetric red peak and an absence of a blue peak. We additionally estimate the intrinsic stacked Ly α profile of our targets by correcting for intergalactic medium (IGM) transmission using a range of neutral hydrogen fractions, finding no significant change in the profile. We measure a velocity offset V _red > 300 km s ^â1 and an asymmetry in our red peak A ⌠3. Using various models and estimators, such as the peak separation, the asymmetry of the red peak, the ratio between Ly α and H ÎČ, and the ÎČ slope, we conclude that the escape fraction in these three UV-bright, massive (âŒ10 ^10 M _â ), z â„ 7 galaxies is f _esc (LyC) †10%