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Cold clouds in the hot intracluster medium

Abstract not available

• Speaker: Ricarda Beckmann (University of Cambridge)
• Friday 23 September 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Ricarda Beckmann.

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The fuzzy dark matter (FDM) scenario has received increased attention in recent years due to the small-scale challenges of the vanilla Lambda cold dark matter ($\Lambda$CDM) cosmological model and the lack of robust experimental evidence for any constituent particle. In this study, we use cosmological $N$-body simulations to investigate the high-redshift cosmic web and its responsiveness to an FDM-like power spectrum cutoff in the primordial density perturbations by looking at three distinct properties of virialised FDM dark matter halos as a function of the particle mass $m$. First, compared to $\Lambda$CDM the concentrations of their mass density profiles are lower, peaking at an $m$-dependent halo mass and thus breaking the approximate universality of density profiles in $\Lambda$CDM even further. The halo profiles of the intermediate-to-major and minor-to-major shape parameters are monotonically increasing with ellipsoidal radius in $N$-body simulations of $\Lambda$CDM, yet become non-monotonic owing to baryonic physics at lower redshifts and an FDM-like power spectrum cutoff at higher redshifts. Finally, intrinsic alignment correlations, stemming from the deformation of initially spherically collapsing halos in an ambient gravitational tidal field, become stronger with decreasing FDM particle mass. At $z\sim 4$, we find a $6.4 \sigma$-significance in the fractional differences between the inferred isotropised linear alignment magnitudes $D_{\text{iso}}$ in $\Lambda$CDM and the rather extreme $m=10^{-22}$ eV FDM model. Such FDM-like imprints on the internal properties of virialised halos are strikingly visible in the pristine high-$z$ cosmic web whose evolution is governed largely by linear structure formation physics.

Cold clouds in the hot intracluster medium

Abstract not available

• Speaker: Ricarda Beckmann (University of Cambridge)
• Friday 19 August 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Ricarda Beckmann.

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The phase transition between galaxies and quasars is often identified with the rare population of hyper-luminous, hot dust-obscured galaxies. Galaxy formation models predict these systems to grow via mergers, that can deliver large amounts of gas toward their centers, induce intense bursts of star formation and feed their supermassive black holes. Here we report the detection of 24 galaxies emitting Lyman-alpha emission on projected physical scales of about 400 kpc around the hyper-luminous hot dust-obscured galaxy W0410-0913, at redshift z = 3.631, using Very Large Telescope observations. While this indicates that W0410-0913 evolves in a very dense environment, we do not find clear signs of mergers that could sustain its growth. Data suggest that if mergers occurred, as models expect, these would involve less massive satellites, with only a moderate impact on the internal interstellar medium of W0410-0913, which is sustained by a rotationally-supported fast-rotating molecular disk, as Atacama Large Millimeter Array observations suggest.

We present the first detailed study comparing the populations of stellar streams in cosmological simulations to observed Milky Way dwarf galaxy streams. In particular, we compare streams identified around Milky Way analogs in the FIRE-2 simulations to stellar streams observed by the Southern Stellar Stream Spectroscopic Survey (S5). For an accurate comparison between the stream populations, we produce mock Dark Energy Survey (DES) observations of the FIRE streams and estimate the detectability of their tidal tails and progenitors. The number and stellar mass distributions of detectable stellar streams is consistent between observations and simulations. However, there are discrepancies in the distributions of pericenters and apocenters, with the detectable FIRE streams, on average, forming at larger pericenters (out to > 110 kpc) and surviving only at larger apocenters (> 40 kpc) than those observed in the Milky Way. We find that the population of high-stellar mass dwarf galaxy streams in the Milky Way is incomplete. Interestingly, a large fraction of the FIRE streams would only be detected as satellites in DES-like observations, since their tidal tails are too low-surface brightness to be detectable. We thus predict a population of yet-undetected tidal tails around Milky Way satellites, as well as a population of fully undetected low surface brightness stellar streams, and estimate their detectability with the Rubin Observatory. Finally, we discuss the causes and implications of the discrepancies between the stream populations in FIRE and the Milky Way, and explore future avenues for tests of satellite disruption in cosmological simulations.

We analyse the chemical properties of three z~8 galaxies behind the galaxy cluster SMACS J0723.3-7327, observed as part of the Early Release Observations programme of the James Webb Space Telescope (JWST). Exploiting [O III]4363 auroral line detections in NIRSpec spectra, we robustly apply the direct Te method for the very first time at such high redshift, measuring metallicities ranging from extremely metal poor (12+log(O/H)~7) to about one-third solar. We also discuss the excitation properties of these sources, and compare them with local strong-line metallicity calibrations. We find that none of the considered diagnostics match simultaneously the observed relations between metallicity and strong-line ratios for the three sources, implying that a proper re-assessment of the calibrations may be needed at these redshifts. On the mass-metallicity plane, the two galaxies at z~7.6 (log(M*/M_sun) = 8.1, 8.7) have metallicities that are consistent with the extrapolation of the mass-metallicity relation at z~2-3, while the least massive galaxy at z~8.5 (log(M*/M_sun) = 7.8) shows instead a significantly lower metallicity . The three galaxies show different level of offset relative to the Fundamental Metallicity Relation, with two of them (at z~7.6) being marginally consistent, while the z~8.5 source deviating significantly, being probably far from the smooth equilibrium between gas flows, star formation and metal enrichment in place at later epochs.

We present a detailed analysis of the rest-frame optical emission line ratios for three spectroscopically confirmed galaxies at $z>7.5$. The galaxies were identified in the \emph{James Webb Space Telescope} (\emph{JWST}) Early Release Observations field SMACS J0723.3$-$7327. By quantitatively comparing Balmer and oxygen line ratios of these galaxies with various low-redshift "analogue" populations (e.g. Green Peas, Blueberries, etc.), we show that no single analogue population captures the diversity of line ratios of all three galaxies observed at $z>7.5$. We find that S06355 at $z=7.67$ and S10612 at $z=7.66$ are similar to local Green Peas and Blueberries. In contrast, S04590 at $z=8.50$ appears to be significantly different from the other two galaxies, most resembling extremely low-metallicity systems in the local Universe. Perhaps the most striking spectral feature in S04590 is the curiously high [O {\small III}]\,$\lambda4363$/[O {\small III}]\,$\lambda5007$ ratio (RO3) of $0.047$ (or $0.059$ when dust-corrected), implying either extremely high electron temperatures, $>3\times10^4$~K, or gas densities $>10^4\ {\rm cm^{-3}}$. Observed line ratios indicate that this galaxy is unlikely to host an AGN. Using photoionization modelling, we show that the inclusion of high-mass X-ray binaries or a high cosmic ray background in addition to a young, low-metallicity stellar population can provide the additional heating necessary to explain the observed high RO3 while remaining consistent with other observed line ratios. Our models represent a first step at accurately characterising the dominant sources of photoionization and heating at very high redshifts, demonstrating that non-thermal processes may become important as we probe deeper into the Epoch of Reionization.

We present the Sherwood-Relics simulations, a new suite of large cosmological hydrodynamical simulations aimed at modelling the intergalactic medium (IGM) during and after the cosmic reionization of hydrogen. The suite consists of over 200 simulations that cover a wide range of astrophysical and cosmological parameters. It also includes simulations that use a new lightweight hybrid scheme for treating radiative transfer effects. This scheme follows the spatial variations in the ionizing radiation field, as well as the associated fluctuations in IGM temperature and pressure smoothing. It is computationally much cheaper than full radiation hydrodynamics simulations and circumvents the difficult task of calibrating a galaxy formation model to observational constraints on cosmic reionization. Using this hybrid technique, we study the spatial fluctuations in IGM properties that are seeded by patchy cosmic reionization. We investigate the relevant physical processes and assess their impact on the z > 4 Lyman-alpha forest. Our main findings are: (i) Consistent with previous studies patchy reionization causes large scale temperature fluctuations that persist well after the end of reionization, (ii) these increase the Lyman-alpha forest flux power spectrum on large scales, and (iii) result in a spatially varying pressure smoothing that correlates well with the local reionization redshift. (iv) Structures evaporated or puffed up by photoheating cause notable features in the Lyman-alpha forest, such as flat-bottom or double-dip absorption profiles.

Nature, Published online: 27 July 2022; doi:10.1038/d41586-022-02056-5

Astronomers are rapidly analysing spectacular snapshots of the faraway Universe.

Pushing the frontiers of gravitational encounters and collisionless dynamics

Abstract not available

• Speaker: Uddipan Banik (Yale)
• Friday 30 September 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Eugene Vasiliev.

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tbc

Abstract not available

• Speaker: Francesca Fragkoudi (Durham)
• Friday 04 November 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Eugene Vasiliev.

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Science, Volume 377, Issue 6604, Page 377-377, July 2022.

Some Surprising Results from the Simba Simulations

Cosmological simulations of galaxy formation have matured rapidly in the last few years, with recent models combining structure formation, hydrodynamics, stellar and black hole growth, and associated feedback processes to accurately reproduce the demographics of the galaxy population. The input physics required to do so can have consequences that run counter to conventional notions of how galaxies form and evolve. In this talk I present our recent Simba simulations, a state-of-the-art suite that uses novel approaches for modelling black hole growth, AGN feedback, and dust to yield a galaxy population that uniquely reproduces key observations. I will then discuss some unexpected outcomes from Simba that offer surprising takes on how galaxies grow and interact with their cosmic ecosystem.

• Speaker: Romeel Dave (IfA Edinburgh)
• Friday 12 August 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Martin Haehnelt.

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The Hubble Frontier Fields represent the opportunity to probe the high-redshift evolution of the main sequence of star-forming galaxies to lower masses than possible in blank fields thanks to foreground lensing of massive galaxy clusters. We use the BEAGLE SED-fitting code to derive stellar masses, $\mathrm{M_{\star}}=\log(M/\mathrm{M_{\odot}})$, SFRs, $\Psi=\log(\psi/\mathrm{M_{\odot}}\,\mathrm{yr}^{-1})$ and redshifts from galaxies within the ASTRODEEP catalogue. We fit a fully Bayesian hierarchical model of the main sequence over $1.25<z<6$ of the form $\Psi = \alpha_\mathrm{9.7}(z) + \beta(\mathrm{M_{\star}}-9.7) + \mathcal{N}(0,\sigma^2)$ while explicitly modelling the outlier distribution. The redshift-dependent intercept at $\mathrm{M_{\star}}=9.7$ is parametrized as $\alpha_\mathrm{9.7}(z) = \log[N (1+z)^{\gamma}] + 0.7$. Our results agree with an increase in normalization of the main sequence to high redshifts that follows the redshift-dependent rate of accretion of gas onto dark matter halos with $\gamma=2.40^{+0.18}_{-0.18}$. We measure a slope and intrinsic scatter of $\beta=0.79^{+0.03}_{-0.04}$ and $\sigma=0.26^{+0.02}_{-0.02}$. We find that the sampling of the SED provided by the combination of filters (Hubble + ground-based Ks-band + Spitzer 3.6 and 4.5 $\mathrm{\mu m}$) is insufficient to constrain $\mathrm{M_{\star}}$ and $\Psi$ over the full dynamic range of the observed main sequence, even at the lowest redshifts studied. While this filter set represents the best current sampling of high-redshift galaxy SEDs out to $z>3$, measurements of the main sequence to low masses and high redshifts still strongly depend on priors employed in SED fitting (as well as other fitting assumptions). Future data-sets with JWST should improve this.

Title to be confirmed

Abstract not available

• Speaker: Romeel Dave (IfA Edinburgh)
• Friday 12 August 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Martin Haehnelt.

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Science, Volume 377, Issue 6603, Page 249-249, July 2022.

We present constraints on primordial B modes from large angular scale cosmic microwave background polarisation anisotropies measured with the Planck satellite. To remove Galactic polarised foregrounds, we use a Bayesian parametric component separation method, modelling synchrotron radiation as a power law and thermal dust emission as a modified blackbody. This method propagates uncertainties from the foreground cleaning into the noise covariance matrices of the maps. We construct two likelihoods: (i) a semi-analytical cross-spectrum-based likelihood-approximation scheme (momento) and (ii) an exact polarisation-only pixel-based likelihood (pixlike). Since momento is based on cross-spectra it is statistically less powerful than pixlike, but is less sensitive to systematic errors correlated across frequencies. Both likelihoods give a tensor-to-scalar ratio, r, that is consistent with zero from low multipole (2 <= ell < 30) Planck polarisation data. From full-mission maps we obtain r_0.05<0.274, at 95 per cent confidence, at a pivot scale of k = 0.05 Mpc^-1, using pixlike. momento gives a qualitatively similar but weaker 95 per cent confidence limit of r_0.05<0.408.

Nature, Published online: 06 July 2022; doi:10.1038/d41586-022-01560-y

State-of-the-art computer simulations show that the first supermassive black holes were born in rare, turbulent reservoirs of gas in the primordial Universe without the need for finely tuned, exotic environments — contrary to what has been thought for almost two decades.

The Sherwood Relics simulations - patchy cosmic reionization and its imprints on the IGM and Lyman-alpha forest

Abstract not available

• Speaker: Ewald Puchwein (AIP Potsdam)
• Friday 15 July 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Martin Haehnelt.

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Title to be confirmed

Abstract not available

• Speaker: Ewald Puchwein (AIP Potsdam)
• Friday 15 July 2022, 11:30-12:30
• Venue: in person & via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Martin Haehnelt.

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