Cosmological Hydrodynamical Simulations Research Articles

How to interpret measurements of diffuse light in stacked observations of groups and clusters of galaxies

ABSTRACT The diffuse light within galaxy groups and clusters provides valuable insight into the growth of massive cosmic structures. Groups are particularly interesting in this context, because they represent the link between galactic haloes and massive clusters. However, low surface brightness makes their diffuse light extremely challenging to detect individually. Stacking many groups is a promising alternative, but its physical interpretation is complicated by possible systematic variations of diffuse light profiles with other group properties. Another issue is the often ambiguous choice of group centre. We explore these challenges using mock observations for 497 galaxy groups and clusters with halo masses from $\sim 10^{12}$ to $1.5 \times 10^{15}\, \textrm {M}_{\odot }$ at redshift 0.1 from the Hydrangea cosmological hydrodynamic simulations. In 18 per cent of groups with at least five galaxies above $10^{9} \, \mathrm{M}_\odot$ in stellar mass, the r-band brightest galaxy is not the one at the centre of the gravitational potential; line-of-sight projections account for half of these cases. Miscentring does not significantly affect the ensemble average mass density profile or the surface brightness profile for our sample: even within ambiguously centred haloes, different centring choices lead to only a 1 per cent change in the total fraction of diffuse intra-group light, fIGL. We find strong correlations of fIGL with the luminosity of the central group galaxy and halo mass. Stacking groups in narrow bins of central galaxy luminosity will therefore make the physical interpretation of the signal more straightforward than combining systems across a wide range of mass.

Revisiting the [C ii] 158 μm line-intensity mapping power spectrum from the EoR using non-uniform line-luminosity scatter

Abstract Detecting the line-intensity mapping (LIM) signal from the galaxies of the epoch of reionization is an emerging tool to constrain their role in reionization. Ongoing and upcoming experiments target the signal fluctuations across the sky to reveal statistical and astrophysical properties of these galaxies via signal statistics, e.g. the power spectrum. Here, we revisit the [C ii]$_{158 \, \mu \text{m}}$ LIM power spectrum under non-uniform line–luminosity scatter, which has a halo-mass variation of statistical properties. Line–luminosity scatter from a cosmological hydrodynamic and radiative transfer simulation of galaxies at $z$ = 6 is considered in this study. We test the robustness of different model frameworks that interpret the impact of the line-luminosity scatter on the signal statistics. We use a simple power-law model to fit the scatter and demonstrate that the mean luminosity–halo mass correlation fit cannot preserve the mean intensity of the LIM signal (hence the clustering power spectrum) under non-uniform scatter. In our case, the mean intensity changes by ∼48 per cent compared to the mean correlation fit in contrast to the general case with semi-analytical scatter. However, we find that the prediction for the mean intensity from the most-probable fit can be modelled robustly, considering the generalized and more realistic non-uniform scatter. We also explore the possibility of diminishing luminosity bias under non-uniform scatter, affecting the clustering power spectrum, although this phenomenon might not be statistically significant. Therefore, we should adopt appropriate approaches that can consistently interpret the LIM power spectrum from observations.

Is the star-formation rate in z ∼ 6 quasars overestimated?

ABSTRACT The large total infrared (TIR) luminosities (LTIR; ≳1012 L⊙) observed in z ∼ 6 quasars are generally converted into high star-formation rates (SFRs; $\gtrsim\!{10}^2~{\rm M}_{\odot }\, {\rm yr}^{-1}$) of their host galaxies. However, these estimates rely on the assumption that dust heating is dominated by stellar radiation, neglecting the contribution from the central active galactic nucleus (AGN). We test the validity of this assumption by combining cosmological hydrodynamic simulations with radiative transfer calculations. We find that, when AGN radiation is included in the simulations, the mass (luminosity)-weighted dust temperature in the host galaxies increases from T ≈ 50 K (T ≈ 70 K) to T ≈ 80 K (T ≈ 200 K), suggesting that AGN effectively heats the bulk of dust in the host galaxy. We compute the AGN-host galaxy SFR from the synthetic spectral energy distribution by using standard SFR – LTIR relations, and compare the results with the ‘true’ values in the simulations. We find that the SFR is overestimated by a factor of ≈3 (≳10) for AGN bolometric luminosities of Lbol ≈ 1012 L⊙ (≳1013 L⊙), implying that the SFRs of z ∼ 6 quasars can be overestimated by over an order of magnitude.

Measurements of the angular momentum–mass relations in the Simba simulation

We present measurements of the specific angular momentum content (j) of galaxies drawn from the Simba cosmological hydrodynamic simulations. For the stellar, Hi and baryonic matter components we demonstrate the existence of extremely tight relations between j and the mass contained within the radius at which the Hi mass surface density decreases to 1 M⊙ pc−2. These relations are broadly consistent with a variety of empirical measurements. We confirm the observational result that the scatter in the stellar j–M relation is driven largely by Hi content, and measure the dependence of its scatter on the deviations of galaxies from other important scaling relations. For a given stellar mass, Hi-rich/poor galaxies have more/less-than-average stellar specific angular momentum. A similar, yet weaker, correlation exists for Hi mass fraction. Overall, our results demonstrate the utility of the Simba simulations as a platform for understanding and contextualising the data and results from forthcoming large galaxy surveys.

Cosmological Simulation of Galaxy Groups and Clusters. II. Studying Different Modes of Feedback through X-Ray Observations

The impact of feedback from active galactic nuclei (AGNs) on the cosmological evolution of the large-scale structure is a long-studied problem. However, it is still not well understood how the feedback energy couples to the ambient medium to influence the properties of AGN host galaxies and dark matter halos. In this work we investigate different modes of AGN feedback and their effect on the surrounding medium by probing the diffuse X-ray emission from the hot gas inside galaxy groups and clusters. For this purpose, we use the cosmological hydrodynamic simulation SIMBA to theoretically calculate the X-ray emission from simulated galaxy clusters/groups with the help of the Astrophysical Plasma Emission Code. We also perform synthetic observations of these systems with the Chandra X-ray telescope using the ray-tracing simulator Model of AXAF Response to X-rays. Our results show that in addition to the radiative wind mode of feedback from the AGNs, jet and X-ray modes of feedback play significant roles in suppressing the X-ray emission from the diffuse gas in the vicinity of the black hole. Our mock observational maps suggest that the signatures of AGN feedback from high-redshift objects may not be detected with the instrumental resolution of current X-ray telescopes like Chandra, but provide promising prospects for detection of these features with potential X-ray missions such as Lynx.

Improved Measurements of Galaxy Star Formation Stochasticity from the Intrinsic Scatter of Burst Indicators

Measurements of short-timescale star formation variations (i.e., “burstiness” or star formation stochasticity) are integral to our understanding of star formation feedback mechanisms and the assembly of stellar populations in galaxies. We expand upon the work of Broussard et al. by introducing a new analysis of galaxy star formation burstiness that accounts for variations in the distribution, a major confounding factor. We use Balmer decrements from the MOSFIRE Deep Evolution Field (MOSDEF) survey to measure Q sg , which we use to construct mock catalogs from the Santa Cruz Semi-Analytic Models and Mufasa cosmological hydrodynamical simulation based on 3D-HST, Fiber Multi-Object Spectrograph (FMOS)-COSMOS, and MOSDEF galaxies with Hα detections. The results of the mock catalogs are compared against observations using the burst indicator , with the standard deviation of the η distribution indicating burstiness. We find decent agreement between mock and observed η distribution shapes; however, the FMOS-COSMOS and MOSDEF mocks show a systematically low median and scatter in η in comparison to the observations. This work also presents the novel approach of analytically deriving the relationship between the intrinsic scatter in η, scatter added by measurement uncertainties, and observed scatter, resulting in an intrinsic burstiness measurement of 0.06–0.16 dex.

A Machine-learning Approach to Enhancing eROSITA Observations

The eROSITA X-ray telescope, launched in 2019, is predicted to observe roughly 100,000 galaxy clusters. Follow-up observations of these clusters from Chandra, for example, will be needed to resolve outstanding questions about galaxy cluster physics. Deep Chandra cluster observations are expensive, and it is unfeasible to follow up every eROSITA cluster, therefore the objects that are chosen for follow-up must be chosen with care. To address this, we have developed an algorithm for predicting longer-duration, background-free observations, based on mock eROSITA observations. We make use of the hydrodynamic cosmological simulation Magneticum, simulate eROSITA instrument conditions using SIXTE, and apply a novel convolutional neural network to output a deep Chandra-like “super observation” of each cluster in our simulation sample. Any follow-up merit assessment tool should be designed with a specific use case in mind; our model produces observations that accurately and precisely reproduce the cluster morphology, which is a critical ingredient for determining a cluster’s dynamical state and core type. Our model will advance our understanding of galaxy clusters by improving follow-up selection, and it demonstrates that image-to-image deep learning algorithms are a viable method for simulating realistic follow-up observations.

Candidate cosmic filament in the GJ526 field, mapped with the NIKA2 camera

Distinctive large-scale structures have been identified in the spatial distribution of optical galaxies up to redshift z ∼ 1. In the more distant universe, the relationship between the dust-obscured population of star-forming galaxies observed at millimetre wavelengths and the network of cosmic filaments of dark matter apparent in all cosmological hydrodynamical simulations is still under study. Using the NIKA2 dual-band millimetre camera, we mapped a field of ∼90 arcmin2 in the direction of the star GJ526 simultaneously in its 1.15-mm and 2.0-mm continuum wavebands to investigate the nature of the quasi-alignment of five sources found ten years earlier with the MAMBO camera at 1.2 mm. We find that these sources are not clumps of a circumstellar debris disc around this star as initially hypothesized. Rather, they must be dust-obscured star-forming galaxies, or sub-millimetre galaxies (SMGs), in the distant background. The new NIKA2 map at 1.15 mm reveals a total of seven SMGs distributed in projection on the sky along a filament-like structure crossing the whole observed field. Furthermore, we show that the NIKA2 and supplemental Herschel photometric data are compatible with a model of the spectral energy distributions (SEDs) of these sources when a common redshift of 2.5 and typical values of the dust parameters for SMGs are adopted. Hence, we speculate that these SMGs might be located in a filament of the distant ‘cosmic web’. The length of this candidate cosmic filament crossing the whole map is at least 4 cMpc (comoving), and the separations between sources are between 0.25 cMpc and 1.25 cMpc at this redshift, in line with expectations from cosmological simulations. Nonetheless, further observations to determine the precise spectroscopic redshifts of these sources are required to definitively support this hypothesis of SMGs embedded in a cosmic filament of dark matter.

Two modes of LyC escape from bursty star formation: implications for [C ii] deficits and the sources of reionization

ABSTRACT We use the SPHINX20 cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR10) or 100 Myr (SFR100). Both have $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ and experienced an extreme burst of star formation, but Bursty Leakers have SFR10 > SFR100, while Remnant Leakers have SFR10 < SFR100. The maximum SFRs in these bursts were typically ∼100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H ii region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H ii region densities. Both types of leaker exhibit [C ii]158μm deficits on the [C ii]–SFR100 relation, while only Bursty Leakers show deficits when SFR10 is used. We predict that [C ii] luminosity and SFR indicators such as H α and M$_{\rm 1500\mathring{\rm A}}$ can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C ii] observations of known z = 3–4 LyC leakers. Finally, we show that leakers with $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with $f_{\rm esc}\lt 5{{\ \rm per\ cent}}$ becomes significant at the tail-end of reionization.

Gas accretion and ram pressure stripping of haloes in void walls

ABSTRACT We conduct hydrodynamical cosmological zoom simulations of 14 voids to study the ability of haloes to accrete gas at different locations throughout the voids at z = 0. Measuring the relative velocity of haloes with respect to their ambient gas, we find that one-tenth of the haloes are expected to be unable to accrete external gas due to its fast flow passed them (so called ‘fast flow haloes’). These are typically located near void walls. We determine that these haloes have recently crossed the void wall and are still moving away from it. Their motion counter to that of ambient gas falling towards the void wall results in fast flows that make external gas accretion very challenging, and often cause partial gas loss via the resultant ram pressures. Using an analytical approach, we model the impact of such ram pressures on the gas inside haloes of different masses. A halo’s external gas accretion is typically cut off, with partial stripping of halo gas. For masses below a few times 109 M⊙, their halo gas is heavily truncated but not completely stripped. We identify numerous examples of haloes with a clear jelly-fish like gas morphology, indicating their surrounding gas is being swept away, cutting them off from further external accretion. These results highlight how, even in the relatively low densities of void walls, a fraction of galaxies can interact with large-scale flows in a manner that has consequences for their gas content and ability to accrete gas.

Unveiling the main sequence of galaxies at z ≥ 5 with the JWST: predictions from simulations

ABSTRACT We use two independent galaxy-formation simulations, flares, a cosmological hydrodynamical simulation, and shark, a semi-analytic model, to explore how well the JWST will be able to uncover the existence and parameters of the star-forming main sequence (SFS) at z = 5 → 10, i.e. shape, scatter, normalization. Using two independent simulations allows us to isolate predictions (e.g. stellar mass, star formation rate, SFR, luminosity functions) that are robust to or highly dependent on the implementation of the physics of galaxy formation. Both simulations predict that JWST can observe ≥70–90 per cent (for shark and flares, respectively) of galaxies up to z ∼ 10 (down to stellar masses of ${\approx}10^{8.3}\rm M_{\odot }$ and SFRs of ${\approx}10^{0.5}{\rm M}_{\odot }\,{\rm yr}^{-1}$) in modest integration times and given current proposed survey areas (e.g. the Web COSMOS 0.6 deg2) to accurately constrain the parameters of the SFS. Although both simulations predict qualitatively similar distributions of stellar mass and SFR. There are important quantitative differences, such as the abundance of massive, star-forming galaxies with flares predicting a higher abundance than shark; the early onset of quenching as a result of black hole growth in flares (at z ≈ 8), not seen in shark until much lower redshifts; and the implementation of synthetic photometry with flares predicting more JWST-detected galaxies (∼90 per cent) than shark (∼70 per cent) at z = 10. JWST observations will distinguish between these models, leading to a significant improvement upon our understanding of the formation of the very first galaxies.

Constraints on dark matter self-interaction from the internal density profiles of X-COP galaxy clusters

The fundamental properties of the postulated dark matter (DM) affect the internal structure of gravitationally bound structures. In the cold dark matter paradigm, DM particles interact only via gravity. Their distribution is well represented by an Einasto profile with shape parameter α ≈ 0.18 in the smallest dwarf galaxies or the most massive galaxy clusters alike. Conversely, if DM particles self-interact via additional forces, we expect the mass density profiles of DM halos to flatten in their central regions, thereby increasing the Einasto shape parameter. We measured the structural properties of 12 massive galaxy clusters from observations of their hot gaseous atmosphere, using the X-ray observatory XMM-Newton, and of the Sunyaev-Zel’dovich effect using the Planck all-sky survey. After removing morphologically disturbed systems, we measured Einasto shape parameters with mean ⟨α⟩=0.19 ± 0.03 and intrinsic scatter σα = 0.06, which is in close agreement with the prediction of the cold dark matter paradigm. We used cosmological hydrodynamical simulations of cluster formation with self-interacting DM (BAHAMAS-SIDM) to determine how the Einasto shape parameter depends on the self-interaction cross section. We used the fitted relation to turn our measurements of α into constraints on the self-interaction cross section, which imply σ/m < 0.19 cm2 g−1 (95% confidence level) at collision velocity vDM − DM ∼ 1000 km s−1. This is lower than the interaction cross section required for DM self-interactions to solve the core-cusp problem in dwarf spheroidal galaxies, unless the cross section is a strong function of velocity.

Constraining the Fluctuating Gunn–Peterson Approximation using Lyα Forest Tomography at z = 2

The fluctuating Gunn–Peterson approximation (FGPA) is a commonly used method to generate mock Lyα forest absorption skewers at Cosmic Noon (z ≳ 2) from the matter density field of N-body simulations without running expensive hydrodynamical simulations. Motivated by recent developments in 3D intergalactic medium (IGM) tomography observations as well as matter density field reconstruction techniques applied to galaxy redshift samples at z ∼ 2, we examine the possibility of observationally testing FGPA by directly examining the relationship between Lyα transmission and the underlying matter density field. Specifically, we analyze the EAGLE, Illustris, IllustrisTNG, and Nyx cosmological hydrodynamic simulations that were run with different codes and sub-grid models. While the FGPA is an excellent description of the IGM in lower-density regions, the slope of the transmission–density distribution at higher densities is significantly affected by feedback processes causing the FGPA to break down in that regime. Even without added feedback, we find significant deviations caused by hydrodynamical effects arising from nonlinear structure growth. We then proceed to make comparisons using realistic mock data assuming the sightline sampling and spectral properties of the recent CLAMATO survey, and find that it would be challenging to discern between the FGPA and hydrodynamical models with current data sets. However, the improved sightline sampling from future extremely large telescopes or large volumes from multiplexed spectroscopic surveys such as Subaru PFS should allow for stringent tests of the FGPA, and make it possible to detect the effect of galaxy feedback on the IGM.

Simulation view of galaxy clusters with low X-ray surface brightness

Context. X-ray selected samples are known to miss galaxy clusters that are gas poor and have a low surface brightness. This is different for the optically selected samples such as the X-ray Unbiased Selected Sample (XUCS). Aims. We characterise the origin of galaxy clusters that are gas poor and have a low surface-brightness by studying covariances between various cluster properties at fixed mass using hydrodynamic cosmological simulations. Methods. We extracted ≈1800 galaxy clusters from a high-resolution Magneticum hydrodynamic cosmological simulation and computed covariances at fixed mass of the following properties: core-excised X-ray luminosity, gas fraction, hot gas temperature, formation redshift, matter density profile concentration, galaxy richness, fossilness parameter, and stellar mass of the bright central galaxy. We also compared the correlation between concentration and gas fractions in non-radiative simulations, and we followed the trajectories of particles inside galaxy clusters to assess the role of AGN depletion on the gas fraction. Results. In simulations and in observational data, differences in surface brightness are related to differences in gas fraction. Simulations show that the gas fraction strongly correlates with assembly time, in the sense that older clusters are gas poor. Clusters that formed earlier have lower gas fractions because the feedback of the active galactic nucleus ejected a significant amount of gas from the halo. When the X-ray luminosity is corrected for the gas fraction, it shows little or no covariance with other quantities. Conclusions. Older galaxy clusters tend to be gas poor and possess a low X-ray surface brightness because the feedback mechanism removes a significant fraction of gas from these objects. Moreover, we found that most of the LX covariance with the other quantities is explained by differences in the gas fraction.

Testing strong lensing subhalo detection with a cosmological simulation

ABSTRACT Strong gravitational lensing offers a compelling test of the cold dark matter paradigm, as it allows for subhaloes with masses of ∼109 M⊙ and below to be detected. We test commonly used techniques for detecting subhaloes superposed in images of strongly lensed galaxies. For the lens we take a simulated galaxy in a ∼1013 M⊙ halo grown in a high-resolution cosmological hydrodynamical simulation, which we view from two different directions. Though the resolution is high, we note the simulated galaxy still has an artificial core which adds additional complexity to the baryon dominated region. To remove particle noise, we represent the projected galaxy mass distribution by a series of Gaussian profiles which precisely capture the features of the projected galaxy. We first model the lens mass as a (broken) power-law density profile and then search for small haloes. Of the two projections, one has a regular elliptical shape, while the other has distinct deviations from an elliptical shape. For the former, the broken power-law model gives no false positives and correctly recovers the mass of the superposed small halo; however, for the latter we find false positives and the inferred halo mass is overestimated by ∼4–5 times. We then use a more complex model in which the lens mass is decomposed into stellar and dark matter components. In this case, we show that we can capture the simulated galaxy’s complex projected structures and correctly infer the input small halo.

First Light and Reionisation Epoch Simulations (FLARES) – VI. The colour evolution of galaxies z = 5–15

ABSTRACT With its exquisite sensitivity, wavelength coverage, and spatial and spectral resolution, the James Webb Space Telescope (JWST) is poised to revolutionize our view of the distant, high-redshift (z > 5) Universe. While Webb’s spectroscopic observations will be transformative for the field, photometric observations play a key role in identifying distant objects and providing more comprehensive samples than accessible to spectroscopy alone. In addition to identifying objects, photometric observations can also be used to infer physical properties and thus be used to constrain galaxy formation models. However, inferred physical properties from broad-band photometric observations, particularly in the absence of spectroscopic redshifts, often have large uncertainties. With the development of new tools for forward modelling simulations, it is now routinely possible to predict observational quantities, enabling a direct comparison with observations. With this in mind, in this work, we make predictions for the colour evolution of galaxies at z = 5–15 using the First Light And Reionisation Epoch Simulations (flares) cosmological hydrodynamical simulation suite. We predict a complex evolution with time, driven predominantly by strong nebular line emission passing through individual bands. These predictions are in good agreement with existing constraints from Hubble and Spitzer as well as some of the first results from Webb. We also contrast our predictions with other models in the literature: While the general trends are similar, we find key differences, particularly in the strength of features associated with strong nebular line emission. This suggests photometric observations alone should provide useful discriminating power between different models and physical states of galaxies.

The neutron-capture and α-elements abundance ratios scatter in old stellar populations: cosmological simulations of the stellar halo

ABSTRACT We investigate the origin of the abundance ratios and scatter of the neutron-capture elements Sr, Ba, and Eu in the stellar halo of a Milky Way-mass galaxy formed in a hydrodynamical cosmological simulation, and compare them with those of α elements. For this, we implement a novel treatment for chemical enrichment of Type II supernovae that considers the effects of the rotation of massive stars on the chemical yields and differential enrichment according to the life-times of progenitor stars. We find that differential enrichment has a significant impact on the early enrichment of the interstellar medium which is translated into broader element ratio distributions, particularly in the case of the oldest, most metal-poor stars. We find that the [element/Fe] ratios of the α-elements O, Mg, and Si have systematically lower scatter compared to the neutron-capture elements ratios Sr, Ba, and Eu at [Fe/H] < −2, which is ∼0.1–0.4 dex for the former and between ∼0.5 and 1 dex for the latter. The different scatter levels found for the neutron-capture and α-elements is consistent with observations of old stars in the Milky Way. Our model also predicts a high scatter for the [Sr/Ba] ratio, which results from the treatment of the fast-rotating stars and the dependence of the chemical yields on the metallicity, mass, and rotational velocities. Such chemical patterns appear naturally if the different ejection times associated with stars of different mass are properly described, without the need to invoke for additional mixing mechanisms or a distinct treatment of the α- and neutron-capture elements.

Revealing the properties of void galaxies and their assembly using the eagle simulation

ABSTRACT We explore the properties of central galaxies living in voids using the eagle cosmological hydrodynamic simulations. Based on the minimum void-centric distance, we define four galaxy samples: inner void, outer void, wall, and skeleton. We find that inner void galaxies with host halo masses $\lt 10^{12}\,\rm M_{\odot }$ have lower stellar mass and stellar mass fractions than those in denser environments, and the fraction of galaxies with star formation (SF) activity and atomic hydrogen (H i) gas decreases with increasing void-centric distance, in agreement with observations. To mitigate the influence of stellar (halo) mass, we compare inner void galaxies to subsamples of fixed stellar (halo) mass. Compared to denser environments, inner void galaxies with $M_{*}= 10^{[9.0-9.5]}\,\rm M_{\odot }$ have comparable SF activity and H i gas fractions, but the lowest quenched galaxy fraction. Inner void galaxies with $M_{*}= 10^{[9.5-10.5]}\,\rm M_{\odot }$ have the lowest H i gas fraction, the highest quenched fraction and the lowest gas metallicities. On the other hand, inner void galaxies with $M_{*}\gt 10^{10.5}\,\rm M_{\odot }$ have comparable SF activity and H i gas fractions to their analogues in denser environments. They retain the highest metallicity gas that might be linked to physical processes that act with lower efficiency in underdense regions such as AGN (active galaxy nucleus) feedback. Furthermore, inner void galaxies have the lowest fraction of positive gas-phase metallicity gradients, which are typically associated with external processes or feedback events, suggesting they have more quiet merger histories than galaxies in denser environments. Our findings shed light on how galaxies are influenced by their large-scale environment.

Understanding the relation between thermal Sunyaev–Zeldovich decrement and halo mass using the simba and TNG simulations

ABSTRACT The relation between the integrated thermal Sunyaev–Zeldovich (tSZ) y-decrement versus halo mass (Y–M) can potentially constrain galaxy formation models, if theoretical and observational systematics can be properly assessed. We investigate the Y–M relation in the simba and IllustrisTNG-100 cosmological hydrodynamic simulations, quantifying the effects of feedback, line-of-sight projection, and beam convolution. We find that simba’s active galactic nucleus (AGN) jet feedback generates strong deviations from self-similar expectations for the Y–M relation, especially at $M_{\rm 500}\lesssim10^{13}M_\odot$. In simba, this is driven by suppressed in-halo y contributions owing to lowered halo baryon fractions. IllustrisTNG results more closely resemble simba without jets. Projections of line-of-sight structures weaken these model differences slightly, but they remain significant – particularly at group and lower halo masses. In contrast, beam smearing at Planck resolution makes the models indistinguishable, and both models appear to agree well with Planck data down to the lowest masses probed. We show that the arcminute resolution expected from forthcoming facilities would retain the differences between model predictions, and thereby provide strong constraints on AGN feedback.

Gastro Library. I. The Simulated Chemodynamical Properties of Several Gaia–Sausage–Enceladus-like Stellar Halos

The Milky Way (MW) stellar halo contains relics of ancient mergers that tell the story of our galaxy’s formation. Some of them are identified due to their similarity in energy, actions, and chemistry, referred to as the “chemodynamical space,” and are often attributed to distinct merger events. It is also known that our galaxy went through a significant merger event that shaped the local stellar halo during its first billion years. Previous studies using N-body only and cosmological hydrodynamical simulations have shown that such a single massive merger can produce several “signatures” in the chemodynamical space, which can potentially be misinterpreted as distinct merger events. Motivated by these, in this work we use a subset of the GASTRO library, which consists of several smoothed particle hydrodynamics+N-body models of a single accretion event in a MW-like galaxy. Here, we study models with orbital properties similar to the main merger event of our galaxy and explore the implications to known stellar halo substructures. We find that (i) supernova feedback efficiency influences the satellite’s structure and orbital evolution, resulting in distinct chemodynamical features for models with the same initial conditions; (ii) very retrograde high-energy stars are the most metal-poor of the accreted dwarf galaxy and could be misinterpreted as a distinct merger; (iii) the most bound stars are more metal-rich in our models, the opposite of what is observed in the MW, suggesting a secondary massive merger; and, finally, (iv) our models can reconcile other known apparently distinct substructures to a unique progenitor.