arXiv subjects

Joakim Rosdahl

Publications and source records attributed to Joakim Rosdahl.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Obelisk simulation: galaxies contribute more than AGN to HI reionization of protoclusters

We present the Obelisk project, a cosmological radiation-hydrodynamics simulation following the assembly and reionization of a protocluster progenitor during the first two billions of years from the big bang, down to $z = 3.5$. The simulation resolves haloes down to the atomic cooling limit, and tracks the contribution of different sources of ionization: stars, active galactic nuclei, and collisions. The Obelisk project is designed specifically to study the coevolution of high redshift galaxies and quasars in an environment favouring black hole growth. In this paper, we establish the relative contribution of these two sources of radiation to reionization and their respective role in establishing and maintaining the high redshift ionizing background. Our volume is typical of an overdense region of the Universe and displays star formation rate and black hole accretion rate densities similar to high redshift protoclusters. We find that hydrogen reionization happens inside-out and is completed by $z \sim 6$ in our overdensity, and is predominantly driven by galaxies, while accreting black holes only play a role at $z \sim 4$.

astro-ph.GA↗

arXiv2022

LyC escape from SPHINX galaxies in the Epoch of Reionization

We measure escape fractions, $f_{\rm esc}$, of ionizing radiation from galaxies in the SPHINX suite of cosmological radiation-hydrodynamical simulations of reionization, resolving halos with $M_{\rm vir} \gtrapprox 7.5 \times 10^7 \ M_{\odot}$ with a minimum cell width of $\approx 10$ pc. Our new and largest $20$ co-moving Mpc wide volume contains tens of thousands of star-forming galaxies with halo masses up to a few times $10^{11} \ M_{\odot}$. The simulated galaxies agree well with observational constraints of the UV luminosity function in the Epoch of Reionization. The escape fraction fluctuates strongly in individual galaxies over timescales of a few Myrs, due to its regulation by supernova and radiation feedback, and at any given time a tiny fraction of star-forming galaxies emits a large fraction of the ionizing radiation escaping into the inter-galactic medium. Statistically, $f_{\rm esc}$ peaks in intermediate-mass, intermediate-brightness, and low-metallicity galaxies ($M_{*} \approx 10^7 \ M_{\odot}$, $M_{1500} \approx -17$, $Z\lesssim 5 \times 10^{-3} \ Z_{\odot}$), dropping strongly for lower and higher masses, brighter and dimmer galaxies, and more metal-rich galaxies. The escape fraction correlates positively with both the short-term and long-term specific star formation rate. According to SPHINX, galaxies too dim to be yet observed, with $M_{1500} \gtrapprox -17$, provide about $55$ percent of the photons contributing to reionization. The global averaged $f_{\rm esc}$ naturally decreases with decreasing redshift, as predicted by UV background models and low-redshift observations. This evolution is driven by decreasing specific star formation rates over cosmic time.

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arXiv2022

Radiation-MagnetoHydrodynamics simulations of cosmic ray feedback in disc galaxies

Cosmic rays (CRs) are thought to play an important role in galaxy evolution. We study their effect when coupled to other important sources of feedback, namely supernovae and stellar radiation, by including CR anisotropic diffusion and radiative losses but neglecting CR streaming. Using the RAMSES-RT code, we perform the first radiation-magnetohydrodynamics simulations of isolated disc galaxies with and without CRs. We study galaxies embedded in dark matter haloes of $10^{10}$, $10^{11}$ and $10^{12}\, \rm M_{\odot}$ with a maximum resolution of $9 \,\rm pc$. We find that CRs reduce star formation rate in our two dwarf galaxies by a factor 2, with decreasing efficiency with increasing galaxy mass. They increase significantly the outflow mass loading factor in all our galaxies and make the outflows colder. We study the impact of the CR diffusion coefficient, exploring values from $κ= 10^{27}$ to $\rm 3\times 10^{29}\, cm^2\, s^{-1}$. With lower $κ$, CRs remain confined for longer on small scales and are consequently efficient in suppressing star formation, whereas a higher diffusion coefficient reduces the effect on star formation and increases the generation of cold outflows. Finally, we compare CR feedback to a calibrated 'strong' supernova feedback model known to sufficiently regulate star formation in high-redshift cosmological simulations. We find that CR feedback is not sufficiently strong to replace this strong supernova feedback. As they tend to smooth out the ISM and fill it with denser gas, CRs also lower the escape fraction of Lyman continuum photons from galaxies.

astro-ph.GA↗

arXiv2022

MgII in the JWST Era: a Probe of Lyman Continuum Escape?

Limited constraints on the evolution of the Lyman Continuum (LyC) escape fraction represent one of the primary uncertainties in the theoretical determination of the reionization history. Due to the intervening intergalactic medium (IGM), the possibility of observing LyC photons directly in the epoch of reionization is highly unlikely. For this reason, multiple indirect probes of LyC escape have been identified, some of which are used to identify low-redshift LyC leakers (e.g. O32), while others are primarily useful at $z>6$ (e.g. [OIII]/[CII] far infrared emission). The flux ratio of the resonant MgII doublet emission at 2796$\dot{\rm A}$ and 2803$\dot{\rm A}$ as well as the MgII optical depth have recently been proposed as ideal diagnostics of LyC leakage that can be employed at $z>6$ with JWST. Using state-of-the-art cosmological radiation hydrodynamics simulations post-processed with CLOUDY and resonant-line radiative transfer, we test whether MgII is indeed a useful probe of LyC leakage. Our simulations indicate that the majority of bright, star-forming galaxies with high LyC escape fractions are expected to be MgII emitters rather than absorbers at $z=6$. However, we find that the MgII doublet flux ratio is a more sensitive indicator of dust rather than neutral hydrogen, limiting its use as a LyC leakage indicator to only galaxies in the optically thin regime. Given its resonant nature, we show that MgII will be an exciting probe of the complex kinematics in high-redshift galaxies in upcoming JWST observations.

astro-ph.GA↗

arXiv2022

Predicting LyC emission of galaxies using their physical and Ly$α$ emission properties

The primary difficulty in understanding the sources and processes that powered cosmic reionization is that it is not possible to directly probe the ionizing Lyman Continuum (LyC) radiation at that epoch as those photons have been absorbed by the intervening neutral hydrogen in the IGM on their way to us. It is therefore imperative to build a model to accurately predict LyC emission using other properties of galaxies in the reionization era. In recent years, studies have shown that the LyC emission from galaxies may be correlated to their Lya emission. Here, we study this correlation by analyzing thousands of galaxies at high-z in the SPHINX cosmological simulation. We post-process these galaxies with the Lya radiative transfer code RASCAS and analyze the Lya - LyC connection. We find that the Lya and LyC luminosities are strongly correlated with each other, although with dispersion. There is a positive correlation between Lya and LyC escape fractions in the brightest Lya emitters (>$10^{41}$ erg/s), similar to the recent observational studies. However, when we also include fainter Lya emitters (LAEs), the correlation disappears, which suggests that the observed relationship may be driven by selection effects. We also find that bright LAEs are dominant contributors to reionization ($> 10^{40}$ erg/s galaxies contribute $> 90\%$ of LyC emission). Finally, we build predictive models using multivariate linear regression where we use the physical and the Lya properties of simulated galaxies to predict their intrinsic and escaping LyC luminosities with a high degree of accuracy. We find that the most important galaxy properties to predict the escaping LyC luminosity of a galaxy are its escaping Lya luminosity, gas mass, gas metallicity, and SFR. These models can be very useful to predict LyC emissions from galaxies and can help us identify the sources of reionization.

astro-ph.GA↗

arXiv2022

The Low-Redshift Lyman Continuum Survey II: New Insights into LyC Diagnostics

The Lyman continuum (LyC) cannot be observed at the epoch of reionization (z {\gtrsim} 6) due to intergalactic H I absorption. To identify Lyman continuum emitters (LCEs) and infer the fraction of escaping LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Lyα, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the [O III]/[O II] flux ratio and star formation rate surface density, predict whether a galaxy is a LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H I column density, ionization parameter, and stellar feedback. We find LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.

astro-ph.GA↗

arXiv2022

AGN-driven outflows and the formation of Ly$α$ nebulae around high-z quasars

The detection of Ly$α$ nebulae around $z\gtrsim 6$ quasars provides evidence for extended gas reservoirs around the first rapidly growing supermassive black holes. Observations of $z > 6$ quasars can be explained by cosmological models provided that the black holes by which they are powered evolve in rare, massive dark matter haloes. Whether these theoretical models also explain the observed extended Ly$α$ emission remains an open question. We post-process a suite of cosmological, radiation-hydrodynamic simulations targeting a quasar host halo at $z>6$ with the Ly$α$ radiative transfer code RASCAS. A combination of recombination radiation from photo-ionised hydrogen and emission from collisionally excited gas powers Ly$α$ nebulae with a surface brightness profile in close agreement with observations. We also find that, even on its own, resonant scattering of the Ly$α$ line associated to the quasar's broad line region can also generate Ly$α$ emission on $\sim 100 \, \rm kpc$ scales, resulting in comparable agreement with observed surface brightness profiles. Even if powered by a broad quasar Ly$α$ line, Ly$α$ nebulae can have narrow line-widths $\lesssim 1000 \, \rm km \, s^{-1}$, consistent with observational constraints. Even if there is no quasar, we find that halo gas cooling produces a faint, extended Ly$α$ glow. However, to light-up extended Ly$α$ nebulae with properties in line with observations, our simulations unambiguously require quasar-powered outflows to clear out the galactic nucleus and allow the Ly$α$ flux to escape and still remain resonant with halo gas. The close match between observations and simulations with quasar outflows suggests that AGN feedback already operates before $z \, = \, 6$ and confirms that high-$z$ quasars reside in massive haloes tracing overdensities.

astro-ph.GA↗

arXiv2022

The Low-Redshift Lyman Continuum Survey I: New, Diverse Local Lyman-Continuum Emitters

The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the epoch of reionization is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with HST/COS for a z=0.2-0.4 sample of 66 galaxies. After careful processing of the FUV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.

astro-ph.GA↗

arXiv2022

The Nature of High ${\rm [OIII]}_{\rm 88μm}$/${\rm [CII]}_{\rm 158μm}$ Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

ALMA observations of $z>6$ galaxies have revealed abnormally high [OIII]$_{\rm 88μm}$/[CII]$_{\rm 158μm}$ ratios and [CII]$_{\rm 158μm}$ deficits compared to local galaxies. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties. In order to elucidate the underlying physics that drives this high-redshift phenomenon, we employ SPHINX$^{20}$, a state-of-the-art, cosmological radiation-hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization. We find that the observed $z>6$ [OIII]-SFR and [CII]-SFR relations can only be reproduced when the C/O abundance ratio is $\sim8\times$ lower than Solar and the total metal production is $\sim4\times$ higher than that of a Kroupa IMF. This implies that high-redshift galaxies are potentially primarily enriched by low-metallicity core-collapse supernovae with a more top-heavy IMF. As AGB stars and type-Ia supernova begin to contribute to the galaxy metallicity, both the [CII]-SFR and [CII] luminosity functions are predicted to converge to observed values at $z\sim4.5$. While we demonstrate that ionisation parameter, LyC escape fraction, ISM gas density, and CMB attenuation all drive galaxies towards higher [OIII]/[CII], observed values at $z>6$ can only be reproduced with substantially lower C/O abundances compared to Solar. The combination of [CII] and [OIII] can be used to predict the values of ionisation parameter, ISM gas density, and LyC escape fraction and we provide estimates of these quantities for nine observed $z>6$ galaxies. Finally, we demonstrate that [OI]$_{\rm 63μm}$ can be used as a replacement for [CII] in high-redshift galaxies where [CII] is unobserved and argue that more observation time should be used to target [OI] at $z>6$. (Abridged)

astro-ph.GA↗

arXiv2021

A systematic study of the escape of LyC and Ly$α$ photons from star-forming, magnetized turbulent clouds

Understanding the escape of Lyman continuum (LyC) and Lyman $α$ (Ly$α$) photons from giant molecular clouds (GMCs) is crucial if we are to study the reionization of the Universe and to interpret spectra of observed galaxies at high redshift. To this end, we perform high-resolution, radiation-magneto-hydrodynamic simulations of GMCs with self-consistent star formation and stellar feedback. We find that a significant fraction (15-70%) of ionizing radiation escapes from the simulated GMCs with different masses ($10^5$ and $10^6\,M_\odot$), as the clouds are dispersed within about $2$-$5\,{\rm Myr}$ from the onset of star formation. The fraction of LyC photons leaked is larger when the GMCs are less massive, metal-poor, less turbulent, and less dense. The most efficient leakage of LyC radiation occurs when the total star formation efficiency of a GMC is about 20%. The escape of Ly$α$ shows a trend similar to that of LyC photons, except that the fraction of Ly$α$ photons escaping from the GMCs is larger ($f_{\rm esc}^{\rm Lyα}\approx f_{900}^{0.27}$) and that a GMC with strong turbulence shows larger $f_{\rm esc}^{\rm Lyα}$. The simulated GMCs show a characteristic velocity separation of $Δv\approx 120 \,{\rm km\,s^{-1}}$ in the time-averaged emergent Ly$α$ spectra, suggesting that Ly$α$ could be useful to infer the kinematics of the interstellar and circumgalactic medium. We show that Ly$α$ luminosities are a useful indicator of the LyC escape, provided the number of LyC photons can be deduced through stellar population modeling. Finally, we find that the correlations between the escape fractions of Ly$α$, ultraviolet photons at 1500A, and the Balmer $α$ line are weak.

astro-ph.GA↗

arXiv2021

Introducing SPHINX-MHD: The Impact of Primordial Magnetic Fields on the First Galaxies, Reionization, and the Global 21cm Signal

We present the first results from SPHINX-MHD, a suite of cosmological radiation-magnetohydrodynamics simulations designed to study the impact of primordial magnetic fields (PMFs) on galaxy formation and the evolution of the intergalactic medium during the epoch of reionization. The simulations are among the first to employ multi-frequency, on-the-fly radiation transfer and constrained transport ideal MHD in a cosmological context to simultaneously model the inhomogeneous process of reionization as well as the growth of PMFs. We run a series of $(5\,\text{cMpc})^3$ cosmological volumes, varying both the strength of the seed magnetic field ($B_0$) and its spectral index ($n_B$). We find that PMFs that have $n_B > -0.562\log_{10}\left(\frac{B_0}{1{\rm n}G}\right) - 3.35$ produce electron optical depths ($τ_e$) that are inconsistent with CMB constraints due to the unrealistically early collapse of low-mass dwarf galaxies. For $n_B\geq-2.9$, our constraints are considerably tighter than the $\sim{\rm n}G$ constraints from Planck. PMFs that do not satisfy our constraints have little impact on the reionization history or the shape of the UV luminosity function. Likewise, detecting changes in the Lya forest due to PMFs will be challenging because photoionisation and photoheating efficiently smooth the density field. However, we find that the first absorption feature in the global 21cm signal is a sensitive indicator of the properties of the PMFs, even for those that satisfy our $τ_e$ constraint. Furthermore, strong PMFs can marginally increase the escape of LyC photons by up to 25\% and shrink the effective radii of galaxies by $\sim44\%$ which could increase the completeness fraction of galaxy surveys. Finally, our simulations show that surveys with a magnitude limit of ${\rm M_{UV,1500}=-13}$ can probe the sources that provide the majority of photons for reionization out to $z=12$.

astro-ph.CO↗

arXiv2021

Collapse of turbulent massive cores with ambipolar diffusion and hybrid radiative transfer I. Accretion and multiplicity

(Abridged) Context. Massive stars form in magnetized and turbulent environments, and are often located in stellar clusters. Their accretion mechanism, as well as the origin of their system's stellar multiplicity are poorly understood. Aims. We study the influence of both magnetic fields and turbulence on the accretion mechanism of massive protostars and their multiplicity. Methods. We present a series of four Radiation-MHD simulations of the collapse of a massive magnetized, turbulent core of 100 $M_\odot$ with the AMR code Ramses, including a hybrid radiative transfer method for stellar irradiation and ambipolar diffusion. We vary the Mach and Alfvenic Mach numbers to probe sub- and superalfvenic turbulence as well as sub- and supersonic turbulence regimes. Results. Subalfvenic turbulence leads to single stellar systems while superalfvenic turbulence leads to binary formation from disk fragmentation following spiral arm collision, with mass ratios of 1.1-1.6. In those runs, infalling gas reaches the individual disks via a transient circumbinary structure. Magnetically-regulated, thermally-dominated (plasma beta $β>1$), Keplerian disks form in all runs, with sizes 100-200 AU and masses 1-8 $M_\odot$. The disks around primary and secondary sink particles share similar properties. We observe higher accretion rates onto the secondary stars than onto their primary star companion. The primary disk orientation is found to be set by the initial angular momentum carried by turbulence. Conclusions. Small (300 AU) massive protostellar disks as those frequently observed nowadays can only be reproduced so far in the presence of (moderate) magnetic fields with ambipolar diffusion, even in a turbulent medium. The interplay between magnetic fields and turbulence sets the multiplicity of stellar clusters. A plasma beta $β>1$ is a good indicator of streamers and disks.

astro-ph.SR↗

arXiv2021

Cosmological Magnetogenesis: The Biermann Battery during the Epoch of Reionization

We investigate the effect of the Biermann battery during the Epoch of Reionization (EoR) using cosmological Adaptive Mesh Refinement simulations within the framework of the SPHINX project. We develop a novel numerical technique to solve for the Biermann battery term in the Constrained Transport method, preserving both the zero divergence of the magnetic field and the absence of Biermann battery for isothermal flows. The structure-preserving nature of our numerical method turns out to be very important to minimise numerical errors during validation tests of the propagation of a Strömgren sphere and of a Sedov blast wave. We then use this new method to model the evolution of a 2.5 and 5 co-moving Mpc cosmological box with a state-of-the-art galaxy formation model within the RAMSES code. Contrary to previous findings, we show that three different Biermann battery channels emerge: the first one is associated with linear perturbations before the EoR, the second one is the classical Biermann battery associated with reionization fronts during the EoR, and the third one is associated with strong, supernova-driven outflows. While the two former channels generate spontaneously volume-filling magnetic fields with a strength on the order or below $10^{-20}$ G, the latter, owing to the higher plasma temperature and a marginally-resolved turbulent dynamo, reaches a field strength as high as $10^{-18}$ G in the intergalactic medium around massive haloes.

astro-ph.CO↗

arXiv2021

Lyman-$α$ as a tracer of cosmic reionisation in the SPHINX radiation-hydrodynamics cosmological simulation

The Ly$α$ emission line is one of the most promising probes of cosmic reionisation but isolating the signature of a change in the ionisation state of the IGM is challenging because of intrinsic evolution and internal radiation transfer effects. We present the first study of the evolution of Ly$α$ emitters (LAE) during the epoch of reionisation based on a full radiation-hydrodynamics cosmological simulation that is able to capture both the large-scale process of reionisation and the small-scale properties of galaxies. We predict the Ly$α$ emission of galaxies in the $10^3$ cMpc$^3$ SPHINX simulation at $6\leq z\leq9$ by computing the full Ly$α$ radiation transfer from ISM to IGM scales. SPHINX is able to reproduce many observational constraints such as the UV/Ly$α$ luminosity functions and stellar mass functions at z $\geq$ 6 for the dynamical range probed by our simulation ($M_{\rm 1500}\gtrsim-18$, $L_{\rm Lyα}\lesssim10^{42}$ erg/s, $M_{\star}\lesssim10^9$ M$_{\odot}$). As intrinsic Ly$α$ emission and internal Ly$α$ escape fractions barely evolve from $z=6$ to 9, the observed suppression of Ly$α$ luminosities with increasing redshift is fully attributed to IGM absorption. For most observable galaxies ($M_{\rm 1500}\lesssim-16$), the Ly$α$ line profiles are slightly shifted to the red due to internal radiative transfer effects which mitigates the effect of IGM absorption. Overall, the enhanced Ly$α$ suppression during reionisation traces the IGM neutral fraction $x_{\rm HI}$ well but the predicted amplitude of this reduction is a strong function of the Ly$α$ peak shift, which is set at ISM/CGM scales. We find that a large number of LAEs could be detectable in very deep surveys during reionisation when $x_{\rm HI}$ is still $\approx 50\%$.

astro-ph.GA↗

arXiv2021

Tracing the simulated high-redshift circum-galactic medium with Lyman alpha emission

With the Multi Unit Spectroscopic Explorer (MUSE), it is now possible to detect spatially extended Lyman alpha emission from individual faint (M_UV ~ -18) galaxies at redshifts, 3 < z < 6, tracing gas out to circum-galactic scales comparable to the dark matter halo virial radius. To explore the implications of such observations, we present a cosmological radiation hydrodynamics simulation of a single galaxy, chosen to be typical of the Lyman alpha-emitting galaxies detected by MUSE in deep fields. We use this simulation to study the origin and dynamics of the high-redshift circum-galactic medium (CGM). We find that the majority of the mass in the diffuse CGM is comprised of material infalling for the first time towards the halo center, but with the inner CGM also containing a comparable amount of mass that has moved past first-pericentric passage, and is in the process of settling into a rotationally supported configuration. Making the connection to Lyman alpha emission, we find that the observed extended surface brightness profile is due to a combination of three components: scattering of galactic Lyman alpha emission in the CGM, in-situ emission of CGM gas (mostly infalling), and Lyman alpha emission from small satellite galaxies. The weight of these contributions vary with distance from the galaxy such that (1) scattering dominates the inner regions (r < 7 kpc), at surface brightness larger than a few 10^-19 cgs, (2) all components contribute equally around r ~ 10 kpc (or SB ~10^-19), and (3) the contribution of small satellite galaxies takes over at large distances (or SB ~10^-20). Our simulation fails to reproduce the characteristic observed Lyman alpha spectral morphology that is red-shifted with respect to the systemic velocity, with the implication that the simulation is missing an important component of neutral outflowing gas.

astro-ph.GA↗

arXiv2020

The Geometry and Dynamical Role of Stellar Wind Bubbles in Photoionised HII Regions

Winds from young massive stars contribute a large amount of energy to their host molecular clouds. This has consequences for the dynamics and observable structure of star-forming clouds. In this paper, we present radiative magnetohydrodynamic simulations of turbulent molecular clouds that form individual stars of 30, 60 and 120 solar masses emitting winds and ultraviolet radiation following realistic stellar evolution tracks. We find that winds contribute to the total radial momentum carried by the expanding nebula around the star at 10 % of the level of photoionisation feedback, and have only a small effect on the radial expansion of the nebula. Radiation pressure is largely negligible in the systems studied here. The 3D geometry and evolution of wind bubbles is highly aspherical and chaotic, characterised by fast-moving "chimneys" and thermally-driven "plumes". These plumes can sometimes become disconnected from the stellar source due to dense gas flows in the cloud. Our results compare favourably with the findings of relevant simulations, analytic models and observations in the literature while demonstrating the need for full 3D simulations including stellar winds. However, more targeted simulations are needed to better understand results from observational studies.

astro-ph.GA↗

arXiv2020

Dual Effects of Ram Pressure on Star Formation in Multi-phase Disk Galaxies with Strong Stellar Feedback

We investigate the impact of ram pressure stripping due to the intracluster medium (ICM) on star-forming disk galaxies with a multi-phase interstellar medium (ISM) maintained by strong stellar feedback. We carry out radiation-hydrodynamics simulations of an isolated disk galaxy embedded in a 10^11 Msun dark matter halo with various ICM winds mimicking the cluster outskirts (moderate) and the central environment (strong). We find that both star formation quenching and triggering occur in ram pressure-stripped galaxies, depending on the strength of the winds. HI and H$_2$ in the outer galactic disk are significantly stripped in the presence of the moderate winds, whereas turbulent pressure provides support against ram pressure in the central region where star formation is active. Moderate ICM winds facilitate gas collapsing, increasing the total star formation rates by ~40% when the wind is oriented face-on or ~80% when it is edge-on. In contrast, strong winds rapidly blow away neutral and molecular hydrogen gas from the galaxy, suppressing the star formation by a factor of two within ~200 Myr. Dense gas clumps with N_H > 10 Msun pc^-2 are easily identified in extraplanar regions, but no significant young stellar populations are found in such clumps. In our attempts to enhance radiative cooling by adopting a colder ICM of T=10^6K, only a few additional stars are formed in the tail region, even if the amount of newly cooled gas increases by an order of magnitude.

astro-ph.GA↗

arXiv2020

On the origin of low escape fractions of ionizing radiation from massive star-forming galaxies at high redshift

The physical origin of low escape fractions of ionizing radiation derived from massive star-forming galaxies at $z\sim3$-$4$ is not well understood. We perform idealised disc galaxy simulations to understand how galactic properties such as metallicity and gas mass affect the escape of Lyman Continuum (LyC) photons using radiation-hydrodynamic simulations with strong stellar feedback. We find that the luminosity-weighted escape fraction from a metal-poor ($Z=0.002$) galaxy embedded in a halo of mass $M_h\simeq10^{11}\,M_\odot$ is $\left \simeq 10\,\%$. Roughly half of the LyC photons are absorbed within scales of 100 pc, and the other half is absorbed in the ISM ($\lesssim 2\, {\rm kpc}$). When the metallicity of the gas is increased to $Z=0.02$, the escape fraction is significantly reduced to $\left \simeq1\%$ because young stars are enshrouded by their birth clouds for a longer time. In contrast, increasing the gas mass by a factor of 5 leads to $\left \simeq 5\, \%$ because LyC photons are only moderately absorbed by the thicker disc. Our experiments suggest that high metallicity is likely more responsible for the low escape fractions observed in massive star-forming galaxies, supporting the scenario in which the escape fraction is decreasing with increasing halo mass. Finally, negligible correlation is observed between the escape fraction and surface density of star formation or galactic outflow rates.

astro-ph.GA↗