arXiv subjects

Max Gronke

Publications and source records attributed to Max Gronke.

At least 91 records · Page 5Linked to original sources

arXiv2019

Gravitational lensing reveals ionizing ultraviolet photons escaping from a distant galaxy

During the epoch of reionisation, neutral gas in the early Universe was ionized by hard ultraviolet radiation emitted by young stars in the first galaxies. To do so, ionizing ultraviolet photons must escape from the host galaxy. We present Hubble Space Telescope observations of the gravitationally lensed galaxy PSZ1-ARC G311.6602-18.4624, revealing bright, multiply-imaged ionizing photon escape from a compact star-forming region through a narrow channel in an optically thick gas. The gravitational lensing magnification shows how ionizing photons escape this galaxy, contributing to the re-ionization of the Universe. The multiple sight lines to the source probe absorption by intergalactic neutral hydrogen on scales of no more than a few hundred, perhaps even less than ten, parsec.

astro-ph.GA↗

arXiv2019

Lyman Radiation Hydrodynamics of Turbulent H II Regions in Molecular Clouds: A Physical Origin of LyC Leakage and the Associated Ly$α$ Spectra

We examine Lyman continuum (LyC) leakage through H II regions regulated by turbulence and radiative feedback in a giant molecular cloud in the context of fully-coupled radiation hydrodynamics (RHD). The physical relations of the LyC escape with H I covering fraction, kinematics, spectral hardness, and the emergent Lyman-$α$ (Ly$α$) line profiles are studied using a series of RHD turbulence simulations performed with RAMSES-RT. The turbulence-regulated mechanism allows ionizing photons to leak out at early times before the onset of supernova feedback. The LyC photons escape through turbulence-generated low column density channels which are evacuated efficiently by radiative feedback via photoheating-induced shocks across the D-type ionization fronts. Ly$α$ photons funnel through the photoionized channels along the paths of LyC escape, resulting in a diverse Ly$α$ spectral morphology including narrow double-peaked profiles. The Ly$α$ peak separation is controlled by the residual H I column density of the channels and the line asymmetry correlates with the porosity and multiphase structure of the H II region. This mechanism through the turbulent H II regions can naturally reproduce the observed Ly$α$ spectral characteristics of some of LyC-leaking galaxies. This RHD turbulence-origin provides an appealing hypothesis to explain high LyC leakage from very young ($\sim3$ Myr) star-forming galaxies found in the local Universe without need of extreme galactic outflows nor supernova feedback. We discuss the implications of the turbulent H II regions on other nebular emission lines and a possible observational test with the Magellanic System and local blue compact dwarf galaxies as analogs of reionization-era systems.

astro-ph.GA↗

arXiv2019

Breaking cosmic degeneracies: Disentangling neutrinos and modified gravity with kinematic information

Searches for modified gravity in the large-scale structure try to detect the enhanced amplitude of density fluctuations caused by the fifth force present in many of these theories. Neutrinos, on the other hand, suppress structure growth below their free-streaming length. Both effects take place on comparable scales, and uncertainty in the neutrino mass leads to a degeneracy with modified gravity parameters for probes that are measuring the amplitude of the matter power spectrum. We explore the possibility to break the degeneracy between modified gravity and neutrino effects in the growth of structures by considering kinematic information related to either the growth rate on large scales or the virial velocities inside of collapsed structures. In order to study the degeneracy up to fully non-linear scales, we employ a suite of $N$-body simulations including both $f(R)$ modified gravity and massive neutrinos. Our results indicate that velocity information provides an excellent tool to distinguish massive neutrinos from modified gravity. Models with different values of neutrino masses and modified gravity parameters possessing a comparable matter power spectrum at a given time have different growth rates. This leaves imprints in the velocity divergence, which is therefore better suited than the amplitude of density fluctuations to tell the models apart. In such models with a power spectrum comparable to $Λ$CDM today, the growth rate is strictly enhanced. We also find the velocity dispersion of virialised clusters to be well suited to constrain deviations from general relativity without being affected by the uncertainty in the sum of neutrino masses.

astro-ph.CO↗

arXiv2018

Probing modified gravity in cosmic filaments

Multiple modifications of general relativity (GR) have been proposed in the literature in order to understand the nature of the accelerated expansion of the Universe. However, thus far all the predictions of GR have been confirmed with constantly increasing accuracy. In this work, we study the imprints of a particular class of models -- "screened" modified gravity theories -- on cosmic filaments. We have utilized the $N$-body code ISIS/RAMSES to simulate the symmetron model and the Hu-Sawicky $f(R)$ model, and we post-process the output with DisPerSE to identify the filaments of the cosmic web. We investigated how the global properties of the filaments -- such as their lengths, masses, and thicknesses -- as well as their radial density and speed profiles change under different gravity theories. We find that filaments are, on average, shorter and denser in modified gravity models compared to in $Λ$CDM. We also find that the speed profiles of the filaments are enhanced, consistent with theoretical expectations. Overall, our results suggest that cosmic filaments can be an effective complementary probe of screened modified gravity theories on Mpc scales.

astro-ph.CO↗

arXiv2018

Confirmation of double peaked Lyman-alpha emission at z=6.593: Witnessing a galaxy directly contributing to the reionisation of the Universe

Distant luminous Lyman-alpha emitters (LAEs) are excellent targets for spectroscopic observations of galaxies in the epoch of reionisation (EoR). We present deep high-resolution (R=5000) VLT/X-SHOOTER observations, along with an extensive collection of photometric data of `COLA1', a proposed double peaked LAE at z=6.6 (Hu et al. 2016). We rule out that COLA1's emission line is an [OII] doublet at z=1.475 on the basis of i) the asymmetric red line-profile and flux ratio of the peaks (blue/red=$0.31\pm0.03$) and ii) an unphysical [OII]/Ha ratio ([OII]/Ha > 22). We show that COLA1's observed B-band flux is explained by a faint extended foreground LAE, for which we detect Lya and [OIII] at z=2.142. We thus conclude that COLA1 is a real double-peaked LAE at z=6.593, the first discovered at z>6, confirming the result from Hu et al. (2016). COLA1 is UV luminous (M$_{1500}=-21.6\pm0.3$), has a high equivalent width (EW$_{0}$~120 Å) and very compact Lya emission (r$_{50} = 0.3$ kpc). Relatively weak inferred Hb+[OIII] line-emission from Spitzer/IRAC indicates an extremely low metallicity of Z<1/20 Z$_{\odot}$ or reduced strength of nebular lines due to high escape of ionising photons. The small Lya peak separation of $220\pm20$ km/s implies a low HI column density and an ionising photon escape fraction of ~15-30 %, providing the first direct evidence that such galaxies contribute actively to the reionisation of the Universe at z>6. Based on simple estimates, we find that COLA1 could have provided just enough photons to reionise its own ~0.3 pMpc bubble, allowing the blue Lya line to be observed. However, we also discuss alternative scenarios explaining the detected double peaked nature of COLA1. Our results show that future high-resolution observations of statistical samples of double peaked LAEs at z>5 are a promising probe of the occurrence of ionised regions around galaxies in the EoR.

astro-ph.GA↗

arXiv2018

The growth and entrainment of cold gas in a hot wind

Both absorption and emission line studies show that cold gas around galaxies is commonly outflowing at speeds of several hundred km$\,\textrm{s}^{-1}$. This observational fact poses a severe challenge to our theoretical models of galaxy evolution since most feedback mechanisms (e.g., supernovae feedback) accelerate hot gas, and the timescale it takes to accelerate a blob of cold gas via a hot wind is much larger than the time it takes to destroy the blob. We revisit this long-standing problem using three-dimensional hydrodynamical simulations with radiative cooling. Our results confirm previous findings, that cooling is often not efficient enough to prevent the destruction of cold gas. However, we also identify regions of parameter space where the cooling efficiency of the mixed, `warm' gas is sufficiently large to contribute new comoving cold gas which can significantly exceed the original cold gas mass. This happens whenever, $t_{\mathrm{cool, mix}}/t_{\mathrm{cc}} < 1$, where $t_{\mathrm{cool,mix}}$ is the cooling time of the mixed warm gas and $t_{\mathrm{cc}}$ is the cloud-crushing time. This criterion is always satisfied for a large enough cloud. Cooling `focuses' stripped material onto the tail where mixing takes place and new cold gas forms. A sufficiently large simulation domain is crucial to capturing this behavior.

astro-ph.GA↗

arXiv2018

The imprint of cosmic ray driven outflow on Lyman-alpha spectra

Recent magneto-hydrodynamic simulations of the star-forming interstellar medium (ISM) with parsec scale resolution indicate that relativistic cosmic rays support the launching of galactic outflows on scales of a few kpc. If these fundamental constituents of the ISM are injected at the sites of supernova (SN) explosions, the outflows are smoother, colder, and denser than the highly structured, hot-phase driven outflows forming, e.g., by thermal SN energy injection alone. In this Letter we present computations of resonant Lyman-$α$ (Ly$α$) radiation transfer through snapshots of a suite of stratified disk simulations from the SILCC project. For a range of thermal, radiative, and kinetic feedback models only simulations including non-thermal cosmic rays produce Ly$α$ spectra with enhanced red peaks and strong absorption at line center -- similar to observed systems. The absence of cosmic ray feedback leads to spectra incompatible with observations. We attribute this to the smoother neutral gas distribution of cosmic ray supported outflows within a few kpc from the disk midplane.

astro-ph.GA↗

arXiv2018

A bright lensed galaxy at $z=5.4$ with strong Ly$α$ emission

We present a detailed study of a unusually bright, lensed galaxy at $z=5.424$ discovered within the CFHTLS imaging survey. With an observed flux of $i_{\rm AB}=23.0$, J141446.82+544631.9 is one of the brightest galaxies known at $z>5$. It is characterized by strong Ly$α$ emission, reaching a peak in (observed) flux density of $>10^{-16}~{\rm erg}~{\rm s}^{-1}~{\rm cm}^{-2}~{\rm Å}^{-1}$. A deep optical spectrum from the LBT places strong constraints on NV and CIV emission, disfavouring an AGN source for the emission. However, a detection of the NIV]~$λ$1486 emission line indicates a hard ionizing continuum, possibly from hot, massive stars. Resolved imaging from HST deblends the galaxy from a foreground interloper; these observations include narrowband imaging of the Ly$α$ emission, which is marginally resolved on $\sim$few~kpc scales and has EW$_0~\sim$ 260Å. The Ly$α$ emission extends over ~2000~${\rm km}~{\rm s}^{-1}$ and is broadly consistent with expanding shell models. SED fitting that includes Spitzer/IRAC photometry suggests a complex star formation history that include both a recent burst and an evolved population. J1414+5446 lies 30" from the centre of a known lensing cluster in the CFHTLS; combined with the foreground contribution this leads to a highly uncertain estimate for the lensing magnification in the range $5 \leq μ\leq 25$. Because of its unusual brightness J1414+5446 affords unique opportunities for detailed study of an individual galaxy near the epoch of reionization and a preview of what can be expected from upcoming wide-area surveys that will yield hundreds of similar objects.

astro-ph.GA↗

arXiv2018

Unlocking the Full Potential of Extragalactic Ly$α$ through Its Polarization Properties

Lyman-$α$ (Ly$α$) is a powerful astrophysical probe. Not only is it ubiquitous at high redshifts, it is also a resonant line, making Ly$α$ photons scatter. This scattering process depends on the physical conditions of the gas through which Ly$α$ propagates, and these conditions are imprinted on observables such as the Ly$α$ spectrum and its surface brightness profile. In this work, we focus on a less-used observable capable of probing any scattering process: polarization. We implement the density matrix formalism of polarization into the Monte Carlo radiative transfer code tlac. This allows us to treat it as a quantum mechanical process where single photons develop and lose polarization from scatterings in arbitrary gas geometries. We explore static and expanding ellipsoids, biconical outflows, and clumpy multiphase media. We find that photons become increasingly polarized as they scatter and diffuse into the wings of the line profiles, making scattered Ly$α$ polarized in general. The degree and orientation of Ly$α$ polarization depends on the kinematics and distribution of the scattering HI gas. We find that it generally probes spatial or velocity space asymmetries and aligns itself tangentially to the emission source. We show that the mentioned observables, when studied separately, can leave similar signatures for different source models. We conclude by revealing how a joint analysis of the Ly$α$ spectra, surface brightness profiles, and polarization can break these degeneracies and help us extract unique physical information on galaxies and their environments from their strongest, most prominent emission line.

astro-ph.GA↗

arXiv2017

The Sunburst Arc: Direct Lyman α escape observed in the brightest known lensed galaxy

We present rest-frame ultraviolet and optical spectroscopy of the brightest lensed galaxy yet discovered, at redshift z = 2.4. This source reveals a characteristic, triple-peaked Lyman α profile which has been predicted by various theoretical works but to our knowledge has not been unambiguously observed previously. The feature is well fit by a superposition of two components: a double-peak profile emerging from substantial radiative transfer, and a narrow, central component resulting from directly escaping Lyman α photons; but is poorly fit by either component alone. We demonstrate that the feature is unlikely to contain contamination from nearby sources, and that the central peak is unaffected by radiative transfer effects apart from very slight absorption. The feature is detected at signal-to-noise ratios exceeding 80 per pixel at line center, and bears strong resemblance to synthetic profiles predicted by numerical models.

astro-ph.GA↗

arXiv2017

The Lyman Alpha Reference Sample. VIII. Characterizing Lyman-Alpha Scattering in Nearby Galaxies

We examine the dust geometry and Lyα scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < $z$ < 0.2) Lyα emitting and starbursting systems with Hubble Space Telescope Lyα, Hα, and Hβ imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media while others of them show significantly lower Lyα emission compared to their Balmer decrement. With the LARS imaging, we present Lyα/Hα and Hα/Hβ maps with spatial resolutions as low as $\sim$ 40 pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Lyα photons only scatter. We show that the dust that affects the escape of Lyα is more restricted to the galaxies' central regions, while the larger Lyα halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Lyα scatters in the halo, and find that this "characteristic" scattering distance correlates with the measured size of the Lyα halo. We note that there exists a slight anti-correlation between the scattering distance of Lyα and global dust properties.

astro-ph.GA↗

arXiv2017

Modelling the gas kinematics of an atypical Lyman-alpha emitting compact dwarf galaxy

Star-forming Compact Dwarf Galaxies (CDGs) resemble the expected pristine conditions of the first galaxies in the Universe and are the best systems to test models on primordial galaxy formation and evolution. Here we report on one of such CDGs, Tololo 1214-277, which presents a broad, single peaked, highly symmetric Ly$α$ emission line that had evaded theoretical interpretation so far. In this paper we reproduce for the first time these line features with two different physically motivated kinematic models: an interstellar medium composed by outflowing clumps with random motions and an homogeneous gaseous sphere undergoing solid body rotation. The multiphase model requires a clump velocity dispersion of $54.3\pm 0.6$ km s$^{-1}$ with outflows of $54.3\pm 5.1$ km s$^{-1}$, while the bulk rotation velocity is constrained to be $348^{+75}_{-48}$ km s$^{-1}$. We argue that the results from the multiphase model provide a correct interpretation of the data. In that case the clump velocity dispersion implies a dynamical mass of $2\times 10^{9}$ M$_{\odot}$, ten times its baryonic mass. If future kinematic maps of Tololo 1214-277 confirm the velocities suggested by the multiphase model, it would provide additional support to expect such kinematic state in primordial galaxies, opening the opportunity to use the models and methods presented in this paper to constrain the physics of star formation and feedback in the early generation of Ly-$α$ emitting galaxies.

astro-ph.GA↗

arXiv2017

Modeling $237$ Lyman-$α$ spectra of the MUSE-Wide survey

We compare $237$ Lyman-$α$ (Ly$α$) spectra of the "MUSE-Wide survey" (Herenz et al. 2017) to a suite of radiative transfer simulations consisting of a central luminous source within a concentric, moving shell of neutral gas, and dust. This six parameter shell-model has been used numerously in previous studies, however, on significantly smaller data-sets. We find that the shell-model can reproduce the observed spectral shape very well - better than the also common `Gaussian-minus-Gaussian' model which we also fitted to the dataset. Specifically, we find that $\sim 94\%$ of the fits possess a goodness-of-fit value of $p(χ^2)>0.1$. The large number of spectra allows us to robustly characterize the shell-model parameter range, and consequently, the spectral shapes typical for realistic spectra. We find that the vast majority of the Ly$α$ spectral shapes require an outflow and only $\sim 5\%$ are well-fitted through an inflowing shell. In addition, we find $\sim 46\%$ of the spectra to be consistent with a neutral hydrogen column density $<10^{17}\,\mathrm{cm}^{-2}$ - suggestive of a non-negligible fraction of continuum leakers in the MUSE-Wide sample. Furthermore, we correlate the spectral against the Ly$α$ halo properties against each other but do not find any strong correlation.

astro-ph.GA↗

arXiv2017

Ly$α$ profile, dust, and prediction of Ly$α$ escape fraction in Green Pea Galaxies

We studied Lyman-$α$ (Ly$α$) escape in a statistical sample of 43 Green Peas with HST/COS Ly$α$ spectra. Green Peas are nearby star-forming galaxies with strong [OIII]$λ$5007 emission lines. Our sample is four times larger than the previous sample and covers a much more complete range of Green Pea properties. We found that about 2/3 of Green Peas are strong Ly$α$ line emitters with rest-frame Ly$α$ equivalent width $>20$ Å. The Ly$α$ profiles of Green Peas are diverse. The Ly$α$ escape fraction, defined as the ratio of observed Ly$α$ flux to intrinsic Ly$α$ flux, shows anti-correlations with a few Ly$α$ kinematic features -- both the blue peak and red peak velocities, the peak separations, and FWHM of the red portion of the Ly$α$ profile. Using properties measured from SDSS optical spectra, we found many correlations -- Ly$α$ escape fraction generally increases at lower dust reddening, lower metallicity, lower stellar mass, and higher [OIII]/[OII] ratio. We fit their Ly$α$ profiles with the HI shell radiative transfer model and found Ly$α$ escape fraction anti-correlates with the best-fit $N_{HI}$. Finally, we fit an empirical linear relation to predict Ly$α$ escape fraction from the dust extinction and Ly$α$ red peak velocity. The standard deviation of this relation is about 0.3 dex. This relation can be used to isolate the effect of IGM scatterings from Ly$α$ escape and to probe the IGM optical depth along the line of sight of each $z>7$ Ly$α$ emission line galaxy in the JWST era.

astro-ph.GA↗

arXiv2017

Resonant line transfer in a fog: Using Lyman-alpha to probe tiny structures in atomic gas

Motivated by observational and theoretical work which both suggest very small scale ($\lesssim 1\,$pc) structure in the circum-galactic medium of galaxies and in other environments, we study Lyman-$α$ (Ly$α$) radiative transfer in an extremely clumpy medium with many "clouds" of neutral gas along the line of sight. While previous studies have typically considered radiative transfer through sightlines intercepting $\lesssim 10$ clumps, we explore the limit of a very large number of clumps per sightline (up to $f_{\mathrm{c}} \sim 1000$). Our main finding is that, for covering factors greater than some critical threshold, a multiphase medium behaves similar to a homogeneous medium in terms of the emergent Ly$α$ spectrum. The value of this threshold depends on both the clump column density and on the movement of the clumps. We estimate this threshold analytically and compare our findings to radiative transfer simulations with a range of covering factors, clump column densities, radii, and motions. Our results suggest that (i) the success in fitting observed Ly$α$ spectra using homogeneous "shell models" (and the corresponding failure of multiphase models) hints towards the presence of very small-scale structure in neutral gas, in agreement within a number of other observations; and (ii) the recurrent problems of reproducing realistic line profiles from hydrodynamical simulations may be due to their inability to resolve small-scale structure, which causes simulations to underestimate the effective covering factor of neutral gas clouds.

astro-ph.GA↗

arXiv2016

Estimates of cluster masses in screened modified gravity

We use cosmological hydrodynamical simulations to study the effect of screened modified gravity models on the mass estimates of galaxy clusters. In particular, we focus on two novel aspects: (i) we study modified gravity models in which baryons and dark matter are coupled with different strengths to the scalar field, and, (ii) we put the simulation results into the greater context of a general screened-modified gravity parametrization. We have compared the mass of clusters inferred via lensing versus the mass inferred via kinematical measurements as a probe of violations of the equivalence principle at Mpc scales. We find that estimates of cluster masses via X-ray observations is mainly sensitive to the coupling between the scalar degree of freedom and baryons - while the kinematical mass is mainly sensitive to the coupling to dark matter. Therefore, the relation between the two mass estimates is a probe of a possible non-universal coupling between the scalar field, the standard model fields, and dark matter. Finally, we used observational data of kinetic, thermal and lensing masses to place constraints on deviations from general relativity on cluster scales for a general parametrization of screened modified gravity theories which contains $f(R)$ and Symmetron models. We find that while the kinematic mass can be used to place competitive constraints, using thermal measurements is challenging as a potential non-thermal contribution is degenerate with the imprint of modified gravity.

astro-ph.CO↗

arXiv2016

Giant Lyman-Alpha Nebulae in the Illustris Simulation

Several `giant' Lyman-$α$ (Ly$α$) nebulae with extent $\gtrsim 300\,$kpc and observed Ly$α$ luminosity of $\gtrsim 10^{44}\,{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^{-2}\,{\rm arcsec}^{-2}$ have recently been detected, and it has been speculated that their presence hints at a substantial cold gas reservoir in small cool clumps not resolved in modern hydro-dynamical simulations. We use the Illustris simulation to predict the Ly$α$ emission emerging from large halos ($M > 10^{11.5}M_{\odot}$) at $z\sim 2$ and thus test this model. We consider both AGN and star driven ionization, and compare the simulated surface brightness maps, profiles and Ly$α$ spectra to a model where most gas is clumped below the simulation resolution scale. We find that with Illustris no additional clumping is necessary to explain the extents, luminosities and surface brightness profiles of the `giant Ly$α$ nebulae' observed. Furthermore, the maximal extents of the objects show a wide spread for a given luminosity and do not correlate significantly with any halo properties. We also show how the detected size depends strongly on the employed surface brightness cutoff, and predict that further such objects will be found in the near future.

astro-ph.GA↗

arXiv2016

From Mirrors to Windows: Lyman-Alpha Radiative Transfer in a Very Clumpy Medium

Lyman-Alpha (Ly$α$) is the strongest emission line in the Universe and is frequently used to detect and study the most distant galaxies. Because Lya is a resonant line, photons typically scatter prior to escaping; this scattering process complicates the interpretation of Ly$α$ spectra, but also encodes a wealth of information about the structure and kinematics of neutral gas in the galaxy. Modeling the Ly$α$ line therefore allows us to study tiny-scale features of the gas, even in the most distant galaxies. Curiously, observed Ly$α$ spectra can be modeled successfully with very simple, homogeneous geometries (such as an expanding, spherical shell), whereas more realistic, multiphase geometries often fail to reproduce the observed spectra. This seems paradoxical since the gas in galaxies is known to be multiphase. In this Letter, we show that spectra emerging from extremely clumpy geometries with many clouds along the line of sight converge to the predictions from simplified, homogeneous models. We suggest that this resolves the apparent discrepancy, and may provide a way to study the gas structure in galaxies on scales far smaller than can be probed in either cosmological simulations or direct (i.e., spatially-resolved) observations.

astro-ph.GA↗