arXiv subjects

Max Gronke

Publications and source records attributed to Max Gronke.

100 records · Page 6Linked to original sources

arXiv2016

The LyA-LyC Connection: Evidence for an Enhanced Contribution of UV-faint Galaxies to Cosmic Reionization

The escape of ionizing Lyman Continuum (LyC) photons requires the existence of low-N_HI sightlines, which also promote escape of Lyman-Alpha (Lya). We use a suite of 2500 Lya Monte-Carlo radiative transfer simulations through models of dusty, clumpy interstellar (`multiphase') media from Gronke & Dijkstra (2016), and compare the escape fractions of Lya [f_esc(Lya)] and LyC radiation [f_esc(LyC)]. We find that f_esc(LyC) and f_esc(Lya) are correlated: galaxies with a low f_esc(Lya) consistently have a low f_esc(LyC), while galaxies with a high f_esc(Lya) exhibit a large dispersion in f_esc(LyC). We argue that there is increasing observational evidence that Lya escapes more easily from UV-faint galaxies. The correlation between f_esc(LyC) and f_esc(Lya) then implies that UV-faint galaxies contribute more to the ionizing background than implied by the faint-end slope of the UV-luminosity function. In multiphase gases, the ionizing escape fraction is most strongly affected by the cloud covering factor, f_cl, which implies that f_esc(LyC) is closely connected to the observed Lya spectral line shape. Specifically, LyC emitting galaxies typically having narrower, more symmetric line profiles. This prediction is qualitatively similar to that for `shell models'.

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arXiv2016

Ly$α$ Spectra from Multiphase Outflows, and their Connection to Shell Models

We perform Lyman-$α$ (Ly$α$) Monte-Carlo radiative transfer calculations on a suite of $2500$ models of multiphase, outflowing media, which are characterized by $14$ parameters. We focus on the Ly$α$ spectra emerging from these media, and investigate which properties are dominant in shaping the emerging Ly$α$ profile. Multiphase models give rise to a wide variety of emerging spectra, including single, double and triple peaked spectra. We find that the dominant parameters in shaping the spectra include (i) the cloud covering factor, $f_c$, in agreement with earlier studies, and (ii) the temperature and number density of residual HI in the hot ionized medium. We attempt to reproduce spectra emerging from multiphase models with `shell models' which are commonly used to fit observed Ly$α$ spectra, and investigate the connection between shell-model parameters and the physical parameters of the clumpy media. In shell models, the neutral hydrogen content of the shell is one of the key parameters controlling Ly$α$ radiative transfer. Because Ly$α$ spectra emerging from multi-phase media depend much less on the neutral hydrogen content of the clumps, the shell model parameters such as HI column density (but also shell velocity and dust content) are generally not well matched to the associated physical parameters of clumpy media.

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arXiv2016

Green Pea Galaxies Reveal Secrets of Ly$α$ Escape

We analyze archival Ly$α$ spectra of 12 "Green Pea" galaxies observed with the Hubble Space Telescope, model their Ly$α$ profiles with radiative transfer models, and explore the dependence of Ly$α$ escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [OIII]$λ$5007 equivalent widths of hundreds of Å. All 12 Green Pea galaxies in our sample show Ly$α$ lines in emission, with a Ly$α$ equivalent width distribution similar to high redshift Ly$α$ emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Ly$α$ escape fractions and find correlations between Ly$α$ escape fraction and kinematic features of Ly$α$ profiles. The escape fraction of Ly$α$ in these galaxies ranges from 1.4% to 67%. We also find that the Ly$α$ escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Ly$α$ profiles with single HI shell radiative transfer models and find that the Ly$α$ escape fraction anti-correlates with the derived HI column densities. Single shell models fit most Ly$α$ profiles well, but not the ones with highest escape fractions of Ly$α$. Our results suggest that low HI column density and low metallicity are essential for Ly$α$ escape, and make a galaxy a Ly$α$ emitter.

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arXiv2016

Lyman Alpha Signatures from Direct Collapse Black Holes

`Direct collapse black holes' (DCBHs) provide possible seeds for supermassive black holes that exist at redshifts as high as z~7. We study Lyman Alpha (Lya) radiative transfer through simplified representations of the DCBH-scenario. We find that gravitational heating of the collapsing cloud gives rise to a Lya cooling luminosity of up to ~ 1e38(M_gas/1e6 Msun)^2 erg/s. The Lya production rate can be significantly larger during the final stages of collapse, but collisional deexcitation efficiently suppresses the emerging Lya flux. Photoionization by a central source boosts the Lya luminosity to L~1e43(M_BH/1e6 M_sun) erg/s during specific evolutionary stages of the cloud, where M_BH denotes the mass of the black hole powering this source. We predict that the width and velocity off-set of the Lya spectral line range from a few tens to few thousands km/s, depending sensitively on the evolutionary state of the cloud. We also compare our predictions to observations of CR7 (Sobral et al. 2015), a luminous Lya emitter at z~7, which is potentially associated with a DCBH. If CR7 is powered by a black hole, then its Lya flux alone requires that M_BH> 1e7 M_sun, which exceeds the mass of DCBHs when they first form. The observed width of the Lya spectrum favors the presence of only a low column density of hydrogen, log [N_HI/cm^-2]~19-20. The shape of the Lya spectrum indicates that this gas is outflowing. These requirements imply that if CR7 harbors a DCBH, then the physical conditions that enabled its formation have been mostly erased, which is in agreement with theoretical expectations.

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arXiv2015

A systematic study of Lyman-Alpha transfer through outflowing shells: Model parameter estimation

Outflows promote the escape of Lyman-$α$ (Ly$α$) photons from dusty interstellar media. The process of radiative transfer through interstellar outflows is often modelled by a spherically symmetric, geometrically thin shell of gas that scatters photons emitted by a central Ly$α$ source. Despite its simplified geometry, this `shell model' has been surprisingly successful at reproducing observed Ly$α$ line shapes. In this paper we perform automated line fitting on a set of noisy simulated shell model spectra, in order to determine whether degeneracies exist between the different shell model parameters. While there are some significant degeneracies, we find that most parameters are accurately recovered, especially the HI column density ($N_{\rm HI}$) and outflow velocity ($v_{\rm exp}$). This work represents an important first step in determining how the shell model parameters relate to the actual physical properties of Ly$α$ sources. To aid further exploration of the parameter space, we have made our simulated model spectra available through an interactive online tool.

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arXiv2015

Universal predictions of screened modified gravity on cluster scales

Modified gravity models require a screening mechanism to be able to evade the stringent constraints from local gravity experiments and, at the same time, give rise to observable astrophysical and cosmological signatures. Such screened modified gravity models necessarily have dynamics determined by complex nonlinear equations that usually need to be solved on a model-by-model basis to produce predictions. This makes testing them a cumbersome process. In this paper, we investigate whether there is a common signature for all the different models that is suitable to testing them on cluster scales. To do this we propose an observable related to the fifth force, which can be observationally related to the ratio of dynamical-to-lensing mass of a halo, and then show that the predictions for this observable can be rescaled to a near universal form for a large class of modified gravity models. We demonstrate this using the Hu-Sawicki $f(R)$, the Symmetron, the nDGP, and the Dilaton models, as well as unifying parametrizations. The universal form is determined by only three quantities: a strength, a mass, and a width parameter. We also show how these parameters can be derived from a specific theory. This self-similarity in the predictions can hopefully be used to search for signatures of modified gravity on cluster scales in a model-independent way.

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arXiv2015

Blue Wings and Bumps via Fermi-like Acceleration of Lyman-alpha photons across Shocks

We explore the impact of Fermi-like acceleration of Lyman-alpha (Lyα) photons across shock fronts on the observed Lyα spectral line shape. We first confirm the result of Neufeld & McKee (1988) that this mechanism gives rise to extended blue wings which may have been observed in some radio galaxies. Our Monte-Carlo radiative transfer calculations further show that in a minor modification of the shell-model, in which we add an additional static shell of hydrogen, this process can naturally explain the small blue bumps observed in a subset of Lyα emitting galaxies, which have been difficult to explain with conventional shell-models. Blue bumps can be produced with an additional column density of static hydrogen as small as $N_{HI}^{static} \ll N_{HI}^{shell}$, and typically occur at roughly the outflow velocity of the shell. In our model the spectra of so-called 'blue-bump objects' might reflect an evolutionary stage in which the outflows regulating the escape of Lyα photons are still engulfed within a static interstellar medium.

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arXiv2015

Connecting Faint End Slopes of the Lyman-$α$ emitter and Lyman-break Galaxy Luminosity Functions

We predict Lyman-$α$ (Ly$α$) luminosity functions (LFs) of Ly$α$-selected galaxies (Ly$α$ emitters, or LAEs) at $z=3-6$ using the phenomenological model of Dijkstra & Wyithe (2012). This model combines observed UV-LFs of Lyman-break galaxies (LBGs, or drop out galaxies), with constraints on their distribution of Ly$α$ line strengths as a function of UV-luminosity and redshift. Our analysis shows that while Ly$α$ LFs of LAEs are generally not Schechter functions, these provide a good description over the luminosity range of $\log_{10}( L_α/{\rm erg}\,{\rm s}^{-1})=41-44$. Motivated by this result, we predict Schechter function parameters at $z=3-6$. Our analysis further shows that (i) the faint end slope of the Ly$α$ LF is steeper than that of the UV-LF of Lyman-break galaxies, (with a median $α_{Lyα} < -2.0$ at $z\gtrsim 4$), and (ii) a turn-over in the Ly$α$ LF of LAEs at Ly$α$ luminosities $10^{40}$ erg s$^{-1} M_{\rm UV}>-14$. We discuss the implications of these results - which can be tested directly with upcoming surveys - for the Epoch of Reionization.

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arXiv2014

Halo velocity profiles in screened modified gravity theories

Screened modified gravity predicts potentially large signatures in the peculiar velocity field that makes it an interesting probe to test gravity on cosmological scales. We investigate the signatures induced by the Symmetron and a Chameleon $f(R)$ model in the peculiar velocity field using $N$-body simulations. By studying fifth force and halo velocity profiles we identify three general categories of effects found in screened modified gravity models: a fully screened regime where we recover $Λ$CDM to high precision, an unscreened regime where the fifth force is in full operation, and, a partially screened regime where screening occurs in the inner part of a halo, but the fifth force is active at larger radii. These three regimes can be pointed out very clearly by analyzing the deviation in the maximum cluster velocity. Observationally, the partially screened regime is of particular interest since an uniform increase of the gravitational force - as present in the unscreened regime - is degenerate with the (dynamical) halo mass estimate, and, thus, hard to detect.

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arXiv2014

Directional Lya equivalent boosting I. Spherically symmetric distributions of clumps

We quantify the directional dependence of the escape fraction of Lyman-$α$ (Ly$α$) and non-ionising UV-continuum photons from a multiphase medium, and investigate whether there exist directional enhancements in the Ly$α$ equivalent width (EW). Our multiphase medium consists of spherically symmetric distributions of cold, dusty clumps embedded within a hot dust-free medium. We focus on three models from the analysis presented by Laursen et al. (2013). We find that for a Ly$α$ and UV-continuum point source, it is possible to find an EW boost $b(θ,ϕ) > 5 \bar{b}$ in a few per cent of sight lines, where $\bar{b}$ denotes the boost averaged over all photons. For spatially extended sources this directional dependence vanishes quickly when the size of the UV emitting region exceeds the mean distance between cold dusty clumps. Our analysis suggests that directional EW boosting can occur, and that this is mostly driven by reduced escape fractions of UV photons (which gives rise to UV-continuum `shadows'), and less due to an enhanced Ly$α$ escape fraction (or beaming), in certain directions.

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