arXiv subjects

Mark Dijkstra

Publications and source records attributed to Mark Dijkstra.

At least 37 records · Page 2Linked to original sources

arXiv2017

ALMA [CII] detection of a redshift 7 lensed galaxy behind RXJ1347.1-1145

We present the results of ALMA spectroscopic follow-up of a $z=6.765$ Lyman-$α$ emitting galaxy behind the cluster RXJ1347-1145. We report the detection of [CII]158$μ$m line fully consistent with the Lyman-$α$ redshift and with the peak of the optical emission. Given the magnification of $μ=5.0 \pm 0.3$ the intrinsic (corrected for lensing) luminosity of the [CII] line is $L_{[CII]} =1.4^{+0.2}_{-0.3} \times 10^7L_{\odot}$, which is ${\sim}5$ times fainter than other detections of $z\sim 7$ galaxies. The result indicates that low $L_{[CII]}$ in $z\sim 7$ galaxies compared to the local counterparts might be caused by their low metallicities and/or feedback. The small velocity off-set ($Δv = 20_{-40}^{+140} \rm km/s$) between the Lyman-$α$ and [CII] line is unusual, and may be indicative of ionizing photons escaping.

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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.

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arXiv2016

Boosting Lya and HeII 1640A Line Fluxes from Pop III Galaxies: Stochastic IMF Sampling and Departures from Case-B

We revisit calculations of nebular hydrogen Lya and HeII 1640A line strengths for population III galaxies, undergoing continuous and bursts of star formation. We focus on initial mass functions (IMFs) motivated by recent theoretical studies, which generally span a lower range of stellar masses than earlier works. We also account for case-B departures and the stochastic sampling of the IMF. In agreement with previous works, we find that departures from case-B can enhance the Lya flux by a factor of a few, but we argue that this enhancement is driven mainly by collisional excitation and ionization, and not due to photoionization from the n = 2 state of atomic hydrogen. The increased sensitivity of the Lya flux to the high-energy end of the galaxy spectrum makes it more subject to stochastic sampling of the IMF. The latter introduces a dispersion in the predicted nebular line fluxes around the deterministic value by as much as a factor of ~4. In contrast, the stochastic sampling of the IMF has less impact on the emerging Lyman Werner (LW) photon flux. When case-B departures and stochasticity effects are combined, nebular line emission from population III galaxies can be up to one order of magnitude brighter than predicted by 'standard' calculations that do not include these effects. This enhances the prospects for detection with future facilities such as JWST and large, groundbased telescopes.

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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

Detection of Lyman-Alpha Emission From a Triple Imaged z=6.85 Galaxy Behind MACS J2129.4-0741

We report the detection of Ly$α$ emission at $\sim9538$Å in the Keck/DEIMOS and \HST WFC3 G102 grism data from a triply-imaged galaxy at $z=6.846\pm0.001$ behind galaxy cluster MACS J2129.4$-$0741. Combining the emission line wavelength with broadband photometry, line ratio upper limits, and lens modeling, we rule out the scenario that this emission line is \oii at $z=1.57$. After accounting for magnification, we calculate the weighted average of the intrinsic Ly$α$ luminosity to be $\sim1.3\times10^{42}~\mathrm{erg}~\mathrm{s}^{-1}$ and Ly$α$ equivalent width to be $74\pm15$Å. Its intrinsic UV absolute magnitude at 1600Å is $-18.6\pm0.2$ mag and stellar mass $(1.5\pm0.3)\times10^{7}~M_{\odot}$, making it one of the faintest (intrinsic $L_{UV}\sim0.14~L_{UV}^*$) galaxies with Ly$α$ detection at $z\sim7$ to date. Its stellar mass is in the typical range for the galaxies thought to dominate the reionization photon budget at $z\gtrsim7$; the inferred Ly$α$ escape fraction is high ($\gtrsim 10$\%), which could be common for sub-$L^*$ $z\gtrsim7$ galaxies with Ly$α$ emission. This galaxy offers a glimpse of the galaxy population that is thought to drive reionization, and it shows that gravitational lensing is an important avenue to probe the sub-$L^*$ galaxy population.

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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

On the contribution of fluorescence to Ly$α$ halos (LAHs) around star forming galaxies

We quantify the contribution of Ly$α$ fluorescence to observed spatially extended Ly$α$ halos around Ly$α$ emitters (LAE) at redshift ${\rm z=3.1}$. The key physical quantities that describe the fluorescent signal include (${\it i}$) the distribution of cold gas in the circum-galactic medium (CGM); we explore simple analytical models and fitting functions to recent hydrodynamical simulations; (${\it ii}$) local variations in the ionizing background due to ionizing sources that cluster around the central galaxy. We account for clustering by boosting the observationally inferred volumetric production rate of ionizing photons, $ε_{\rm LyC}$, by a factor of $1+ξ_{\rm LyC}(r)$, in which $ξ_{\rm LyC}(r)$ quantifies the clustering of ionizing sources around the central galaxy. We compute $ξ_{\rm LyC}(r)$ by assigning an 'effective' bias parameter to the ionizing sources. This novel approach allows us to quantify our ignorance of the population of ionizing sources in a simple parametrized form. We find a maximum enhancement in the local ionizing background in the range $50-200$ at $r \sim 10$ physical kpc. For spatially uncorrelated ionizing sources and fluorescing clouds we find that fluorescence can contribute up to $\sim 50-60\%$ of the observed spatially extended Ly$α$ emission. We briefly discuss how future observations can shed light on the nature of Ly$α$ halos around star forming galaxies.

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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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arXiv2016

Spectro-polarimetry confirms central powering of a Ly$α$ nebula at z=3.09

We present a follow-up study to the imaging polarimetry performed by Hayes et. al. 2011 on LAB1 in the SSA22 protocluster region. Arguably the most well-known Lyman-$α$ "blob", this radio-quiet emission-line nebula likely hosts a galaxy which is either undergoing significant star formation or hosts an AGN, or both. We obtain deep, spatially resolved spectro-polarimetry of the Ly$α$ emission and detect integrated linear polarization of $9$-$13\%\pm2$-$3\%$ at a distance of approximately 15 kpc north and south of the peak of the Lyman-$α$ surface brightness with polarization vectors lying tangential to the galactic central source. In these same regions, we also detect a wavelength dependence in the polarization which is low at the center of the Ly$α$ line profile and rises substantially in the wings of the profile. These polarization signatures are easily explained by a weak out-flowing shell model. The spectral dependence of the polarization presented here provide a framework for future observations and interpretations of the southern portion of LAB1 in that any model for this system must be able to reproduce this particular spectral dependence. However, questions still remain for the northern-most spur of LAB1. In this region we detect total linear polarization of between $3$ and $20\%$ at the $5\%$ significance level. Simulations predict that polarization should increase with radius for a symmetric geometry. That the northern spur does not suggests either that this region is not symmetric (which is likely) and exhibits variations in columns density, or that it is kinematically distinct from the rest of LAB1 and powered by another mechanism altogether.

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arXiv2016

3-cm Fine Structure Masers: A Unique Signature of Supermassive Black Hole Formation via Direct Collapse in the Early Universe

The direct collapse black hole (DCBH) scenario describes the isothermal collapse of a pristine gas cloud directly into a massive, M_BH=10^4-10^6 M_sun black hole. In this paper we show that large HI column densities of primordial gas at T~10^4 K with low molecular abundance - which represent key aspects of the DCBH scenario - provide optimal conditions for pumping of the 2p-level of atomic hydrogen by trapped Lyman alpha (Lya) photons. This Lya pumping mechanism gives rise to inverted level population of the 2s_1/2-2p_3/2 transition, and therefore to stimulated fine structure emission at 3.04 cm (rest-frame). We show that simplified models of the DCBH scenario amplify the CMB by up to a factor of 10^5, above which the maser saturates. Hyperfine splitting of the 3-cm transition gives rise to a characteristic broad (FWHM ~ tens of MHz in the observers frame) asymmetric line profile. This signal subtends an angular scale of ~ 1-10 mas, which translates to a flux of ~ 0.3-3 microJy, which is detectable with ultra-deep surveys being planned with SKA1-MID. While challenging, as the signal is visible for a fraction of the collapse time of the cloud, the matching required physical conditions imply that a detection of the redshifted 3-cm emission line would provide direct evidence for the DCBH scenario.

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arXiv2015

Constraining Reionization with Lyman Alpha Emitting Galaxies

Neutral diffuse intergalactic gas that existed during the Epoch of Reionization (EoR) suppresses Lyman Alpha (Lya) flux emitted by background galaxies. In this chapter I summarise the increasing observational support for the claim that Lya photons emitted by galaxies at z>6 are suppressed by intervening HI gas. I describe key physical processes that affect Lya transfer during the EoR. I argue that in spite of the uncertainties associated with this complex multiscale problem, the data on Lya emitting galaxies at z=0-6 strongly suggests that the observed reduction in Lya flux from galaxies at z>6 is due to additional intervening HI gas. The main question is what fraction of this additional HI gas is in the diffuse neutral IGM. I summarise how future surveys on existing and upcoming instruments are expected to reduce existing observational uncertainties enormously. With these improved data we will likely be able to nail down reionization with Lya emitting galaxies.

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arXiv2015

Lyman-Alpha Emitting Galaxies as a Probe of Reionization: Large-Scale Bubble Morphology and Small-Scale Absorbers

The visibility of LyA emitting galaxies during the Epoch of Reionization is controlled by both diffuse HI patches in large-scale bubble morphology and small-scale absorbers. To investigate the impact on LyA photons, we apply a novel combination of analytic and numerical calculations to three scenarios: (i) the `bubble' model, where only diffuse HI outside ionized bubbles is present; (ii) the `web' model, where HI exists only in overdense self-shielded gas; and (iii) the more realistic 'web-bubble' model, which contains both. Our analysis confirms that there is a degeneracy between the ionization structure of the intergalactic medium (IGM) and the HI fraction inferred from LyA surveys, as the three models suppress LyA flux equally with very different HI fractions. We argue that a joint analysis of the LyA luminosity function and the rest-frame equivalent width distribution/LyA fraction can break this degeneracy and provide constraints on the reionization history and its topology. We further show that constraints can improve if we consider the full shape of the M_UV-dependent redshift evolution of the LyA fraction of Lyman break galaxies. Contrary to conventional wisdom, we find that (i) a drop of LyA fraction larger for UV-faint than for UV-bright galaxies can be reproduced with web and web-bubble models and therefore does not provide exclusive evidence of patchy reionization, and (ii) the IGM-transmission PDF is unimodal for bubble models and bimodal in web models. We further highlight the importance of galaxy-absorber cross-correlation. Comparing our models to observations, the neutral fraction at z~7 is likely to be of order of tens of per cent when interpreted with bubble or web-bubble models. Alternatively, we obtain a conservative lower limit ~1% in the web models, if we allow for a drop in the photoionization rate by a factor of ~100 from the post-reionized universe. [abridged]

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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

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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arXiv2015

The LyAlpha Line Profiles of Ultraluminous Infrared Galaxies: Fast Winds and Lyman Continuum Leakage

We present new Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet (far-UV) spectroscopy and Keck Echellete optical spectroscopy of 11 ultraluminous infrared galaxies (ULIRGs), a rare population of local galaxies experiencing massive gas inflows, extreme starbursts, and prominent outflows. We detect H Lyman alpha emission from 8 ULIRGs and the companion to IRAS09583+4714. In contrast to the P Cygni profiles often seen in galaxy spectra, the H Lyman alpha profiles exhibit prominent, blueshifted emission out to Doppler shifts exceeding -1000 km/s in three HII-dominated and two AGN-dominated ULIRGs. To better understand the role of resonance scattering in shaping the H Lyman alpha line profiles, we directly compare them to non-resonant emission lines in optical spectra. We find that the line wings are already present in the intrinsic nebular spectra, and scattering merely enhances the wings relative to the line core. The H Lyman alpha attenuation (as measured in the COS aperture) ranges from that of the far-UV continuum to over 100 times more. A simple radiative transfer model suggests the H Lyman alpha photons escape through cavities which have low column densities of neutral hydrogen and become optically thin to the Lyman continuum in the most advanced mergers. We show that the properties of the highly blueshifted line wings on the H Lyman alpha and optical emission-line profiles are consistent with emission from clumps of gas condensing out of a fast, hot wind. The luminosity of the H Lyman alpha emission increases non-linearly with the ULIRG bolometric luminosity and represents about 0.1 to 1% of the radiative cooling from the hot winds in the HII-dominated ULIRGs.

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arXiv2014

The impact of gas bulk rotation on the lyman-alpha line

We present results of radiative transfer calculations to measure the impact of gas bulk rotation on the morphology of the Lyman $α$ emission line in distant galaxies. We model a galaxy as a sphere with an homogeneous mixture of dust and hydrogen at a constant temperature. These spheres undergo solid-body rotation with maximum velocities in the range 0-300 \kms and neutral hydrogen optical depths in the range $τ_{\rm H}=10^{5}-10^{7}$. We consider two types of source distributions in the sphere: central and homogeneous. Our main result is that rotation introduces a dependence of the line morphology with viewing angle and rotational velocity. Observations with a line of sight parallel to the rotation axis yield line morphologies similar to the static case. For lines of sight perpendicular to the rotation axis both the intensity at the line center and the line width increase with rotational velocity. Along the same line of sight, the line becomes single peaked at rotational velocities close to half the line width in the static case. Notably, we find that rotation does not induce any spatial anisotropy in the integrated line flux, the escape fraction or the average number of scatterings. This is because Lyman α scattering through a rotating solid-body proceeds identical as in the static case. The only difference is the doppler shift from the different regions in the sphere that move with respect to the observer. This allows us to derive an analytic approximation for the viewing-angle dependence of the emerging spectrum, as a function of rotational velocity.

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