arXiv subjects

Mark Dijkstra

Publications and source records attributed to Mark Dijkstra.

At least 73 records · Page 4Linked to original sources

arXiv2009

Lyman Alpha Blobs as an Observational Signature of Cold Accretion Streams into Galaxies

Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T ~ 1e4 K) gas flowing into the centers of dark matter halos as massive as 1e12-1e13.5 M_sun at redshifts z~1-3. In this paper we show that if > 20% of the gravitational binding energy of the gas is radiated away, then the simulated cold flows are spatially extended Lyman Alpha (Lya) sources with luminosities, Lya line widths, and number densities that are comparable to those of observed Lya blobs. Furthermore, the filamentary structure of the cold flows can explain the wide range of observed Lya blob morphologies. Since the most massive halos form in dense environments, the association of Lya blobs with overdense regions arise naturally. We argue that Lya blobs - even those which are clearly associated with starburst galaxies or quasars - provide direct observational support for the cold accretion mode of galaxies. We discuss various testable predictions of this association.

astro-ph.CO↗

arXiv2009

Upper Limit on Dimming of Cosmological Sources by Intergalactic Grey Dust from the Soft X-ray Background

Active Galactic Nuclei (AGN) produce a dominant fraction (~80%) of the Soft X-ray background (SXB) at photon energies 0.5<E<2 keV. If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray halos around AGN. Taking account of known galaxies and galaxy clusters, only a fraction F_halo <10% of the SXB can be in the form of diffuse X-ray halos around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains (with radii in the range a=0.2-2.0 mum) to a level tau_GD<0.15(F_halo/10%) to a redshift z~1. Our results are only weakly dependent on the grain size distribution or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state.

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arXiv2009

On the Detectability of the Hydrogen 3-cm Fine Structure Line from the EoR

A soft ultraviolet radiation field, 10.2 eV < E <13.6 eV, that permeates neutral intergalactic gas during the Epoch of Reionization (EoR) excites the 2p (directly) and 2s (indirectly) states of atomic hydrogen. Because the 2s state is metastable, the lifetime of atoms in this level is relatively long, which may cause the 2s state to be overpopulated relative to the 2p state. It has recently been proposed that for this reason, neutral intergalactic atomic hydrogen gas may be detected in absorption in its 3-cm fine-structure line (2s_1/2 -> 2p_3/2) against the Cosmic Microwave Background out to very high redshifts. In particular, the optical depth in the fine-structure line through neutral intergalactic gas surrounding bright quasars during the EoR may reach tau~1e-5. The resulting surface brightness temperature of tens of micro K (in absorption) may be detectable with existing radio telescopes. Motivated by this exciting proposal, we perform a detailed analysis of the transfer of Lyman beta,gamma,delta,... radiation, and re-analyze the detectability of the fine-structure line in neutral intergalactic gas surrounding high-redshift quasars. We find that proper radiative transfer modeling causes the fine-structure absorption signature to be reduced tremendously to tau< 1e-10. We therefore conclude that neutral intergalactic gas during the EoR cannot reveal its presence in the 3-cm fine-structure line to existing radio telescopes.

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arXiv2009

Acceleration of Galactic Supershells by Lyman Alpha Radiation

Scattering of Lyman Alpha (hereafter Lya) photons by neutral hydrogen gas in a single outflowing 'supershell' around star forming regions often explains the shape and offset of the observed Lya emission line from galaxies. We compute the radiation pressure that is exerted by this scattered Lya radiation on the outflowing material. We show that for reasonable physical parameters, Lya radiation pressure alone can accelerate supershells to velocities in the range v_sh=200-400 km/s. These supershells possibly escape from the gravitational potential well of their host galaxies and contribute to the enrichment of the intergalactic medium. We compute the physical properties of expanding supershells that are likely to be present in a sample of known high-redshift (z=2.7-5.0) galaxies, under the assumption that they are driven predominantly by Lya radiation pressure. We predict ranges of radii r_sh=0.1-10 kpc, ages t_sh=1-100 Myr, and energies E_sh=1e53-1e55 ergs, which are in reasonable agreement with the properties of local galactic supershells. Furthermore, we find that the radius, r_sh, of a Lya-driven supershell of constant mass depends uniquely on the intrinsic Lya luminosity of the galaxy, L_alpha, the HI column density of the supershell, N_HI, and the shell speed, v_sh, through the scaling relation r_sh ~ L_alpha/(N_HI v_sh^2). We derive mass outflow rates in supershells that reach ~10-100% of the star formation rates of their host galaxies.

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arXiv2008

Fluctuations in the High-Redshift Lyman-Werner Background: Close Halo Pairs as the Origin of Supermassive Black Holes

The earliest generation of stars and black holes must have established an early 'Lyman-Werner' background (LWB) at high redshift, prior to the epoch of reionization. Because of the long mean free path of photons with energies E<13.6 eV, the LWB was nearly uniform. However, some variation in the LWB is expected due to the discrete nature of the sources, and their highly clustered spatial distribution. In this paper, we compute the probability distribution function (PDF) of the LW flux that irradiates dark matter (DM) halos collapsing at high-redshift (z~10). Our model accounts for (i) the clustering of DM halos, (ii) Poisson fluctuations in the number of corresponding star forming galaxies, and (iii) scatter in the LW luminosity produced by halos of a given mass (calibrated using local observations). We find that > 99% of the DM halos are illuminated by a LW flux within a factor of 2 of the global mean value. However, a small fraction, ~1e-8 to 1e-6, of DM halos with virial temperatures above 1e4 K have a close luminous neighbor within < 10 kpc, and are exposed to a LW flux exceeding the global mean by a factor of > 20, or to J_(21,LW)> 1e3 (in units of 1e-21 erg/s/Hz/sr/cm^2). This large LW flux can photo--dissociate H_2 molecules in the gas collapsing due to atomic cooling in these halos, and prevent its further cooling and fragmentation. Such close halo pairs therefore provide possible sites in which primordial gas clouds collapse directly into massive black holes (M_BH~ 1e4 - 1e6 M_sun), and subsequently grow into supermassive (M_BH > 1e9 M_sun) black holes by z~6.

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arXiv2008

Continuum Emission by Cooling Clouds

The collapse of baryons into the center of a host dark matter halo is accompanied by radiation that may be detectable as compact (< 10 kpc) UV-continuum and Lyman Alpha (hereafter Lya) emission with Lya luminosities as high as ~1e42-1e43 erg/s in halos of mass M=1e11-10e12 solar masses. We show that the observed equivalent width (EW) of the Lya line emitted by these cooling clouds is EW 400 Angstrom (restframe). These luminosities and EWs are comparable to those detected in narrowband surveys for redshifted Lya emission. The rest-frame ultraviolet of Lya emitting cooling clouds radiation may be dominated by two-photon transitions from 2s->1s. The resulting spectrum can distinguish cooling clouds from a broad class of young star forming galaxies.

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arXiv2008

Lyman Alpha Driven Outflows Around Star Forming Galaxies

We present accurate Monte-Carlo calculations of Lyman Alpha (hereafter Lya) radiation pressure in a range of models which represent galaxies during various epochs of our Universe. We show that the radiation force that Lya photons exert on hydrogen gas in the neutral intergalactic medium (IGM), that surrounds minihalos that host the first stars, may exceed gravity by orders of magnitude and drive supersonic winds. Lya radiation pressure may also dominate over gravity in the neutral IGM that surrounds the HII regions produced by the first galaxies. However, the radiation force is likely too weak to result in supersonic outflows in this case. Furthermore, we show that Lya radiation pressure may drive outflows in the interstellar medium of star forming galaxies that reach hundreds of km/s. This mechanism could also operate at lower redshifts z<6, and may have already been indirectly detected in the spectral line shape of observed Lya emission lines.

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arXiv2008

The Polarization of Scattered Lyman Alpha Radiation Around High-Redshift Galaxies

The high-redshift Universe contains luminous Lyman Alpha (hereafter Lya) emitting sources such as galaxies and quasars. The emitted Lya radiation is often scattered by surrounding neutral hydrogen atoms. We show that the scattered Lya radiation obtains a high level of polarization for a wide range of likely environments of high-redshift galaxies. For example, the back-scattered Lya flux observed from galaxies surrounded by a superwind-driven outflow may reach a fractional polarization as high as ~40%. Equal levels of polarization may be observed from neutral collapsing protogalaxies. Resonant scattering in the diffuse intergalactic medium typically results in a lower polarization amplitude (<7%), which depends on the flux of the ionizing background. Spectral polarimetry can differentiate between Lya scattering off infalling gas and outflowing gas; for an outflow the polarization should increase towards longer wavelengths while for infall the opposite is true. Our numerical results suggest that Lya polarimetry is feasible with existing instruments, and may provide a new diagnostic of the distribution and kinematics of neutral hydrogen around high-redshift galaxies. Moreover, polarimetry may help suppress infrared lines originating in the Earth's atmosphere, and thus improve the sensitivity of ground-based observations to high-redshift Lya emitting galaxies outside the currently available redshift windows.

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arXiv2008

UV Radiative Feedback During the Advanced Stages of Reionization

The ionizing ultraviolet background (UVB) during reionization can suppress the gas content of low-mass galaxies, even those capable of efficient atomic cooling, and thus lead to an extended reionization epoch. In this work, we explore the importance of negative UV radiative feedback on Tvir > 10^4 K halos during the middle and late stages of reionization. We do not try to self-consistently model reionization; instead, we explore a large parameter space in an attempt to draw general, robust conclusions. We do this using a tiered approach. Using 1-D hydrodynamical simulations, we model the collapse of gas onto halos of various masses under UVBs of various intensities. We then generate realistic, parametrized maps of the inhomogeneous UVB, using large-scale semi-numeric simulations. By combining these results, we find that under all reasonably conservative scenarios, UV feedback on atomically-cooled halos is not strong enough to notably delay the bulk of reionization. Such a delay is only likely if ionizing efficiencies of z > 10 sources are much higher (~ two orders of magnitude) than z ~ 6 data seem to imply. We also find that feedback is very strongly dependent on halo mass. Our results suggest that the natural time-scale for the bulk of reionization is the growth of the global collapsed fraction contained in Tvir > 10^4 K halos. Finally, our results underscore the importance of taking into account extended dynamical ranges when modeling reionization.

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arXiv2007

Requirements for Cosmological 21-cm Masers

We perform Monte-Carlo calculations of the radiative transfer of Lyman Alpha (hereafter Lya) photons emitted by a source embedded in a neutral collapsing gas cloud. This represents a young galaxy or quasar during the early stages of the epoch of reionisation (EoR). After computing the Lya spectrum as function of radius and time, we find that the Lya color temperature may be negative in large volumes surrounding the central source. Motivated by this result, we explore the prospects for a population inversion in the hyperfine levels of atomic hydrogen via the Wouthuysen-Field (WF) effect. The reason for this exploration is clear: if 21-cm masers exist during the EoR, they could greatly boost the expected 21-cm flux from this epoch. We find that population inversion is unlikely to occur for several reasons: (1) the required Lya luminosities of the central source exceed ~10e45 erg/s. The radiation pressure exerted by such a large Lya flux likely halts the collapse of the cloud; (2) When quantum corrections to the WF-coupling strength are applied, the required Lya luminosities are (even) larger by orders of magnitude; (3) A relatively low flux of Lya photons that is produced via other channels (x-ray heating, collisional excitation of hydrogen) prevents the Lya color temperature from becoming negative.

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arXiv2007

Very Massive Stars in High-Redshift Galaxies

A significant fraction of Lyman Alpha (Lya) emitting galaxies (LAEs) at z> 5.7 have rest-frame equivalent widths (EW) greater than ~100 Angstrom. However only a small fraction of the Lya flux produced by a galaxy is transmitted through the IGM, which implies intrinsic Lya EWs that are in excess of the maximum allowed for a population-II stellar population having a Salpeter mass function. In this paper we study characteristics of the sources powering Lya emission in high redshift galaxies. We propose a simple model for Lya emitters in which galaxies undergo a burst of very massive star formation that results in a large intrinsic EW, followed by a phase of population-II star formation with a lower EW. We confront this model with a range of high redshift observations and find that the model is able to simultaneously describe the following eight properties of the high redshift galaxy population with plausible values for parameters like the efficiency and duration of star formation: i-iv) the UV and Lya luminosity functions of LAEs at z=5.7 and 6.5, v-vi) the mean and variance of the EW distribution of Lya selected galaxies at z=5.7, vii) the EW distribution of i-drop galaxies at z~6, and viii) the observed correlation of stellar age with EW. Our modeling suggests that the observed anomalously large intrinsic equivalent widths require a burst of very massive star formation lasting no more than a few to ten percent of the galaxies star forming lifetime. This very massive star formation may indicate the presence of population-III star formation in a few per cent of i-drop galaxies, and in about half of the Lya selected galaxies.

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arXiv2007

Luminosity Functions of Lyman Alpha Emitting Galaxies and Cosmic Reionization of Hydrogen

Recent observations imply that the observed number counts of Lya Emitters (LAEs) evolved significantly between z=5.7 and z=6.5. It has been suggested that this was due to a rapid evolution in the ionisation state, and hence transmission of the IGM which caused Lya flux from z=6.5 galaxies to be more strongly suppressed. In this paper we consider the joint evolution of the Lya and UV luminosity functions (LFs) and show that the IGM transmission evolved between z=6.5 and z=5.7 by a factor 1.1 0.5.

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arXiv2007

The Impact of The IGM on High-Redshift Lyman Alpha Emission Lines

We calculate the impact of the intergalactic medium (IGM) on the observed Lyman alpha lines (hereafter Lya) emitted by galaxies in an ionised IGM at z>4. Our model accounts for gas clumping in the IGM and for the fact that high-redshift galaxies reside in overdense regions, which causes the velocity field of the IGM to depart from the Hubble flow. The observed shape of the Lya line varies widely, with dependence on the intrinsic width and systemic velocity of the line, a galaxies star formation rate and the local extra-galactic UV-background. For large star formation rates and levels of the UV-background, absorption in the IGM does not result in a Lya line that is asymmetric as is common among known high-redshift Lya emitters. For models in which the lines do show the observed strong asymmetry, the IGM typically transmits only 10-30% of the Lya flux. The increase in the ionising background that accompanied the completion of reionisation barely increased the IGM transmission, which suggests that LAEs of comparable luminosity should not appear to be significantly dimmer prior to overlap. In this light, we briefly discuss the potential of Lya emitters as a probe into the epoch of reionisation.

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arXiv2006

Lyman Alpha Constraints on Very Low Luminosity AGN

Recent surveys have detected Lya emission from z=4.5-6.5 at luminosities as low as 10^41 erg/s. There is good evidence that low numbers of AGN are among observed faint Lya emitters. Combining these observations with an empirical relation between the intrinsic Lya and B-band luminosities of AGN, we obtain an upper limit on the number density of AGN with absolute magnitudes M_B=[-16,-19] at z=4.5-6.5. These AGN are up to two orders of magnitude fainter than those discovered in the Chandra Deep Field, resulting in the faintest observational constraints to date at these redshifts. At z=4.5, the powerlaw slope of the very faint end of the luminosity function of AGN is shallower than the slope observed at lower redshifts, beta <1.6, at the 98% confidence level. In fact, we find marginal evidence that the luminosity function rises with luminosity, corresponding to a powerlaw slope beta <0, at magnitudes fainter than M_B~-20 (75% confidence level). These results suggest either that accretion onto lower mass black holes is less efficient than onto their more massive counterparts, or that the number of black holes powering AGN with M_B >-20 is lower than expected from the M_BH-sigma relation by one-two orders of magnitude. Extrapolating from reverberation-mapping studies suggests that these black holes would have M_BH=10^6-10^7 Msun. To facilitate the identification of AGN among observed Lya emitters, we derive observational properties of faint AGN in the Lya line, as well as in the X-ray and optical bands.

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arXiv2006

Lyman Alpha Radiation From Collapsing Protogalaxies I: Characteristics of the Emergent Spectrum

We present Monte Carlo calculations of Lyman alpha (Lya) radiative transfer through collapsing gas clouds, representing proto-galaxies that are caught in the process of their assembly. Such galaxies produce Lya flux over an extended solid angle from a spatially extended Lya emissivity and/or from scattering effects. We study the effect of the gas distribution and kinematics, and of the Lya emissivity profile, on the emergent spectrum and surface brightness distribution. The emergent Lya spectrum is typically double-peaked and asymmetric. In practice, the blue peak is significantly enhanced and the red peak, in most cases, will be undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, will render extended Lya emission from collapsing protogalaxies difficult to detect beyond redshift z=4. The surface brightness distribution is typically flat, and a strong wavelength dependence of its slope (with preferential flattening at the red side of the line) would be a robust indication that Lya photons are being generated (rather than just scattered) in a spatially extended region around the galaxy. We also find that for self-ionized clouds whose effective Lya optical depth is less than 10^3, infall and outflow models can produce nearly identical spectra and surface brightness distributions, and are difficult to distinguish from one another. The presence of deuterium with a cosmic abundance may produce a narrow but detectable dip in the spectra of systems with moderate hydrogen column densities, in the range 10^18-10^20 cm^-2. Finally, we present a new analytic solution for the emerging Lya spectrum in the limiting case of a static uniform sphere, extending previous solutions for static plane-parallel slabs.

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arXiv2006

Lyman Alpha Radiation From Collapsing Protogalaxies II: Observational Evidence for Gas Infall

We model the spectra and surface brightness distributions for the Lyman alpha (Lya) radiation expected from protogalaxies that are caught in the early stages of their assembly. We use the results of a companion paper to characterize the radiation emerging from spherically collapsing gas clouds. We then modify the intrinsic spectra to incorporate the effect of subsequent resonant scattering in the intergalactic medium (IGM). Using these models, we interpret a number of recent observations of extended Lya blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, as well as the relatively shallow surface brightness profiles, and double-peaked spectra, that several of these LABs may be associated with collapsing protogalaxies. We suggest two follow-up observations to diagnose the presence of gas infall. High S/N spectra of LABs should reveal a preferential flattening of the surface brightness profile at the red side of the line. Complementary imaging of the blobs at redshifted Balmer alpha wavelengths should reveal the intrinsic Lya emissivity and allow its separation from radiative transfer effects. We show that Lya scattering by infalling gas can reproduce the observed spectrum of Steidel et al's LAB2 as accurately as a recently proposed outflow model. Finally, we find similar evidence for infall in the spectra of point-like Lyman alpha emitters. The presence of scattering by the infalling gas implies that the intrinsic Lya luminosities, and derived quantities, such as the star-formation rate, in these objects may have been underestimated by about an order of magnitude.

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arXiv2005

On Detecting the X-ray Silhouette of a Damped Lyman alpha System

We explore the possibility of resolving an image of a damped Lyman alpha (DLA) system in absorption against an extended, diffuse background X-ray source. Typical columns of neutral hydrogen in DLAs are high enough to block out up to ~30% of the soft X-ray flux at an observed photon energy of 0.5 keV, and we find that ~ 1% of the area of extended X-ray sources at z > 1 have their 0.5 keV flux reduced by at least 20%. We discuss the observability of such absorption and find that < 2 arcsecond resolution, and > 300 photons per angular resolution element are required in the 0.3-8 keV band for its detection, and in order to distinguish it from intrinsic surface brightness fluctuations. For the surface brightness of the currently known high-redshift extended X-ray sources, this requires an integration time of a few Msec on Chandra. The detection will be within the reach of a routine observation with a next generation X-ray telescope such as XEUS or Generation X.

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arXiv2004

The Growth of the Earliest Supermassive Black Holes and Their Contribution to Reionization

We discuss currently available observational constraints on the reionization history of the intergalactic medium (IGM), and the extent to which accreting black holes (BHs) can help explain these observations. We show new evidence, based on the combined statistics of Lyman alpha and beta absorption in quasar spectra, that the IGM contains a significant amount of neutral hydrogen, and is experiencing rapid ionization around redshift z=6. However, we argue that quasar BHs, even faint ones that are below the detection thresholds of existing optical surveys, are unlikely to drive the evolution of the neutral fraction around this epoch, because they would over-produce the present-day soft X-ray background. On the other hand, the seeds of the quasar BHs around z=6 likely appeared at much earlier epochs (around z=20), and produced hard ionizing radiation by accretion. These early BHs are promising candidates to account for the high redshift ionization (around z=15) implied by the recent cosmic microwave anisotropy data from WMAP. Using a model for the growth of BHs by accretion and mergers in a hierarchical cosmology, we suggest that the early growth of quasars must include a super-Eddington growth phase, and that, although not yet optically identified, the FIRST radio survey may have already detected several thousand BHs at z>6 with Mbh>10^8 Msun.

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