Mark Dijkstra

Photoionization Feedback in Low-Mass Galaxies at High Redshift

The cosmic ultraviolet ionizing background impacts the formation of dwarf galaxies in the low-redshift universe (z 3) by suppressing gas infall into galactic halos with circular velocities up to vcirc ~ 75 km s-1. Using a one-dimensional, spherically symmetric hydrodynamics code (Thoul & Weinberg), we examine the effect of an ionizing background on low-mass galaxies forming at high redshifts (z 10). We find that the importance of photoionization feedback is greatly reduced because (1) at high redshift, dwarf galaxy-sized objects can self-shield against the ionizing background, (2) collisional cooling processes at high redshift are more efficient, (3) the amplitude of the ionizing background at high redshift is lower, and (4) the ionizing radiation turns on when the perturbation that will become the dwarf galaxy has already grown to a substantial overdensity. We find that because of these reasons, gas can collect inside halos with circular velocities as low as vcirc ~ 10 km s-1 at z > 10. This result has important implications for the reionization history of the universe.

Ly alpha radiation from collapsing protogalaxies. I. Characteristics of the emergent spectrum

We present Monte Carlo calculations of Ly alpha radiative transfer through optically thick, spherically symmetric, collapsing gas clouds. These represent simplified models of protogalaxies in the process of their assembly. Such galaxies produce Ly alpha flux over an extended solid angle, either from a spatially extended Ly alpha emissivity, or from scattering effects, or both. We present a detailed study of the effect of the gas distribution and kinematics and of the Ly alpha emissivity profile on the emergent spectrum and surface brightness distribution. The emergent Ly alpha spectrum is typically double peaked and asymmetric. In practice, however, we find energy transfer from the infalling gas to the Ly alpha photons to significantly enhance the blue peak and to render the red peak, in most cases, undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, renders extended Ly alpha emission from collapsing protogalaxies difficult to detect beyond redshift z greater than or similar to 4. We find a strong wavelength dependence of the slope of the surface brightness distribution (with preferential flattening at the red side of the line) to be a robust indication that Ly alpha photons are being generated (rather than just scattered) in a spatially extended, collapsing region around the galaxy. For self-ionized clouds whose effective Ly alpha optical depth is less than or similar to 10(3), infall and outflow models can produce nearly identical spectra and surface brightness distributions and are practically indistinguishable. The presence of cosmic abundance of deuterium may produce a detectable dip in the spectra of systems with moderate hydrogen column densities, N(H) = 10(18)-10(20) cm(-2). Finally, we present a new analytic solution for the emerging Ly alpha spectrum in the limiting case of a static uniform sphere, extending previous solutions for slabs.

Lyα blobs as an observational signature of cold accretion streams into galaxies

Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T ∼ 10 4 K) gas flowing into the centres of dark matter haloes as massive as 10 12-13.5 M ⊙ at redshifts z ∼ 1-3. In this paper, we show that if ≳20 per cent of the gravitational binding energy of the gas is radiated away then the simulated cold flows are spatially extended Lyα sources with luminosities, Lyα linewidths and number densities that are comparable to those of observed Lyα blobs. Furthermore, the filamentary structure of the cold flows can explain the wide range of observed Lyα blob morphologies. Since the most massive haloes form in dense environments, the association of Lyα blobs with overdense regions arise naturally. We argue that Lyα 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.

New Observations of z ~ 7 Galaxies: Evidence for a Patchy Reionization

We present new results from our search for z ~ 7 galaxies from deep spectroscopic observations of candidate z dropouts in the CANDELS fields. Despite the extremely low flux limits achieved by our sensitive observations, only two galaxies have robust redshift identifications, one from its Lyα emission line at z = 6.65, the other from its Lyman break, i.e., the continuum discontinuity at the Lyα wavelength consistent with a redshift of 6.42 but with no emission line. In addition, for 23 galaxies we present deep limits in the Lyα equivalent width derived from the nondetections in ultradeep observations. Using this new data as well as previous samples, we assemble a total of 68 candidate z ~ 7 galaxies with deep spectroscopic observations, of which 12 have a line detection. With this much enlarged sample we can place solid constraints on the declining fraction of Lyα emission in z ~ 7 Lyman-break galaxies compared to z ~ 6, both for bright and faint galaxies. Applying a simple analytical model, we show that the present data favor a patchy reionization process rather than a smooth one.

Lyα emitting galaxies as a probe of reionisation

The Epoch of Reionization (EoR) represents a milestone in the evolution of our Universe. Star-forming galaxies that existed during the EoR likely emitted a significant fraction (∼ 5− 40%) of their bolometric luminosity as Lyα line emission. However, neutral intergalactic gas that existed during the EoR was opaque to Lyα emission that escaped from galaxies during this epoch, which makes it difficult to observe. The neutral intergalactic medium (IGM) may thus reveal itself by suppressing the Lyα flux from background galaxies. Interestingly, a ‘sudden’ reduction in the observed Lyα flux has now been observed in galaxies at z > 6. This review contains a detailed summary of Lyα radiative processes: I describe (i) the main Lyα emission processes, including collisional-excitation & recombination (and derive the origin of the famous factor ‘0.68’), and (ii) basic radiative transfer concepts, including e.g. partially coherent scattering, frequency diffusion, resonant versus wing scattering, optically thick versus ’extremely’ optically thick (static/outflowing/collapsing) media, and multiphase media. Following this review, I derive expressions for the Gunn-Peterson optical depth of the IGM during (inhomogeneous) reionization and post-reionization. I then describe why current observations appear to require a very rapid evolution of volume-averaged neutral fraction of hydrogen in the context of realistic inhomogeneous reionization models, and discuss uncertainties in this interpretation. Finally, I describe how existing & futures surveys and instruments can help reduce these uncertainties, and allow us to fully exploit Lyα emitting galaxies as a probe of the EoR.

Feedback-regulated supermassive black hole seed formation

The nature of the seeds of high-redshift supermassive black holes (SMBHs) is a key question in cosmology. Direct collapse black holes (DCBH) that form in pristine, atomic-line cooling halos, illuminated by a Lyman-Werner (LW) UV flux exceeding a critical threshold J_crit, represent an attractive possibility. We investigate when and where these conditions are met during cosmic evolution. For the LW intensity, J_LW, we account for departures from the background value in close proximity to star forming galaxies. For the pristine halo fraction, we account for both (i) supernova driven outflows, and (ii) the inherent pollution from progenitor halos. We estimate the abundance of DCBH formation sites, n_DCBH(z), and find that it increases with cosmic time from n_DCBH(z=20) ~ 1e-12 -1e-7 cMpc^-3 to n_DCBH(z=10) ~ 1e-10 - 1e-5 cMpc^-3. Our analysis shows the possible importance of galactic winds, which can suppress the predicted n_DCBH by several orders of magnitude, and cause DCBH formation to preferentially occur around the UV-brightest (M_UV ~ -22 to -20) star forming galaxies. Our analysis further highlights the dependence of these predictions on (i) the escape fraction of LW photons, (ii) J_crit, and (iii) the galactic outflow prescription.

Lyα Radiation from Collapsing Protogalaxies. II. Observational Evidence for Gas Infall

We model the spectra and surface brightness distributions for the Lyα 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 these spectra to incorporate the effect of subsequent resonant scattering in the IGM. Using these models, we interpret a number of recent observations of extended Lyα blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, 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 Hα wavelengths should reveal the intrinsic Lyα emissivity and allow its separation from radiative transfer effects. We show that Lyα scattering by infalling gas can reproduce the observed spectrum of the Steidel et al. LAB2 as accurately as a recently proposed outflow model. Finally, we find similar evidence for infall in the spectra of pointlike Lyα emitters. The presence of scattering by the infalling gas implies that the intrinsic Lyα luminosities and derived quantities, such as the star formation rate, in these objects may have been underestimated by about an order of magnitude.

Line transfer through clumpy, large‐scale outflows: Ly α absorption and haloes around star‐forming galaxies

We present constrained radiative transfer calculations of Lyα photons propagating through clumpy, dusty, large scale outflows, and explore whether we can quantitatively explain the Lyα halos that have been observed around Lyman Break Galaxies. We construct phenomenological models of large-scale outflows which consist of cold clumps that are in pressure equilibrium with a constant–velocity hot wind. First we consider models in which the cold clumps are distributed symmetrically around the galaxy, and in which the clumps undergo a continuous acceleration in its ‘circumgalactic’ medium (CGM). We constrain the properties of the cold clumps (radius, velocity, H I column density, & number density) by matching the observed Lyα absorption strength of the CGM in the spectra of background galaxies. We then insert a Lyα source in the center of this clumpy outflow, which consists of 10 5 6 clumps, and compute observable properties of the scattered Lyα photons. In these models, the scattered radiation forms halos that are significantly more concentrated than is observed. In order to simultaneously reproduce the observed Lyα absorption line strengths and the Lyα halos, we require – preferably bipolar – outflows in which the clumps decelerate after their initial acceleration. This deceleration is predicted naturally in ‘momentum–driven’ wind models of clumpy outflows. In models that simultaneously fit the absorption and emission line data, the predicted linear polarization is � 30 40% at a surface brightness contour of S = 10 18 erg s 1 cm 2 arcsec 2 . Our work illustrates clearly that Lyα emission line halos around starforming galaxies provide valuable constraints on the cold gas distribution & kinematics in their circumgalactic medium, and that these constraints complement those obtained from absorption line studies alone.

The grism lens-amplified survey from space (glass). i. survey overview and first data release

We give an overview of the Grism Lens Amplified Survey from Space (GLASS), a large Hubble Space Telescope program aimed at obtaining grism spectroscopy of the fields of ten massive clusters of galaxies at redshift z=0.308-0.686, including the Hubble Frontier Fields (HFF). The Wide Field Camera 3 yields near infrared spectra of the cluster cores, covering the wavelength range 0.81-1.69mum through grisms G102 and G141, while the Advanced Camera for Surveys in parallel mode provides G800L spectra of the infall regions of the clusters. The WFC3 spectra are taken at two almost orthogonal position angles in order to minimize the effects of confusion. After summarizing the scientific drivers of GLASS, we describe the sample selection as well as the observing strategy and data processing pipeline. We then utilize MACSJ0717.5+3745, a HFF cluster and the first one observed by GLASS, to illustrate the data quality and the high-level data products. Each spectrum brighter than H_AB=23 is visually inspected by at least two co-authors and a redshift is measured when sufficient information is present in the spectra. Furthermore, we conducted a thorough search for emission lines through all the GLASS WFC3 spectra with the aim of measuring redshifts for sources with continuum fainter than H_AB=23. We provide a catalog of 139 emission-line based spectroscopic redshifts for extragalactic sources, including three new redshifts of multiple image systems (one probable, two tentative). In addition to the data itself we also release software tools that are helpful to navigate the data.

Through the Looking GLASS: HST Spectroscopy of Faint Galaxies Lensed by the Frontier Fields Cluster MACSJ0717.5+3745

The Grism Lens-Amplified Survey from Space (GLASS) is a Hubble Space Telescope (HST) Large Program, which will obtain 140 orbits of grism spectroscopy of the core and infall regions of 10 galaxy clusters, selected to be among the very best cosmic telescopes. Extensive HST imaging is available from many sources including the CLASH and Frontier Fields programs. We introduce the survey by analyzing spectra of faint multiply-imaged galaxies and z ≳ 6 galaxy candidates obtained from the first 7 orbits out of 14 targeting the core of the Frontier Fields cluster MACSJ0717.5+3745. Using the G102 and G141 grisms to cover the wavelength range 0.8-1.7 μm, we confirm four strongly lensed systems by detecting emission lines in each of the images. For the 9 z ≳ 6 galaxy candidates clear from contamination, we do not detect any emission lines down to a 7 orbit 1σ noise level of ∼5 × 10 -18 erg s-1 cm-2. Taking lensing magnification into account, our flux sensitivity reaches ∼0.2-5 × 10-18 erg s-1cm-2. These limits over an uninterrupted wavelength range rule out the possibility that the high-z galaxy candidates are instead strong line emitters at lower redshift. These results show that by means of careful modeling of the background - and with the assistance of lensing magnification - interesting flux limits can be reached for large numbers of objects, avoiding pre-selection and the wavelength restrictions inherent to ground-based multi-slit spectroscopy. These observations confirm the power of slitless HST spectroscopy even in fields as crowded as a cluster core.