Search for: All records

We present a new semi-analytical formalism for modelling metal absorption lines that emerge from a clumpy galactic environment, ALPACA. We predict the “down-the-barrel” (DTB) metal absorption line profiles and the equivalent width (EW) of absorption at different impact parameters (b) as a function of the clump properties, including clump kinematics, clump volume filling factor, clump number density profile, and clump ion column densities. With ALPACA, we jointly model the stacked DTB C ii λ1334 spectrum of a sample of z ∼ 3 Lyman break galaxies and the EW versus b profile of a sample of z ∼ 2 star-forming galaxy–galaxy pairs. ALPACA successfully reproduced two data sets simultaneously, and the best fit prefers a low clump volume filling factor (∼3 × 10−3). The radial velocities of the clumps are a superposition of a rapidly accelerated outflow with a maximum velocity of $\sim 400 \, {\mathrm{km}\, \mathrm{s}^{-1}}$ and a velocity dispersion of $\sigma \sim 120 \, {\mathrm{km}\, \mathrm{s}^{-1}}$. The joint modelling reveals a physical scenario where the absorption observed at a particular velocity is contributed by the clumps distributed over a fairly broad range of radii. We also find that the commonly adopted Sobolev approximation is at best only applicable within a narrow range of radii where the clumps are undergoing rapid acceleration in a non-volume-filling clumpy medium. Lastly, we find that the clump radial velocity profile may not be fully constrained by the joint modelling and spatially resolved Ly α emission modelling may help break the degeneracy.

 

ABSTRACT Existing ubiquitously in the Universe with the highest luminosity, the Lyman-α (Lyα) emission line encodes abundant physical information about the gaseous medium it interacts with. Nevertheless, the resonant nature of the Lyα line complicates the radiative transfer (RT) modelling of the line profile. We revisit the problem of deciphering the Lyα emission line with RT modelling. We reveal intrinsic parameter degeneracies in the widely used shell model in the optically thick regime for both static and outflowing cases, which suggest the limitations of the model. We also explore the connection between the more physically realistic multiphase, clumpy model, and the shell model. We find that the parameters of a ‘very clumpy’ slab model and the shell model have the following correspondences: (1) the total column density, the effective temperature, and the average radial clump outflow velocity of the clumpy slab model are equal to the H i column density, effective temperature, and expansion velocity of the shell model, respectively; (2) large intrinsic linewidths are required in the shell model to reproduce the wings of the clumpy slab models; (3) adding another phase of hot interclump medium increases peak separation, and the fitted shell expansion velocity lies between the outflow velocities of two phases of gas. Our results provide a viable solution to the major discrepancies associated with Lyα fitting reported in previous literature, and emphasize the importance of utilizing information from additional observations to break the intrinsic degeneracies and interpreting the model parameters in a more physically realistic context. 

We present new spectroscopic observations of Ly α (Ly α) Blob 2 (z ∼ 3.1). We observed extended Ly α emission in three distinct regions, where the highest Ly α surface brightness (SB) centre is far away from the known continuum sources. We searched through the MOSFIRE slits that cover the high Ly α SB regions, but were unable to detect any significant nebular emission near the highest SB centre. We further mapped the flux ratio of the blue peak to the red peak and found it is anticorrelated with Ly α SB with a power-law index of ∼ –0.4. We used radiative transfer models with both multiphase, clumpy, and shell geometries and successfully reproduced the diverse Ly α morphologies. We found that most spectra suggest outflow-dominated kinematics, while 4/15 spectra imply inflows. A significant correlation exists between parameter pairs, and the multiphase, clumpy model may alleviate previously reported discrepancies. We also modelled Ly α spectra at different positions simultaneously and found that the variation of the inferred clump outflow velocities can be approximately explained by line-of-sight projection effects. Our results support the ‘central powering  + scattering’ scenario, i.e. the Ly α photons are generated by a central powering source and then scatter with outflowing, multiphase H  i gas while propagating outwards. The infalling of cool gas near the blob outskirts shapes the observed blue-dominated Ly α profiles, but its energy contribution to the total Ly α luminosity is less than 10 per cent, i.e. minor compared to the photoionization by star-forming galaxies and/or AGNs.

 

ABSTRACT We present new observations of Lyman-α (Lyα) Blob 1 (LAB1) in the SSA22 protocluster region (z = 3.09) using the Keck Cosmic Web Imager and Keck Multi-object Spectrometer for Infrared Exploration. We have created a narrow-band Lyα image and identified several prominent features. By comparing the spatial distributions and intensities of Lyα and Hβ, we find that recombination of photo-ionized H i gas followed by resonant scattering is sufficient to explain all the observed Lyα/Hβ ratios. We further decode the spatially resolved Lyα profiles using both moment maps and radiative transfer modelling. By fitting a set of multiphase, ‘clumpy’ models to the observed Lyα profiles, we manage to reasonably constrain many parameters, namely the H i number density in the interclump medium (ICM), the cloud volume filling factor, the random velocity and outflow velocity of the clumps, the H i outflow velocity of the ICM, and the local systemic redshift. Our model has successfully reproduced the diverse Lyα morphologies, and the main results are: (1) the observed Lyα spectra require relatively few clumps per line of sight as they have significant fluxes at the line centre; (2) the velocity dispersion of the clumps yields a significant broadening of the spectra as observed; (3) the clump bulk outflow can also cause additional broadening if the H i in the ICM is optically thick; (4) and the H i in the ICM is responsible for the absorption feature close to the Lyα line centre. 

ABSTRACT We present the first statistical analysis of kinematically resolved, spatially extended $\rm Ly\alpha$ emission around z = 2–3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (zmed = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse $\rm Ly\alpha$ emission with respect to the stellar continuum within projected galactocentric distances of ≲30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged $\rm Ly\alpha$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of $\rm Ly\alpha$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended $\rm Ly\alpha$ emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated $\rm Ly\alpha$ emission. The symmetry implies that the $\rm Ly\alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of $\rm Ly\alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak $\rm Ly\alpha$ emission. We speculate that this weak directional dependence of $\rm Ly\alpha$ emission becomes discernible only when the $\rm Ly\alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with $\rm Ly\alpha$ radiative transfer modelling. 

ABSTRACT We explore the survival of cool clouds in multiphase circumgalactic media. We revisit the ‘cloud-crushing problem’ in a large survey of simulations including radiative cooling, self-shielding, self-gravity, magnetic fields, and anisotropic Braginskii conduction and viscosity (with saturation). We explore a wide range of parameters including cloud size, velocity, ambient temperature and density, and a variety of magnetic field configurations and cloud turbulence. We find that realistic magnetic fields and turbulence have weaker effects on cloud survival; the most important physics is radiative cooling and conduction. Self-gravity and self-shielding are important for clouds that are initially Jeans-unstable, but largely irrelevant otherwise. Non-self-gravitating, realistically magnetized clouds separate into four regimes: (1) at low column densities, clouds evaporate rapidly via conduction; (2) a ‘failed pressure confinement’ regime, where the ambient hot gas cools too rapidly to provide pressure confinement for the cloud; (3) an ‘infinitely long-lived’ regime, in which the cloud lifetime becomes longer than the cooling time of gas swept up in the leading bow shock, so the cloud begins to accrete and grow; and (4) a ‘classical cloud destruction’ regime, where clouds are eventually destroyed by instabilities. In the final regime, the cloud lifetime can exceed the naive cloud-crushing time owing to conduction-induced compression. However, small and/or slow-moving clouds can also evaporate more rapidly than the cloud-crushing time. We develop simple analytic models that explain the simulated cloud destruction times in this regime.