分类： 天文学 >> 天文学 提交时间： 2023-02-19
Nicole Arulanantham
Max Gronke
Eleonora Fiorellino
Jorge Filipe Gameiro
Antonio Frasca
Joel Green
Seok-Jun Chang
Rik A. B. Claes
Catherine C. Espaillat
Kevin France
Gregory J. Herczeg
Carlo F. Manara
Laura Venuti
Péter Ábrahám
Richard Alexander
Jerome Bouvier
Justyn Campbell-White
Jochen Eislöffel
William J. Fischer
Ágnes Kóspál
摘要： T Tauri stars produce broad Lyman-alpha emission lines that contribute $\sim$88% of the total UV flux incident on the inner circumstellar disks. Lyman-alpha photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyman-alpha emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high velocity emission line wings, providing estimates of the average velocities within the bulk intervening H I. The model velocities are significantly correlated with the K band veiling, indicating a turnover from Lyman-alpha profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with red-shifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyman-alpha photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyman-alpha irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present new spectroscopic observations of Lyman-$\alpha$ (Ly$\alpha$) Blob 2 ($z \sim$ 3.1). We observed extended Ly$\alpha$ emission in three distinct regions, where the highest Ly$\alpha$ surface brightness (SB) center is far away from the known continuum sources. We searched through the MOSFIRE slits that cover the high Ly$\alpha$ SB regions, but were unable to detect any significant nebular emission near the highest SB center. We further mapped the flux ratio of the blue peak to the red peak and found it is anti-correlated with Ly$\alpha$ SB with a power-law index of $\sim$ -0.4. We used radiative transfer models with both multiphase, clumpy and shell geometries and successfully reproduced the diverse Ly$\alpha$ 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 modeled Ly$\alpha$ 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$\alpha$ photons are generated by a central powering source and then scatter with outflowing, multiphase HI gas while propagating outwards. The infalling of cool gas near the blob outskirts shapes the observed blue-dominated Ly$\alpha$ profiles, but its energy contribution to the total Ly$\alpha$ luminosity is less than 10%, i.e. minor compared to the photo-ionization by star-forming galaxies and/or AGNs.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Existing ubiquitously in the Universe with the highest luminosity, the Lyman-$\alpha$ emission line encodes abundant physical information about the gaseous medium it interacts with. Nevertheless, the resonant nature of Ly$\alpha$ complicates the radiative transfer (RT) modeling of the line profile, making the extraction of physical properties of the surrounding gaseous medium notoriously difficult. In this paper, we revisit the problem of deciphering the Ly$\alpha$ emission line with RT modeling. 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 have also explored 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 of the clumpy slab model is equal to the HI column density of the shell model; (2) the effective temperature of the clumpy slab model, which incorporates the clump velocity dispersion, is equal to the effective temperature of the shell model; (3) the average radial clump outflow velocity is equal to the shell expansion velocity; (4) large intrinsic line widths are required in the shell model to reproduce the wings of the clumpy slab models; (5) adding another phase of hot inter-clump medium will increase 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 Ly$\alpha$ fitting reported in previous literature, and emphasize the importance of utilizing information from additional observations to break the intrinsic degeneracies as well as interpreting the model parameters in a more physically realistic context.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Understanding the survival, growth and dynamics of cold gas is fundamental to galaxy formation. While there has been a plethora of work on `wind tunnel' simulations that study such cold gas in winds, the infall of this gas under gravity is at least equally important, and fundamentally different since cold gas can never entrain. Instead, velocity shear increases and remains unrelenting. If these clouds are growing, they can experience a drag force due to the accretion of low momentum gas, which dominates over ram pressure drag. This leads to sub-virial terminal velocities, in line with observations. We develop simple analytic theory and predictions based on turbulent radiative mixing layers. We test these scalings in 3D hydrodynamic simulations, both for an artificial constant background, as well as a more realistic stratified background. We find that the survival criterion for infalling gas is more stringent than in a wind, requiring that clouds grow faster than they are destroyed ($t_{\rm grow} < 4\,t_{\rm cc} $). This can be translated to a critical pressure, which for Milky Way like conditions is $P \sim 3000 {\rm k}_B {\rm K}\,{\rm cm}^{-3}$ . Cold gas which forms via linear thermal instability ($t_{\rm cool}/t_{\rm ff} < 1$) in planar geometry meets the survival threshold. In stratified environments, larger clouds need only survive infall until cooling becomes effective. We discuss applications to high velocity clouds and filaments in galaxy clusters.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Rongxin Luo
Ming Sun
Pavel Jáchym
Will Waldron
Matteo Fossati
Michele Fumagalli
Alessandro Boselli
Francoise Combes
Jeffrey D. P. Kenney
Yuan Li
Max Gronke
摘要： Ram pressure stripping (RPS) is an important process to affect the evolution of cluster galaxies and their surrounding environment. We present a large MUSE mosaic for ESO 137-001 and its stripped tails, and studied the detailed distributions and kinematics of the ionized gas and stars. The warm, ionized gas is detected to at least 87 kpc from the galaxy and splits into three tails. There is a clear velocity gradient roughly perpendicular to the stripping direction, which decreases along the tails and disappears beyond $\sim45$ kpc downstream. The velocity dispersion of the ionized gas increases to $\sim80$ km s$^{-1}$ at $\sim20$ kpc downstream and stays flat beyond. The stars in the galaxy disk present a regular rotation motion, while the ionized gas is already disturbed by the ram pressure. Based on the observed velocity gradient, we construct the velocity model for the residual galactic rotation in the tails and discuss the origin and implication of its fading with distance. By comparing with theoretical studies, we interpreted the increased velocity dispersion as the result of the oscillations induced by the gas flows in the galaxy wake, which may imply an enhanced degree of turbulence there. We also compare the kinematic properties of the ionized gas and molecular gas from ALMA, which shows they are co-moving and kinematically mixed through the tails. Our study demonstrates the great potential of spatially resolved spectroscopy in probing the detailed kinematic properties of the stripped gas, which can provide important information for future simulations of RPS.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Dawn K. Erb
Zhihui Li
Charles C. Steidel
Yuguang Chen
Max Gronke
Allison L. Strom
Ryan F. Trainor
Gwen C. Rudie
摘要： The resonantly scattered Lyman-$\alpha$ line illuminates the extended halos of neutral hydrogen in the circumgalactic medium of galaxies. We present integral field Keck Cosmic Web Imager observations of double-peaked, spatially extended Ly$\alpha$ emission in 12 relatively low-mass ($M_{\star} \sim10^9 \, M_{\odot}$) $z\sim2$ galaxies characterized by extreme nebular emission lines. Using individual spaxels and small bins as well as radially binned profiles of larger regions, we find that for most objects in the sample the Ly$\alpha$ blue-to-red peak ratio increases, the peak separation decreases, and the fraction of flux emerging at line center increases with radius. We use new radiative transfer simulations to model each galaxy with a clumpy, multiphase outflow with radially varying outflow velocity, and self-consistently apply the same velocity model to the low ionization interstellar absorption lines. These models reproduce the trends of peak ratio, peak separation and trough depth with radius, and broadly reconcile outflow velocities inferred from Ly$\alpha$ and absorption lines. The galaxies in our sample are well-described by a model in which neutral, outflowing clumps are embedded in a hotter, more highly ionized inter-clump medium (ICM), whose residual neutral content produces absorption at the systemic redshift. The peak ratio, peak separation and trough flux fraction are primarily governed by the line-of-sight component of the outflow velocity, the HI column density, and the residual neutral density in the ICM respectively. Azimuthal asymmetries in the line profile further suggest non-radial gas motions at large radii and variations in the HI column density in the outer halos.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Xinfeng Xu
Alaina Henry
Timothy Heckman
John Chisholm
Gábor Worseck
Max Gronke
Anne Jaskot
Stephan R. McCandliss
Sophia R. Flury
Mauro Giavalisco
Zhiyuan Ji
Ricardo O. Amorín
Danielle A. Berg
Sanchayeeta Borthakur
Nicolas Bouche
Cody Carr
Dawn K. Erb
Harry Ferguson
Thibault Garel
Matthew Hayes
摘要： Star-forming galaxies are considered the likeliest source of the H I ionizing Lyman Continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z >~ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the Epoch of Reionization. Emission from Mg II \ly\ly 2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for 8 LyC emitter candidates, selected to have strong Mg II emission lines. We securely detect LyC emission in 50% (4/8) galaxies with 2$\sigma$ significance. This high detection rate suggests that strong Mg II emitters might be more likely to leak LyC than similar galaxies without strong Mg II. Using photoionization models, we constrain the escape fraction of Mg II as ~ 15 -- 60%. We confirm that the escape fraction of Mg II correlates tightly with that of Lyman-alpha (LyA), which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg II emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg II emission can be adopted to estimate the escape fraction of LyA and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the Epoch of Reionization.
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