اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدExcitation temperature of the warm neutral medium as a new probe of the Lyman-{alpha} radiation field
49
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We use the Karl G. Jansky Very Large Array (VLA) to conduct a high-sensitivity survey of neutral hydrogen (HI) absorption in the Milky Way. In combination with corresponding HI emission spectra obtained mostly with the Arecibo Observatory, we detect a widespread warm neutral medium (WNM) component with excitation temperature <Ts>= 7200 (+1800,-1200) K (68% confidence). This temperature lies above theoretical predictions based on collisional excitation alone, implying that Ly-{alpha} scattering, the most probable additional source of excitation, is more important in the interstellar medium (ISM) than previously assumed. Our results demonstrate that HI absorption can be used to constrain the Ly-{alpha} radiation field, a critical quantity for studying the energy balance in the ISM and intergalactic medium yet notoriously difficult to model because of its complicated radiative transfer, in and around galaxies nearby and at high redshift.
قيم البحث
اقرأ أيضاً
Galactic outflows are critical to our understanding of galaxy formation and evolution. However the details of the underlying feedback process remain unclear. We compare Ly$alpha$ observations of the circumgalactic medium (CGM) of Lyman Break Galaxies
(LBGs) with mock observations of their simulated CGM. We use cosmological hydrodynamical `zoom-in simulations of an LBG which contains strong, momentum-driven galactic outflows. Simulation snapshots at $z=2.2$ and $z=2.65$ are used, corresponding to the available observational data. The simulation is post-processed with the radiative transfer code textsc{crash} to account for the impact of ionising photons on hydrogen gas surrounding the simulated LBG. We generate mock absorption line maps for comparison with data derived from observed close galaxy-galaxy pairs. We perform calculations of Ly$alpha$ photons scattering through the CGM with our newly developed Monte-Carlo code textsc{slaf}, and compare to observations of diffuse Ly$alpha$ halos around LBGs. Our fiducial galactic outflow model comes closer to reproducing currently observed characteristics of the CGM in Ly$alpha$ than a reference inefficient feedback model used for comparison. Nevertheless, our fiducial model still struggles to reproduce the observed data of the inner CGM (at impact parameter $b<30$kpc). Our results suggest that galactic outflows affect Ly$alpha$ absorption and emission around galaxies mostly at impact parameters $b<50$ kpc, while cold accretion flows dominate at larger distances. We discuss the implications of this result, and underline the potential constraining power of CGM observations - in emission and absorption - on galactic outflow models.
The Ly$alpha$ emission line is one of the most promising probes of cosmic reionisation but isolating the signature of a change in the ionisation state of the IGM is challenging because of intrinsic evolution and internal radiation transfer effects. W
e present the first study of the evolution of Ly$alpha$ emitters (LAE) during the epoch of reionisation based on a full radiation-hydrodynamics cosmological simulation that is able to capture both the large-scale process of reionisation and the small-scale properties of galaxies. We predict the Ly$alpha$ emission of galaxies in the $10^3$ cMpc$^3$ SPHINX simulation at $6leq zleq9$ by computing the full Ly$alpha$ radiation transfer from ISM to IGM scales. SPHINX is able to reproduce many observational constraints such as the UV/Ly$alpha$ luminosity functions and stellar mass functions at z $geq$ 6 for the dynamical range probed by our simulation ($M_{rm 1500}gtrsim-18$, $L_{rm Lyalpha}lesssim10^{42}$ erg/s, $M_{star}lesssim10^9$ M$_{odot}$). As intrinsic Ly$alpha$ emission and internal Ly$alpha$ escape fractions barely evolve from $z=6$ to 9, the observed suppression of Ly$alpha$ luminosities with increasing redshift is fully attributed to IGM absorption. For most observable galaxies ($M_{rm 1500}lesssim-16$), the Ly$alpha$ line profiles are slightly shifted to the red due to internal radiative transfer effects which mitigates the effect of IGM absorption. Overall, the enhanced Ly$alpha$ suppression during reionisation traces the IGM neutral fraction $x_{rm HI}$ well but the predicted amplitude of this reduction is a strong function of the Ly$alpha$ peak shift, which is set at ISM/CGM scales. We find that a large number of LAEs could be detectable in very deep surveys during reionisation when $x_{rm HI}$ is still $approx 50%$.
We present 21-cm absorption measurements towards 12 radio continuum sources with previously identified thermally-unstable warm neutral medium (WNM). These observations were obtained with the Expanded Very Large Array (EVLA) and were complemented with
the HI emission spectra obtained with the Arecibo Observatory. Out of 12 sources, HI absorption was detected along 5 lines of sight (seven new absorption features in total), resulting in a detection rate of ~42%. While our observations are sensitive to the WNM with a spin temperature T_s<3000 K, we detected only two wide absorption lines with T_s=400-900 K. These temperatures lie above the range allowed for the cold neutral medium (CNM) by the thermal equilbrium models and signify the thermally unstable WNM. Several absorption features have an optical depth of only a few x10^{-3}. While this is close or lower than what is theoretically expected for the CNM, we show that these weak lines are important for constraining the fraction of the thermally unstable WNM. Our observations demonstrate that, for the first time, high bandpass stability can be achieved with the VLA, allowing detection of absorption lines with a peak optical depth of ~10^{-3}.
We report a deep Giant Metrewave Radio Telescope (GMRT) search for Galactic H{sc i} 21cm absorption towards the quasar B0438$-$436, yielding the detection of wide, weak H{sc i} 21cm absorption, with a velocity-integrated H{sc i} 21cm optical depth of
$0.0188 pm 0.0036$~km~s$^{-1}$. Comparing this with the H{sc i} column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of $3760 pm 365$~K, one of the highest measured in the Galaxy. This is consistent with most of the H{sc i} along the sightline arising in the stable warm neutral medium (WNM). The low peak H{sc i} 21cm optical depth towards B0438$-$436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H{sc i} 21cm absorption and emission spectra. This yields a gas kinetic temperature of $rm T_k leq (4910 pm 1900)$~K, and a spin temperature of $rm T_s = (1000 pm 345)$~K for the gas that gives rise to the H{sc i} 21cm absorption. Our data are consistent with the H{sc i} 21cm absorption arising from either the stable WNM, with $rm T_s ll T_k$, $rm T_k approx 5000$~K, and little penetration of the background Lyman-$alpha$ radiation field into the neutral hydrogen, or from the unstable neutral medium, with $rm T_s approx T_k approx 1000;K$.
The transition from the diffuse warm neutral medium (WNM) to the dense cold neutral medium (CNM) is what set the initial conditions to the formation of molecular clouds. The properties of the turbulent cascade in the WNM, essential to describe this r
adiative condensation process, have remained elusive in part due to the difficulty to map out the structure and kinematics of each H I thermal phases. Here we present an analysis of a 21 cm hyper-spectral data cube from the GHIGLS HI survey where the contribution of the WNM is extracted using ROHSA, a Gaussian decomposition tool that includes spatial regularization. The distance and volume of the WNM emission is estimated using 3D dust extinction map information. The thermal and turbulent contributions to the Doppler line width of the WNM were disentangled using two techniques, one based on the statistical properties of the column density and centroid velocity fields, and another on the relative motions of CNM structures as a probe of turbulent motions. We found that the volume of WNM sampled here, located at the outer edge of the Local Bubble, shows thermal properties in accordance with expected values for heating and cooling processes typical of the Solar neighbourhood. The WNM has the properties of sub/trans-sonic turbulence, with a turbulent Mach number at the largest scale probed here (l = 130 pc) of Ms = 0.87 +- 0.15, a density contrast of 0.6 +- 0.2, and velocity and density power spectra compatible with k-11/3. The low Mach number of the WNM provides dynamical conditions that allows the condensation mode of thermal instability (TI) to grow freely and form CNM structures, as predicted by theory.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
