اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدThe Anomalous 21-cm Absorption at High Redshifts
76
0
0.0
(
0
)
تأليف
Fulvio Melia
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The EDGES collaboration has reported the detection of a global 21-cm signal with a plateau centered at 76 MHz (i.e., redshift 17.2), with an amplitude of 500^(+200)_(-500) mK. This anomalous measurement does not comport with standard cosmology, which can only accommodate an amplitude < 230 mK. Nevertheless, the line profiles redshift range (15 < z < 20) suggests a possible link to Pop III star formation and an implied evolution out of the `dark ages. Given this tension with the standard model, we here examine whether the observed 21-cm signal is instead consistent with the results of recent modeling based on the alternative Friedmann-Lemaitre-Robertson-Walker cosmology known as the R_h=ct universe, showing that--in this model--the CMB radiation might have been rethermalized by dust ejected into the IGM by the first-generation stars at redshift z < 16. We find that the requirements for this process to have occurred would have self-consistently established an equilibrium spin temperature T_s~3.4 K in the neutral hydrogen, via the irradiation of the IGM by deep penetrating X-rays emitted at the termination shocks of Pop III supernova remnants. Such a dust scenario has been strongly ruled out for the standard model, so the spin temperature (~3.3 K) inferred from the 21-cm absorption feature appears to be much more consistent with the R_h=ct profile than that implied by LCDM, for which adiabatic cooling would have established a spin temperature T_s(z=17.2)~6 K.
قيم البحث
اقرأ أيضاً
We introduce simulations aimed at assessing how well weak gravitational lensing of 21cm radiation from the Epoch of Reionization ($z sim 8$) can be measured by an SKA-like radio telescope. A simulation pipeline has been implemented to study the perfo
rmance of lensing reconstruction techniques. We show how well the lensing signal can be reconstructed using the three-dimensional quadratic lensing estimator in Fourier space assuming different survey strategies. The numerical code introduced in this work is capable of dealing with issues that can not be treated analytically such as the discreteness of visibility measurements and the inclusion of a realistic model for the antennae distribution. This paves the way for future numerical studies implementing more realistic reionization models, foreground subtraction schemes, and testing the performance of lensing estimators that take into account the non-Gaussian distribution of HI after reionization. If multiple frequency channels covering $z sim 7-11.6$ are combined, Phase 1 of SKA-Low should be able to obtain good quality images of the lensing potential with a total resolution of $sim 1.6$ arcmin. The SKA-Low Phase 2 should be capable of providing images with high-fidelity even using data from $zsim 7.7 - 8.3$. We perform tests aimed at evaluating the numerical implementation of the mapping reconstruction. We also discuss the possibility of measuring an accurate lensing power spectrum. Combining data from $z sim 7-11.6$ using the SKA2-Low telescope model, we find constraints comparable to sample variance in the range $L<1000$, even for survey areas as small as $25mbox{ deg}^2$.
Observations in the lowest MWA band between $75-100$ MHz have the potential to constrain the distribution of neutral hydrogen in the intergalactic medium at redshift $sim 13-17$. Using 15 hours of MWA data, we analyse systematics in this band such as
radio-frequency interference (RFI), ionospheric and wide field effects. By updating the position of point sources, we mitigate the direction independent calibration error due to ionospheric offsets. Our calibration strategy is optimized for the lowest frequency bands by reducing the number of direction dependent calibrators and taking into account radio sources within a wider field of view. We remove data polluted by systematics based on the RFI occupancy and ionospheric conditions, finally selecting 5.5 hours of the cleanest data. Using these data, we obtain two sigma upper limits on the 21 cm power spectrum in the range of $0.1lessapprox k lessapprox 1 ~rm ~h~Mpc^{-1}$ and at $z$=14.2, 15.2 and 16.5, with the lowest limit being $6.3times 10^6 ~rm mK^2$ at $rm k=0.14 rm ~h~Mpc^{-1}$ and at $z=15.2$ with a possibility of a few % of signal loss due to direction independent calibration.
A recent observation points to an excess in the expected 21-cm brightness temperature from cosmic dawn. In this paper, we present an alternative explanation of this phenomenon, an interaction in the dark sector. Interacting dark energy models have be
en extensively studied recently and there is a whole variety of such in the literature. Here we particularize to a specific model in order to make explicit the effect of an interaction.
We present the results of a survey for intervening HI 21-cm absorbers at intermediate and low redshift (0<z<1.2). For our total sample of 24 systems, we obtained high quality data for 17 systems, the other seven being severely affected by radio frequ
ency interference (RFI). Five of our targets are low redshift (z<0.17) optical galaxies with small impact parameters (<20 kpc) toward radio-bright background sources. Two of these were detected in 21-cm absorption, showing narrow, high optical depth absorption profiles, the narrowest having a velocity dispersion of only 1.5 km/s, which puts an upper limit on the kinetic temperature of T_k<270 K. Combining our observations with results from the literature, we measure a weak anti-correlation between impact parameter and integral optical depth in local (z<0.5) 21-cm absorbers. Of eleven CaII and MgII systems searched, two were detected in 21-cm absorption, and six were affected by RFI to a level that precludes a detection. For these two systems at z~0.6 we measure spin temperatures of T_s=(65+/-17) K and T_s>180 K. A subset of our systems were also searched for OH absorption, but no detections were made.
We present the results of Giant Metrewave Radio Telescope (GMRT) observations to detect H{sc i} in absorption towards the cores of a sample of radio galaxies. From observations of a sample of 16 sources, we detect H{sc i} in absorption towards the co
re of only one source, the FR,II radio galaxy 3C,452 which has been reported earlier by Gupta & Saikia (2006a). In this paper we present the results for the remaining sources which have been observed to a similar optical depth as for a comparison sample of compact steep-spectrum (CSS) and giga-hertz peaked spectrum (GPS) sources. We also compile available information on H{sc i} absorption towards the cores of extended radio sources observed with angular resolutions of a few arcsec or better. The fraction of extended sources with detection of H{sc i} absorption towards their cores is significantly smaller (7/47) than the fraction of H{sc i} detection towards CSS and GPS objects (28/49). For the cores of extended sources, there is no evidence of a significant correlation between H{sc i} column density towards the cores and the largest linear size of the sources. The distribution of the relative velocity of the principal absorbing component towards the cores of extended sources is not significantly different from that of the CSS and GPS objects. However, a few of the CSS and GPS objects have blue-shifted components $gapp$1000 km s$^{-1}$, possibly due to jet-cloud interactions. With the small number of detections towards cores, the difference in detection rate between FR,I (4/32) and FR,II (3/15) sources is within the statistical uncertainties.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
