Повышение яркости космического фонового радиоизлучения в направлении на скопления галактик

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Abstract

Исследована возможность регистрации в направлении скоплений галактик избытка космического фонового радиоизлучения из-за его комптоновского рассеяния на электронах горячего межгалактического газа. При картографировании флуктуаций фона на частотах ниже ≤ 800 МГц этот эффект ведет к появлению на месте скопления радиоисточника. На более высоких частотах, где в космическом фоне доминирует микроволновое (реликтовое) излучение, на месте скопления наблюдается “отрицательный” источник (“тень” на карте флуктуаций фона), что связано с переносом при рассеянии части реликтовых фотонов вверх по оси частот (в область ν ≥ 217 ГГц, Сюняев, Зельдович, 1970, 1972). В работе рассчитаны спектры ожидаемых искажений фонового радиоизлучения для разных параметров скоплений, показано, что во многих случаях в широком диапазоне частот 30 МГц ≤ ν ≤ 3 ГГц измерению искажений будет препятствовать собственное тепловое (тормозное) излучение межгалактического газа, а также рассеянное радиоизлучение галактик скоплений, связанное с их былой активностью, включая синхротронное излучение выброшенных релятивистских электронов. Ниже ~20 МГц эффект рассеяния всегда преобладает над тепловым излучением газа из-за общего роста интенсивности космического радиофона, однако высокоточные измерения на таких частотах становятся сложными. Ниже ~5 МГц эффект подавляется индуцированным рассеянием. В работе найдены диапазоны частот, оптимальные для поиска и измерения комптоновского избытка фонового радиоизлучения. Показано, что наиболее перспективны для его наблюдения горячие (kTe ≥ 8 кэВ) скопления, находящиеся на больших (z ≥ 0.5) красных смещениях. Из-за сильной концентрации тормозного излучения к центру скопления периферийные наблюдения комптоновского избытка должны быть предпочтительнее центральных. Более того, благодаря тепловому излучению газа и его концентрации к центру, отмеченный выше переход от “отрицательного” источника на карте флуктуаций фона к “положительному” при движении вниз по оси частот должен происходить не плавно, а через стадию “гибридного источника” – появления яркого пятна, окруженного темным кольцом. Такой вид источника в проекции объясняется его необычной трехмерной формой в виде узкого пика тормозного радиоизлучения, поднимающегося из центра широкой глубокой ямы, связанной с комптоновским рассеянием реликтового излучения. Рассеянное излучение активной в прошлом центральной галактики скопления может усилить эффект. Аналогичный “гибридный источник” появляется на карте флуктуаций фона и вблизи частоты 217.5 ГГц – при переходе от дефицита реликтового изучения к избытку (за счет фотонов, испытавших рассеяние). Необычная форма источника при этом вновь связана с тепловым излучением газа. Одновременные измерения потока тормозного радиоизлучения газа и амплитуды искажений из-за рассеяния фонового радио- и реликтового излучения позволят определять важнейшие параметры скопления.

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С. А. Гребенев

Институт космических исследований РАН

Author for correspondence.
Email: grebenev@iki.rssi.ru
Russian Federation, Москва

Р. А. Сюняев

Институт космических исследований РАН; Институт астрофизики Общества им. Макса Планка

Email: grebenev@iki.rssi.ru
Russian Federation, Москва; Гархинг, Германия

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2. Fig. 1. (a) The spectrum of the background radio and microwave emission (red dashed curve) and the corresponding distorted spectrum due to scattering on electrons of the hot gas of the galaxy cluster (green long dashes), as well as the contribution from the braking emission of this gas (blue solid line, the spectrum of the braking emission itself is shown as a dashed straight line). Demonstration calculation for a hypothetical cluster with a homogeneous density distribution, radius Rc = 350 kpc, temperature kTe = 5 keV, and Compton yC = = 0.15 and braking yB = 2 × 1023 cm-5 parameters, which actually determine the distortion amplitudes (real clusters have yC and yB with three orders of magnitude smaller values). (b) Relative distortions of the background emission spectrum in the direction towards the cluster (the solid blue curve takes into account the stopping gas emission). The frequencies ν2 ≃ 802 MHz (equality in absolute value of the Compton distortions of the radio and microwave backgrounds), ν1 and ν3 (equality of the flux of braking radiation and Compton excess in the spectrum of the radiophone, or Compton failure in the spectrum of relic radiation), and ν0 = 217 GHz (transition from photon deficiency to excess in this spectrum) are indicated.

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3. Fig. 2. Comparison of distortions of the background radio emission due to its scattering on electrons of the hot gas of a galaxy cluster (green long dashes) and due to the contribution of the braking radiation of this gas (solid blue lines corresponding to different radii Rc of the cluster). In both cases, distortions of the relic emission due to scattering on electrons are taken into account (shown separately by red short dashes). The green short dashes show the reduction of distortions due to induced scattering, and the green dashed line also shows the reduction of distortions due to braking absorption of radio emission in the gas. Calculation for a close (z 1) cluster with a homogeneous density distribution, temperature kTe = 5 keV, and Thomson thickness along the line of sight at its center τT = 6×10-3.

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4. Fig. 3. The same as in Fig. 2, but for distortions in the spectrum ν Fν(ν), which allows us to better study the contribution of the braking radiation of intergalactic gas in the centimeter-decimeter wavelength region, where the contribution of Compton distortions of the relic emission spectrum is strong. It can be seen that for the gas temperature kTe = 5 keV, the braking radiation compensates the Compton falloff of the radiation flux up to λ ~ 6 cm (ν ∼ 5 GHz). The optical thickness of the gas is τT = 1.2 × 10-2 (twice as large as in Fig. 2).

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5. Fig. 4. Same as Fig. 2, but for clusters with parameters (a) kTe = 7 keV, τT = 1 × 10-2, (b) kTe = 15 keV, τT = 1.4 × 10-2, (c) kTe = 3 keV, τT = 8 × 10-3, and (d) kTe = 2 keV, τT = 8 × 10-3. The braking emission (solid blue lines) dominates in the broader region of the spectrum in cold, strongly relaxed clusters. At the same time, it fully compensates for the brightness drop of the relic emission up to frequencies 5-8 GHz (λ 4-6 cm) and weakens it at higher frequencies. The distortions in the background radio spectrum, although becoming stronger in hot young clusters with a large optical thickness, are comparable to the braking emission only at very low frequencies ν  20 MHz (λ  15 m).

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6. Fig. 5. Boundary frequencies ν1, above which the braking radiation flux of the intergalactic gas of the cluster dominates over the Compton amplification of the background radio emission in the total distortion spectrum (solid green curves), and frequency ν3, below which the braking radiation dominates in absolute value over the Compton attenuation of the relic flux (dashed blue curves). The curves for different gas temperatures in the cluster are shown. The vertical dashed red line shows the frequency ν2 at which the Compton distortion of the radio and relic radiation is compared. The positions of the frequencies ν1 and ν3 are given as a function of the temperature kTe and density Ne (more precisely, of the value σT yB/yC [kTe/mec2] 1.18Ne) of the intergalactic gas.

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7. Fig. 6. Minimum electron density Ne,min (more precisely, the value σT yB/ τT ≈ 1.18Ne,min) of the hot intergalactic gas, which is necessary for a frequency interval with a dominant contribution from its braking radiation to exist in the distortion spectrum of the background radio emission in the direction toward the cluster. The density is higher for distant (z > 0) clusters. The radius Rc of the cluster with critical density and Thomson thickness centered at τT = 6 × 10-3 is plotted on the y-axis on the right. The dashed line marks a typical cluster radius Rc ≃ 350 kpc.

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8. Fig. 7. The same as in Fig. 5, but for distant clusters at redshifts z = 0.5 and 1. Because of the decrease of the brightness of the braking emission with z, the frequency interval in which this emission dominates in the background distortion spectrum in the direction toward the cluster is noticeably narrower.

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9. Fig. 8. Same as in Fig. 2, but for a cluster with β-distribution of gas density (β = 2/3). The Thomson thickness at the center of the cluster is τT = 6 × 10-3, the gas is isothermal kTe = 5 keV, and the cluster is close (z 1). The contribution of the braking radiation is shown in solid blue lines for different values of the radius Rc of the cluster core. We consider central (sighting distance ρ = 0, top) and peripheral (ρ = 0.5 Rc, bottom) observations.

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10. Fig. 9. Comparison of the distortions of the background radio emission due to its scattering on the electrons of the hot gas of the galaxy cluster (green dashed lines), as well as the braking emission of this gas (solid blue lines) at different frequencies as a function of the aiming distance ρ/Rc from the direction to the cluster center. Distortions of the relic emission due to scattering on the electrons of the gas (solid red lines) are taken into account. Calculation for a close (z 1) cluster with a β-density distribution (β = 2/3), core radius Rc = 350 kpc, temperature kTe = 7 keV, and Thomson thickness at the center τT = 1×10-2 (see Fig. 4a).

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11. Fig. 10. Distortion map of the cosmic microwave and radiophone distortions in the direction of the galaxy cluster (the same as in Fig. 9) at different frequencies (blue lines are “negative” deviations, red lines are “positive” deviations). The outer contours (at distances ρ  6 Rc) are not shown.

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12. Fig. 11. Evolution of the distortion map of the background radio and microwave emission in the direction of the galaxy cluster during the transition between the reduced and increased brightness regimes (blue lines - “negative” background distortions, red lines - “positive”). The distortions are caused by electron scattering, but the appearance of a hybrid source (a bright spot surrounded by a dark ring) as well as a compact “positive” source in the map corresponding to ν = 900 MHz is largely due to thermal emission from the cluster's hot gas. The outer (ρ  6 Rc) contours are not shown in the maps with ν ≤ 1.0 GHz. Calculation for a close (z 1) cluster with a β-density distribution (β = 2/3) and the same parameters as the cluster in Figs. 9 and 10.

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13. Fig. 12. Relative (in %) distortions of the spectrum of cosmic background radio and microwave radiation expected due to its scattering on hot gas electrons in several known galaxy clusters (green long dashes). The blue lines also account for thermal (braking) emission from the gas. The decrease in the brightness of the microwave emission due to scattering on electrons is shown by the dashed red lines. The distortions observed at the center of the cluster (left) and at a sighting distance close to the radius of its nucleus ρ = 0.8Rc (right) are given. The gas is assumed to be isothermal and to have a β-density distribution (β = 2/3); the parameters of the clusters are given in Table 1.

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14. Fig. 13. Integral radio emission flux within radius Rb from a galaxy cluster characterized by a β-distribution of the intergalactic gas density (β = 2/3), core radius Rc = 350 kpc, and Thomson thickness at the center τT = 0.01 (see Figs. 4a and 9). Such a flow will be registered by a telescope with angular resolution b = Rb/dA, where dA is the angular distance to the cluster. The gas is assumed to be isothermal with temperature kTe = 7 keV, and the cluster itself is assumed to be located at redshift z = 0.11 (left) or 0.3 (right). The radio emission is due to 1). scattering of the cosmic background radio emission by the electrons of the hot gas (green long dashed lines) or 2). thermal braking emission from this gas (solid blue lines). In both cases, distortions of the relic emission due to Compton scattering are taken into account (red dashed lines). Single-type lines correspond to different radio frequencies.

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15. Fig. 14. Distortions of the spectrum of the cosmic background radio emission due to its scattering on the electrons of the hot gas of the galaxy cluster (green dashes), the braking emission of this gas (solid blue line), and the scattered (diffuse) emission of the central galaxy (histograms). The red curve (long dashes) shows the absolute value of the decrease in the brightness of the relic background due to its scattering in the cluster gas. The integral radiation flux (from the whole cluster) is presented everywhere. It is assumed that the galaxy was active for a long time in the radio band with spectral index γ = 0.4 and luminosity LR = 1 × 1041 erg c-1 in the frequency range 10 MHz - 100 GHz, but turned off tso million years ago. The histograms are obtained by Monte Carlo simulations. The case tso = 0 is also calculated analytically (formula (13), maroon curve). We consider a close (z = 0.005) cluster with a homogeneous density of isothermal (kTe = 5 keV) gas, radius Rc = 350 kpc, and optical thickness at the beam of view τT = 0.006 (the same as in Fig. 2).

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16. Fig. 15. Dependence of the excess cosmic background radio emission in the direction of the galaxy cluster on the aiming distance ρ (solid green line). The blue solid line shows distortions due to scattering in the hot gas of the cluster, the short blue dashes show the contribution of the braking gas emission, and the dashed black line shows the scattered emission from the central galaxy. Everywhere, except for the galaxy radiation, the decrease in the brightness of the relic radiation due to its Compton scattering is taken into account (red long dashes). The galaxy is considered to have been active for a long time, having a spectral index γ = 0.4 and a luminosity LR = 1 × 1041 erg c-1 in the frequency range 10 MHz - 100 GHz, but recently (tso ≃ 0) turned off. The cluster is considered with the same parameters as in Fig. 14.

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17. Fig. 16. Same as Fig. 14, but for central galaxies of different luminosities in the frequency range 10 MHz - 100 GHz, LR = L41 × 1041 erg s-1, where L41 = 2, 1, or 0.4 (shown in red numbers), turned off only recently tso = 0. The radio emission scattered in the gas of the galaxies is shown by the black dashed lines. The black solid lines show the scattered emission of galaxies taking into account the induced Compton scattering. It is considered that inside the radius R0 = 1-3 kpc (blue figures) near active galaxy nuclei there is no scattering gas.

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