The Physical State of Interstellar Hydrogen.

Strömgren, Bengt

doi:10.1086/144074

Cited by 461 publications

(283 citation statements)

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“…Strömgren 1939), which is defined by equating the rate of ionizing photonsṄ γ to the number of recombinations per unit time: Here, the number density of free electrons is taken to be n e ≈ n H within the ionized zone, and the rate of ionizing photons iṡ…”

Section: Discussionmentioning

confidence: 99%

A lepto-hadronic model for high-energy emission from FR I radiogalaxies

Reynoso

Medina

Romero

2011

A&A

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Context. The well known radiogalaxy Cen A has been recently detected as a source of very high energy (VHE) γ-rays by the HESS experiment just before Fermi/LAT detected it at high energies (HE). The detection, together with that of M 87, established radiogalaxies as VHE γ-ray emitters. Aims. The aim of this work is to present a lepto-hadronic model for the VHE emission from the relativistic jets in FR I radiogalaxies. Methods. We consider that protons and electrons are accelerated in a compact region near the base of the jet, and they cool emitting multi wavelength radiation as propagating along the jet. The proton and electron distributions are obtained through steady-state transport equation taking into account acceleration, radiative and non-radiative cooling processes, as well as particle transport by convection. Results. Considering the effects of photon absorption at different wavelengths, we calculate the radiation emitted by the primary protons and electrons, as well as the contribution of secondaries particles (e ± , πs and μs). The expected high-energy neutrino signal is also obtained and the possibility of detections with KM3NeT and IceCube is discussed. Conclusions. The spectral energy distribution obtained in our model with an appropriate set of parameters for an extended emission zone can account for much of the observed spectra for both AGNs.

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Section: Discussionmentioning

confidence: 99%

A lepto-hadronic model for high-energy emission from FR I radiogalaxies

Reynoso

Medina

Romero

2011

A&A

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“…We ignore recombinations straight into the ground state by invoking the on-the-spot approximation (Osterbrock 1974). Strømgren (1939) was the first to show that the transition from a state of almost completely ionized material to a state of almost completely neutral material occurs in a very short distance compared with the dimensions of the Hii region. For example, for a spherically-symmetric Hii region expanding into a cloud of uniform density ρ n , the radius of the ionization front is given by the Strømgren radius …”

Section: Spitzer Solutionmentioning

confidence: 99%

Smoothed particle hydrodynamics simulations of expanding H II regions

Bisbas¹,

Wünsch²,

Whitworth³

et al. 2009

A&A

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Context. Ionizing radiation plays a crucial role in star formation at all epochs. In contemporary star formation, ionization abruptly raises the pressure by more than three orders of magnitude; the temperature increases from ∼10 K to ∼10 4 K, and the mean molecular weight decreases by a factor of more than 3. This may result in positive feedback, either by compressing pre-existing clouds and rendering them unstable, or by sweeping up gravitationally unstable shells. It may also result in negative feedback (by dispersing residual dense gas). Ionizing radiation from OB stars is also routinely invoked as a means of injecting kinetic energy into the interstellar medium and as a driver of sequential self-propagating star formation in galaxies. Aims. We describe a new algorithm for including the dynamical effects of ionizing radiation in SPH simulations, and we present several examples of how the algorithm can be applied to problems in star formation. Methods. We use the HEALPix software to tessellate the sky and to solve the equation of ionization equilibrium along a ray towards each of the resulting tesserae. We exploit the hierarchical nature of HEALPix to make the algorithm adaptive, so that fine angular resolution is invoked only where it is needed, and the computational cost is kept low. Results. We present simulations of (i) the spherically symmetric expansion of an Hii region inside a uniform-density, non-selfgravitating cloud; (ii) the spherically symmetric expansion of an Hii region inside a uniform-density, self-gravitating cloud; (iii) the expansion of an off-centre Hii region inside a uniform-density, non-self-gravitating cloud, resulting in rocket acceleration and dispersal of the cloud; and (iv) radiatively driven compression and ablation of a core overrun by an Hii region. Conclusions. The new algorithm provides the means to explore and evaluate the role of ionizing radiation in regulating the efficiency and statistics of star formation.

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“…The ultraviolet (UV) photons from quasars are sufficiently energetic to ionize the neutral hydrogen in a volume around them, creating a Strömgren (1939) sphere. The extent of the ionization region depends in part on the UV luminosity of the quasar and the neutral fraction of the surrounding medium, potentially providing a complementary probe of reionization.…”

Section: Quasar Ionization Regionmentioning

confidence: 99%

Discovery of a redshift 6.13 quasar in the UKIRT infrared deep sky survey

Mortlock¹,

Patel²,

Warren³

et al. 2009

A&A

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Optical and near-infrared (NIR) spectra are presented for ULAS J131911.29+095051.4 (hereafter ULAS J1319+0950), a new redshift z = 6.127 ± 0.004 quasar discovered in the Third Data Release (DR3) of the UKIRT Infrared Deep Sky Survey (UKIDSS). The source has Y Vega = 19.10 ± 0.03, corresponding to M 1450,AB = −27.12, which is comparable to the absolute magnitudes of the z 6 quasars discovered in the Sloan Digital Sky Survey (SDSS). ULAS J1319+0950 was, in fact, registered by SDSS as a faint source with z AB = 20.13 ± 0.12, just below the signal-to-noise ratio limit of the SDSS high-redshift quasar survey. The faint z-band magnitude is a consequence of the weak Ly α/N v emission line, which has a rest-frame equivalent width of ∼20 Å and provides only a small boost to the z-band flux. Nevertheless, there is no evidence of a significant new population of high-redshift quasars with weak emission lines from this UKIDSS-based search. The Ly α optical depth to ULAS J1319+0950 is consistent with that measured towards similarly distant SDSS quasars, implying that results from optical-and NIR-selected quasars may be combined in studies of cosmological reionization. Also presented is a new NIR-spectrum of the previously discovered UKIDSS quasar ULAS J020332.38+001229.2, which reveals the object to be a broad absorption line quasar. The new spectrum shows that the emission line initially identified as Ly α is actually N v, leading to a revised redshift of z = 5.72, rather than z = 5.86 as previously estimated.

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The Physical State of Interstellar Hydrogen.

Cited by 461 publications

References 0 publications

A lepto-hadronic model for high-energy emission from FR I radiogalaxies

A lepto-hadronic model for high-energy emission from FR I radiogalaxies

Smoothed particle hydrodynamics simulations of expanding H II regions

Discovery of a redshift 6.13 quasar in the UKIRT infrared deep sky survey

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