High Resolution Imaging of Sagittarius A*

Bower, Geoffrey C.

doi:10.1088/1742-6596/54/1/058

Cited by 7 publications

(11 citation statements)

References 21 publications

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“…An important issue is that the galaxy merger history can, and does, depend strongly on the galaxy formation model used. We show this in detail in Figure 4 where four different realizations of the Millennium simulation are shown from Bertone et al (2007), Bower et al (2006), De Lucia et al (2006 and Guo et al (2010). As can be seen, these models do not agree with the observations.…”

Section: Introductionmentioning

confidence: 96%

Galaxy formation as a cosmological tool – I. The galaxy merger history as a measure of cosmological parameters

Conselice

Bluck

Mortlock

et al. 2014

Monthly Notices of the Royal Astronomical Society

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As galaxy formation and evolution over long cosmic time-scales depends to a large degree on the structure of the universe, the assembly history of galaxies is potentially a powerful approach for learning about the universe itself. In this paper we examine the merger history of dark matter halos based on the Extended Press-Schechter formalism as a function of cosmological parameters, redshift and halo mass. We calculate how major halo mergers are influenced by changes in the cosmological values of Ω m , Ω Λ , σ 8 , the dark matter particle temperature (warm vs. cold dark matter), and the value of a constant and evolving equation of state parameter w(z). We find that the merger fraction at a given halo mass varies by up to a factor of three for halos forming under the assumption of Cold Dark Matter, within different underling cosmological parameters. We find that the current measurements of the merger history, as measured through observed galaxy pairs as well as through structure, are in agreement with the concordance cosmology with the current best fit giving 1 − Ω m = Ω Λ = 0.84 +0.16 −0.17 . To obtain a more accurate constraint competitive with recently measured cosmological parameters from Planck and WMAP requires a measured merger accuracy of δf m ∼ 0.01, implying surveys with an accurately measured merger history over 2 -20 deg 2 , which will be feasible with the next generation of imaging and spectroscopic surveys such as Euclid and LSST.

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Section: Introductionmentioning

confidence: 96%

Galaxy formation as a cosmological tool – I. The galaxy merger history as a measure of cosmological parameters

Conselice

Bluck

Mortlock

et al. 2014

Monthly Notices of the Royal Astronomical Society

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“…In radio, Sgr A* has an inverted spectrum (i.e., rising flux density with increasing frequency) that peaks at the 'submm bump', around 350 GHz, beyond which the spectrum steeply drops in the infrared regime. The radio emission is thought to originate mostly from partially self-absorbed synchrotron radiation emitted farther out from the black hole, while emission at frequencies corresponding to the submm bump (Falcke et al, 1998) of the Sgr A* spectrum is commonly associated with the optically thin emission closest to the black hole (Falcke et al, 1998;Shen et al, 2005;Bower, 2006;Doeleman et al, 2008). In the mm regime and at longer wavelengths, the flux density variation is thought to arise from local bulk properties (magnetic field strength, gas density, temperature) of the plasma, while the variability seen in infrared and X-rays is mostly attributed to changes in the population of the high-energy tail of the local electron energy distribution ( Özel et al, 2000;Markoff et al, 2001;Yuan et al, 2003;Dodds-Eden et al, 2010;Dibi et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

ALMA and VLA measurements of frequency-dependent time lags in Sagittarius A*: evidence for a relativistic outflow

Brinkerink

Falcke

Law

et al. 2015

A&A

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Context. Radio and mm-wavelength observations of Sagittarius A* (Sgr A*), the radio source associated with the supermassive black hole at the center of our Galaxy, show that it behaves as a partially self-absorbed synchrotron-emitting source. The measured size of Sgr A* shows that the mm-wavelength emission comes from a small region and consists of the inner accretion flow and a possible collimated outflow. Existing observations of Sgr A* have revealed a time lag between light curves at 43 GHz and 22 GHz, which is consistent with a rapidly expanding plasma flow and supports the presence of a collimated outflow from the environment of an accreting black hole. Aims. Here we wish to measure simultaneous frequency-dependent time lags in the light curves of Sgr A* across a broad frequency range to constrain direction and speed of the radio-emitting plasma in the vicinity of the black hole. Methods. Light curves of Sgr A* were taken in May 2012 using ALMA at 100 GHz using the VLA at 48, 39, 37, 27, 25.5, and 19 GHz. As a result of elevation limits and the longitude difference between the stations, the usable overlap in the light curves is approximately four hours. Although Sgr A* was in a relatively quiet phase, the high sensitivity of ALMA and the VLA allowed us to detect and fit maxima of an observed minor flare where flux density varied by ∼10%. Results. The fitted times of flux density maxima at frequencies from 100 GHz to 19 GHz, as well as a cross-correlation analysis, reveal a simple frequency-dependent time lag relation where maxima at higher frequencies lead those at lower frequencies. Taking the observed size-frequency relation of Sgr A* into account, these time lags suggest a moderately relativistic (lower estimates: 0.5c for two-sided, 0.77c for one-sided) collimated outflow.

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“…We used for observations the set of Sgr A* images simulated with the use of the BJPL2013 physical model developed by A. E. Broderick, T. Johannsen, D. Psaltis, and A. Loeb (Broderick et al 2013). In order to imitate the scattering by the turbulent ionized interstellar medium, the images were smoothed by convolving with an elliptical Gaussian kernel with a FWHM of 22 µas along the major axis and 11 µas along the minor axis, with a position angle of 78 • (see Bower 2006;Shen 2005;Bower et al 2004;Falcke et al 2000). The elliptical locus of the 2D Gaussian FWHM is sketched in Fig.…”

Section: Simulation Setupmentioning

confidence: 99%

Reconstruction of Static Black Hole Images Using Simple Geometric Forms

Benkevitch,

Akiyama,

et al. 2016

Preprint

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General Relativity predicts that the emission close to a black hole must be lensed by its strong gravitational field, illuminating the last photon orbit. This results in a dark circular area known as the black hole 'shadow'. The Event Horizon Telescope (EHT) is a (sub)mm VLBI network capable of Schwarzschild-radius resolution on Sagittarius A* (or Sgr A*), the 4 million solar mass black hole at the Galactic Center. The goals of the Sgr A* observations include resolving and measuring the details of its morphology. However, EHT data are sparse in the visibility domain, complicating reliable detailed image reconstruction. Therefore, direct pixel imaging should be complemented by other approaches. Using simulated EHT data from a black hole emission model we consider an approach to Sgr A* image reconstruction based on a simple and computationally efficient analytical model that produces images similar to the synthetic ones. The model consists of an eccentric ring with a brightness gradient and a two-dimensional Gaussian. These elemental forms have closed functional representations in the visibility domain, which lowers the computational overhead of fitting the model to the EHT observations. For model fitting we use a version of the Markov chain Monte-Carlo (MCMC) algorithm based on the Metropolis-Hastings sampler with replica exchange. Over a series of simulations we demonstrate that our model can be used for determining geometric measures of a black hole, thus providing information on the shadow size, linking General Relativity with accretion theory.

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High Resolution Imaging of Sagittarius A*

Cited by 7 publications

References 21 publications

Galaxy formation as a cosmological tool – I. The galaxy merger history as a measure of cosmological parameters

Galaxy formation as a cosmological tool – I. The galaxy merger history as a measure of cosmological parameters

ALMA and VLA measurements of frequency-dependent time lags in Sagittarius A*: evidence for a relativistic outflow

Reconstruction of Static Black Hole Images Using Simple Geometric Forms

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