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Register a new userSynchrotron Emission from Elliptical Galaxies Consequent to AGN outbursts
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Both radiative and mechanical feedback from Active Galactic Nuclei have been found to be important for the evolution of elliptical galaxies. We compute how a shock may be driven from a central black hole into the gaseous envelope of an elliptical galaxy by mechanical as well as radiative feedback (in the form of nuclear winds) using high resolution 1-D hydrodynamical simulations. We calculate the synchrotron emission from the electron cosmic rays accelerated by the shocks (not the jets) in the central part of elliptical galaxies, and we also study the synchrotron spectrums evolution using the standard diffusive shock acceleration mechanism, which is routinely applied to the scaled volume case of supernova remnants. We find good agreement quantitatively between the synchrotron radio emission produced via this mechanism with extant observations of elliptical galaxies which are undergoing outbursts. Additionally, we also find that synchrotron optical and X-ray emission can co-exist inside elliptical galaxies during a certain phase of evolution subsequent to central outbursts. In fact, our calculations predict a synchrotron luminosity of $sim 1.3times 10^6 L_{odot}$ at the frequency 5 GHz (radio band), of $sim 1.1times 10^6 L_{odot}$ at $4.3times10^{14}$ Hz (R band, corresponding to the absolute magnitude -10.4), and of $sim 1.5times 10^{7} L_{odot}$ at $2.4times10^{17}$ Hz (soft X-ray, 0.5 -- 2.0 keV band).
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The ISM evolution of elliptical galaxies experiencing feedback from accretion onto a central black hole was studied recently with high-resolution 1D hydrodynamical simulations including radiative heating and pressure effects, a RIAF-like radiative efficiency, mechanical input from AGN winds, and accretion-driven starbursts. Here we focus on the observational properties of the models in the X-ray band (nuclear luminosity; hot ISM luminosity and temperature; temperature and brightness profiles during quiescence and during outbursts). The nuclear bursts last for ~10^7 yr, with a duty-cycle of a few X (10^-3-10^-2); the present epoch bolometric nuclear emission is very sub-Eddington. The ISM thermal luminosity lx oscillates in phase with the nuclear one; this helps reproduce statistically the observed large lx variation. In quiescence the temperature profile has a negative gradient; thanks to past outbursts, the brightness profile lacks the steep shape typical of inflowing models. Outbursts produce disturbances in these profiles. Most significantly, a hot bubble from shocked hot gas is inflated at the galaxy center; the bubble would be conical in shape, and show radio emission. The ISM resumes a smooth appearance on a time-scale of ~200 Myr; the duty-cycle of perturbances in the ISM is of the order of 5-10%. From the present analysis, additional input physics is important in the ISM-black hole coevolution, to fully account for the properties of real galaxies, as a confining external medium and a jet. The jet will reduce further the mass available for accretion (and then the Eddington ratio $l$), and may help, together with an external pressure, to produce flat or positive temperature gradient profiles (observed in high density environments). Alternatively, $l$ can be reduced if the switch from high to low radiative efficiency takes place at a larger $l$ than routinely assumed.
Outbursts from active galactic nuclei (AGN) affect the hot atmospheres of isolated giant elliptical galaxies (gEs), as well as those in groups and clusters of galaxies. Chandra observations of a sample of nearby gEs show that the average power of AGN outbursts is sufficient to stop their hot atmospheres from cooling and forming stars, consistent with radio mode feedback models. The outbursts are intermittent, with duty cycles that increases with size.
The class of Double-Double Radio Galaxies (DDRGs) relates to episodic jet outbursts. How various regions and components add to the total intensity in radio images is less well known. In this paper we synthesize synchrotron images for DDRGs based on special relativistic hydrodynamic simulations, making advanced approximations for the magnetic fields. We study the synchrotron images for: Three different radial jet profiles; Ordered, entangled or mixed magnetic fields; Spectral ageing from synchrotron cooling; The contribution from different jet components; The viewing angle and Doppler (de-)boosting; The various epochs of the evolution of the DDRG. To link our results to observational data, we adopt to J1835+6204 as a reference source. In all cases the synthesized synchrotron images show two clear pairs of hotspots, in the inner and outer lobes. The best resemblance is obtained for the piecewise isochoric jet model, for a viewing angle of approximately $vartheta sim -71^{circ}$, i.e. inclined with the lower jet towards the observer, with predominantly entangled ($gtrsim 70$ per cent of the magnetic pressure) in turbulent, rather than ordered fields. The effects of spectral ageing become significant when the ratio of observation frequencies and cut-off frequency $ u_{rm obs}/ u_{infty,0} gtrsim 10^{-3}$, corresponding to $sim 3 cdot 10^2$ MHz. For viewing angles $vartheta lesssim -30^{circ}$, a DDRG morphology can no longer be recognized. The second jets must be injected within $lesssim$ 4 per cent of the lifetime of the first jets for a DDRG structure to emerge, which is relevant for Active Galactic Nuclei feedback constraints.
We present 5.5 GHz observations with the VLA of a sample of nearby galaxies with energetic nuclear outbursts at mid-infrared (MIR) bands. These observations reach a uniform depth down to a median rms of ~10 uJy, representing one of most sensitive searches for radio emission associated with nuclear transients. We detect radio emission in 12 out of 16 galaxies at a level of >5sigma, corresponding to a detection rate of 75%. Such a high detection is remarkably different from previous similar searches in stellar tidal disruption events. The radio emission is compact and not resolved for the majority of sources on scales of ~<0.5 (<0.9 kpc at z<0.1). We find the possibility of the star-formation contributing to the radio emission is low, but an AGN origin remains a plausible scenario, especially for sources that show evidence of AGN activity in their optical spectra. If the detections could represent radio emission associated with nuclear transient phenomenon such as jet or outflow, we use the blast wave model by analogy with the GRB afterglows to describe the evolution of radio light curves. In this context, the observations are consistent with a decelerating jet with an energy of ~10^{51-52} erg viewed at 30degree-60degree off-axis at later times, suggesting that powerful jets may be ubiquitous among MIR-burst galaxies. Future continuous monitoring observations will be crucial to decipher the origin of radio emission through detections of potential flux and spectral evolution. Our results highlight the importance of radio observations to constrain the nature of nuclear MIR outbursts in galaxies.
Elliptical galaxies today appear aligned with the large-scale structure of the Universe, but it is still an open question when they acquire this alignment. Observational data is currently insufficient to provide constraints on the time evolution of intrinsic alignments, and hence existing models range from assuming that galaxies gain some primordial alignment at formation, to suggesting that they react instantaneously to tidal interactions with the large-scale structure. Using the cosmological hydrodynamical simulation Horizon-AGN, we measure the relative alignments between the major axes of galaxies and eigenvectors of the tidal field as a function of redshift. We focus on constraining the time evolution of the alignment of the main progenitors of massive $z=0$ elliptical galaxies, the main weak lensing contaminant at low redshift. We show that this population, which at $z=0$ has a stellar mass above $10^{10.4}$ M$_odot$, transitions from having no alignment with the tidal field at $z=3$, to a significant alignment by $z=1$. From $z=0.5$ they preserve their alignment at an approximately constant level until $z=0$. We find a mass-dependence of the alignment signal of elliptical progenitors, whereby ellipticals that are less massive today ($10^{10.4}<M/{rm M}_odot<10^{10.7}$) do not become aligned till later redshifts ($z<2$), compared to more massive counterparts. We also present an extended study of progenitor alignments in the parameter space of stellar mass and galaxy dynamics, the impact of shape definition and tidal field smoothing.
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