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15 votes

Why can't 'dead' galaxies start forming stars again if they merge with another, 'active' galaxy?

It is not true that "dead", or quenched galaxies cannot start forming stars again. It is called rejuvenation and is observed occasionally (e.g. Chauke et al. 2019), although not commonly (...
pela's user avatar
  • 38.9k
13 votes
Accepted

Who is Seyfert, and what is "the so-called Seyfert flare"?

As has now been corrected, BBC has misunderstood term "Seyfert flare", instead calling it a "Sifter flare". A "Seyfert flare", is not really a common term, but the authors refer to an energetic ...
pela's user avatar
  • 38.9k
11 votes

How close would a quasar have to be to be seen with the naked eye?

Let's do the math: the equation for absolute magnitude $M$ in relation to apparent magnitude $m$ in relation to distance $d$ is: $m - M = -5 + 5 \log(d)$ (You do not need to know how that works as I ...
ArchiveOfStars's user avatar
10 votes

How are neutrinos produced in blazar jets?

Neutrinos are typically produced in AGN jets through what we refer to as hadronic processes. Protons are accelerated to relativistic speeds and interact with nearby photons. Depending on the ...
HDE 226868's user avatar
  • 37.3k
8 votes
Accepted

On the "periodic" lightcurves of Active Galactic Nuclei

I don't think what you're seeing is periodicity; my bet is that it's just stochastic red noise. Searches for periodicity or quasi-periodic oscillations (QPOs) in AGN light curves is a difficult and at ...
HDE 226868's user avatar
  • 37.3k
8 votes
Accepted

Is the appearance of accretion disks of M87 and Sgr A* in EHT image, related to the nucleus being active or non-active?

The rings are not direct images of accretion disks. They are blurred images of the gravitationally lensed light from all around the black holes, with a central "shadow" due to photon orbits ...
ProfRob's user avatar
  • 156k
7 votes
Accepted

Do any particles in AGN jets escape the galaxy?

Yes, definitely. While some matter returns to the galaxy as a so-called "galactic fountain" (e.g. Biernacki & Teyssier 2018), some material is ejected at super-escape velocity, becoming ...
pela's user avatar
  • 38.9k
6 votes
Accepted

Why is the Milky Way's SMBH inactive?

I think the answer to your question is 1) because of the general smaller probability of hosting an AGN with time, and 2) stochasticity. In general, the fraction of galaxies with active galactic ...
pela's user avatar
  • 38.9k
6 votes

Why does dust affect UV light more severe than it affects X-rays?

Ordinary solid materials, even very thin, are almost perfectly opaque to UV. On the other hand, we use hard x-rays to image the interiors of macroscopic solid objects. Hard x-rays pass right through ...
John Doty's user avatar
  • 2,587
5 votes

Why does dust affect UV light more severe than it affects X-rays?

tl;dr, it’s the sizes of the grains of dust and how many there are that create wavelength dependent extinction. Based off of the additional context you provided and reading the article you gave, it ...
Justin T's user avatar
  • 3,404
3 votes

On the "periodic" lightcurves of Active Galactic Nuclei

AGN's are divided into roughly 3 types, Quasars, Seyfert galaxies and radio galaxies. All of these produce light, each in their own electromagnetic spectrum and intensity like quasar emits across all, ...
Arjun's user avatar
  • 1,624
3 votes
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Optical emission line data archives

A few sources come to mind to add to MAST and Sloan. Data releases from the SAMI Galaxy survey, which contain (or will eventually contain) emission line spectra for about 3500 galaxies. The papers for ...
Joseph Farah's user avatar
3 votes
Accepted

An image for the unified model of AGN

I agree with your supervisor: If your paper is going to a peer-reviewed journal, you should cite peer-reviewed material. I also agree with you that the NASA version is clearer than the original figure,...
pela's user avatar
  • 38.9k
2 votes
Accepted

What experiment does this "Tracing the History of Starlight" video try to explain?

Blazar sequence Let's first look at which kind of sources are studied. Blazars are special type of radio-loud AGN. One of their special properties is that their spectral energy distributions (SED) ...
samcarter_is_at_topanswers.xyz's user avatar
2 votes

How to shift an AGN X-ray spectrum to rest frame

The frequency is related to the redshift by $$\frac{\nu_{\rm obs}}{\nu_{\rm emit}} = \frac{1}{1+z}$$ Another useful relation is the fact that, for any redshift, $$\frac{I_{\nu}}{\nu^3} = {\rm ...
kleingordon's user avatar
2 votes

Are quasars simply AGNs that are viewed from a particular angle?

There is total confusion in the answers you refer to. Quasars were first defined because of their star-like appearance - quasi-stellar objects or QSOs. The fact that they are Galaxy-sized objects with ...
ProfRob's user avatar
  • 156k
2 votes
Accepted

How much light comes from quasars in an AGN?

Only a few percent of galaxies in the local universe have active galactic nuclei (e.g., Mishra & Dai 2020). The fraction with optically visible AGN (e.g., Seyfert nuclei) is lower than this, since ...
Peter Erwin's user avatar
  • 17.3k
2 votes

How much light comes from quasars in an AGN?

Active Galactic Nuclei (AGNs) are very luminous galactic nuclei. In Quasars, a type of an AGN, the nucleus outshines the host galaxy. Therefore the main source of radiation is the nucleus, not the ...
AstroTeen's user avatar
  • 149
2 votes
Accepted

Meaning of "blue continuum spectrum"?

The continuum spectrum could be described as "blue" as it is skewed towards shorter wavelengths. There is an excess of blue light compared to red. You can see this in the way that the ...
James K's user avatar
  • 126k
2 votes

What are the applications of cross matching astronomical catalogues?

I did work in the 90s on cross-matching optical catalogues with catalogues of X-ray emitting sources. The main reason for doing this was to figure out which object was responsible for the X-ray ...
ProfRob's user avatar
  • 156k
1 vote

Can the Hubble expansion inject energy in AGN and quasar outflows?

The paper says At the onset of shell expansion, there are two energy reservoirs: the input energy of the quasar itself, $E_0$, and the kinetic energy of the preexisting Hubble flow within a distance $...
benrg's user avatar
  • 3,904
1 vote

What are the applications of cross matching astronomical catalogues?

If I’m understanding what you’re asking, then it boils down to two things: resource management and different physics. Let’s say you’re looking at a given variable star. Maybe you just started ...
Justin T's user avatar
  • 3,404
1 vote

What fraction of galaxies host active galactic nuclei?

As Pela said in the comment, it is highly redshift dependent so there is no straight answer. "Some" is the safest option.
Sean Mooney's user avatar
1 vote

Who is Seyfert, and what is "the so-called Seyfert flare"?

Answering the who is part of the question. Carl Seyfert (1911-1960) was a US astronomer. He is best known for his 1943 research paper on high-excitation line emission from the centers of some ...
Fred's user avatar
  • 2,169
1 vote

What is the mass and momentum of a relativistic jet?

I am not expert in that field, but I found a number and a reference: A paper by E. Memola et al. on Theoretical thermal X-ray spectra of relativistic MHD jets assumes a mass flow of $\dot{M}_{\rm jet}...
B--rian's user avatar
  • 5,636
1 vote
Accepted

Regions and size of an AGN

pc is the abbreviation for parsec, a common unit of distance objects outside the Solar system. In order to compress the scales, they have also taken the logarithm (...
astrosnapper's user avatar
  • 8,452
1 vote

Are quasars simply AGNs that are viewed from a particular angle?

The consensus view is that Quasars are a type of AGN, and depending on precisely what meaning you put in "Quasar" it can also mean it is strictly a type defined by viewing angle. I think the ...
Tobychev's user avatar
1 vote

Structure of the torus of the Active Galactic Nucleus

This question was asked more roughly 8 years ago, and for 2 years there is a review article around which I found worth mentioning and summarizing. The manuscript is called Redefining the torus: A ...
B--rian's user avatar
  • 5,636

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