# Proper motion of a Quasar?

• A
quasarLie
Hello,
Does quasars has a proper motion? I searched in different article and sometimes they are considered as objects having a proper motion and other times not. So I want some clarification. in which case can we say that a quasar has a proper motion?
Thanks

Mentor
Everything has proper motion. The motion can be very small, and for objects far away it is simply too small to detect it, but it won't be exactly zero.

Staff Emeritus
2021 Award
I searched in different article and sometimes they are considered as objects having a proper motion and other times not.

Where? Which articles exactly?

Quasars are so far away that their proper motion is unmeasurably small. This is why Gaia uses them as a fixed system of reference. In fact, in this recent paper, they are searching through the Gaia data release to find objects with zero proper motion (within the error of the measurement of course) as a way to find new quasars.

nnunn
CygnusX-1
Yes, exactly. If you think you've measured the proper motion of a quasar, you've probably done something wrong. :)

Mentor
Just as order of magnitude estimate: 500 km/s relative velocity at ~5 Gly (?) angular diameter distance gives a proper motion of 70 nas (nano-arcseconds) per year as very fast motion. Gaia hopes to measure positions of bright sources down to 7 μas for very bright sources, and somewhere between 100 to 300 μas for weaker sources - more than a factor 1000 above the expected proper motion.

nnunn
quasarLie
Quasars are so far away that their proper motion is unmeasurably small. This is why Gaia uses them as a fixed system of reference. In fact, in this recent paper, they are searching through the Gaia data release to find objects with zero proper motion (within the error of the measurement of course) as a way to find new quasars.
Thanks this is why am working on, i am using DR2 to study the proper motion of quasars. I found that the most of quasars does not have a proper motion, but few of them have a proper motion. how can t his be explained (are they quasars with small redshift?)

Thanks this is why am working on, i am using DR2 to study the proper motion of quasars. I found that the most of quasars does not have a proper motion, but few of them have a proper motion. how can t his be explained (are they quasars with small redshift?)

(a) they are not quasars
or
(b) measurement errors

quasarLie
(a) they are not quasars
or
(b) measurement errors
No they are considered as quasars by SDSS. and it s not measurement error

No they are considered as quasars by SDSS. and it s not measurement error

Oh. Then they have transverse velocity on the order of 500 000 km/s. ;)

nnunn
quasarLie
Oh. Then they have transverse velocity on the order of 500 000 km/s. ;)

You did not like my honest answer, so I attempted to guess the answer which you look for.

Mentor
Are you sure you got the right object from SDSS? How do you compare them?
What are their redshift values?
Is the proper motion significantly different from zero, or compatible within the uncertainties?

No they are considered as quasars by SDSS. and it s not measurement error

I agree with nikkkom. How do you know SDSS didn't make an error? And how do you know it's not measurement error? Try calculating the lateral motion you would need to give the proper motion you see at the distance inferred from the redshift. It's probably unbelievably large.

snorkack
What is the definition of a quasar?

Quasars are supposed to be active galactic nuclei.
Galactic nuclei at various degrees of activity are found in many galaxies, including ours (but not all, Magellanic Clouds and Triangulum conspicuously lack nuclei).

A distant AGN should not have proper motion (that would require a large transverse speed).
But how about a nearby AGN? How is the distinction made between a distant AGN, and a nearby AGN that happens to have low transverse velocity?
Many quasars have redshifts, usually high, suggesting that those are distant. Are there also any quasars that have blank continuum spectra and therefore no redshift?

What is the definition of a quasar?

Quasars are supposed to be active galactic nuclei.
Galactic nuclei at various degrees of activity are found in many galaxies, including ours (but not all, Magellanic Clouds and Triangulum conspicuously lack nuclei).

A distant AGN should not have proper motion (that would require a large transverse speed).
But how about a nearby AGN?

There is no nearby AGN which would be active enough to be classified as quasar. The nearest quasars are all more than 2 Gly distant.

quasarLie
There is no nearby AGN which would be active enough to be classified as quasar. The nearest quasars are all more than 2 Gly distant.
if you take a look at quasars catalogue, you will see that there are many quasars with redshift<2.

if you take a look at quasars catalogue, you will see that there are many quasars with redshift<2.
An object at a redshift of 2 is at a comoving distance of ~ 5 Gpc or ~ 17 Gly. nikkkom's statement was that the nearest quasars are more than 2 Gly distant. A comoving distance of 2 Gly equates to a redshift of about 0.15.

snorkack
Markarian 231´s status is in dispute. A quasar or a Seyfert galaxy? If a quasar, it´s just 580 million light years.
The next is 3C 273, at 2400 million lightyears.

What is the defining line between quasars and Seyfert galaxies?
What is the most distant Seyfert galaxy not qualified as a quasar?

Gold Member
Could the handle "quasarLie" be a clue here?

Bandersnatch
nnunn
Oh. Then they have transverse velocity on the order of 500 000 km/s. ;)

Thanks nikkkom for helping to put this in context. However, you stir up an unexpected thought.

Given the assumptions of special relativity (recall all those undergrad exercises on "space-ship frame" vs. "rest frame"), then your (playful) transverse velocity of 500,000 km/s has interesting implications for current consensus assumptions (i.e. that large redshifts imply purely radial recession).

For a fun intro to some implications, see e.g., "Transverse Doppler Effect simplified"

Nigel

Gold Member
The video is completely wrong. You really can't believe just anything you find on the internet, people who know nothing can still use fancy graphics.

alantheastronomer
Does your inquiry into the proper motion of quasars have anything to do with Arp's conjecture that quasars are expelled from the centers of galaxies?