<p>Yes, a blueshift here would mean relative to the systemic velocity of the galaxy - which means that, <em>relative to the galaxy</em>, the material is moving at least partly in the direction of Earth.</p>
<p>In many cases the outflow may be <a href="https://www.eso.org/public/usa/news/eso0903/?lang" rel="nofollow noreferrer">a pair of jets being ejected in opposite directions out of the nucleus of the galaxy</a>, but we are less likely to see absorption lines from the jet on the far side of the galaxy (relative to Earth) because the disk of the galaxy may be blocking our view of it, and in any case it won't produce absorption lines because it isn't between us and the rest of the galaxy.</p>

CC BY-SA 4.0
2020-07-11T20:09:10.883Z
Cool thanks! How do you know that the blueshifted material you're observing is on the side of the galaxy closer to us, instead of the far side -- in the latter case, the blueshifted gas would be moving toward the galaxy itself, right? Edit: I guess we would never observe blueshifted absorption for gas on the far side of the galaxy because the galaxy's starlight would be in the foreground. We would need another background light source to illuminate gas on the farside of the galaxy.
quantumflash
CC BY-SA 4.0
2020-07-11T20:16:09.543Z
Wait hold up, can I also ask about gas in emission? If you detect blueshifted gas emission, how do you distinguish whether it's on the side closer to us (flowing out of the galaxy, towards us) vs. on the far side of the galaxy (flowing into/toward the galaxy itself)?
quantumflash
CC BY-SA 4.0
2020-07-12T00:04:48.597Z
If the system is unresolved (no spatial information, just a spectrum), then you are right that we can’t determine whether blueshifted gas is flowing out or in.  But in many cases, we can resolve the emission and see which side of the galaxy it is on, and thus determine which way it is going, either from Doppler shift, or in some cases, from time series observations where we see knots (e.g. in a jet) moving on the sky over time.  Here’s an example of [jets from M87](https://apod.nasa.gov/apod/ap190427.html).
Eric Jensen
CC BY-SA 4.0
2020-07-11T19:51:06.863Z
2020-07-11T19:51:06.863Z
Eric Jensen
<p>I know that galaxies can eject gas due to supernovae, accreting black holes, etc. These galactic &quot;winds/outflows&quot; are often defined/detected observationally using blueshifted absorption lines in spectra. But why is it always blueshifted absorption -- what about redshifted absorption? Does it have something to do with the outflow velocities being measured relative to the galaxy central/systemic velocity, such that redshifted absorption implies inflow instead of outflow? But what's the geometrical picture?</p>

<galaxy><supernova><star-formation><interstellar-medium><gas>
Galactic winds/outflows: why and how are they detected via blueshifted absorption lines in spectra?
quantumflash
# Galactic winds/outflows: why and how are they detected via blueshifted absorption lines in spectra?

(Asked by: username_0 on 2020-07-11)

I know that galaxies can eject gas due to supernovae, accreting black holes, etc. These galactic "winds/outflows" are often defined/detected observationally using blueshifted absorption lines in spectra. But why is it always blueshifted absorption -- what about redshifted absorption? Does it have something to do with the outflow velocities being measured relative to the galaxy central/systemic velocity, such that redshifted absorption implies inflow instead of outflow? But what's the geometrical picture?

**Tags:** galaxy, supernova, star-formation, interstellar-medium, gas

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## 1 answers

### Answer by username_1 on 2020-07-11. Score: 3 | Accepted answer

Yes, a blueshift here would mean relative to the systemic velocity of the galaxy - which means that, *relative to the galaxy*, the material is moving at least partly in the direction of Earth.

In many cases the outflow may be [a pair of jets being ejected in opposite directions out of the nucleus of the galaxy](https://www.eso.org/public/usa/news/eso0903/?lang), but we are less likely to see absorption lines from the jet on the far side of the galaxy (relative to Earth) because the disk of the galaxy may be blocking our view of it, and in any case it won't produce absorption lines because it isn't between us and the rest of the galaxy.

> **Comments:**
> **username_0**: Cool thanks! How do you know that the blueshifted material you're observing is on the side of the galaxy closer to us, instead of the far side -- in the latter case, the blueshifted gas would be moving toward the galaxy itself, right? Edit: I guess we would never observe blueshifted absorption for gas on the far side of the galaxy because the galaxy's starlight would be in the foreground. We would need another background light source to illuminate gas on the farside of the galaxy.  
> **username_0**: Wait hold up, can I also ask about gas in emission? If you detect blueshifted gas emission, how do you distinguish whether it's on the side closer to us (flowing out of the galaxy, towards us) vs. on the far side of the galaxy (flowing into/toward the galaxy itself)?  
> **username_1**: If the system is unresolved (no spatial information, just a spectrum), then you are right that we can’t determine whether blueshifted gas is flowing out or in.  But in many cases, we can resolve the emission and see which side of the galaxy it is on, and thus determine which way it is going, either from Doppler shift, or in some cases, from time series observations where we see knots (e.g. in a jet) moving on the sky over time.  Here’s an example of [jets from M87](https://apod.nasa.gov/apod/ap190427.html).  

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