How Close do the Z's have to be for it to be a merger?

[dreamslaughter]

I have two spirals that look like they could be merging, on has a z of 19.22, the other has a z of 18.98. Can these be merging?

How do you translate z to actual distance. As I understand it, z indicates how far the object is away from us. Is this correct? And if so, how does actual distance relate to the z?

What's a typical distance in z that a galaxy is from edge to edge?

[zookeeperKevin]

Hey, it's an extremely difficult questions and the easy answer is: there's no good cutoff.

The ONLY way you can be SURE you're seeing a merger is if you see the galaxies actually interacting.

[dreamslaughter]

Hey, it's an extremely difficult questions and the easy answer is: there's no good cutoff.

The ONLY way you can be SURE you're seeing a merger is if you see the galaxies actually interacting.

Cool, that makes sense, although the two spirals were edge on so you can't see the interaction.

Also, where can I find how z relates to actual distance?

Thanks

[fjgiie]

The other Z

Use the Z at the bottom of the page, usually less than one. (sometimes)

Look at this link, the z is 0.065 down at bottom.
http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587736478661017762

[altymczuk]

Right!  The Z you quoted was the luminance Z.  The red shift z is in the lower table (if it is there at all).  There has been an on-going discussion in various postings concerning the accuracy of the z in complex objects, especially overlaps.  Targeting errors can lead to two measurements on the same object, and identical z's.

See reply #9 by Edd.

[suprtrkr]

Hey, it's an extremely difficult questions and the easy answer is: there's no good cutoff.

No fertilizer! I've seen spirals (easier to tell, since they've such defined structure) with identical z, clearly overlapped in projection, but with no sign of disturbance. And others that were obviously merging with z's differing by (max so far) .008.

Just how much measurement error is there in this, anyway?

[zookeeperKevin]

Just how much measurement error is there in this, anyway?

It's not so much the error that causes the problem.

Redshift z is essentially a velocity. It's the sum of the recession velocity due to the expansion of the universe and whatever local "peculiar velocity" a galaxy has; motion relative to its neighbours. Thus, if two galaxies are physically apart but moving relative to each other, their redshifts might give the impression that they're at the same distance.

[suprtrkr]

Redshift z is essentially a velocity. It's the sum of the recession velocity due to the expansion of the universe and whatever local "peculiar velocity" a galaxy has; motion relative to its neighbours. Thus, if two galaxies are physically apart but moving relative to each other, their redshifts might give the impression that they're at the same distance.

Ah! So much to learn! Like I didn't know that redshift was a vector sum. And idiot me had been thinking of it in terms of scalar distance, I guess just because we don't have any other way to judge it.

Thank you, Kevin: you are so good at explaining things to us non-scientific people.

[makrogabor]

wait, wait, wait a moment please!!!!
what? z means the distance???

so if i can understand good, if an object's z is e.g.: 16 and the other's is 10 than it means that
object (z=10) is closer than other??!!

[Edd]

No, the z that sits with the u, g, r, and i in the tables is a measure of the brightness in the z band.

The z that sits next to spectra at the bottom sometimes is a measure of the distance though (subject to the caveats Kevin mentioned).

The first will be somewhere above 10. The second will be below 7.

So, your 10 and 16 are just measures of a particular kind of brightness, not distance.

[zookeeperKevin]

Yes, it's a bit confusing...

The SDSS filters are u,g,r,i, and z. And of course z also stands for redshift. It's happened more than once that I suddenly got impossibly bright galaxies because I used the wrong z to compute their absolute magnitude....

Also note that the common varaible for redshift is little z. Capital Z generally stands for metallicity (another astro-misnomer: all elements other than hydrogen and helium are "metals").

[makrogabor]

wao, it's fantastic, thanks!
and which object is closer to us? z is lesser or bigger?

[suprtrkr]

wait, wait, wait a moment please!!!!
what? z means the distance???

so if i can understand good, if an object's z is e.g.: 16 and the other's is 10 than it means that
object (z=10) is closer than other??!!

Not exactly (sigh). Kevin did it better, but I'll try.

Redshift, technically, is a measure of the displacement of light colors toward the lower or red end of the spectrum. This is a function of Doppler shift (try google or wikipedia for a full explaination). Basically, it means that if a light emitting object is coming toward you, the light waves "bunch up" by the amount of the velocity of the object--this is blueshift--and going away they "stretch out" by the same factor a/k/a redshift.

Now, we believe (sorta) in an expanding universe; that is, pretty much everything in the universe is moving away from us. And, for reasons that nobody understands, it seems like the farther away they are, the faster they go.

So, ok, they measure the spectrum of a distant galaxy--and they know, for reasons too complex to explain and that I don't fully understand myself, what the spectrum should be. They compare what they actually get with what they expect to see and the amount that these differ is the redshift-- the amount the actual spectrum colors are shifted toward the red end of the expected spectrum.

This gives us an indication of the velocity it's moving from us: By doppler effect, the greater the velocity, the greater the redshift. And that gives us--by the expanding universe thing, and the apparent increase in velocity over distance--a hint about how far away it is.

It's more complex than that--see Kevin's post above where he corrected me--because the spectrum can't measure just the recessional velocity of the universe. It can only measure the total velocity of the object, which is a vector sum of the recessional velocity plus whatever motion the object has on it's own. Thus it can't be treated as an absolute: a far-away object moving toward us and a close object moving away might exhibit the same redshift because the redshift is the sum of the velocities.

Having now confused myself more than you, the simple answer is it's an indication, not a guarantee.

But you were right about one thing    The larger the number, the farther away it is.

You're not likely to see a redshift greater than one, however, or at least not in this survey. The telescope used to take it can't see things far enough away to have a redshift greater than 1. I'm no expert, but reading the forum has brought me some info and I offer it FWIW:

if z=
0.000, it's in this galaxy, and maybe out to .002 or so.
0.050, medium distant, outside the local cluster.
0.150, getting pretty far out there, far enough for that QSO to really be a quasar.
0.400, way the heck out there.
0.5 or 0.6, approaching the limits of the telescope.

If anybody can refine these numbers, i could use the help myself.

[Edd]

Redshift, technically, is a measure of the displacement of light colors toward the lower or red end of the spectrum. This is a function of Doppler shift (try google or wikipedia for a full explaination). Basically, it means that if a light emitting object is coming toward you, the light waves "bunch up" by the amount of the velocity of the object--this is blueshift--and going away they "stretch out" by the same factor a/k/a redshift.

I prefer to think of the cosmological redshift as a stretching of the light, rather than a Doppler shift. There's a useful but rather heavy going FAQ out there on whether or not you should agree with me on this point, but I find the stretching along with the expansion of space the most intuitive and trouble-free explanation.

Basically, the light gets emitted from some distant galaxy and by the time it gets here space has expanded, and the light has expanded along with it.

This still gets mixed up with the true Doppler shift from the object's own velocity (the 'peculiar velocity') in the way that's been described though.

[suprtrkr]

whether or not you should agree with me on this point,

Lol! I understood what you said-- in English. As far as agreeing or not, I don't have the math.

but I find the stretching along with the expansion of space the most intuitive and trouble-free explanation.

And so it is. I can actually understand the concept. If I'm getting this: The object has doppler redshift because it's moving via the universe expanding plus more expansion redshift because the light has stretched (plus or minus peculiar motion)? Or is it all because of the stretching effect?

This still gets mixed up with the true Doppler shift from the object's own velocity

And that was the point I was trying to make.