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"z=0" galaxies



 
 
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  #1  
Old February 21st 18, 11:08 PM posted to sci.astro
Michael F. Stemper
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Posts: 6
Default "z=0" galaxies

For a class I'm in, our team is supposed to "Characterize the
star formation history of z=0 satellite and central galaxies."

However, I'm not sure what the term "z=0 galaxy" means. To me,
"z=0" means "zero redshift", or "right here". Since the only
galaxy that's "right here" is the Milky Way, this obviously
demonstrates some mis-understanding on my part.

Some searching has revealed stuff such as this Arxiv paper:
https://arxiv.org/pdf/1502.07747.pdf, which discusses some
attributes of "z=0 galaxies". However, it appears to be aimed
(as one would expect) at people who are already familiar with
some aspects of "z=0 galaxies".

Any suggestions on where I could find out what they are? Or, if
it's trivial to explain, would somebody please tell me?

Thanks,
--
Michael F. Stemper
Always use apostrophe's and "quotation marks" properly.
  #2  
Old February 21st 18, 11:58 PM posted to sci.astro
dlzc
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Posts: 1,426
Default "z=0" galaxies

Dear Michael F. Stemper:

On Wednesday, February 21, 2018 at 3:08:19 PM UTC-7, Michael F. Stemper wrote:
For a class I'm in, our team is supposed to
"Characterize the star formation history of
z=0 satellite and central galaxies."

However, I'm not sure what the term "z=0 galaxy"
means. To me, "z=0" means "zero redshift", or
"right here".


Not a bad assumption.

Since the only galaxy that's "right here" is
the Milky Way, this obviously demonstrates
some mis-understanding on my part.

....
Any suggestions on where I could find out what they are? Or, if
it's trivial to explain, would somebody please tell me?


I'd recommend confining yourself to the Virgo supercluster, which is gravitationally bound (not yet expanding with the rest of the Universe). You should find a published list of contents of this supercluster. Any red or blue shifting (not quite z=0) is due to peculiar motion, rather than expansion recession.

David A. Smith
  #3  
Old February 22nd 18, 02:39 PM posted to sci.astro
Michael F. Stemper
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Posts: 6
Default "z=0" galaxies

On 2018-02-21 16:58, dlzc wrote:
On Wednesday, February 21, 2018 at 3:08:19 PM UTC-7, Michael F. Stemper wrote:


For a class I'm in, our team is supposed to
"Characterize the star formation history of
z=0 satellite and central galaxies."

However, I'm not sure what the term "z=0 galaxy"
means. To me, "z=0" means "zero redshift", or
"right here".


Not a bad assumption.


Nice to know that I'm not totally unhinged.

Since the only galaxy that's "right here" is
the Milky Way, this obviously demonstrates
some mis-understanding on my part.

...
Any suggestions on where I could find out what they are? Or, if
it's trivial to explain, would somebody please tell me?


I'd recommend confining yourself to the Virgo supercluster, which is gravitationally bound (not yet expanding with the rest of the Universe). You should find a published list of contents of this supercluster. Any red or blue shifting (not quite z=0) is due to peculiar motion, rather than expansion recession.


That makes sense. Thanks!

--
Michael F. Stemper
Deuteronomy 24:17
  #4  
Old February 23rd 18, 06:20 PM posted to sci.astro
Steve Willner
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Posts: 1,172
Default "z=0" galaxies

In article ,
"Michael F. Stemper" writes:
However, I'm not sure what the term "z=0 galaxy" means. To me,
"z=0" means "zero redshift", or "right here".


Right. Specifically, it means galaxies so nearby that their
redshifts don't matter. That means the observed and emitted
wavelengths are effectively equal, and the lookback time is small
compared to any evolutionary timescale. Depending on what you are
trying to do, that could mean less than a few tens of megaparsecs to
z0.1 or even larger.

It probably wasn't an ideal term for an instructor to use without
clarification, but for the exercise "Characterize the star formation
history of z=0 satellite and central galaxies," you should be safe
out to Coma and maybe farther if the relevant data exist.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #5  
Old February 24th 18, 08:23 PM posted to sci.astro
Michael F. Stemper
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Posts: 6
Default "z=0" galaxies

On 2018-02-23 11:20, Steve Willner wrote:
In article ,
"Michael F. Stemper" writes:
However, I'm not sure what the term "z=0 galaxy" means. To me,
"z=0" means "zero redshift", or "right here".


Right. Specifically, it means galaxies so nearby that their
redshifts don't matter. That means the observed and emitted
wavelengths are effectively equal, and the lookback time is small
compared to any evolutionary timescale. Depending on what you are
trying to do, that could mean less than a few tens of megaparsecs to
z0.1 or even larger.


Why would the characteristics of z=0 galaxies be different from those
of other galaxies? After all, there's no special place in the universe
(other than the brewpub around the corner from my house).

It probably wasn't an ideal term for an instructor to use without
clarification, but for the exercise "Characterize the star formation
history of z=0 satellite and central galaxies," you should be safe
out to Coma and maybe farther if the relevant data exist.


Actually (and I should probably have mentioned this in my OP), we're
working with an Illustris model. As I understand it, nothing in
Illustris corresponds to any place in our universe. Of course, this
causes me to wonder how the folks who coded up the Illustris simulation
decided to label some point (for exceeding large values of "point") as
the place where z=0.


--
Michael F. Stemper
Always remember that you are unique. Just like everyone else.
  #6  
Old March 1st 18, 08:37 PM posted to sci.astro
Steve Willner
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Posts: 1,172
Default "z=0" galaxies

Specifically, it means galaxies so nearby that their
redshifts don't matter. That means the observed and emitted
wavelengths are effectively equal, and the lookback time is small
compared to any evolutionary timescale.


In article ,
"Michael F. Stemper" writes:
Why would the characteristics of z=0 galaxies be different from those
of other galaxies?


I gave you two reasons in the quote above. Specifically:

1. if you observe a z=0 galaxy at, say, 550 nm, the light was emitted
at that wavelength. For a z=0.1 galaxy, though, the light was
emitted at 500 nm. Depending on what you are doing, the difference
may or may not be significant. If it is, you have to make what is
known as a "K correction." For a z=1 galaxy, the light was emitted
at 275 nm and represents an entirely different stellar population (or
perhaps an active nucleus and not stars at all).

2. if you observe a z=1 galaxy, you are observing it as it was when
the age of the universe was 5.9 Gyr as opposed to 13.7 Gyr for a z=0
galaxy. Galaxies change a lot in 7.8 Gyr! Even z=0.1 corresponds to
a lookback time of 1.3 Gyr, and for some purposes even that much
evolution can be important.

After all, there's no special place in the universe


It's not "where," it's "when." Galaxies change as they get older,
and the galaxy population is a lot different now than it was when the
universe was younger.

we're working with an Illustris model. As I understand it, nothing
in Illustris corresponds to any place in our universe.


The model will show enormous changes over time. Try looking at the
median mass or luminosity of galaxies as a function of time, for
example.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
 




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