<p>Elliptical galaxies have no regions of high density, so they are not forming stars any more.  So all the stars in an elliptical galaxy are old stars, and as you say, tend to have low metallicity (actually, as pointed out, that doesn't seem to be true-- they have higher metallicity, presumably from old star formation no longer occurring).  You are right that they might have had active star formation in the distant past, but it ended long ago.  Spiral galaxies like the Milky Way have been forming new stars for their entire history, so have more opportunity for current metallicity increase, but seem to be starting out behind, as pointed out above.</p>

CC BY-SA 4.0
2018-11-22T10:50:55.660Z
First line of the abstract of  Pipino & Matteucci (2006)   "Elliptical galaxies probably host the most metal rich stellar populations in the Universe. " https://arxiv.org/abs/astro-ph/0610831
ProfRob
CC BY-SA 4.0
2018-11-22T07:36:13.770Z
2018-11-22T11:09:39.500Z
2018-11-22T11:09:39.500Z
Ken G
<p>It isn't true that elliptical (or early-type) galaxies are metal-poor. This sounds like a pop-sci myth being propagated. The first line of the abstract of <a href="https://arxiv.org/abs/astro-ph/0610831" rel="nofollow noreferrer">Pipino &amp; Matteucci (2006)</a> states that "Elliptical galaxies probably host the most metal rich stellar populations in the Universe".  Thus your suspicion of the idea that "ellipticals are metal-poor" is well-founded.</p>

<p>Measurements of galaxy metallicities and metallicity gradients have been around for decades and show that ellipticals obey fairly simple scaling relationships between mass, luminosity, velocity dispersion and metallicity.</p>

<p>In terms of metallicity, massive ellipticals with bigger velocity dispersions are a bit more metal-rich than the Sun, whilst smaller galaxies may get down half the solar metallicity. The range is <em>much</em> larger for spiral galaxies. In addition, there is a tendency for ellipticals that are <em>older</em> to have higher metallicities. Examples of these trends can be seen in <a href="https://arxiv.org/abs/1005.0014" rel="nofollow noreferrer">Graves &amp; Faber (2010)</a> and <a href="https://arxiv.org/abs/1802.01819" rel="nofollow noreferrer">Li et al. (2018)</a>.</p>

<p><em>The central plot here (taken from Graves &amp; Faber 2010) shows how the metallicity of galaxies varies with their stellar velocity dispersion (an indicator of their mass). [Z/H] is a logarithmic base 10 scale where 0.0 means the metallicity of the Sun and -1 would a tenth the metallicity of the Sun. You can see that the elliptical galaxies (red) have a relatively narrow range of metallicities, that most are as metal-rich as the Sun, that less massive ellipticals have lower metallicity and that spirals (blue) have a much bigger range of metallicity.</em></p>

<p><a href="https://i.sstatic.net/RWXX6.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/RWXX6.png" alt="The fundamental plane from Graves &amp; Faber (2010)"></a></p>

<p>The reasons behind these trends are still debated. But as you rightly point out, ellipticals likely form from mergers of objects that have already been actively forming stars. A low metallicity is therefore not necessarily expected. Much depends on how much enriched gas from the ejecta of previous (higher mass) stellar populations is <em>retained</em> within the galaxy. Higher mass galaxies with deeper gravitational potentials are able to retain more of that enriched gas.</p>

CC BY-SA 4.0
2018-11-22T13:15:07.630Z
Minor quibble: the "Fundamental Plane" is a relation between luminosity (or mass), stellar velocity dispersion, and *size* (usually half-light radius). It does not involve metallicity.
Peter Erwin
CC BY-SA 4.0
2018-11-22T13:20:34.960Z
There *is* a well-studied "mass-metallicity" relation, though studies of spiral/star-forming galaxies usually focus on gas-phase metallicity, while studies of ellipticals focus on stellar metallicity. (But your general points stand, of course.)
Peter Erwin
CC BY-SA 4.0
2018-11-22T08:50:37.353Z
2018-11-22T14:41:39.427Z
2018-11-22T14:41:39.427Z
ProfRob
<p>As far as I understand, metallicities of old stars are usually low and those of new stars are usually high.</p>

<p>If elliptical galaxies form after merger of galaxies, then surely that means the elliptical galaxy passed through a stage where it was an irregular galaxy. Now, irregular galaxies are said to be sites of rapid, new star formation.</p>

<p>Why then do we say that elliptical galaxies have low metallicity, if the star formation went on for a long time there?</p>

<p>[New to astronomy, so a detailed explanation would help]</p>

CC BY-SA 4.0
2018-11-21T23:18:28.603Z
Why do you think it is? e.g. https://arxiv.org/abs/astro-ph/0610831 https://link.springer.com/article/10.1023/A:1002195607719 http://adsbit.harvard.edu//full/1996MNRAS.278..841D/0000841.000.html
ProfRob
CC BY-SA 4.0
2018-12-30T22:09:20.173Z
@RobJeffries, In the astronomy lecture, the professor told us that the stars of elliptical galaxy are old and metal-poor.
S R Maiti
<star><galaxy><metallicity>
Why is the metallicity of elliptical galaxies low?
S R Maiti
# Why is the metallicity of elliptical galaxies low?

(Asked by: username_0 on 2018-11-21)

As far as I understand, metallicities of old stars are usually low and those of new stars are usually high.

If elliptical galaxies form after merger of galaxies, then surely that means the elliptical galaxy passed through a stage where it was an irregular galaxy. Now, irregular galaxies are said to be sites of rapid, new star formation.

Why then do we say that elliptical galaxies have low metallicity, if the star formation went on for a long time there?

[New to astronomy, so a detailed explanation would help]

**Tags:** star, galaxy, metallicity
> **Comments:**
> **username_1**: Why do you think it is? e.g. https://arxiv.org/abs/astro-ph/0610831 https://link.springer.com/article/10.1023/A:1002195607719 http://adsbit.harvard.edu//full/1996MNRAS.278..841D/0000841.000.html  
> **username_0**: @RobJeffries, In the astronomy lecture, the professor told us that the stars of elliptical galaxy are old and metal-poor.  

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## 2 answers

### Answer by username_1 on 2018-11-22. Score: 7 | Accepted answer

It isn't true that elliptical (or early-type) galaxies are metal-poor. This sounds like a pop-sci myth being propagated. The first line of the abstract of [Pipino & Matteucci (2006)](https://arxiv.org/abs/astro-ph/0610831) states that "Elliptical galaxies probably host the most metal rich stellar populations in the Universe". Thus your suspicion of the idea that "ellipticals are metal-poor" is well-founded.

Measurements of galaxy metallicities and metallicity gradients have been around for decades and show that ellipticals obey fairly simple scaling relationships between mass, luminosity, velocity dispersion and metallicity.

In terms of metallicity, massive ellipticals with bigger velocity dispersions are a bit more metal-rich than the Sun, whilst smaller galaxies may get down half the solar metallicity. The range is *much* larger for spiral galaxies. In addition, there is a tendency for ellipticals that are *older* to have higher metallicities. Examples of these trends can be seen in [Graves & Faber (2010)](https://arxiv.org/abs/1005.0014) and [Li et al. (2018)](https://arxiv.org/abs/1802.01819).

*The central plot here (taken from Graves & Faber 2010) shows how the metallicity of galaxies varies with their stellar velocity dispersion (an indicator of their mass). [Z/H] is a logarithmic base 10 scale where 0.0 means the metallicity of the Sun and -1 would a tenth the metallicity of the Sun. You can see that the elliptical galaxies (red) have a relatively narrow range of metallicities, that most are as metal-rich as the Sun, that less massive ellipticals have lower metallicity and that spirals (blue) have a much bigger range of metallicity.*

[![The fundamental plane from Graves & Faber (2010)](https://i.sstatic.net/RWXX6.png)](https://i.sstatic.net/RWXX6.png)

The reasons behind these trends are still debated. But as you rightly point out, ellipticals likely form from mergers of objects that have already been actively forming stars. A low metallicity is therefore not necessarily expected. Much depends on how much enriched gas from the ejecta of previous (higher mass) stellar populations is *retained* within the galaxy. Higher mass galaxies with deeper gravitational potentials are able to retain more of that enriched gas.

> **Comments:**
> **username_3**: Minor quibble: the "Fundamental Plane" is a relation between luminosity (or mass), stellar velocity dispersion, and *size* (usually half-light radius). It does not involve metallicity.  
> **username_3**: There *is* a well-studied "mass-metallicity" relation, though studies of spiral/star-forming galaxies usually focus on gas-phase metallicity, while studies of ellipticals focus on stellar metallicity. (But your general points stand, of course.)  

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### Answer by username_2 on 2018-11-22. Score: -1

Elliptical galaxies have no regions of high density, so they are not forming stars any more. So all the stars in an elliptical galaxy are old stars, and as you say, tend to have low metallicity (actually, as pointed out, that doesn't seem to be true-- they have higher metallicity, presumably from old star formation no longer occurring). You are right that they might have had active star formation in the distant past, but it ended long ago. Spiral galaxies like the Milky Way have been forming new stars for their entire history, so have more opportunity for current metallicity increase, but seem to be starting out behind, as pointed out above.

> **Comments:**
> **username_1**: First line of the abstract of  Pipino & Matteucci (2006)   "Elliptical galaxies probably host the most metal rich stellar populations in the Universe. " https://arxiv.org/abs/astro-ph/0610831  

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