What type of galaxy dominates in the low-z universe?

[Ann]

How common are elliptical galaxies in the moderately nearby, relatively low-z universe, and how common are spirals?

Obviously dwarf galaxies are more common than large galaxies. Personally I think that the really tiny galaxies are not so interesting. Galaxies smaller than half the size of the Small Magellanic Cloud should perhaps not be counted here.

How dominant are the large galaxies when it comes to the number of stars they contain? How many galaxies like the Fornax Dwarf would it take to "fill" the Milky Way with the required number of stars (that is, how many more stars are there in the Milky Way than in the Fornax Dwarf)? And how many galaxies like the Fornax Dwarf would it take to provide the total mass of the Milky Way?

Imagine a kind of galactic roulette wheel. Each number of this wheel represent different types of galaxies. There are the monstrously large elliptical galaxies like M 87, although they would be rare. There are the considerably smaller elliptical galaxies, and I'm sure there are several of those in the Virgo cluster. There are the quite small elliptical galaxies like NGC 205. There are the really large spiral galaxies like M 88 and NGC 1961, which are very rare. There are the much smaller spiral galaxies like M 33. And there are galaxies like the Milky Way.

Let's play the roulette in two different ways. In the first round, each type of galaxy is assigned a one or more numbers, depending on how common this galaxy type is. Galaxies like M 87 would certainly get no more than one number, but galaxies like NGC 205 might get so many numbers that there wouldn't be many numbers left for the spirals! Perhaps we need a larger roulette wheel with more numbers?

If we play the roulette wheel this way, how likely is it, statistically speaking, that "our" number (corresponding to spiral galaxies the size of the Mily Way) would "win" the round?

Let's play the roulette in a different way. Now let's consider the total number of stars in the low-z universe and consider what kind of galaxy stars are more likely to find themselves in. How many of the stars of the moderately local universe are inside galaxies like M 87? How many stars are inside galaxies like NGC 205? How many stars are inside galaxies like M 88 and NGC 1961? How many stars are inside galaxies like M 33? And how many stars are inside spiral galaxies more or less the size of the Milky Way?

To put it differently, how ordinary is the Milky Way in the moderately low-z universe? And how likely is it, statistically speaking, that the Sun would be found inside a spiral galaxy the size of the Milky Way?



How likely is it that the Milky Way would win?

Ann

[Beyond]

How likely is it that the Milky Way would win?

Ann, it "won" for us. Isn't that enough? Besides, I like snickers better!!

[Ann]

Any takers? Chris?

Ann

[alter-ego]

... And how likely is it, statistically speaking, that the Sun would be found inside a spiral galaxy the size of the Milky Way?

I'll start with the easier one: To a very high degree of precision, 100%

How likely is it that the Milky Way would win?

I think the better question is how likely is it that the Milky Way would lose?

[Beyond]

Ok Ann, you started this "z" thing, now explain it please. You must be smarter than yahoo or Bing. They didn't have anything.

[bystander]

Ok Ann, you started this "z" thing, now explain it please. You must be smarter than yahoo or Bing. They didn't have anything.

See Wikipedia: redshift

Basically, z is a measure of the redshift of light (EM radiation). This tells how fast an object is moving away from us. The more distant an object is, the higher it's redshift. So, low z objects are objects that are relatively close.

[Beyond]

Ok Ann, you started this "z" thing, now explain it please. You must be smarter than yahoo or Bing. They didn't have anything.

See Wikipedia: redshift

Basically, z is a measure of the redshift of light (EM radiation). This tells how fast an object is moving away from us. The more distant an object is, the higher it's redshift. So, low z objects are objects that are relatively close.

Well that's nice If i know that z is redshift, i don't have to go and look it up. If i don't know that z is redshift, then i can't look it up because i won't know to look up redshift and there are (as far as i know) no mentions of z having to do with redshift, except under the definition of redshift. Geez talk about your catch 22 situations. But it doesen't really matter anyway. I'll most likely forget it by tomorrow.

[Ann]

... And how likely is it, statistically speaking, that the Sun would be found inside a spiral galaxy the size of the Milky Way?

I'll start with the easier one: To a very high degree of precision, 100%

How likely is it that the Milky Way would win?

I think the better question is how likely is it that the Milky Way would lose?

Well, obviously we are inside the Milky Way. But would an elliptical galaxy the size of the MIlky Way offer similar hospitable conditions as the Milky Way obviously does, since we are here?

My question is, are most galaxies the size of the MIlky Way in the low-z (that means moderately nearby) universe likely to be as hospitable as the Milky Way? Are elliptical galaxies in the nearby universe as good as the Milky Way as incubators of life?

Let me put it like this. If most stars in the nearby universe are inside a spiral galaxy relatively similar to the Milky Way, then we are inside such a galaxy because that's where most stars are "now" (that is, "now" as opposed to billions of years ago). But if most stars "today" are inside elliptical galaxies, then we are probably in a spiral galaxy because galaxies like the Milky Way are better for life than ellipticals.

That's why I want to know if we are inside a galaxy like the Milky Way because most stars are inside such galaxies, or if we are here because we probably couldn't exist in many other galaxies.

Ann

[JeanTate]

One quick way to answer the question is to look at the galaxy color-magnitude diagram^WP (CMD).

This plots the estimated absolute luminosity (or magnitude) against color; most galaxies fall onto what is called the red sequence (mostly elliptical and lenticulars, though there are some red spirals) or the blue cloud (mostly spirals and irregulars, though there are some blue ellipticals). The region between them is called the green valley, and is poorly populated.

Here is an example of such a diagram:


It's from Wyder et al. (2007) - link is to arXiv preprint - Figure 9:

The volume density of galaxies as a function of (NUV −r)_0.1 and M_r,0.1 using the V_max method. The density is determined in 0.2 mag wide bins in color and 0.5 mag wide bins in absolute magnitude. There are twelve logarithmically spaced contours ranging from 10^−5.5 to 10^−2.3 Mpc⁻³ mag⁻². The dashed lines indicate the ridge lines of the red and blue sequences as determined from the Gaussian fits shown in Figure 15 below.

In terms of numbers, clearly faint blue galaxies dominate!

PS Another very good paper on the bimodality of galaxies in a CMD is Baldry et al (2004).

[neufer]

Well, obviously we are inside the Milky Way. But would an elliptical galaxy the size of the MIlky Way offer similar hospitable conditions as the Milky Way obviously does, since we are here?

My question is, are most galaxies the size of the MIlky Way in the low-z (that means moderately nearby) universe likely to be as hospitable as the Milky Way? Are elliptical galaxies in the nearby universe as good as the Milky Way as incubators of life?

Let me put it like this. If most stars in the nearby universe are inside a spiral galaxy relatively similar to the Milky Way, then we are inside such a galaxy because that's where most stars are "now" (that is, "now" as opposed to billions of years ago). But if most stars "today" are inside elliptical galaxies, then we are probably in a spiral galaxy because galaxies like the Milky Way are better for life than ellipticals.

That's why I want to know if we are inside a galaxy like the Milky Way because most stars are inside such galaxies, or if we are here because we probably couldn't exist in many other galaxies.

I don't think that anyone really knows enough about these matters to answer your question satisfactorily, Ann.

About all we can say, perhaps, is that:

1) active galactic nucleus galaxies probably would NOT be favorable for advanced life

2) while galaxies like the Milky Way probably would be favorable for advanced life.

[JeanTate]

To add to neufer's answer: if a galaxy has a super-massive black hole (SMBH) in its nucleus, then it will be, or have been, an AGN galaxy.

The 'duty cycle' of AGNs is not well known. However, the massive ellipticals in, or near, the center of rich clusters very often have AGNs with jets and double radio source lobes (they are DRAGNs). M87 is an example. These seem to be quite different from the AGNs in Seyferts, for example; the former have a high duty cycle (they are nearly always 'on'), the latter a low one (they turn 'on' only every so often, and turn off after a period perhaps as short as a few million years).

A star close to the nucleus of a galaxy with a SMBH is very unlikely to have life (as we know it) on any Earth-like planet that it may have; however, a dozen kpc or so out, in the disk, things are a lot more tranquil, so perhaps life would be possible.

A big danger for complex, multi-cellular life as we know it, on the surface of an Earth-like planet, is supernovae. When disk galaxies with lots of cold gas merge or collide, starbursts often result; these are not healthy for such life ... too many supernovae, throughout much of the galaxy.

[Chris Peterson]

Well, obviously we are inside the Milky Way. But would an elliptical galaxy the size of the MIlky Way offer similar hospitable conditions as the Milky Way obviously does, since we are here?

We don't have enough information to answer that question. I'd argue that any type of galaxy probably offers conditions that are both hospitable and inhospitable to life as we understand it (that is, any galaxy type would allow systems similar to ours to form). What does it take to create a hospitable environment? I think a reasonable case could be made for nothing more than regions of high metallicity, occasional star formation, and low stellar density regions where planetary systems can exist for a long time without significant gravitational perturbation from other stars. I doubt there is anything particularly unusual about the Solar System.

Let me put it like this. If most stars in the nearby universe are inside a spiral galaxy relatively similar to the Milky Way, then we are inside such a galaxy because that's where most stars are "now" (that is, "now" as opposed to billions of years ago). But if most stars "today" are inside elliptical galaxies, then we are probably in a spiral galaxy because galaxies like the Milky Way are better for life than ellipticals.

Ellipticals represent something on the order of 20% of low-z galaxies, and they are smaller than spirals. Most stars are in spiral galaxies, so from a statistical standpoint, it is unsurprising to find ourselves in one. To me, your argument treads dangerously close to the Anthropic Principle; we are still the only system we know about that is the way it is, so it is pretty much impossible to draw any serious conclusions about why we are where we are.

[Ann]

Chris wrote:

Ellipticals represent something on the order of 20% of low-z galaxies, and they are smaller than spirals. Most stars are in spiral galaxies, so from a statistical standpoint, it is unsurprising to find ourselves in one.

Thanks, Chris. That is exactly what I was wondering about, so I appreciate the answer.

To me, your argument treads dangerously close to the Anthropic Principle; we are still the only system we know about that is the way it is, so it is pretty much impossible to draw any serious conclusions about why we are where we are.

Was my question controversial? I didn't know that the Weak Anthropic Principle was seriously disputed at all (although it can certainly be argued that the Weak Anthropic Principle isn't interesting in itself, since explains extremely little). As for the Strong Anthropic Principle, I have never championed it in any way.

However, I do believe that we have been extremely lucky. My question was meant to try to find out a bit more about just how lucky we have been. It appears that as far as we know, we haven't been extraordinarily lucky when it comes to the galaxy that we find ourselves in. Nevertheless, would you say that my belief that we have been very lucky is a controversial view and a controversial approach to astronomy and cosmology, in your opinion, Chris?

Ann

[Chris Peterson]

Was my question controversial? I didn't know that the Weak Anthropic Principle was seriously disputed at all (although it can certainly be argued that the Weak Anthropic Principle isn't interesting in itself, since explains extremely little).

The latter is my viewpoint, and is what I was hinting at.

However, I do believe that we have been extremely lucky. My question was meant to try to find out a bit more about just how lucky we have been. It appears that as far as we know, we haven't been extraordinarily lucky when it comes to the galaxy that we find ourselves in. Nevertheless, would you say that my belief that we have been very lucky is a controversial view and a controversial approach to astronomy and cosmology, in your opinion, Chris?

I don't know about "controversial". I just think that there is nothing that really suggest a system like ours should be unusual. What is "lucky"? It's a big, diverse universe we live in. Maybe conditions for a hospitable solar system only occur once in a million times. Does that make us "lucky"? I wouldn't say so, but to each their own.

[neufer]

However, I do believe that we have been extremely lucky. My question was meant to try to find out a bit more about just how lucky we have been. It appears that as far as we know, we haven't been extraordinarily lucky when it comes to the galaxy that we find ourselves in. Nevertheless, would you say that my belief that we have been very lucky is a controversial view and a controversial approach to astronomy and cosmology, in your opinion, Chris?

I don't know about "controversial". I just think that there is nothing that really suggest a system like ours should be unusual. What is "lucky"? It's a big, diverse universe we live in. Maybe conditions for a hospitable solar system only occur once in a million times. Does that make us "lucky"? I wouldn't say so, but to each their own.

http://asterisk.apod.com/vie ... ky#p129905

[Beyond]

view topic.php?f=31&t=20696&p=129905&hilit=lucky#p129905

[neufer]

http://asterisk.apod.com/vie ... ky#p129905

You should talk

[Ann]

So I guess Lucky comes with Essy. And Vlad. So we are both Lucky, Essy and Vlad, then? I'm Vlad we are so Lucky!

Ann

[neufer]

So I guess Lucky comes with Essy. And Vlad.
so we are both Lucky, Essy and Vlad, then? I'm Vlad we are so Lucky!

What type of galaxy dominates in the lousy universe?

Lousy, a. 1. Infested with lice. 2. Mean; contemptible. [Vulgar]

[Beyond]

http://asterisk.apod.com/vie ... ky#p129905

You should talk

Yes, i should talk, HOWEVER, NO-ONE can hold a LOQUACIOUS candle to the resident QUOTIDIAN QUOTATIONIST So any talking that i do is really miniscule in comparison. Plus i don't know how to type so my miniscule takes longer, also.
So, with that being said it's time for me to exit stage right from my miniscule post. Hahaha-Hehehe

[Ann]

I googled "louse universe" and got this book. Weird. I guess the author's name is really "Louse B. Young":



Ann

[Beyond]

Ann, that's Louise NOT louse. That's Louise B. Young. A young Louise is usually called "louie."

[Ann]

Ann, that's Louise NOT louse. That's Louise B. Young. A young Louise is usually called "louie."

You're right! I don't want to insult Louise. She hasn't bothered me or my universe!

So I googled "lousy universe" instead of "louse universe". I got, among other things, this image.

Click to view full size image

Art, here is a lousy universe for you!

Ann

[JeanTate]

Well, obviously we are inside the Milky Way. But would an elliptical galaxy the size of the MIlky Way offer similar hospitable conditions as the Milky Way obviously does, since we are here?

We don't have enough information to answer that question. I'd argue that any type of galaxy probably offers conditions that are both hospitable and inhospitable to life as we understand it (that is, any galaxy type would allow systems similar to ours to form). What does it take to create a hospitable environment? I think a reasonable case could be made for nothing more than regions of high metallicity, occasional star formation, and low stellar density regions where planetary systems can exist for a long time without significant gravitational perturbation from other stars. I doubt there is anything particularly unusual about the Solar System.

Let me put it like this. If most stars in the nearby universe are inside a spiral galaxy relatively similar to the Milky Way, then we are inside such a galaxy because that's where most stars are "now" (that is, "now" as opposed to billions of years ago). But if most stars "today" are inside elliptical galaxies, then we are probably in a spiral galaxy because galaxies like the Milky Way are better for life than ellipticals.

Ellipticals represent something on the order of 20% of low-z galaxies, and they are smaller than spirals. Most stars are in spiral galaxies, so from a statistical standpoint, it is unsurprising to find ourselves in one. To me, your argument treads dangerously close to the Anthropic Principle; we are still the only system we know about that is the way it is, so it is pretty much impossible to draw any serious conclusions about why we are where we are.

Ellipticals represent something on the order of 20% of low-z galaxies

I'm not sure where Chris got his 20% from, but I decided to check out a recent, rather extensive, as-close-to-objective-as-you-could-wish source: "Galaxy Zoo 1 : Data Release of Morphological Classifications for nearly 900,000 galaxies*" (Lintott et al. (2010) "Accepted by MNRAS" - link is to arXiv abstract).

The abstract is worth quoting:

Morphology is a powerful indicator of a galaxy's dynamical and merger history. It is strongly correlated with many physical parameters, including mass, star formation history and the distribution of mass. The Galaxy Zoo project collected simple morphological classifications of nearly 900,000 galaxies drawn from the Sloan Digital Sky Survey, contributed by hundreds of thousands of volunteers. This large number of classifications allows us to exclude classifier error, and measure the influence of subtle biases inherent in morphological classification. This paper presents the data collected by the project, alongside measures of classification accuracy and bias. The data are now publicly available and full catalogues can be downloaded in electronic format from this http URL

Table 2 from the paper "contains the data for all galaxies with measured redshifts in the range 0.001 < z < 0.25 and u and r photometry in SDSS DR7, excluding those with extreme absolute magnitudes or sizes given by the SDSS pipeline. 667,945 galaxies are included. This table includes the raw votes, the weighted votes in elliptical (E) and combined spiral (CS) categories, and flags indicating the inclusion of the galaxy in a clean, debiased catalogue. The flags take into account not only the redshift dependence of the spiral/elliptical ratio as described in Section 3.1 but also the redshift dependence of the ratio of spirals to ellipticals in the clean catalogue." (the "clean" catalog contains all galaxies for which the debiased classification yields an 80+% vote for either spiral or elliptical).

How do the numbers stack up then?

In the first 50,000 entries in Table 2, 13,513 are clean debiased spirals, 4,863 are clean debiased ellipticals, and 31,624 are uncertain; (clean, debiased) ellipticals thus comprise 26.5% of all the clean, debiased galaxies (in this subset).

Oh, and the * in the paper's title? "This publication has been made possible by the participation of more than 100,000 volunteers in the Galaxy Zoo project. Their contributions are individually acknowledged at http://www.galaxyzoo.org/Volunteers.aspx"

[Ann]

Thanks a lot for that answer, JeanTate!

Ann