by JeanTate » Thu Sep 23, 2010 9:04 pm
Chris Peterson wrote:Ann wrote: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"
[quote="Chris Peterson"][quote="Ann"]Well, obviously we [i]are[/i] 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?[/quote]
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.
[quote]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.[/quote]
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.[/quote]
[b]Ellipticals represent something on the order of 20% of low-z galaxies[/b]
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*" ([url=http://arxiv.org/abs/1007.3265]Lintott et al. (2010)[/url] "Accepted by MNRAS" - link is to arXiv abstract).
The abstract is worth quoting:
[quote]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 [url=http://data.galaxyzoo.org./]this http URL[/url][/quote]
Table 2 from the paper "[i]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.[/i]" (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? "[i]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[/i]"