Galaxy Zoo Talk

"Spiral galaxies (and why Galaxy Zoo is perfect for their study)" - new GZ blog post

  • JeanTate by JeanTate

    Link.

    The title is pretty self-explanatory, and as an introduction to the topic, it's a good blog post. The author - Ross Hart - intends to write more blog posts on this topic, so I thought we might have a thread, here in GZT, in which to discuss this.

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  • mlpeck by mlpeck in response to JeanTate's comment.

    Jean:

    A cursory search for "catalogs of local group galaxies" turned up this one that seems to be most nearly up to date. They also ventured to assign Hubble classes to everything in their list. They show 5 spirals in the LG including the LMC and SMC and 4 Irr(regular). I'd say all 4 of those are probably disk galaxies and NGC 3109 at least might show very loose spiral structure (I think it's sometimes classified Sm). Out of the 10 brightest members (per this table) there are only 2 spheroidal galaxies, namely M32 and NGC 205 - both satellites of M31. An SDSS depth survey would only target NGC 3109 for spectroscopy out to about z=0.015.

    Blanket statements about the percent of spiral galaxies seem a little off to me since it's been known for decades that the distribution of morphologies is highly dependent on the environment and it also depends on how faint you look. But if you count irregulars as honorary spirals it might be true that ∼75% of brighter galaxies in the local universe are spirals.

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  • JeanTate by JeanTate in response to mlpeck's comment.

    Thanks mlpeck.

    Here's part of the comment I wrote in the blog (a question addressed to Ross Hart):

    1. you write “Spiral galaxies are the most numerous type of galaxy in the local Universe, with approximately 2/3 of local galaxies exhibiting spiral arms.” Yet, in the Local Group of ~50 galaxies, only three or four are spirals, right?

    I have a particular interest in "spirals", and have read up on them. One thing I've learned is that there are many different definitions, and little overall consensus among (today's) astronomers. For example, some - like Ross Hart (apparently) - regard the presence of a 'spiral arm' as an essential criterion; others consider the existence of a (~kpc+ scale) disc sufficient. And so on.

    Blanket statements about the percent of spiral galaxies seem a little off to me since it's been known for decades that the distribution of morphologies is highly dependent on the environment and it also depends on how faint you look.

    And surface brightness!

    There has been a trickle (or more) of papers these last few years, reporting the discovery of quite a lot of low surface brightness (LSB) galaxies. E.g. (links are to arXiv preprint abstracts) Davies+ (2015), van Dokkum+ (2015), Koda+ (2015), Merritt+ (2014), van der Berg+ (2016). Certainly many of these are dwarfs, with even total estimated mass (i.e. including DM) well below that of even M33. However, many are apparently quite massive. And big, with effective radii >1.5 kpc. I don't know how many have been classified, morphologically, but I doubt many are the kind of spiral we were on the lookout for, in any GZ project.

    I hope Ross Hart - or another astronomer associated with GZ - will drop by soon! 😃

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  • zutopian by zutopian in response to JeanTate's comment.

    I hope Ross Hart - or another astronomer associated with GZ - will drop by soon! 😃

    You might want to inform about this Talk discussion by posting a comment in the GZ blog post! 😃

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  • JeanTate by JeanTate in response to zutopian's comment.

    Thanks zutopian.

    In fact I thought I'd already done so! 😮 However, I hadn't 😦 but now have 😃

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  • zutopian by zutopian

    I noticed, that there is following response* (GZ blog post) by KWillett now available.:

    Kyle Willett says : March 31, 2016 at 4:39 pm

    1. (...)
    1. I’ll let Ross answer his own words more specifically, but will note that the fraction of galaxies with spiral arms depends really strongly on what mass range you’re looking at. More massive galaxies are much more likely to be elliptical, while galaxies the size of the Milky Way are more often spirals. The low mass end has more irregulars (and it’s not clear that there’s a strict lower bound in mass for what’s considered a “galaxy”).

    http://blog.galaxyzoo.org/?_ga=1.10354037.131079670.1459276820

    '* Response to following comment:

    Jean Tate says : March 18, 2016 at 7:30 pm

    (...)

    Two questions, if I may:

    1. (...)
    2. you write “Spiral galaxies are the most numerous type of galaxy in the local Universe, with approximately 2/3 of local galaxies exhibiting spiral arms.” Yet, in the Local Group of ~50 galaxies, only three or four are spirals, right?

    http://blog.galaxyzoo.org/?_ga=1.10354037.131079670.1459276820

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  • JeanTate by JeanTate in response to zutopian's comment.

    Thanks for posting this, zutopian.

    Still no sign of Ross Hart though 😦

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  • ross_e_hart by ross_e_hart

    Hi everyone!

    Sorry I didn't really check my blog post after I put it up. I'll try to look at my post/threads more often to see if there are any questions etc. that I can answer.

    As to my comment about most of the local galaxies, this was referring to the classifications of GZ1. Galaxy structure is highly dependent on local environment, with spirals being less common in groups and clusters. If we look at the SDSS, which should be representative of the local universe as a whole, we see more spiral galaxies than S0s or ellipticals. I hope that clears things up a bit!

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  • JeanTate by JeanTate in response to ross_e_hart's comment.

    Welcome! 😃

    Thanks for clearing up things about 'spiral galaxies'.

    As to my comment about most of the local galaxies, this was referring to the classifications of GZ1. Galaxy structure is highly dependent on local environment, with spirals being less common in groups and clusters. If we look at the SDSS, which should be representative of the local universe as a whole, we see more spiral galaxies than S0s or ellipticals.

    What is known about the biases which relying on SDSS images introduce, in terms of SDSS GZ1 "spirals" being representative of the local universe? Can you please give references (papers) which report the results of investigations into these biases?

    From your blog post:

    most of the star-formation in the local Universe occurs in spiral galaxies

    How is this determined? For example, how does the star-formation in irregulars and dwarfs compare with that in spirals?

    Looking forward to interesting discussions!

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  • ross_e_hart by ross_e_hart

    Hi Jean,

    The Sloan Digital Sky Survey is the largest survey of its kind, covering 1/3 of the entire sky, making it very 'representative' of the entire local universe in that sense- it should be sensitive to 'all' galaxies around us. However, your point about the local group is an interesting one. When we talk about 'completeness', we mean above a certain detection limit. So the SDSS is complete and representative of the local universe above a certain brightness that is detectable by the telescope.

    The statement about star-formation follows this logic:

    most of the local universe's galaxies are spirals + spiral galaxies are bluer than ellipticals and S0s (which means more young stars are being formed in them) = more star-formation is happening in spirals than other types of galaxy.

    As for dwarf galaxies, we don't really know a huge amount about them- they are much smaller and fainter than spiral and elliptical galaxies, so constraining how much star-formation is occurring in them compared to 'normal galaxies' as a whole would be difficult. However, taking the LMC as a typical example, it is about 1/100th of the size of the Milky Way. From that, I would infer that you would need a lot of dwarf galaxies (of order 100 or so) to account for the star-formation going on in a typical spiral galaxy. However, there are a number of caveats that go into that calculation, so that is only a very rough estimate!

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  • JeanTate by JeanTate in response to ross_e_hart's comment.

    Thanks Ross.

    So the SDSS is complete and representative of the local universe above a certain brightness that is detectable by the telescope.

    Integrated magnitude (flux) is certainly one factor for a galaxy being detectable in SDSS, but so is surface brightness: a bright (integrated r-band mag < 18, say) but large (petroRad_r > 30" perhaps) and diffuse galaxy would be invisible in SDSS, wouldn't it? And as I noted in an earlier post in this thread, it seems there are rather a lot more of these LSB (low-surface brightness) galaxies than originally thought.

    most of the local universe's galaxies are spirals + spiral galaxies are bluer than ellipticals and S0s (which means more young stars are being formed in them) = more star-formation is happening in spirals than other types of galaxy.

    Hmm ... agreed over the apparent dominance of spirals as a Hubble type in SDSS, but the "more star-formation is happening in spirals than other types of galaxy" is true only at some 'global' level surely? For example, there's an awful lot of star-formation taking place in (most) Irregular (Irr) galaxies, and many of those are not dwarfs (they're not spirals, by definition). Also, mergers can produce some spectacular star-bursts, but while they are fairly rare in the local universe, they are not (usually) classed as spirals, are they?

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  • KWillett by KWillett scientist, admin, translator

    Classification of a merger is somewhat of a chicken-and-egg scenario - it depends so strongly on the stage of the merger. By the time it's finished happening, it's almost always an elliptical. For the intermediate stages, it doesn't have to fall into either spiral or elliptical.

    That being said, there's definitely scientific merit in looking at the merger progenitors (usually before the nuclei have combined) and assessing their morphological type. The Darg et al. (2010a,b,2011) papers label mergers as S+S, E+E, or E+S and look at how that correlates with things like mass ratio and merger stage.

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  • JeanTate by JeanTate in response to KWillett's comment.

    By the time it's finished happening, it's almost always an elliptical.

    I know that's the commonly held view, but how well is that grounded in physical reality?

    For example, doesn't it depend on the kind of merger? And what about 'fly-bys' (two galaxies which interact but do not merge)?

    On the former, it's accepted that our own, spiral, galaxy has cannibalized several dwarf galaxies over its lifetime (leaving evidence such as the many stellar stream discovered from analysis of SDSS data), and is in the process of 'eating' several others, right?

    Isn't it still an at least somewhat open question, as to what mergers will leave behind what looks like a spiral?

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  • JeanTate by JeanTate

    Something on LSB galaxies, and tangentially spirals.

    In 2012, Mike Disney and Huw Lang published a paper called "The galaxy ancestor problem" in MNRAS (ADS link; the PDF is not behind a paywall).

    As is Disney's wont, he puts forward a controversial idea. While it's not too hard to find at least one fatal flaw (how many can you find?), he and Lang's general point that there could well be a large population of LSB galaxies we are simply unaware of is well made, IMHO.

    How confidently can we put lower limits on the rates of 'current' star-formation, in the local universe, by studying only SDSS spirals?

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  • JeanTate by JeanTate

    This post is for readers who are not professional astronomers (or PhD students), and aims to describe the physical, quantitative basis for "brightness", and then what "surface brightness" means.

    Almost everyone who reads things in GZ Talk will quickly come across "magnitudes", and surmise (or read) that it's a quantitative measure of "the brightness" of an astronomical object such as a star or galaxy. This unit has a very long history (WP), but it's only fairly recently that it's become firmly tied to the system of physical units universally used in the physical sciences (the SI system, WP).

    So how do "magnitudes" relate to things like energy?

    Intuitively you'd expect an apparently "brighter"star to emit more energy than a "fainter" one, and you'd be right. Light is one form of electromagnetic radiation, as is gamma rays, x-rays, UV, infrared, microwaves, and radio; astronomers often call all these forms "light", which can sometimes lead to considerable confusion.

    If light is absorbed, as happens when you see a star, energy is converted from one form (electromagnetic radiation) to another (e.g. heat, an electro-chemical signal).

    So brightness is related to energy. But it's more than that: it's not the total energy, but rather its rate: two stars of different (apparent) brightness appear different because the energy per second your eye receives differs.

    And there's more: a star looks brighter in a telescope than when you look at it with just your 'naked eyes'. Yet it's the same star, with the same brightness. To account this 'size dependence', brightness includes an 'area'; the quantitative physical unit of brightness has energy per unit of time, and per unit of area.

    Yet more: how to compare the brightness of an obviously red star with a white one? The energy a photon or electromagnetic wave carries (or has) depends the wavelength of the wave (or photon; the two descriptions are just two sides of the same coin). Or its frequency. But they're related by the speed of light (c). The shorter the wavelength (higher the frequency), the more energy a photon has.

    So magnitude includes frequency (or wavelength); magnitudes based on frequency are called AB magnitudes (WP), and I think those based on wavelength are called STMAGs ("ST" stands for "Space Telescope", i.e. the Hubble).

    (to be continued)

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  • JeanTate by JeanTate in response to JeanTate's comment.

    (continued)

    Time to introduce the jansky (Jy)! It's an SI unit, with 1 Jy = 10^-26 watt m^-2 Hz^-1 (watts per square meter per hertz).

    A flux, f, in Jy is easily converted to an AB magnitude (m): m = -2.5 log (f) + 8.90

    Actually, that's not quite correct; "f" is not a "flux", but rather a "spectral flux density"! 😮 "Spectral" because it's "per frequency", and "density" because it's "per square metre". "Flux" and "flux density" are easily confused. And to make matters worse, in at least some astronomy and astrophysics papers, "flux" is used when it's clear that it should be "flux density" (I don't think the reverse happens much, if at all). 😦

    Anyway, on to "surface brightness", or how bright a chunk of a source appears.

    Perhaps the most counter-intuitive aspect of surface brightness is that is independent of magnification. So the surface brightness of the central region of an elliptical galaxy (say, 10" in radius) is the same whether you observe it with binoculars, your 40cm backyard telescope, the 2.5m SDSS one, the 2.4m Hubble, or one of the 8.2m VLTs, no matter what the magnification is (there are some, minor, caveats; holler if you're interested)! 😮 😮

    Or, to take perhaps a more familiar example, the Sun: a 5" across piece of the Sun, say, has the same surface brightness, no matter how big (or small) the telescope you use, nor how much you magnify it.

    One reason this seems counter-intuitive is that stars don't seem to follow this pattern: you magnify one, and it seems to get brighter. What's happening is that a star is (with just a handful of exceptions, if you use the Hubble) a 'point source'; no matter how much you magnify it, it remains a single point (that 'point' is a few arcmins wide for your eyes, ~1-5" for a telescope on the ground, and as small as 0.05" for the Hubble).

    Even more counter-intuitive, perhaps, is the fact that surface brightness is independent of distance: the Sun, for example has the same surface brightness observed from Mercury, from here on Earth, from Pluto, ... and from Alpha Cen. Of course, when the Sun becomes indistinguishable from a point, surface brightness loses its meaning.

    There is a very important 'exception' to 'surface brightness is independent of distance', which I'll cover in a later post.

    Next: the surface brightness of galaxies.

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  • zutopian by zutopian

    The previous posts had been done in March/April. Now the subsequent GZ blog post is available.:

    A new paper about spiral galaxies in Galaxy Zoo
    https://blog.galaxyzoo.org/2016/07/22/a-new-paper-about-spiral-galaxies-in-galaxy-zoo/

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