Milky Way arms and cluster density

[selden]

After Fridger reminded us of how good a job pulsar density does of revealing the galaxy's arms, I started wondering if there were other tracers one could use. Another substantial database of objects is the list of open clusters compiled by Diaz et al. and available at http://www.astro.iag.usp.br/~wilton/ It lists 2140 clusters, 1309 of which have distances specified.

Here's a plot of those 1309 clusters, marked in Celestia. The brightness of each marker roughly indicates the number of other clusters within 400 ly, with white indicating that there are 20 or more within that distance.

(I'm working on providing this as an Addon. Some packaging needs to be done.)

[ajtribick]

I guess any object with sufficiently short stellar lifetimes should do the trick of revealing the stellar arms, e.g. O-type stars.

Data from the Gaia astrometry mission will be really useful for this kind of thing...

[selden]

Wouldn't "clusters" and "O stars" be almost equivalent for this purpose? O stars don't last very long, and thus are likely to still be in their original clusters, while clusters tend to dissipate with age. An advantage is that cluster distances can be estimated to much greater values than the Hipparcos star parallax measurements, although those large cluster distance values also are much less accurate, of course.

I am surprised at the relatively high density of clusters core-ward of the Sun.

[t00fri]

Globular clusters of our galaxy, for example, seem to be unrelated to the location of the spiral arms. They are known to be relatives of Dwarf galaxies. See e.g.

http://www.astro.lsa.umich.edu/~ognedin/dwarf/

and plenty of papers that I have.

I am not sure about the argument in case of open clusters. O-type stars make more sense to me.

Fridger

[ajtribick]

Not sure about open clusters as there are lots of examples which have ages that imply they can survive for multiple galactic orbits, which is not good for tracing the spiral arms. E.g. the Hyades has an age of about 625 million years, versus the Sun's galactic orbit of ~250 million years.

[t00fri]

Not to forget another familiar albeit somewhat indirect evidence for the spiral arm structure and size:

(see my old 2006/2009 post: viewtopic.php?f=7&t=13388&p=111594)

original paper by E.S. Levine and friends,
http://arxiv.org/pdf/astro-ph/0605728

Here the outer spiral structure of the MilkyWay is
inferred from a map of the perturbed surface density of
neutral hydrogen! These data indicate that our Milkyway
is about a factor of two larger in size than what was
commonly thought.

Here is a plot from that paper, with the location of our sun marked as well.

In the bottom image (with median divided out), the spiral arm pattern can be clearly seen far outside the usual MilkyWay disk in this map of the atomic hydrogen (HI) surface density perturbations!!



The interesting aspect is that there are many pulsars
to be found in arc-type arrangements much further out
than the rim of our Celestia template. This again matches
perfectly with this interesting paper

Fridger

[t00fri]

Not sure about open clusters as there are lots of examples which have ages that imply they can survive for multiple galactic orbits, which is not good for tracing the spiral arms. E.g. the Hyades has an age of about 625 million years, versus the Sun's galactic orbit of ~250 million years.

Andrew,

So we agree. See my corresponding remark just above your post.

Fridger

[selden]

The cluster data file includes estimates of the clusters' ages.

I would have expected the density of younger clusters to have some relationship to the arms, but I don't see a strong correlation. In the screengrab below, lighter color markers indicate younger clusters.

[kevinjardine]

The Diaz list is officially called the "Catalog of Optically Visible Open Clusters and Candidates".

I think that the key phrase here is "Optically Visible". As we know from the Spitzer results, many and perhaps most young clusters are embedded in obscuring dust clouds.

I think that younger clusters are no doubt spiral tracers, but we will need to wait for a reasonably complete list of infrared clusters before we can really use them to trace the full extent of the spiral arms.