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Fraser
2005-Aug-01, 04:08 PM
SUMMARY: Cepheid variables are one of the most accurate tools astronomers can use to measure the distance to objects in space because the period of their pulsation is directly tied to their brightness. An international team of astronomers have used this method to calculate the distance to galaxy NGC 300 which is in the Sculptor Group. The team found more than 100 Cepheid variables, and then used them to calculate the distance to be 6.13 million light-years away; give or take 3%.

View full article (http://www.universetoday.com/am/publish/most_accurate_distance_measuring_for_ngc300.html)

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wstevenbrown
2005-Aug-01, 09:01 PM
That's 1.88 Mpc-- down from 2.56 Mpc as measured by the Tip of the Red Giant Branch method. About a 25% reduction. Both methods are sensitive to extinction, as are all of the resolved-star methods of distance determination. It would be nice to see the same extinction criteria applied to the TRGB data to establish a correlation between the two methods. S

suitti
2005-Aug-02, 02:18 PM
Wasn't the whole distance ladder scale reduced by about 20% when the hipparcos data came out? Also, it seems to me that there was a proposal to revisit the space based astrometry mission. Was that cancelled in the wake of Mars fever? While hipparcos achieved 2-4 milliarcseconds, HST's fine guidance sensors routinely achieve 3 milliarcseconds and with effort has achieved something like 200 microarcseconds. I don't think this can be done anywhere but space.

antoniseb
2005-Aug-02, 02:36 PM
Originally posted by suitti@Aug 2 2005, 02:18 PM
Also, it seems to me that there was a proposal to revisit the space based astrometry mission. Was that cancelled in the wake of Mars fever?
The US has postponed indefinitely the Space Interferometry Mission (SIM), but as far as astrometry goes, the ESA's Gaia mission will do some amazing things, far beyond what Hipparcos did.

http://www.esa.int/esaSC/120377_index_0_m.html

You can expect some additional refinement in the distance ladder (and a lot of other things) when the Gaia data comes in.

wstevenbrown
2005-Aug-02, 05:24 PM
Looking back at the history of these observations, some facts new to me have emerged. Butler et. al. Had a revised TRGB distance in late ’03 :

http://arxiv.org/abs/astro-ph/0312007

They note very carefully that:


We have also determined the first distance modulus estimate based on the tip of the red giant branch method: on the Cepheid distance scale of Ferrarese et al. (2000) we find (m-M)o = 26.56+/-0.07 (+/-0.13) mag; and a similar value from the Cepheid-independent empirical method by Lee et al. (1993), both in good agreement with the Cepheid distance determined by Freedman et al. (2001). A discrepancy between this and the theoretical calibration of the red giant branch tip magnitude method remains.

Italics mine. The work of the Araucaria Group centered on resolving this discrepancy:

http://arxiv.org/astro-ph/0503626


These guys are the real deal. Keep an eye on them at their homepage:

http://cluster.cfm.udec.cl/wgieren/

Edit: removed AG homepage (currently disabled), substituted Wolfgang Gieren's homepage. S


The correlations I spoke of earlier in this string had already been done in the 05/03 paper, tho this did not come out in the journalistic article. The Araucaria Group’s focus is on obtaining better correlations across the resolved-star distance measures, and obtaining tighter constraints on the uncertainties inherent in those measures. Forging better links between the steps of the distance ladder is high-priority work. One of the hidden variables is this: if you are using a secondary distance measure (any of the resolved-star measures), there are two sources of error. First, the uncertainties inherent in the method, and observational limits. Second, never to be forgotten, the uncertainties carried forward from the primary method (parallax) which established the secondary methods as valid. The probable ‘actual’ error is the root-mean-square of the two. Upcoming missions like Gaia and TPF should reduce the primary uncertainties substantially.

Next is the really tricky part—establishing the link between secondary and teriary measures, such as the Hubble Rule for redshift. Remember that Hubble worked in an era when the world’s largest telescope (the Hooker at Mt. Palomar) was only 2.5m. The first observations establishing the Hubble Flow were taken from relatively nearby galaxies, where the probable error is largest, due to their peculiar motions. As telescope size increased, the Hubble 'constant' has been repeatedly revised, and still has a large uncertainty (20%). There are many dangers lurking in drawing conclusions about faraway galaxies in ‘rich’ clusters based on observations of close-up galaxies in ‘sparse’ clusters (we live in one). My working notes on distance scales, heavily loaded with opinions, is posted over in the Arp string:

http://www.universetoday.com/forum/index.p...340&#entry79811 (http://www.universetoday.com/forum/index.php?showtopic=5972&st=340&#entry79811)

I may have to recant my unkind assessment of the Cepheid methods, based on this new infrared back door proposed by the Araucaria Group. That’s progress. My hat is off to them! Best regards-- Steve

Duane
2005-Aug-02, 08:34 PM
I don't suppose this is a galaxy that Arp et al associate with a nearby AGN? Probably not, that would be too easy :(

wstevenbrown
2005-Aug-02, 09:11 PM
AFAIK off the top of my head, the only two Arp objects in the Local Volume are NGC 1569 (Arp 210) and M82 (Arp337). There could be others, but I don't pay them any especial attention-- and I have no idea whether any quasar ejection claims were made for them by Arp. Both are within reach of resolved-star distance methods. NGC 1569 = 1.95 Mpc (TRGB) and M82 = 3.53 Mpc (TRGB), subject to change without notice. It would be convenient if testable claims were made about them. S