Keywords: DataAccWG
Hi All,
I'm trying to do a bottom-up estimate of the catalog sizes, using Tim's
numbers and input from Kem and Sergei. Attached, please find the latest
draft. It is worrisome that the numbers we are getting are much smaller
than the bottom-up estimate of 300 TB for the first year (I'm currently
getting 15 TB).
Below I am including relevant comments from Kem. Any help with
correcting these numbers would be appreciated!
thanks,
Jacek
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150,000 objects/square degree seems pretty high for a single image at a
reasonable galactic latitude. I believe this is the final number of
galaxies in the 10 year stack and I don't think the first DR will have
quite that many.
There are only about 8 unique square degrees per image for hexagonal
close packing, but there will probably be a few more than 3000 fields,
maybe as many as 3100.
There is something wrong with the scaling on the number of galaxies.
The formula must not be accurate across the whole range. The
(20DR)**(1.25*.45) says that compared to a single visit,
the final stack will have 29x as many galaxies. A single visit gives
about 20 galaxies per square arcminute (if I remember correctly), which
means the final stack will have 580! per square arcminute or 42 billion
for the 20,000 square degree survey. This is a factor of 10 too high.
There is confusion in the accounting between visits and exposures.
There are going to be two exposures per visit, so there will be twice
the number of 'observations' which are relevant to the time dependent db.
I think the fraction of variable objects per field is way too high,
particularly as the number of objects increases with increasing depth.
Most of the deeper objects will be galaxies and the fraction of agn, SN
whatever in galaxies is more like .01 than 0.1.
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I just checked the Subaru Deep field for some numbers (Kashikawa etal
astroph/0410005 table 2)
I find that the total number of galaxies per square degree to R=24 which
is r = 24.5 for a (g-r) =0 galaxy (( R = r - 0.51 -.15(g-r))) is roughly
62836 which yields 17 per square arcminute, but their data only start at
18.25, so this may be a (very) small underestimate.
I interpolate to about 190,000 galaxies per square degree for DR1
(R=25.6 or r= 26.1)
I find that the d(logN)/dmag over the range 24 - 27 varies from .27 -
.35 and that the cumulative galaxy counts to R=27.25 (as deep as they
go) is 804329 per square degree which is about the final depth of the
ten year survey (SRD has r=27.8).
So, my earlier email using numbers from memory was not right, but then
the table numbers are not right either.
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I realize that those numbers may not be quite right depending on how the
bins in table 2 are labeled. Those numbers assume the bin label refers
to the bin maximum, but the labels probably refer to the middle of the
bin. So those numbers are a little high. using the middle of the bin,
I get:
R = 24: 47097 / square degree (r = 24.5)
DR1: 150,000 / square degree (hmm, guess the number in the table is for
DR1)
10 year survey gives about 760,000 / square degree
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