Sep 3, 2008NVOSS 2008 1

VO Analysis UsingLocal Utilities

Mike FitzpatrickNOAO

Sep 3, 2008NVOSS 2008 2

Introduction

• We’ll use various VO tools in an attempt to re-create a Tully-Fisher relation using data queries.

• We’ll do this using command-line and desktop tools.– When presented in the 2006 NVOSS this was done

using web-based toolshttp://nvotwiki.stsci.edu/twiki/pub/Main/NVOSS3CourseNotes/scienceApplications3.html

• We’ll skip the detailed work needed to make real science out of this.

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What’s a Tully-Fisher Relation?

• A (empirical) correlation of the luminosity and maximum rotation velocity of a spiral galaxy.

• The bigger a galaxy is, the faster it rotates.• So, if you know the rotation velocity you can

make assumptions using T-F about its intrinsic brightness.

• You compare this with the apparent magnitude and you have a measure of its distance.

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What’s a Tully-Fisher Relation?

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Top: Abell 1367

Bottom: Fornax

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What Do We Need?

1. Some measure of the rotational velocity– HI emission lines can give us this

2. A selection of spiral galaxies with photometry

3. A method to correct for the inclination4. A method to convert magnitude to

luminosities using the disance modulus:

M = constant - 10log10Vmax

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Finding Velocities

• Do a Registry query for ‘HI’% voregistry -R -t catalog HI

• The ‘digitalHIarchive’ looks like what we need, let’s see if it has the columns we require:

% voregistry -meta digitalHIarchive

Note: The RC3 Catalog also contains HI data, see how thisexercise would differr using that as the resource.

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Finding Velocities

• This is a fairly small catalog, so let’s download it using an all-sky query:

% vodata -O HI -V digitalHIarchive 0.0 0.0 180

• From the metadata we know this table has positions, morph type, velocities, and semi-major/minor axis (needed for inclination), save only these columns:

% stilts tpipe HI.xml \ cmd=‘select “morphtype==1”’ \

cmd=‘keepcols “ra dec a b vhelio wf50”’ \ out=spirals.xml

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Finding Photometry

• The SDSS is a good resource for photometry of galaxies• We can start with a query for SDSS resources:

% voregistry -R -t catalog sdss

• We notice a Russian service offering the PhotObjAll and PhotTag tables as a Cone Service. These are nice, but not fast enough for this demo. Let’s see if one of the SDSS Cone searches gives us a magnitude:

% voregistry -meta SDSS-DR4

• From this service we can get ugriz magnitudes

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Finding Photometry

• Now let’s query for the SDSS data given the positions we have for the Sa galaxies:

% vodata -O dr4 -1 SDSS_DR4 spirals.xml -rs 3 -mt 3

• Why the ‘-mt 3’ flag?– The task runs multiple threads and we can overwhelm

the server and get bogus results. This flag says run only three threads.

• For our 327 positions we get 125 results from SDSS• Why the ‘-1’ flag?

– We want to collect the results into one final table

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What Do We Have Now?

• The file ‘spirals.xml’ is a VOTable that has our Sa galaxies with the velocity information.

• The file ‘dr4.csv’ has photometry of those galaxies that overlap SDSS.

• We need to combine these, and add information (e.g. inclination)

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Combining Tables

• We use STILTS to match the tables and produce a combined output table:

% stilts tmatch2 in1=spirals.xml in2=dr4.csv ifmt2=csv\ matcher=sky \

values1="RA Dec" \ values2="POS_EQ_RA_MAIN

POS_EQ_DEC_MAIN" params="2" \ out=combined.xml

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Adding Columns

We use STILTS once again:

% stilts tpipe in=combined.xml out=final.xml \

cmd="addcol axratio 'b/a'" \

cmd="addcol cossqi '(axratio*axratio-0.4)/(0.96)'" \ cmd='addcol inclination "acos ( sqrt ( cossqi ) )"' \ cmd='addcol logcorW "log10(wf50/sin(inclination))"' \ cmd="addcol distance 'vhelio/70.0'" \ cmd="addcol absMag 'phot_sdss_i-

5*log10(distance*1000000/10.0 )'"

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Plotting Results

We now have all the columns necessary to plot a very simply Tully-Fisher relation

To plot the results, use TOPCAT to load the final table

% topcat final.xml

Bring up the graph window, and plot absMag vs logcorW

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Plotting Results