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Galaxy depth and completeness using Mangle Mask » History » Version 1

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Diego Capozzi, 11/26/2013 06:18 PM


Galaxy depth and completeness using Mangle Mask

Here I (Diego) describe the results of a study aiming at identifying homogeneous depth regions using the information contained in the Mangle mask. These regions will be constructed in the molygon space, so they won't necessarily be constituted by contiguous molygons. In addition, relations between depths estimates measured with various types of magnitude (the one used for constructing the Mangle mask is a 2"-aperture magnitude, MAG_APER4) is explored so to be useful for data selection purposes, which can be different according to the science carried out. A first attempt to study these issues in detail has been carried out and described here: Some tests on depth with Mangle Mask. Here I implement the old tests with the selection cuts that have been discussed during the SVA1 telecons and in particular among Eli, Nacho and myself. These cuts also include the latest star/galaxy separation criteria described here: A Modest Proposal for Preliminary Star/Galaxy Separation.

I point out that the study described below is carried out only on the following fields: SPT (E & W); El Gordo; RXJ2248. As of today, there is no SVA1 data-based Mangle Mask for tew Bullet Cluster field. SN fields (as of now, a mangle mask is available only for SN_S) have been excluded on purpose, as my aim is to characterise a region with homogeneous depth for the standard 10-tilings Survey (from now on referred to as Wide Survey). In addition, I only considered

in addition to the cuts that will be described below, I have used only tiles satisfyinbg

The tests shown below are carried out on the latest Mangle mask available for SVA1 data (SN fields are excluded) and considering only the tiles where the zero-point offsets are minimised according to Huan Lin's criteria. Molygons with area>3 str and with i-band 2"-aperture mag=0 (the latter corresponding to regions that are masked by bright stars) have been excluded. The plots are quite rough but they should give an idea of the current situation. The aim is to make sure to identify areas with homogeneous depth (i.e. with a minimum value of depth) in the molygon space so to be able to select a galaxy sample out to the minimum depth value found in the identified region. After this is achieved, galaxy completeness studies will be carried out within the identified region and using the identified galaxy catalogue. Comments on these tests would be really appreciated!

3. Studying galaxy depth via galaxy counts, using Mangle mask: comparing MAG_AUTO, MAG_DETMODEL & MAG_APER4 depths against molygon ones.

Note: SVA1 molygons in SPT (E & W)/El Gordo/RXJ2248 fields (i-band). SVA1 data only!

  • Data Selection
    • Galaxy selection step 1 (from SVA1_COADD_GRIZY table):
      1. Contained in SPT (E & W), El Gordo & RXJ2248 fields
      2. Contained in good tiles according to Huan Lin's "galaxy locus" zeropoint offsets criteria
      3. 10<MAG_AUTO_I<25
      4. MAGERR_AUTO_I<0.11
      5. NITER_MODEL_I>0
      6. SPREAD_MODEL_I>0.0015
    • Molygon selection step 1 (from MOLYGON table):
      1. I-band molygons contained in SPT (E & W), El Gordo & RXJ2248 fields
      2. I-band molygons in tiles according to Huan Lin's "galaxy locus" zeropoint offsets criteria
      3. MAG_LIMIT (i-band)>0 (Avoiding regions masked by bright stars)
      4. AREA_STR<3 [Avoiding molygons with strangely large area (these are few outliers and reducing the cut to 1 or less won't change the extracted molygons)]
    • Galaxy selection step 2 (from COADD_OBJECTS_MOLYGON table):
      1. Contained in molygons selected in previous point (step necessary to extract information linking galaxies to the molygons they belong to)
    • Galaxy selection step 3:
      1. Selecting properties from SVA1_COADD_GRIZY of galaxies identified in step 1 which are contained in the identified molygons using the information obtained in "Galaxy selection step 2"
    • Molygon selection step 2:
      1. Checking the actual molygons used by identified the unique molygons ID values associated with the galaxy sample identified in "Galaxy selection step 3"
  • How do plots in paragraph 2 look like for the identified molygons associated with the galaxies in the identified sample?

  • For the identified galaxy sample, values of MAG_AUTO, MAG_DETMODEL & MU_MAX_MODEL have been corrected for zeropoint offset according to Huan Lin's tables. This was not done for APER_MAG4. Question: Has mangle mask depth estimation taken zeropoint offsets into account?
  • I-band depth comparison. What has been done:
    • Binning molygons in mag_limit (2-arcsec aperture-magnitude depth associated with molygons as output of the Mangle masking process) bins of 0.2 mag.
    • In each bin, distributions of MAG_APER4, MU_MAX_MODEL, MAG_AUTO and MAG_DETMODEL are analysed. Bin size used for all these distributions: 0.05 mag
    • For each distribution the depth is found as the peak in the number counts [if the peak is not unique, but more than 1 with the same value are seen, then the first peak (corresponding to the brightest magnituded) is taken]. The presence of more peaks should only happen with smaller galaxy samples, i.e. for the brighter molygon-depth bins. In general, the peak should be one and at magnitude fainter than this peak, galaxy number counts should decrease.
    • Galaxy depth is measured. In general, the process for defining galaxy depth for galaxies in molygons within a given mag_limit bin, works as follows:
      1. Find peak in the surface brigthness (in this case MU_MAX MODEL) distribution. NOTE: the shape of this distribution is always found weirdly gaussian-like!
      2. Select galaxies with: reasonable lower limit<surface brightness values<surface brightness peak. In this study, reasonable lower limit=16.
      3. Find peak in the magnitude distribution. This peak is defined as the galaxy magnitude depth. Note that the surface brightness does not strongly influence the galaxy depth definition (peak of the magnitude number counts), but the galaxy completeness definition (shape+peak of the magnitude number counts).

The following plots show the results obtained for molygons with 22.6<MAG_LIMIT<24.2.