Galaxy depth and completeness using Mangle Mask » History » Version 3
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, using SVA1 data. 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.
Galaxy completeness cannot be studied at the moment, but I use the surface brightness information to identify what could possibly be the magnitude at which the survey is complete. However, without having a deeper reference for a standard 10=tilings field, at which completeness level this magnitude value corresponds to cannot be inferred.
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 tiles where the zero-point offsets are minimised according to Huan Lin's "galaxy-locus criteria":.
Finally, 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 (see Some tests on depth with Mangle Mask for more details).
1. Studying galaxy depth via galaxy counts, using Mangle mask: comparing MAG_AUTO, MAG_DETMODEL & MAG_APER4 depths against molygon ones.¶
- Data Selection
- Galaxy selection step 1 (from SVA1_COADD_GRIZY table):
- Contained in SPT (E & W), El Gordo & RXJ2248 fields
- Contained in good tiles according to Huan Lin's "galaxy locus" zeropoint offsets criteria
- DEC>-61 (avoid LMC)
- (((SPREAD_MODEL_I+(3*SPREADERR_MODEL_I))>0.003) and ((MAG_AUTO_I>12 and MAG_AUTO_I<18 and CLASS_STAR_I<0.3) or (MAG_AUTO_I>18 and MAG_AUTO_I<25))) and MAG_PSF<30
- Molygon selection step 1 (from MOLYGON table):
- I-band molygons contained in SPT (E & W), El Gordo & RXJ2248 fields
- I-band molygons in tiles according to Huan Lin's "galaxy locus" zeropoint offsets criteria
- MAG_LIMIT (i-band)>0 (Avoiding regions masked by bright stars)
- 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):
- Contained in molygons selected in previous point (step necessary to extract information linking galaxies to the molygons they belong to)
- Galaxy selection step 3:
- 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:
- Checking the actual molygons used to identify the unique molygons ID values associated with the galaxy sample identified in "Galaxy selection step 3"
- Galaxy selection step 1 (from SVA1_COADD_GRIZY table):
- For the identified galaxy sample, values of magnitude (APER_MAG4 excluded, for which corrections are not available) and surface brightness have been corrected for zeropoint offset according to Huan Lin's tables.
- The importance of what surface brightness measurement is used.
In the previous study, the surface brightness measure that was used was model surface brightness [either effective (MU_EFF_MODEL) or that at the brightness peak (MU_MAX_MODEL)]. This measurement was always found to show a gausian-like distribution rather than the common steeply increasing distribution with a sharp drop at the faint end. This was proven to be independent on the depth inhomogeneity over the sky (see GEWG report on SVA1 for details). For this study I used MU_MAX, which is a model-independent measure of surface brightness. In the plot in Figure 1 below, I compare the distributions of MU_EFF_MODEL (whose shape is equivalent to the one of MU_MAX_MODEL) and MU_MAX for the galaxy catalogue identified above. One can see the great difference between the two distributions, and that the MU_MAX distribution is definitely closer to the expected distribution, despite its drop still lacking the expected sharpness.
This comparison indicates that there is something going on with model-based measures of surface brightness. I point out that this wasn't the case for DC6B, whose data did not show a gaussian-like shape of model-dependent surface brightness measures, as shown in Figure 2 below.
- 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, 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:
- Find peak in the magnitude distribution. This peak is defined as the galaxy magnitude depth.
- Approximate value for galaxy magnitude limit at which the sample is complete at an unknown completeness level is measured. In general, the process for defining this value for galaxies in molygons within a given mag_limit bin, works as follows:
- Find peak in the surface brigthness (in this case MU_MAX) distribution. NOTE: the shape of this distribution does not show the typical sharp drop after the peak. The situation is definitely better than when using MU_MAX_MODEL (see below for more details).
- Select galaxies with: reasonable lower limit<surface brightness values<surface brightness peak. In this study, reasonable lower limit=16.
Note that the surface brightness strongly influences the galaxy completeness limit.
The following plots show the results obtained for molygons with 22.6<MAG_LIMIT<24.2.