Galaxy depth and completeness using Mangle Mask » History » Version 4

Diego Capozzi, 11/26/2013 07:24 PM

1 1 Diego Capozzi
h1. Galaxy depth and completeness using Mangle Mask
2 1 Diego Capozzi
3 2 Diego Capozzi
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]].
4 1 Diego Capozzi
5 2 Diego Capozzi
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.
6 1 Diego Capozzi
7 1 Diego Capozzi
8 2 Diego Capozzi
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":.
9 1 Diego Capozzi
10 2 Diego Capozzi
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).
11 1 Diego Capozzi
12 2 Diego Capozzi
h2. 1. Studying galaxy depth via galaxy counts, using Mangle mask: comparing MAG_AUTO, MAG_DETMODEL & MAG_APER4 depths against molygon ones. 
13 1 Diego Capozzi
14 1 Diego Capozzi
15 1 Diego Capozzi
16 1 Diego Capozzi
* Data Selection
17 1 Diego Capozzi
** Galaxy selection step 1 (from SVA1_COADD_GRIZY table):  
18 1 Diego Capozzi
### Contained in SPT (E & W), El Gordo & RXJ2248 fields
19 1 Diego Capozzi
### Contained in good tiles according to Huan Lin's "galaxy locus" zeropoint offsets criteria 
20 2 Diego Capozzi
### MAGERR_APER4_I<0.11
21 2 Diego Capozzi
22 2 Diego Capozzi
23 2 Diego Capozzi
24 2 Diego Capozzi
### DEC>-61 (avoid LMC)
25 2 Diego Capozzi
### FLAGS_I<4
26 2 Diego Capozzi
### (((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
27 1 Diego Capozzi
** Molygon selection step 1 (from MOLYGON table):
28 1 Diego Capozzi
### I-band molygons contained in SPT (E & W), El Gordo & RXJ2248 fields
29 1 Diego Capozzi
### I-band molygons in tiles according to Huan Lin's "galaxy locus" zeropoint offsets criteria 
30 1 Diego Capozzi
### MAG_LIMIT (i-band)>0 (Avoiding regions masked by bright stars)
31 1 Diego Capozzi
### 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)]
32 1 Diego Capozzi
** Galaxy selection step 2 (from COADD_OBJECTS_MOLYGON table): 
33 1 Diego Capozzi
### Contained in molygons selected in previous point (step necessary to extract information linking galaxies to the molygons they belong to)
34 1 Diego Capozzi
** Galaxy selection step 3:
35 1 Diego Capozzi
### 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"
36 1 Diego Capozzi
** Molygon selection step 2:
37 2 Diego Capozzi
### Checking the actual molygons used to identify the unique molygons ID values associated with the galaxy sample identified in "Galaxy selection step 3"   
38 1 Diego Capozzi
39 2 Diego Capozzi
* 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.
40 1 Diego Capozzi
41 1 Diego Capozzi
42 3 Diego Capozzi
* The importance of what surface brightness measurement is used.
43 3 Diego Capozzi
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. 
44 3 Diego Capozzi
45 3 Diego Capozzi
46 1 Diego Capozzi
47 4 Diego Capozzi
Figure 1
48 3 Diego Capozzi
49 3 Diego Capozzi
50 3 Diego Capozzi
51 3 Diego Capozzi
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. 
52 1 Diego Capozzi
53 1 Diego Capozzi
54 4 Diego Capozzi
55 1 Diego Capozzi
56 2 Diego Capozzi
57 1 Diego Capozzi
* I-band depth comparison. What has been done:
58 1 Diego Capozzi
** 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. 
59 2 Diego Capozzi
** 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
60 1 Diego Capozzi
** 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.
61 1 Diego Capozzi
** 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:
62 2 Diego Capozzi
### Find peak in the magnitude distribution. This peak is defined as the galaxy magnitude depth. 
63 2 Diego Capozzi
64 2 Diego Capozzi
** 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:
65 2 Diego Capozzi
### 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).
66 1 Diego Capozzi
### Select galaxies with: reasonable lower limit<surface brightness values<surface brightness peak. In this study,  reasonable lower limit=16.
67 2 Diego Capozzi
68 2 Diego Capozzi
69 2 Diego Capozzi
Note that the surface brightness strongly influences the galaxy completeness limit.
70 2 Diego Capozzi
71 2 Diego Capozzi
72 2 Diego Capozzi
73 2 Diego Capozzi
74 2 Diego Capozzi
75 2 Diego Capozzi
76 1 Diego Capozzi
77 2 Diego Capozzi
78 1 Diego Capozzi
79 4 Diego Capozzi
The following plots show the results obtained for molygons with 22.6<MAG_LIMIT<24.2.