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Some tests on depth with Mangle Mask

-Diego Capozzi

1. Preliminary tests on depth.

NOTE: I have discovered that data from more than one data release were included in the data used, so despite the general trend should be real, the plots in this paragraph 1 should be taken with caution.

Only good tiles according to Hun Lin criteria on zero-point offsets.

Molygons i-band 2-arcsec aperture mag depth maps:

Zoom in mag:

Molygons i-band 2-arcsec aperture mag depth histogram:

Comments:

Selection of areas with homogeneous depth:

Area we would have for LF studies as function of molygons depth:

2-arcsec i-band magnitude limit (mag) Area (sq. deg)
23.3 ~273
23.5 ~224
23.6 ~193
23.8 (~peak of distribution) ~110

2. More preliminary tests on SVA1 molygons in SPT (E & W)/El Gordo/RXJ2248 fields (i-band). SVA1 data only!

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!

I've found some suspicious behaviour of the molygons. Some of them have incredibly high depth (i-band 2" aperture mag>25) with exposure time<500 s

Focusing on those extreme molygons with i-band 2" aperture mag>26 (~4000), one can notice that all of them, independently of the total exposure time associated with them, reside in the same "small" region (black region) in the middle of the SPT-E field.

So i decided to check the issue globally at all magnitude values, in particular by constructing a rough molygon map of exposure time as a function of magnitude and area. The depth reached in the molygons sounds suspiciously too deep when taking into account the total exposure time associated with them. Below I report the expected stellar magnitude in the i-band at several S/N levels (what we want to consider though is S/N~10 level) for two values of total exposure time. These are calculated by using the DECam Exposure Time Calculator for stellar photometry available on the CTIO DECam webpage. I have considered the case of best observing conditions, i.e. when the sky brightness is the lowest possible (so to speak, with new moon). This choice is to show that even in the best observing scenario, the depth reached in some molygons doesn't seem realisitic. One has to take into account that the Exposure Calculator is made for stellar photometry, so one should roughly subtract 1 mag from the stellar magnitude value it provides, to get a realistic value for galaxies.

Case of Total exposure time fixed at 540 s (i.e. 6 x pointings/90 s exposures)

Stellar i-band magnitude limit (mag) Galaxy i-band magnitude limit (mag) S/N
26 ~25 1.9
25.5 ~24.5 3.1
25 ~24 4.9
24.5 ~23.5 7.7
24 ~23 12.1
23.5 ~22.5 19.1
23 ~22 30.0

Case of Total exposure time fixed at 450 s (i.e. 5 x pointings/90 s exposures)

Stellar i-band magnitude limit (mag) Galaxy i-band magnitude limit (mag) S/N
26 ~25 1.8
25.5 ~24.5 2.8
25 ~24 4.4
24.5 ~23.5 7.0
24 ~23 11.1
23.5 ~22.5 17.4
23 ~22 27.4

The plot below gives some evidence of suspiciously deep molygons. Keep in mind that the expectation (R4 requirement) is a galaxy depth of i-band 1.5"-aperture magnitude=24.3 at 10 sigma level for a 900 s total exposure time (10 x 90 s exposures). This value will be slightly shallower when using 2" aperture magnitude. However, independently of the requirement, some molygons do seem to have too high a depth given the total exposure time associated with them. This is true also when not considering the most extreme cases (depth>25 and total exposure<400).

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. FLAGS_I<4
      6. NITER_MODEL_I>0
      7. 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.

PLOT CAPTION
upper left panel: 2-arcsec aperture-magnitude distribution for the identified galaxy sample within the selected molygons
upper centre panel: MU_MAX_MODEL distributions for the identified galaxies in: a) all the molygons selected in the "Data Selection" section; b) molygons in the the MAG_LIMIT bin analysed
upper right panel: MAG_AUTO distributions for the identified galaxies in the molygons contained in the analysed MAG_LIMIT bin: with and without MU_MAX_MODEL selection
lower left panel: MAG_DETMODEL distributions for the identified galaxies in the molygons contained in the analysed MAG_LIMIT bin: with and without MU_MAX_MODEL selection
lower centre panel: MAG_APER4 distributions for the identified galaxies in the molygons contained in the analysed MAG_LIMIT bin: with and without MU_MAX_MODEL selection
lower right panel: comparison between MAG_AUTO, MAG_DETMODEL & MAG_APER4 distribution for galaxies selected also in MU_MAX_MODEL

22.6<MAG_LIMIT<22.8

22.8<MAG_LIMIT<23.0

23.0<MAG_LIMIT<23.2

23.2<MAG_LIMIT<23.4

23.4<MAG_LIMIT<23.6

23.6<MAG_LIMIT<23.8

23.8<MAG_LIMIT<24.0

24.0<MAG_LIMIT<24.2

Galaxy depth comparison:

The galaxy depths measured via number counts in each MAG_LIMIT bin for galaxies selected also in MU_MAX_MODEL, are compared with those given as the output of the mangle masking process in the following plot. Note that, as suspected in Paragraph 2, the mangle mask depths seem to be deeper than what exposure time (see paragraph 2) and number counts (see this paragraph and following plot) would suggest. Note that the "Reliable" region in the following plot represents MAG_LIMIT bins where the galaxy sample is large enough not to show multiple peaks in the magnitude distributions.