Inquiry About DR10 Mask Pixel Locations and Mask Radii for Galaxy Density Analysis

[Vignesh V V]

Dear Legacy Surveys Team,

I am Vignesh Vavillakula Venkataramana Rao, a PhD scholar from National Chung Hsing University, Taiwan. I am working on understanding the galactic environments of fast radio bursts, and for calculating galaxy number density, I use the Legacy Surveys DR10 release data.

While reviewing the DR10 bitmask documentation, I understood that the MASKBITS images provide the masked pixel footprints for flags such as BRIGHT, MEDIUM, GALAXY, CLUSTER, SATUR, and ALLMASK. However, I would like to know whether the corresponding geometric properties of these masks—particularly the pixel locations of the mask centres and the effective mask radii (or ellipse parameters)—are available in any catalogue or auxiliary data product.

This information would be very useful for my analysis, as it would allow me to exclude those masked regions while calculating galaxy number density and thereby avoid biases in the measurements.

Could you please clarify the following:

1. Are the central pixel positions of masks such as BRIGHT, MEDIUM, GALAXY, and CLUSTER available in any DR10 catalogue or mask-related file?

2. Are the mask radii or ellipse parameters for these masked regions stored anywhere, or are only the final per-pixel mask footprints available?

3. If only footprints are available, is there a recommended method to reconstruct the effective masked area for statistical studies such as galaxy number density estimation?

4. For GALAXY masks, would the ellipse parameters from the SGA catalogue be the correct quantities to use for excluding these regions?

5. If these mask centre positions and radius/ellipse parameters are not publicly available in DR10, would it be possible to obtain this information from the Legacy Surveys team?

My goal is to accurately estimate the usable sky area around each target and avoid contaminated regions during the density calculations.

Thank you very much for your time and assistance.

Best regards,
Vignesh Vavillakula Venkataramana Rao
PhD Scholar
National Chung Hsing University

[Dustin Lang]

Hi,

Apologies for the slow reply!

The catalogs we use for masking are listed and linked here

https://www.legacysurvey.org/dr10/external/#external-catalogs-used-for-masking

cheers,

dustin

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[Vignesh V V]

Hi there,

Thank you for your reply. I have already included this information in my analysis. However, even after incorporating the masking location and radius information, I still observe some large circular empty regions in the sky.

These empty regions do not appear to resemble astrophysical structures such as voids; rather, they look like nearly perfect circular gaps. I would also like to mention that these large circular empty regions appear in my figure only when I apply the condition `MASKBITS != 0`.

Could you please advise whether this behavior is expected, or if there may be an additional masking-related issue that I should consider?

With Regards,

Vignesh Vavillakula Venkataramana Rao

[Dustin Lang]

Hi,

It would probably be informative to see which MASKBITS are set -- instead of just checking for MASKBITS != 0, try each bit and see which one is responsible.

Or if you want to send an example RA,DEC and approximate radius, I can take a look.

cheers,

dustin

[Vignesh V V]

Hi there, 

Sorry for the delayed response. I am attaching an example figure for reference.

This figure shows a 100 × 100 Mpc sky region centred on an FRB source at a redshift of approximately 0.25. In this plot, I have already incorporated the available mask information from the NGC, SGA, and other catalogues listed on the website. These catalogue-based masked regions are shown using the red, orange, and black circles.

However, I still find some circular empty regions that do not appear to be associated with any of these catalogue masks. I have highlighted these apparently unassociated regions using magenta circles. My main concern is that these empty regions are very circular in shape, which suggests that they may be related to some additional masking or selection effect, rather than representing a real astrophysical underdensity.

Also, I have attached a CSV file containing the RA, Dec, and radius information for the unexplained regions highlighted by the magenta circles. I would be very grateful if you could kindly have a look at it.

With Regards,

Vignesh Vavillakula Venkataramana Rao

[Dustin Lang]

Those are Tycho-2 stars

https://www.legacysurvey.org/viewer/?ra=191.8934&dec=66.7896&layer=ls-dr9&zoom=12&tycho2

https://www.legacysurvey.org/dr10/external/#tycho-2

these are also used for masking, as described in the BRIGHT paragraph.

--dstn

[Vignesh V V]

Hi there,

Thank you for the clarification and your quick response. I will try to incorporate this information into my mask-fit analysis. If the issue is resolved, I will definitely let you know.

With Regards,

Vignesh Vavillakula Venkataramana Rao