Narrow Band Magic

Galaxy morphology changes once you go to a different color. You are more sensitive to different stellar populations. Blue filters pick up young, massive stars for preference and redder filters the older population of galaxies, one is more sensitive to star-formation, and the other overall stellar mass (something the S4G survey used to great effect). 

This brings me to the narrow-band magic. Narrow filters are only sensitive to a short wavelength range. But if an emission line happens to lie in that range, the contrast for those images will be fantastic. 

This is the idea behind the Merian Survey and what this week’s paper is all about:

A Nonparametric Morphological Analysis of Hα Emission in Bright Dwarfs Using the Merian Survey

Mintz+

[astroph]

The two filters used, each capturing either Halpha or Hbeta+OII. So instead of a morphology estimate that is dominated by stellar populations, the morphology of your images is almost exclusive the emission line. These emission lines, especially Halpha, is powered by new star-formation. So this survey maps new star-formation in nearby galaxies and where it occurs. They combine their observations with a local estimate of the stellar continuum from z-band. 

The optical image, narrow-band image, the continuum contribution and the line emission image of galaxies of this survey. This is a neat way to map lots of galaxies fast!

This allows for a clean segmentation of the image. The issue with Halpha imaging is often that it is very fractured. Individual HII regions are not inter-connected. So it is hard to define the part of the image over which to compute…drumroll please…morphometrics! 

some of the segmantations of the images. Continuum defines the area, and then the morphometrics can be calculated over the Halpha flux.

One can then start exploring the HII morphology space and its dependence on inferred galaxy properties; stellar mass and overall star-formation rate or combine these into specific star-formation rate (SSFR). 

The Gini-M20 plane with Asymmetry color-coded to show where Halpha and sdss-r contunuum light morphology lie for the sample. Note that the continuum lies in the disk galaxy space, but Halpha shows a much greater range.

This is the Gini-M20 space that Lotz+ has used to identify disks, spheroids and interacting galaxies. The divisions look to be very different in Halpha morphology though! 

The distributions of morphometrics as a function of stellar mass. These seem to not change much with mass.

The morphology in Halpha does seem to change a lot with specific star-formation. No surprise since Halpha is driven by star-formation. If there relatively a lot of it, the morphology changes. Even if the underlying disk is not very perturbed. 

The potential weakness is that the view of Halpha is skewed by dust. The authors address this and correct for this some. But to correct the morphology completely for that, commensurate hot dust (e.g. 20 micron imaging) would be necessary. Peter Kamphuis used something like that on…you guessed it NGC 891. 

The correlations with SSFR is a first good exploration. I am curious to see what the survey team is going to be working on next!