1. Fundamentals of Finding Faint Field Dwarfs Photometrically

Title: Hunting Faint Dwarf Galaxies in the Field Using Integrated Light Surveys
Status: Published in ApJ, 2018
Authors: Shany Danieli, Pieter van Dokkum, and Charlie Conroy 

Integrated Light vs. Star Counting Surveys

There are two basic means of identifying dwarf galaxies in observations:

1. treating galaxies as bodies that are made up of many individual point sources

2. considering galaxies as their own essentially indivisible integrated light objects 

In the first scenario (star counting), one looks for over-densities of stars relative to the Galactic background. Locally, this is a very effective strategy, which can detect dwarfs down to the very low surface brightness regime, but fails at greater distances where the brightness of the individual stars is attenuated.

At greater distances, the second scenario (using integrated light) is preferable, taking advantage of surface brightness being distance independent at low redshift. The drawback here is that, ultimately, studies interested in the resolved stellar populations of a candidate system will require follow up. 

Because these two methods act in essentially different regimes, they can be considered complementary. As integrated light surveys join the efforts of existing star counting surveys, we should expect to improve our completeness with respect to local dwarf detections.

Dwarfs in the Local Volume

Using empirical relations based on the properties of Local Group (d < 3 Mpc) dwarf galaxies and extrapolations of the stellar mass-halo mass (SMHM) relation at low stellar masses, Danieli et al. infer the number density of dwarfs at various stellar masses with limiting surface brightnesses and spatial resolutions. There are two primary sources of uncertainty in this estimation:

1. The assumption that Local Group dwarfs are representative of dwarfs throughout the Local Volume (d ≲ 10 Mpc), where different formation and environmental mechanisms might be in play introduces certain biases. Until there's a comprehensive sample of low mass dwarf galaxies in the Local Volume that is comparable in completeness to the Local Group sample, this uncertainty cannot be easily constrained.

2. The stellar mass-halo mass relation is poorly constrained at low masses (due to lack of observations). The authors adopt results from Behroozi et al. 2013 as the upper limit of the SMHM relation and Rodriguez-Puebla et al. 2017 as the lower limit. The anticipated number of detected dwarf galaxies differs by an order of magnitude depending upon which of these relations is used in the calculation. Once again, this uncertainty can only be improved by increasing the number of observations of low mass galaxies.

The contours show the minimum mass detected within the observational (spatial resolution, surface brightness) parameter space. Within the Local Group, the limiting factor for observations is simply the surface brightness, while at larger distances (~8 Mpc), the spatial resolution begins to play a role. Figure from Danieli et al. 2018. 

Because the limiting distance for both integrated light and star counting surveys is well understood as a function of galaxy's stellar mass (for integrated light surveys this is dominated by the constraints of spatial resolution, while star count surveys are hobbled by the inverse square law), the authors assess the number density of dwarf galaxies detected by various instruments. Using the Dragonfly Telephoto Array (Dragonfly, integrated light), Dark Energy Camera (DECam, star counting with a limiting g-band magnitude of 24.6), and the Hyper Suprime Camera (HSC, star counting with a limiting g-band magnitude of 26.5), they construct detection rates for both SMHM relations.

The cumulative number of dwarf galaxies expected to be detected via the complementary approaches of integrated light and star counting surveys as a function of stellar mass. The left panel is representative of the upper limit on the SMHM relation, while the right panel is representative of the lower limit on the SMHM relation, corresponding to an order of magnitude difference in the number of anticipated dwarf galaxy detections. Dragonfly (shown in blue) is representative of the integrated light surveys where the limiting factor is spatial resolution. DECam (limiting magnitude 24.6) and HSC (limiting magnitude 26.5), shown in orange, are examples of star counting surveys. Figure from Danieli et al. 2018.