# DiskMass

## Contact

Kyle Westfall
University of California, Santa Cruz
westfall@ucolick.org

## Summary

Galaxies from the DiskMass Survey have been observed in MaNGA to (1) compare stellar-population-synthesis modeling obtained with MaNGA spectra and the dynamical stellar masses obtained from the DiskMass Survey and (2) calibrate the velocity-dispersion and asymmetric-drift measurements from MaNGA against the higher-spectral-resolution results from the DiskMass Survey.

## Finding Targets

An object whose MANGA_TARGET3 or MNGTARG3 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Targets
DISKMASS 16 DiskMass Survey Target 15

## Description

Dynamical masses and stellar mass-to-light ratios in dynamically cold systems, such as spiral disks, remain challenging to measure, even for the Milky Way (MW). Recent measurements of the MW (Bovy & Rix 2013) and external, MW-like galaxies (e.g., the DiskMass Survey; Bershady et al. 2011, Westfall et al. 2011, Martinsson et al. 2013b; Swaters et al. 2014) appear at odds concerning how much mass is in their disks relative to their halos. Bovy & Rix find that the MW has a very small and massive (maximal) disk. However, the DiskMass Survey finds stellar mass-to-light ratios in spiral galaxies are quite low, with a value of 0.3 (solar) in the K band, consistent with relatively light (submaximal) disks and an abundance of cool, luminous stars (e.g., TP-AGB). It is possible that the MW is unusual, or that our vantage point from within the MW or outside other galaxies unwittingly biases our dynamical measurements. Resolving this tension is basic to our understanding of how baryons settle and are processed in galaxies.

To begin this resolution, and to aid in connecting MW and MaNGA surveys, we will obtain standard MaNGA observations of 15 galaxies in the DiskMass Survey (DMS; Bershady et al. 2010a,b; Martinsson et al. 2013a) sample to achieve the following goals:

1. Test the predictions of stellar-population-synthesis models (e.g., Conroy et al. 2009; Wilkinson et al. 2015) against dynamical measurements of stellar mass-to-light ratios in both bulge- and disk-dominated regions of galaxies in the DMS. This analysis will combine existing DMS data with MaNGA spectroscopic measurements of the stellar content in spiral disks similar to the Milky Way. For the latter, we specifically ask, do we see spectroscopic signatures of an abundance of cool (TP-AGB) giant stars in galaxies with relatively young (light-weighted) stellar populations?
2. Calibrate stellar and gas line-of-sight velocity dispersions (σLOS) near and below MaNGA’s instrumental resolution for the purpose of measuring kinematics of dynamically cold systems.
3. Calibrate asymmetric drift, i.e., the lag of the stellar tangential speed relative to gas, as a surrogate for estimating stellar velocity dispersions in radial regions with intrinsic values that are well below the MaNGA instrumental resolution.

Our calibration efforts will extend MaNGA observations and measurements into the dynamically cold regime of MW-like disks, thereby connecting the high-resolution measurements of the MW from SEGUE and APOGEE-1/2 with the rich, statistical ensemble of the MaNGA sample.

DMS galaxies are nearly face-on, intermediate-type spirals. They are typically at lower redshifts than either the 1.5 Re or 2.5 Re MaNGA samples for their total stellar mass (I-band absolute magnitude). The 127-fiber MaNGA IFUs cover 0.5-1 Re, or 1 disk scale-length on average, providing ample coverage of the bulge and the inner disk regions. Based on our photometric decomposition of the light profiles (Martinsson et al. 2013a), the bulge/pseudo-bulge regions for this sample contribute less than 10% in surface brightness beyond 5-8 arcsec in radius. We will reassess this decomposition using the more sophisticated, spectral techniques developed by Johnston et al. (2012, 2014).

Existing fiber IFU data from the DiskMass Survey have spectral resolutions of (sigma) 12-16 km/s in the Mgb region near 510nm and the H-alpha region near 670 nm. The sample has a wealth of additional ancillary data including Spitzer 4.5, 8, 24, and 70 micron images and 21 cm aperture-synthesis maps with spatial resolutions of approximately 15 arcsec. The DiskMass Survey IFU data is unique for a sample of this size and for galaxies at an angular scale reasonably matched to MaNGA. The high spectral resolution enables direct measurements of the dynamical mass of the spiral disks by probing the line-of-sight velocity dispersion. MaNGA’s higher angular resolution will improve the DMS dynamical analysis of the inner-disk and bulge regions of these galaxies.

Critically, MaNGA is the only facility capable of providing the well-calibrated, broad wavelength range (360-1000 nm) spectra needed for a comparison the DiskMass dynamical mass estimates with detailed stellar-population-synthesis models, particularly given the red extension where the presence of cool stars, perhaps responsible for the dynamically-inferred low M/L, would be detected.

## Target Selection

Targets are selected from the DMS sample (Bershady et al. 2010ab; Martinsson et al. 2013ab) to meet the following criteria: (a) in the SDSS DR7 footprint; (b) have high-quality PPak stellar spectroscopy near 510 nm; (c) have high-quality SparsePak spectroscopy near 670 nm; (d) have Spitzer and HI aperture synthesis measurements; and (e) line-of-sight stellar velocity dispersions (sigma_los) at or less than the MaNGA instrumental resolution (65 km/s) at a radius of 10 arcsec. The latter criterion removes the few large-bulge galaxies for which we will not be able to probe the disk-dominated regions, given the MaNGA IFU spatial coverage. This yields a total of 15 galaxies.