The Hosts of Megamaser Disk Galaxies
We propose to obtain multi-wavelength imaging of a new sample of exquisite megamaser disk galaxies. Much like in NGC 4258, the megamasers in our sample trace sub-pc, circumnuclear gas in Keplerian rotation around the central supermassive black holes (BHs). We are thus able to derive remarkably precise BH masses in these systems. As the megamasers are found in spiral galaxies with BH masses ~10^7 M_sun, they offer a unique opportunity to study BH-bulge scaling relations in later-type galaxies. Existing ground-based imaging is inadequate to disentangle the many structural components (star clusters, bars, spirals, and dust lanes) found in the nuclei of these galaxies. Thus, we propose F336W, F438W, F814W, F110W, and F160W observations of these galaxies with WFC3 to simultaneously model the stellar populations and structures of their inner regions. With these data we will constrain BH-bulge scaling relations at low mass, study the relation between nuclear star clusters and supermassive BHs, and look for evidence of coincident star formation and accretion. The mass-to-light ratio profiles will provide important inputs to complementary stellar- and gas-dynamical modeling of the BH masses. The high-precision masses afforded by this sample of new megamaser galaxies provides a tremendous resource for our community that can only be adequately exploited with complementary HST imaging.
We take as our sample the nine galaxies with megamaser disks and VLBI observations in hand. With distances of 18-150 Mpc, the galaxies have apparent B-band magnitudes of 15.4-12.5 mag and apparent sizes ranging from 1-4 arcmin (~15 kpc at 50 Mpc).
Choice of Filters and Camera:
In order to disentangle, both spectrally and spatially, variations in stellar populations, dust absorption, and dust emission from the AGN, we require multiple filters. In the UV we take F336W (broad U), which allows us to probe the near UV part of the spectrum with reasonable throughput, while in the blue we choose F438W to avoid contamination from Hbeta emission. In the red we take F814W (broad I) because of its high throughput and to avoid contamination from emission lines such as [O III] 5007. In the near-infrared, we anticipate some contamination from dust heated by the active galaxy continuum in the F160W filter, while we expect galaxy light to dominate the F110W filter. We thus request both F110W and F160W to ensure that we have an AGN-free filter in the red to constrain the nuclear cluster mass. WFC3 is the camera of choice, as it will allow us to cover the entire wavelength range with the highest sensitivity and finest pixel scale.
Observational Plan:
Our exposure times are driven by a number of considerations. Firstly, to model the nuclear star cluster masses accurately, we require ~ 3% accuracy in our color measurements, requiring S/N>50 for the faintest expected cluster (V=21.8 mag). Secondly, we must be sensitive to classical bulges with muV ~ 22 mag/ sq. arcsec at ~ 5 arcsec, which is typically a factor of a few below the total surface brightness at these scales (Fig. 3). Finally, because we are interested in decoupling various non-axisymmetric components, such as bars, and to facilitate combining with ground-based data, we require S/N>5 at ~ 20arcsec scales (as faint as muV ~ 24.5 mag/ sq arcsec; Fig. 3) in our deepest filters (IH). These goals can be accomplished in two ~ 51 min orbits per galaxy.
In the UVIS, we will use a three-point dither pattern to facilitate cosmic-ray removal. In the IR, we will use the WFC3-IR-DITHER-BOX in order to properly sample the PSF. For filters with short exposures (F438W, F110W, F160W) we will use subarrays (2k$\times$2k and IRSUB512 respectively) to avoid buffer dumps.
In summary, the exposure times for each filter are as follows:
F336W: for nuclear cluster and bulge stellar populations, require S/N>50 for all nuclear clusters; 3 x 400 sec.
F438W: the same as F336W; 3 x 130 sec.
F814W: for galaxy structural measurements, requires S/N>5 at muV=24.5 mag/sq arcsec; 3 x 640 sec.
F110W: for stellar populations, dust, and structural measurements, requires S/N>5 at muV=24.5
mag/sq arcsec; 4 x 30 sec.
F160W: combination of AGN and nuclear star cluster at the center, also our deep near-infrared structural image; requires 4 x 100 sec.
We divide the exposures as follows. Orbit 1 will proceed as F110W (4 x 30 sec)-F438W (1 x 130 sec)-F336W
(3 x 400 sec)-F438 (2 x 130 sec). Since the first two filters use subarrays, we incur no buffer dumps, but spend 22 min on overheads (6 for guide-star acquisition, 12.4 min for readout, 2.5 min for dithering, and 1 min to change from IR to UVIS). Orbit 2 will be the remaining filters: F160W (4 x 100 sec)-F814W (3 x 690 sec). The long exposures in F814W allow us to read out the full array with no buffer dumps. It is worth noting that we are in danger of saturating in F814W, F110W, and F160W. In the former case we thus include a fourth 5 sec exposure. For the NIR filters, we use the cluster luminosity of NGC 4388 (F160W=14.8) as a guide. In order to
avoid saturation of this cluster, we take SPARS10, NSAMP=5 for F110W and SPARS25, NSAMP=8 for F160W. Using fewer samples increases the readnoise slightly, but this will not impact our bright targets. Finally, we note that there is some danger for a low-level jump in the background level for the F110W and F160W observations, which will hopefully be removed in software, but should not significantly impact our observations.
We request a total of 18 orbits to image the entire sample.
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