Low surface brightness galaxies (LSBGs) represent a significant percentage oflocal galaxies but their formation and evolution remain elusive. They may holdcrucial information for our understanding of many key issues (i.e., census ofbaryonic and dark matter, star formation in the low density regime, massfunction). The most massive examples - the so called giant LSBGs - can be asmassive as the Milky Way, but with this mass being distributed in a much largerdisk. Malin 1 is an iconic giant LSBG, perhaps the largest disk galaxy known.We attempt to bring new insights on its structure and evolution on the basis ofnew images covering a wide range in wavelength. We have computed surfacebrightness profiles (and average surface brightnesses in 16 regions ofinterest), in six photometric bands (FUV, NUV, u, g, i, z). We compared thesedata to various models, testing a variety of assumptions concerning theformation and evolution of Malin 1. We find that the surface brightness andcolor profiles can be reproduced by a long and quiet star-formation history dueto the low surface density; no significant event, such as a collision, isnecessary. Such quiet star formation across the giant disk is obtained in adisk model calibrated for the Milky Way, but with an angular momentumapproximately 20 times larger. Signs of small variations of the star-formationhistory are indicated by the diversity of ages found when different regionswithin the galaxy are intercompared.For the first time, panchromatic images ofMalin 1 are used to constrain the stellar populations and the history of thisiconic example among giant LSBGs. Based on our model, the extreme disk of Malin1 is found to have a long history of relatively low star formation (about 2Msun/yr). Our model allows us to make predictions on its stellar mass andmetallicity.

We present the unexpected discovery of four ultra diffuse galaxies (UDGs) ina group environment. We recently identified seven extremely low surfacebrightness galaxies in the vicinity of the spiral galaxy M101, using data fromthe Dragonfly Telephoto Array. The galaxies have effective radii of $10"-38"$and central surface brightnesses of $25.6-27.7$ mag arcsec$^{-2}$ in g-band. Wesubsequently obtained follow-up observations with $HST$ to constrain thedistances to these galaxies. Four remain persistently unresolved even with thespatial resolution of $HST$/ACS, which implies distances of $D > 17.5$ Mpc. Weshow that the galaxies are most likely associated with a background group at$\sim 27$ Mpc containing the massive ellipticals NGC 5485 and NGC 5473. At thisdistance, the galaxies have sizes of $2.6-4.9$ kpc, and are classified as UDGs,similar to the populations that have been revealed in clusters such as Coma,Virgo and Fornax, yet even more diffuse. The discovery of four UDGs in a galaxygroup demonstrates that the UDG phenomenon is not exclusive to clusterenvironments. Furthermore, their morphologies seem less regular than those ofthe cluster populations, which may suggest a different formation mechanism orbe indicative of a threshold in surface density below which UDGs are unable tomaintain stability.

We examine ultraviolet halos around a sample of highly inclined galaxieswithin 25 Mpc to measure their morphology and luminosity. Despite contaminationfrom galactic light scattered into the wings of the point-spread function, wefind that UV halos occur around each galaxy in our sample. Around most galaxiesthe halos form a thick, diffuse disk-like structure, but starburst galaxieswith galactic superwinds have qualitatively different halos that are moreextensive and have filamentary structure. The spatial coincidence of the UVhalos above star-forming regions, the lack of consistent association withoutflows or extraplanar ionized gas, and the strong correlation between thehalo and galaxy UV luminosity suggest that the UV light is an extragalacticreflection nebula. UV halos may thus represent 1-10 million solar masses ofdust within 2-10 kpc of the disk, whose properties may change with height instarburst galaxies.

Aiming at deriving a statistically well-justified Galactic Center distance,$R_0$, and reducing any occurrence of publication bias, we compiled the mostcomprehensive and most complete database of Galactic Center distances availableto date, containing 273 new or revised $R_0$ estimates published since recordsbegan in October 1918 until June 2016. We separate our $R_0$ compilation intodirect and indirect distance measurements. The latter include a large body ofestimates that rely on centroid determinations for a range of tracerpopulations as well as measurements based on kinematic observations of objectsat the solar circle, combined with a mass and/or rotational model of the MilkyWay. Careful assessment of the Galactic Center distances resulting from orbitalmodeling and statistical parallax measurements in the Galactic nucleus yieldsour final Galactic Center distance recommendation of $R_0 = 8.3 \pm 0.2 \mbox{(statistical)} \pm 0.4 \mbox{ (systematic)}$ kpc. The centroid-based distancesare in good agreement with this recommendation. Neither the direct measurementsnor the post-1990 centroid-based distance determinations suggest thatpublication bias may be important. The kinematics-based distance estimates areaffected by significantly larger uncertainties, but they can be used toconstrain the Galaxy's rotation velocity at the solar Galactocentric distance,$\Theta_0$. Our results imply that the International AstronomicalUnion-recommended Galactic Center distance ($R_0^{\rm IAU} = 8.5$ kpc) needs adownward adjustment, while its $\Theta_0$ recommendation ($\Theta_0 = 220$ kms$^{-1}$) requires a substantial upward revision.