We investigate the lowest-mass quiescent galaxies known to exist in isolatedenvironments ($\mathrm{M^* = 10^{9.0-9.5} \ M_\odot}$; 1.5 Mpc from a moremassive galaxy). This population may represent the lowest stellar mass galaxiesin which internal feedback quenches galaxy-wide star formation. We presentKeck/ESI long-slit spectroscopy for 27 isolated galaxies in this regime: 20quiescent galaxies and 7 star-forming galaxies. We measure emission linestrengths as a function of radius and place galaxies on the Baldwin PhillipsTerlevich (BPT) diagram. Remarkably, 16 of 20 quiescent galaxies in our samplehost central AGN-like line ratios. Only 5 of these quiescent galaxies wereidentified as AGN-like in SDSS due to lower spatial resolution andsignal-to-noise. We find that many of the quiescent galaxies in our sample havespatially-extended emission across the non-SF regions of BPT-space. Whenconsidering only the central 1$^{\prime\prime}$, we identify a tightrelationship between distance from the BPT star-forming sequence and hostgalaxy stellar age as traced by $\mathrm{D_n4000}$, such that older stellarages are associated with larger distances from the star-forming locus. Ourresults suggest that the presence of hard ionizing radiation (AGN-like lineratios) is intrinsically tied to the quenching of what may be the lowest-massself-quenched galaxies.

The Milky Way's neutral hydrogen (HI) disk is warped and flared. However, adearth of accurate HI-based distances has thus far prevented the development ofan accurate Galactic disk model. Moreover, the extent to which our Galaxy'sstellar and gas disk morphologies are mutually consistent is also unclear.Classical Cepheids, primary distance indicators with distance accuracies of3-5%, offer a unique opportunity to develop an intuitive and accuratethree-dimensional picture. Here, we establish a robust Galactic disk modelbased on 1339 classical Cepheids. We provide strong evidence that the warp'sline of nodes is not oriented in the Galactic Center-Sun direction. Instead, itsubtends a mean angle of 17.5 \pm 1 (formal) \pm 3 (systematic) and exhibits aleading spiral pattern. Our Galaxy thus follows Briggs' rule for spiralgalaxies, which suggests that the origin of the warp is associated with torquesforced by the massive inner disk. The stellar disk traced by Cepheids followsthe gas disk in terms of their amplitudes; the stellar disk extends to at least20 kpc. This morphology provides a crucial, updated map for studies of thekinematics and archaeology of the Galactic disk.

Using integral field spectroscopic data of 24 nearby spiral galaxies obtainedwith the Multi-Unit Spectroscopic Explorer (MUSE), we derive empiricalcalibrations to determine the metallicity of the diffuse ionized gas (DIG)and/or of the low-ionisation emission region (LI(N)ER) in passive regions ofgalaxies. To do so, we identify a large number of HII--DIG/LIER pairs that areclose enough to be chemically homogeneous and we measure the metallicitydifference of each DIG/LIER region relative to its HII region companion whenapplying the same strong line calibrations. The O3N2 diagnostic ($=$log [([OIII]/H$\beta$)/([N II]/H$\alpha$)]) shows a minimal offset (0.01--0.04 dex)between DIG/LIER and HII regions and little dispersion of the metallicitydifferences (0.05 dex), suggesting that the O3N2 metallicity calibration forHII regions can be applied to DIG/LIER regions and that, when used on poorlyresolved galaxies, this diagnostic provides reliable results by sufferinglittle from DIG contamination. We also derive second-order corrections whichfurther reduce the scatter (0.03--0.04 dex) in the differential metallicity ofHII-DIG/LIER pairs. Similarly, we explore other metallicity diagnostics such asO3S2 ($=$log([O III]/H$\beta$+[S II]/H$\alpha$)) and N2S2H$\alpha$ ($=$ log([NII]/[S II]) + 0.264log([N II]/H$\alpha$)) and provide corrections for O3S2 tomeasure the metallicity of DIG/LIER regions. We propose that the corrected O3N2and O3S2 diagnostics are used to measure the gas-phase metallicity in quiescentgalaxies or in quiescent regions of star-forming galaxies.

The distributions of the pairwise line-of-sight velocity between galaxies andtheir host clusters are segregated according to the galaxy's colour andmorphology. We investigate the velocity distribution of red-spiral galaxies,which represents a rare population within galaxy clusters. We find that theprobability distribution function of the pairwise line-of-sight velocity$v_{\rm{los}}$ between red-spiral galaxies and galaxy clusters has a dip at$v_{\rm{los}} = 0$, which is a very odd feature, at 93\% confidence level. Tounderstand its origin, we construct a model of the phase space distribution ofgalaxies surrounding galaxy clusters in three-dimensional space by usingcosmological $N$-body simulations. We adopt a two component model that consistsof the infall component, which corresponds to galaxies that are now fallinginto galaxy clusters, and the splashback component, which corresponds togalaxies that are on their first (or more) orbit after falling into galaxyclusters. We find that we can reproduce the distribution of the line-of-sightvelocity of red-spiral galaxies with the dip with a very simple assumption thatred-spiral galaxies reside predominantly in the infall component, regardless ofthe choice of the functional form of their spatial distribution. Our resultsconstrain the quenching timescale of red-spiral galaxies to a few Gyrs, and thetime when the morphological transformation finishes to $\sim$1.4 Gyr after thequenching completes.