Nelson Caldwell

We present high-resolution profiles of the H α , [N II ], and [O III ] lines of supernova remnants (SNRs) in M33. The 8 km s ⁻¹ resolution allows us to separate emission from the photoionized shock precursors from the emission from shocked gas. We find that there is considerable scatter in the ratios of precursor intensity to shock intensity and that the precursors account for up to 26% of the total H α emission in SNRs in our sample and 35% of the total [N II ] and [O III ] emission. This is generally similar to what is predicted by steady-state precursor models from MAPPINGS for the shock speeds in the 150–180 km s ⁻¹ range, but it is unlikely that all the observed SNRs have shock speeds in this narrow interval. The ratios of precursor to shock emission in H α indicate faster shocks (though selection effects may play a role). The discrepancies between observed and model ratios of precursor to shock emission may be because the precursors are not in steady state, because our fibers select only a portion of the SNRs, so that accidents of geometry can affect the ratio of precursor to shocked emission that we see, or because the plane-parallel MAPPINGS models do not account for geometrical dilution of the radiation field outside of the SNR. Regardless of the modeling issues, these observations point out that precursor emission needs to be taken into account when analyzing spectra obtained on SNRs, especially in galaxies more distant than the local group.

Compact elliptical (cE) galaxies are compact stellar systems with stellar masses of 10 ⁸ ≤ M _* / M _⊙ ≤ 10 ¹⁰ and radii typically g -band Coma images and validate our photometric measurements through comparison with Hubble Space Telescope data. We also analyze spectroscopic data from the Dark Energy Spectroscopic Instrument. We significantly expand the known cE population in the Coma cluster through joint photometric and spectroscopic selection. We identify a subpopulation of cEs that likely formed in infalling groups, through their association with host galaxies, their positions on the caustic diagram, and their projected phase-space trajectories. We estimate that the central cE population will increase by up to 30% within the next 0.4 Gyr, highlighting the important role of preprocessing in cE evolution.

We assess the diagnostic for shock wave ram pressure, ρ V s 2 ∝ I [ O III ] V Dopp , and apply it to high-resolution multifiber spectra of the Cygnus Loop. We find that, while there are many caveats, and while high-resolution profiles are important for controlling some sources of error, the diagnostic is reliable if one selects positions with simple line profiles that are offset away from zero velocity. The Cygnus Loop shows a range of conditions from ≈400 km s⁻¹ nonradiative shocks that produce X-rays to ≈100 km s⁻¹ radiative shocks that produce bright optical emission. We find a range of ram pressures in agreement with previous estimates from [O iii] and from X-ray measurements. The ram pressure is also consistent with recent analysis of the evolutionary stage of the Cygnus Loop.

A subset of galaxies have dense nuclei, and when these galaxies are accreted and tidally stripped, the nuclei can masquerade as globular clusters in the halos of large galaxies. If these nuclei contain massive central black holes, some may accrete gas and become observable as active galactic nuclei. Previous studies have found that candidate stripped nuclei rarely host luminous X-ray sources, but these studies were typically restricted to both the most massive candidate nuclei and the most luminous X-ray sources. Here we use new and archival Chandra and XMM-Newton data to search for X-ray emission in a near-complete sample of massive globular clusters and candidate stripped nuclei in the nearest accessible elliptical galaxy, NGC 5128. This sample has the unique advantage that the candidate stripped nuclei are identified dynamically via elevated mass-to-light ratios (M/L _V ). Our central result is that 5/22 ( 2 3 − 6 + 11 %) of the candidate stripped nuclei have X-ray sources down to a typical limit of L _X ∼ 5 × 10³⁶ erg s⁻¹, a fraction lower than or comparable to that among massive clusters with normal M/L _V (16/41; 3 9 − 7 + 8 %). Hence we confirm and extend the result that nearly all X-ray sources in stripped nuclei are likely to be X-ray binaries rather than active galactic nuclei. If the candidate stripped nuclei have black holes of typical masses ∼2 × 10⁵ M _⊙ needed to explain their elevated M/L _V , then they have typical Eddington ratios of ≲ 2 × 10⁻⁶. This suggests that it will be challenging to conduct an accretion census of wandering black holes around even nearby galaxies.

With a star formation rate of order 0.4 M _⊙ yr⁻¹, M31 should have a significant population of supernova remnants (SNRs) and, in fact, 156 SNRs and SNR candidates have been suggested by J. H. Lee & M. G. Lee, by searching for nebulae with elevated [S ii]/Hα ratios in narrowband images. Here, we use a combination of low- and high-resolution optical spectroscopy obtained with Hectospec on the MMT to characterize 152 of these nebulae. Of these candidates, we find 93 nebulae that have [S ii]/Hα ratios that exceed 0.4, the traditional ratio used to separate SNRs from H ii regions, strongly suggesting that at least these objects are SNRs. Our high-resolution spectroscopy reveals 108 nebulae that have velocity widths in Hα (full width at 20% peak flux) that exceed 50 km s⁻¹, significantly larger than found in H ii regions. There are 72 objects that satisfy both tests. Here, we discuss the spectroscopic characteristics of all of the objects in our sample, and the likelihood that other objects in the sample of J. H. Lee & M. G. Lee are also SNRs, and we briefly consider confirmation by X-ray, radio, and UV observations. We also discuss several new candidates that have been identified serendipitously in the course of examining a large amount of archival Hectospec data.

In the absence of a parallax distance to a pulsar or a surviving binary in a supernova remnant (SNR), distances to Galactic SNRs are generally very uncertain. However, by combining Gaia data with wide-field, multifiber echelle spectroscopy, it is now possible to obtain accurate distances to many SNRs with limited extinction by searching for the appearance of high-velocity Ca ii or Na i absorption lines in hot stars as a function of distance. We demonstrate this for the SNR S147 using the spectra of 259 luminous blue stars. We obtain a median distance of 1.37 kpc (1.30–1.47 kpc at 90% confidence), which is consistent with the median parallax distance to the pulsar of 1.46 kpc (1.12–2.10 kpc at 90% confidence) but with significantly smaller uncertainties. Our distance is also consistent with the distance to the candidate unbound binary companion in this SNR, HD 37424 at a photogeometric distance of 1.45 kpc (1.40–1.50 kpc at 1σ). The presence of high-velocity absorption lines is correlated with the Hα/O [iii] emission-line flux of the SNR but not with the radio flux.

We have conducted a systematic search around the Milky Way (MW) analog NGC 253 (D = 3.5 Mpc), as a part of the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS)—a Magellan+Megacam survey to identify dwarfs and other substructures in resolved stellar light around MW-mass galaxies outside of the Local Group. In total, NGC 253 has five satellites identified by PISCeS within 100 kpc with an absolute V-band magnitude of M _V < −7. We have additionally obtained deep Hubble Space Telescope imaging of four reported candidates beyond the survey footprint: Do III, Do IV, and dw0036m2828 are confirmed to be satellites of NGC 253, while SculptorSR is found to be a background galaxy. We find no convincing evidence for the presence of a plane of satellites surrounding NGC 253. We construct its satellite luminosity function, which is complete down to M _V ≲ −8 out to 100 kpc and M _V ≲ −9 out to 300 kpc, and compare it to those calculated for other Local Volume galaxies. Exploring trends in satellite counts and star-forming fractions among satellite systems, we find relationships with host stellar mass, environment, and morphology, pointing to a complex picture of satellite formation, and a successful model has to reproduce all of these trends.