Mark A. Norris

We catalog active galactic nuclei (AGN) in the volume- and mass-limited Resolved Spectroscopy of a Local Volume (RESOLVE) and Environmental Context (ECO) surveys using (i) the BPT plot with standard and alternate demarcations, (ii) the BPT plot and Veilleux– Osterbrock plots (using [O I ] or [S II ]) to find “SF-AGN” with mixed star-forming and AGN classifications, and (iii) mid-IR WISE color selection. Dividing RESOLVE+ECO into dwarf, transitional, and giant galaxies by baryonic mass, we find AGN occupation fractions of 2%–5%, 16%–27%, and 20%–48%, respectively, indicating a steep ∼5× increase across baryonic mass ∼10 ^9.7 M _⊙ . These percentages are computed relative to all galaxies in each mass division, including those not searchable for AGN, with numerical ranges representing large systematics between crossmatched emission-line measurements from different Sloan Digital Sky Survey (SDSS) catalogs. Stellar-mass divisions yield similar results. Adding (incomplete) archival X-ray/broadline AGN detections, the dwarf AGN occupation fraction reaches 3%–6%. WISE AGN are detected in <1% of galaxies at all masses with optimized WISE photometry. We show that standard WISE catalog photometry yields high rates of both false positives and false negatives. AGN demographics shift with mass due to correlations with gas content, star formation, and group environment, forming a continuum: SF-AGN at the high-SF end, then “Bonus AGN” (defined by alternate literature BPT demarcations), then Composites, and finally Conventional AGN at the low-SF end. Typical (metal-poor, star-forming) dwarfs most often host SF-AGN, whereas typical transitional galaxies most often host Bonus AGN and Composites. We release homogenized SDSS line measurements, reprocessed WISE data, and UV+mid-IR star formation rates for RESOLVE and ECO.

Most galaxies follow a well-defined scaling relation between metallicity and stellar mass; however, some outliers at the low-mass end of the observed galaxy population exhibit unusually high metallicity for their mass. Understanding how these objects get to be so metal-rich is vital for understanding the role of feedback in galaxy formation. Using the TNG50 simulation, we explore the origins of this phenomenon. We identify 227 metal-rich, compact stellar systems (CSSs) that deviate significantly from this scaling relation. These CSSs are satellites located in the vicinity of massive host galaxies, with stellar masses ranging from 10⁸ to 10¹⁰ M _⊙ (including six systems that are close analogs of the M31–M32 system). Contrary to the previously assumed scenario that such objects are predominantly products of tidal stripping, we find that more often ram pressure plays a major role in their formation. Indeed, 76% (173) of these CSSs are formed through a burst of star formation occurring around the time of the first pericentric passage, typically at redshifts z ≲ 1, aided by strong ram pressure and tidal forces. The high ram pressure, resulting from the CSSs’ rapid motion near the host halo center, facilitates metal enrichment, producing high-metallicity CSSs by confining the metal-rich gas from bursty star formation, which leads to distinct stellar populations characterized by enhanced metallicity and high α-abundance. The other 24% (54) of metal-rich CSSs are generated through the tidal stripping of massive progenitors. Our results further indicate that M32 is more likely to have formed through intense star formation events rather than through gradual tidal stripping.

Intermediate-mass black holes (IMBHs) are expected to exist in globular clusters (GCs) and compact stellar systems (CSSs) in general, but none have been conclusively detected. Tidal disruption events (TDEs), where a star is tidally disrupted by the gravitational field of a black hole, have been observed to occur around the supermassive black holes (SMBHs) found at the centres of galaxies, and should also arise around IMBHs, especially in the dense stellar cores of CSSs. However, to date none have been observed in such environments. Using data from the Zwicky Transient Facility, we search for TDEs associated with CSSs, but none are found. This non-detection allows us to set an upper limit on the TDE rate in CSSs of |$n_\text{TDE,Total}\lessapprox 10^{-7} \, \mathrm{ CSS}^{-1}\, \text{yr}^{-1}$|⁠, which is 2 dex below the observed TDE rate involving SMBHs interacting with 1 M_⊙ main-sequence stars in the nuclei of massive galaxies. We also consider ultracompact dwarfs (UCDs) formed through a tidal stripping process in the surveyed volume. On the assumption that these CSSs contain SMBHs and TDE rates are comparable to current observed optical rates in galactic nuclei (⁠|$\approx 3.2 \times 10^{-5}\, \text{gal}^{-1}\, \text{yr}^{-1}$|⁠), we determine an upper limit for the number of UCDs formed through a tidal stripping process in the surveyed volume to be N_GC,Strip < 1.4 × 10⁴, which we estimate represents |$\lt 6~{{\ \rm per\ cent}}$| of the population of GCs >10⁶ M_⊙.

We present a four-step group-finding algorithm for the Gas in Galaxy Groups (G3) initiative, a spin-off of the z ∼ 0 REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntext (ECO) surveys. In preparation for future comparisons to intermediate redshift (e.g., the LADUMA survey), we design the group finder to adapt to incomplete, shallow, or nonuniform data. We use mock catalogs to optimize the group finder’s performance. Compared to friends-of-friends (with false-pair splitting), the G3 algorithm offers improved completeness and halo-mass recovery with minimal loss of purity. Combining it with the volume-limited H i census data for RESOLVE and ECO, we examine the H i content of galaxy groups as a function of group halo mass. Group-integrated H i mass M _{H I,grp} rises monotonically over halo masses M _halo ∼ 10¹¹–10^14.5 M _⊙, pivoting in slope at M _halo ∼ 10^11.4 M _⊙, the gas-richness threshold scale. We present the first measurement of the scatter in this relation, which has a median of ∼0.3 dex and is asymmetric toward lower M _{H I,grp}. We discuss interesting tensions with theoretical predictions and prior measurements of the M _{H I,grp}–M _halo relation. In an appendix, we release RESOLVE DR4 and ECO DR3, including updates to survey redshifts, photometry, and group catalogs, as well as a major expansion of the ECO H i inventory with value-added data products.

Automotive Noise Vibration and Harshness (NVH) continues to be critical to address in passenger vehicles, especially as new vehicles today emphasize the use of lighter weight materials that create even more challenging NVH issues. NVH can negatively impact drivability and comfort, as well as perceived vehicle quality. Moreover, with the development of increased driver ergonomic technologies and capabilities (audible or otherwise), NVH continues to be key in allowing these technologies to be fully effective. Both passive and active control technologies are used extensively to improve automotive NVH. The use of active noise and vibration control has become a standard solution approach in vehicles, especially to mitigate low-frequency noise and vibration inside the vehicle. Linear actuators have been used in active engine mounts as well as frame vibration control to mitigate passenger cabin NVH. This paper introduces a new lightweight approach for active vibration control for automotive applications. Controllable Circular Force Generator (CFG) technology is introduced to displace the use of larger, heavy linear electromagnetic technology, and is shown to provide improved performance with lower weight and cost.

The short-lived buckling instability is responsible for the formation of at least some box/peanut (B/P) shaped bulges, which are observed in most massive, z = 0, barred galaxies. Nevertheless, it has also been suggested that B/P bulges form via the slow trapping of stars onto vertically extended resonant orbits. The key difference between these two scenarios is that when the bar buckles, symmetry about the mid-plane is broken for a period of time. We use a suite of simulations (with and without gas) to show that when the buckling is sufficiently strong, a residual mid-plane asymmetry persists for several Gyrs after the end of the buckling phase, and is visible in simulation images. On the other hand, images of B/P bulges formed through resonant trapping and/or weak buckling remain symmetric about the mid-plane. We develop two related diagnostics to identify and quantify mid-plane asymmetry in simulation images of galaxies that are within 3^○ of edge-on orientation, allowing us to test whether the presence of a B/P-shaped bulge can be explained by a past buckling event. We apply our diagnostics to two nearly edge-on galaxies with B/P bulges from the Spitzer Survey of Stellar Structure in Galaxies, finding no mid-plane asymmetry, implying these galaxies formed their bulges either by resonant trapping or by buckling more than ∼5 Gyr ago. We conclude that the formation of B/P bulges through strong buckling may be a rare event in the past ∼5 Gyr.

Existing star-forming vs. active galactic nucleus (AGN) classification schemes using optical emission-line diagnostics mostly fail for low-metallicity and/or highly star-forming galaxies, missing AGN in typical z ∼ 0 dwarfs. To recover AGN in dwarfs with strong emission lines (SELs), we present a classification scheme optimizing the use of existing optical diagnostics. We use Sloan Digital Sky Survey emission-line catalogs overlapping the volume- and mass-limited REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntex (ECO) surveys to determine the AGN percentage in SEL dwarfs. Our photoionization grids show that the [O iii]/Hβ versus [S ii]/Hα diagram (S ii plot) and [O iii]/Hβ versus [O i]/Hα diagram (O i plot) are less metallicity sensitive and more successful in identifying dwarf AGN than the popular [O iii]/Hβ versus [N ii]/Hα diagnostic (N ii plot or “BPT diagram”). We identify a new category of “star-forming AGN” (SF-AGN) classified as star-forming by the N ii plot but as AGN by the S ii and/or O i plots. Including SF-AGN, we find the z ∼ 0 AGN percentage in dwarfs with SELs to be ∼3%–16%, far exceeding most previous optical estimates (∼1%). The large range in our dwarf AGN percentage reflects differences in spectral fitting methodologies between catalogs. The highly complete nature of RESOLVE and ECO allows us to normalize strong emission-line galaxy statistics to the full galaxy population, reducing the dwarf AGN percentage to ∼0.6%–3.0%. The newly identified SF-AGN are mostly gas-rich dwarfs with halo mass 11.5 M _⊙, where highly efficient cosmic gas accretion is expected. Almost all SF-AGN also have low metallicities (Z ≲ 0.4 Z _⊙), demonstrating the advantage of our method.

Galaxy mergers are instrumental in dictating the final mass, structure, stellar populations, and kinematics of galaxies. Cosmological galaxy simulations indicate that the most massive galaxies at z = 0 are dominated by high fractions of ‘ex-situ’ stars, which formed first in distinct independent galaxies, and then subsequently merged into the host galaxy. Using spatially resolved MUSE spectroscopy we quantify and map the ex-situ stars in thirteen massive early-type galaxies. We use full spectral fitting together with semi-analytic galaxy evolution models to isolate the signatures in the galaxies’ light which are indicative of ex-situ populations. Using the large MUSE field of view we find that all galaxies display an increase in ex-situ fraction with radius, with massive and more extended galaxies showing a more rapid increase in radial ex-situ fraction (reaching values between ∼30 per cent and 100 per cent at 2 effective radii) compared to less massive and more compact sources (reaching between ∼5 per cent and 40 per cent ex-situ fraction within the same radius). These results are in line with predictions from theory and simulations which suggest ex-situ fractions should increase significantly with radius at fixed mass for the most massive galaxies.