arXiv subjects

Knox S. Long

Publications and source records attributed to Knox S. Long.

At least 37 records · Page 2Linked to original sources

arXiv2024

The SDSS-V Local Volume Mapper (LVM): Scientific Motivation and Project Overview

We present the Sloan Digital Sky Survey V (SDSS-V) Local Volume Mapper (LVM). The LVM is an integral-field spectroscopic survey of the Milky Way, Magellanic Clouds, and of a sample of local volume galaxies, connecting resolved pc-scale individual sources of feedback to kpc-scale ionized interstellar medium (ISM) properties. The 4-year survey covers the southern Milky Way disk at spatial resolutions of 0.05 to 1 pc, the Magellanic Clouds at 10 pc resolution, and nearby large galaxies at larger scales totaling $>4300$ square degrees of sky, and more than 55M spectra. It utilizes a new facility of alt-alt mounted siderostats feeding 16 cm refractive telescopes, lenslet-coupled fiber-optics, and spectrographs covering 3600-9800A at R ~ 4000. The ultra-wide field IFU has a diameter of 0.5 degrees with 1801 hexagonally packed fibers of 35.3 arcsec apertures. The siderostats allow for a completely stationary fiber system, avoiding instability of the line spread function seen in traditional fiber feeds. Scientifically, LVM resolves the regions where energy, momentum, and chemical elements are injected into the ISM at the scale of gas clouds, while simultaneously charting where energy is being dissipated (via cooling, shocks, turbulence, bulk flows, etc.) to global scales. This combined local and global view enables us to constrain physical processes regulating how stellar feedback operates and couples to galactic kinematics and disk-scale structures, such as the bar and spiral arms, as well as gas in- and out-flows.

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arXiv2023

State-of-the-art simulations of line-driven accretion disc winds: realistic radiation-hydrodynamics leads to weaker outflows

Disc winds are a common feature in accreting astrophysical systems on all scales. In active galactic nuclei (AGN) and accreting white dwarfs (AWDs), specifically, radiation pressure mediated by spectral lines is a promising mechanism for driving these outflows. Previous hydrodynamical simulations have largely supported this idea, but relied on highly approximate treatments of ionization and radiative transfer. Given the sensitivity of line driving to the ionization state and radiation field in the outflow, here we present a new method for carrying out 2.5D radiation-hydrodynamic simulations that takes full account of the frequency-dependent radiative transfer through the wind, the corresponding ionization state and the resulting radiative accelerations. Applying our method to AWDs, we find that it is much harder to drive a powerful line-driven outflow when the interaction between matter and radiation is treated self-consistently. This conclusion is robust to changes in the adopted system parameters. The fundamental difficulty is that discs luminous enough to drive such a wind are also hot enough to over-ionize it. As a result, the mass-loss rates in our simulations are much lower than those found in earlier, more approximate calculations. We also show that the ultraviolet spectra produced by our simulations do not match those observed in AWDs. We conclude that, unless the over-ionization problem can be mitigated (e.g. by sub-grid clumping or a softer-than-expected radiation field), line driving may not be a promising mechanism for powering the outflows from AWDs. These conclusions are likely to have significant implications for disc winds in AGN also.

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arXiv2023

Supernova Remnants in the Irregular Galaxy NGC4449

The nearby irregular galaxy NGC4449 has a star formation rate of about 0.4 solar masses/yr and should host of order 70 SNRs younger than 20,000 years, a typical age for SNRs expanding into to an ISM with unit density to reach the radiative phase. We have carried out an optical imaging and spectroscopic survey in an attempt to identify these SNRs. This task is challenging because diffuse gas with elevated ratios of [SII]:H-alpha is omnipresent in NGC4449, {causing confusion when using this common diagnostic for SNRs. Using narrow-band interference-filter images, we first identified 49 objects that have elevated [SII]:H-alpha ratios compared to nearby HII regions. Using Gemini-N and GMOS, we then obtained high-resolution spectra of 30 of these SNR candidates, 25 of which have [SII]:H-alpha ratios greater than 0.5. Of these, 15 nebulae are almost certainly SNRs, based on a combination of characteristics: higher [OI]:H-alpha ratios and broader line widths than observed from HII regions. The remainder are good candidates as well, but need additional confirmation. Surprisingly, despite having superior imaging and spectroscopic data sets to examine, we are unable to confirm most of the candidates suggested by Leonidaki (2013). While NGC4449 is likely an extreme case because of the high surface brightness and elevated [SII]:H-alpha ratio of diffuse gas, it highlights the need for sensitive high-resolution optical spectroscopy, or high spatial resolution radio or X-ray observations that can ensure accurate SNR identifications in external galaxies.

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arXiv2023

A disc wind model for blueshifts in quasar broad emission lines

Blueshifts - or, more accurately, blue asymmetries - in broad emission lines such as CIV $λ$1550 are common in luminous quasars and correlate with fundamental properties such as Eddington ratio and broad absorption line (BAL) characteristics. However, the formation of these blueshifts is still not understood, and neither is their physical connection to the BAL phenomenon or accretion disc. In this work, we present Monte Carlo radiative transfer and photoionization simulations using parametrized biconical disc-wind models. We take advantage of the azimuthal symmetry of a quasar and show that we can reproduce CIV blueshifts provided that (i) the disc-midplane is optically thick out to radii beyond the line formation region, so that the receding wind bicone is obscured; and (ii) the system is viewed from relatively low (that is, more face-on) inclinations ($\lesssim40^\circ$). We show that CIV emission line blueshifts and BALs can form in the same wind structure. The velocity profile of the wind has a significant impact on the location of the line formation region and the resulting line profile, suggesting that the shape of the emission lines can be used as a probe of wind-driving physics. While we are successful at producing blueshifts/blue asymmetries in outflows, we struggle to match the detailed shape or skew of the observed emission line profiles. In addition, our models produce redshifted emission-line asymmetries for certain viewing angles. We discuss our work in the context of the CIV $λ$1550 emission blueshift versus equivalent-width space and explore the implications for quasar disc wind physics.

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arXiv2022

High-Resolution Spectra of Supernova Remnants in M83

In order to better characterize the rich supernova remnant (SNR) population of M83 (NGC 5236), we have obtained high-resolution (about 85 km/s) spectra of 119 of the SNRs and SNR candidates in M83 with Gemini/GMOS, as well as new spectra of the young SNRs B12-174a and SN1957D. Most of the SNRs and SNR candidates have [S II]:Hα ratios that exceed 0.4. Combining these results with earlier studies we have carried out with MUSE and at lower spectroscopic resolution with GMOS, we have confirmed a total of 238 emission nebulae to be SNRs on the basis of their [S II]:Hα ratios, about half of which have emission lines that show velocity broadening greater than 100 km/s, providing a kinematic confirmation that they are SNRs and not H II regions. Looking at the entire sample, we find a strong correlation between velocity widths and the line ratios of [O I]λ6300:Hα, [N II]λ6584:Hα and [S II]λλ6716,6731:Hα. The density-sensitive [S II]λ6716:λ6731 line ratio is strongly correlated with SNR diameter, but not with the velocity width. We discuss these results in the context of previously published shock models.

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arXiv2022

Stellar feedback in M83 as observed with MUSE -- II. Analysis of the HII region population: ionisation budget and pre-SN feedback

We study pre-supernova feedback in a sample of $\sim$ 4700 HII regions in the nearby spiral galaxy M83, identified on their H$α$ emission. We pectroscopically identify Wolf-Rayet (WR) stars populating the star-forming regions. For each HII region, we compute the pressure of ionised gas ($P_{\rm ion}$) and the direct radiation pressure ($P_{\rm dir}$) acting in the region, and investigate how they vary with galactocentric distance, with the physical properties of the region, and with the pressure of the galactic environment ($P_\mathrm{DE}$). For a subset of $\sim$ 500 regions, we also investigate the link between the pressure terms and the properties of the cluster population (age, mass, and LyC flux). We find that $P_{\rm ion}$ dominates over $P_{\rm dir}$ by at least a factor of 10 on average over the disk. Both pressure terms are strongly enhanced and become almost comparable in the central starburst region. In the disk ($R \geq 0.15~R_e$), we observe that $P_{\rm dir}$ stays approximately constant with galactocentric distance. $P_{\rm dir}$ is positively correlated with an increase in radiation field strength (linked to the negative metallicity gradient in the galaxy), while it decreases in low extinction regions. $P_{\rm ion}$ decreases constantly for increasing galactocentric distances. In general, we observe that HII regions near the center are underpressured with respect to their surroundings, whereas regions in the disk are overpressured and hence expanding. We find that regions hosting younger clusters or having more mass in young star clusters have a higher internal pressure, indicating that clustered star formation is likely playing a dominant role in setting the pressure. Finally, we estimate that only 13 % of HII regions hosting young clusters and WR stars have $f_{\rm esc} \geq 0$.[Abridged]

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arXiv2022

Far-ultraviolet investigation into the galactic globular cluster M30 (NGC 7099): II. Potential X-ray counterparts and variable sources

We present a far-ultraviolet (FUV) study of the globular cluster M30 (NGC 7099). The images were obtained using the Advanced Camera for Surveys (ACS/SBC, F150LP, FUV) and the Wide Field Planetary Camera 2 (WFPC2, F300W, UV) on board the Hubble Space Telescope (HST). We compare the catalogue of FUV objects to ten known X-ray sources and find six confident matches of two cataclysmic variables (CVs), one RS CVn, one red giant with strong FUV emission and two sources only detected in the FUV. We also searched for variable sources in our dataset and found a total of seven blue stragglers (BSs), four horizontal branch (HB) stars, five red giant branch stars, 28 main sequence stars and four gap objects that demonstrated variability. One BS star is a known W-UMa contact binary, one of the gap objects is a known CV identified in this work to be a dwarf nova, and the three other gap sources are weak variables. The periods and positions of two of the variable HB stars match them to two previously known RR Lyrae variables of types RRab and RRc.

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arXiv2022

Supernova Remnants in M83 as Observed with MUSE

Here we describe a new study of the SNRs and SNR candidates in nearby face-on spiral galaxy M83, based primarily on MUSE integral field spectroscopy. Our revised catalog of SNR candidates in M83 has 366 objects, 81 of which are reported here for the first time. Of these, 229 lie within the MUSE observation region, 160 of which have spectra with [SII]:Halpha ratios exceeding 0.4, the value generally accepted as confirmation that an emission nebula is shock-heated. Combined with 51 SNR candidates outside the MUSE region with high [SII]:Halpha ratios, there are 211 spectroscopically-confirmed SNRs in M83, the largest number of confirmed SNRs in any external galaxy. MUSE's combination of relatively high spectral resolution and broad wavelength coverage has allowed us to explore two other properties of SNRs that could serve as the basis of future SNR searches. Specifically, most of the objects identified as SNRs on the basis of [SII]:Halpha ratios exhibit more velocity broadening and lower ratios of [SIII]:[SII] emission than HII regions. A search for nebulae with the very broad emission lines expected from young, rapidly expanding remnants revealed none, except for the previously identified B12-174a. The SNRs identified in M83 are, with few exceptions, middle-aged ISM-dominated ones. Smaller diameter candidates show a larger range of velocity broadening and a larger range of gas densities than the larger diameter objects, as expected if the SNRs expanding into denser gas brighten and then fade from view at smaller diameters than those expanding into a more tenuous ISM

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arXiv2022

Far-ultraviolet investigation into the galactic globular cluster M30 (NGC 7099): I. Photometry and radial distributions

We present a far-ultraviolet (FUV) study of the globular cluster M30 (NGC 7099). The images were obtained using the Advanced Camera for Surveys (ACS/SBC, F150LP, FUV) and the Wide Field Planetary Camera 2 (WFPC2, F300W, UV) which were both on board the Hubble Space Telescope (HST). The FUV-UV colour-magnitude diagram (CMD) shows a main sequence (MS) turnoff at FUV $\approx$ 22 mag and FUV-UV $\approx$ 3 mag. The MS extends 4 mag below the turnoff, and a prominent horizontal branch (HB) and blue straggler (BS) sequence can be seen. A total of 1218 MS stars, 185 red giant branch stars, 47 BS stars and 41 HB stars are identified, along with 78 sources blueward of the MS which consist of white dwarfs (WDs) and objects in the gap between the WDs and the MS that include potential cataclysmic variable (CV) candidates. The radial distribution of the BS population is concentrated towards the cluster centre, indicating that mass segregation has occurred. The blue and red sub-populations of the double BS sequence appear mixed in the ultraviolet CMD, and no significant central concentration of CV candidates is seen in this cluster.

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arXiv2022

Locating the CSM Emission within the Type Ia Supernova Remnant N103B

We present results from deep Chandra observations of the young Type Ia supernova remnant (SNR) 0509-68.7, also known as N103B, located in the Large Magellanic cloud (LMC). The remnant displays an asymmetry in brightness, with the western hemisphere appearing significantly brighter than the eastern half. Previous multi-wavelength observations have attributed the difference to a density gradient and suggested circumstellar material origins, drawing similarities to Kepler's SNR. We apply a clustering technique combined with traditional imaging analysis to spatially locate various emission components within the remnant. We find that O and Mg emission is strongest along the blast wave, and coincides with Spitzer observations of dust emission and optical emission from the non-radiative shocks. The abundances of O and Mg in these regions are enhanced relative to the average LMC abundances and appear as a distinct spatial distribution compared to the ejecta products, supporting the circumstellar medium (CSM) interpretation. We also find that the spatial distribution of Cr is identical to that of Fe in the interior of the remnant, and does not coincide at all with the O and Mg emission.

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arXiv2022

Optical line spectra of tidal disruption events from reprocessing in optically thick outflows

A significant number of tidal disruption events (TDEs) radiate primarily at optical and ultraviolet (UV) wavelengths, with only weak soft X-ray components. One model for this optical excess proposes that thermal X-ray emission from a compact accretion disc is reprocessed to longer wavelengths by an optically thick envelope. Here, we explore this reprocessing scenario in the context of an optically thick accretion disc wind. Using state-of-the-art Monte Carlo radiative transfer and ionization software, we produce synthetic UV and optical spectra for wind and disc-hosting TDEs. Our models are inspired by observations, spanning a realistic range of accretion rates and wind kinematics. We find that such outflows can efficiently reprocess the disc emission and produce the broad Balmer and helium recombination features commonly seen in TDEs and exhibit asymmetric red wings. Moreover, the characteristic colour temperature of the reprocessed spectral energy distribution (SED) is much lower than that of the accretion disc. We show explicitly how changes in black hole mass, accretion rate and wind properties affect the observed broadband SED and line spectrum. In general, slower, denser winds tend to reprocess more radiation and produce stronger Balmer emission. Most of the outflows we consider are too highly ionized to produce UV absorption features, but this is sensitive to the input SED. For example, truncating the inner disc at just 4 $R_{ISCO}$ lowers the wind ionization state sufficiently to produce UV absorption features for sight lines looking into the wind

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arXiv2021

Efficiencies of Magnetic-Field Amplification and Electron Acceleration in Young Supernova Remnants: Global Averages and Kepler's Supernova Remnant

Particle acceleration to suprathermal energies in strong astrophysical shock waves is a widespread phenomenon, generally explained by diffusive shock acceleration. Such shocks can also amplify upstream magnetic field considerably beyond simple compression. The complex plasma physics processes involved are often parameterized by assuming that shocks put some fraction $ε_e$ of their energy into fast particles, and another fraction $ε_B$ into magnetic field. Modelers of shocks in supernovae, supernova remnants, and gamma-ray bursters, among other locations, often assume typical values for these fractions, presumed to remain constant in time. However, it is rare that enough properties of a source are independently constrained that values of the epsilons can be inferred directly. Supernova remnants (SNRs) can provide such circumstances. Here we summarize results from global fits to spatially integrated emission in six young SNRs, finding $10^{-4} \le ε_e \le 0.05$ and $0.001 \le ε_B \le 0.1$. These large variations might be put down to the differing ages and environments of these SNRs, so we conduct a detailed analysis of a single remnant, that of Kepler's supernova. Both epsilons can be determined at seven different locations around the shock, and we find even larger ranges for both epsilons, as well as for their ratio (thus independent of the shock energy itself). We conclude that unknown factors have a large influence on the efficiency of both processes. Shock obliquity, upstream neutral fraction, or other possibilities need to be explored, while calculations assuming fixed values of the epsilons should be regarded as provisional.

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arXiv2021

The Masses of Supernova Remnant Progenitors in NGC 6946

We constrained the progenitor masses for 169 supernova remnants, 8 historically observed supernovae, and the black hole formation candidate in NGC 6946, finding that they are consistent with originating from a standard initial mass function. Additionally, there were 16 remnants that showed no sign of nearby star formation consistent with a core-collapse supernova, making them good Type Ia candidates. Using $Hubble$ $Space$ $Telescope$ broadband imaging, we measured stellar photometry of ACS/WFC fields in F435W, F555W, F606W, and F814W filters as well as WFC3/UVIS fields in F438W, F606W, and F814W. We then fitted this photometry with stellar evolutionary models to determine the ages of the young populations present at the positions of the SNRs and SNe. We then infer a progenitor mass probability distribution from the fitted age distribution. For 37 SNRs we tested how different filter combinations affected the inferred masses. We find that filters sensitive to H$α$, [N II], and [S II] gas emission can bias mass estimates for remnants that rely on our technique. Using a KS-test analysis on our most reliable measurements, we find the progenitor mass distribution is well-matched by a power-law index of $-2.6^{+0.5}_{-0.6}$, which is consistent with a standard initial mass function.

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arXiv2021

An Improved Model for the Spectra of Disks of Nova-like Variables

The spectra arising from the disks of nova-like variables show many of the features seen in stellar atmospheres. They are typically modelled either from an appropriated weighted set of stellar atmospheres or a disk atmosphere with energy is dissipated near the disk plane, with the effective temperature distribution expected from a steady state accretion disk. However these models generally over-predict the depth of the Balmer jump and the slope of the spectrum in the ultraviolet. The problem is likely due to energy dissipation in the disk atmosphere, which produces a flatter vertical temperature profile than is observed in stars. Here, we provide validation for this hypothesis in the form of spectra generated using the stellar atmosphere code TLUSTY using a parametric prescription for energy dissipation as a function of depth and closely match the spectrum of the nova-like IX Vel over the wavelength range 1150-6000 Å.

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arXiv2021

Optical Identification and Spectroscopy of Supernova Remnants in the Galaxy M51

Using a combination of ground-based and HST imaging, we have constructed a catalog of 179 supernova remnants (SNRs) and SNR candidates in the nearby spiral galaxy M51. Follow-up spectroscopy of 66 of the candidates confirms 61 of these as SNRs, and suggests that the vast majority of the unobserved objects are SNRs as well. A total of 55 of the candidates are coincident with (mostly soft) X-ray sources identified in deep Chandra observations of M51; searching the positions of other soft X-ray sources resulted in several additional possible optical candidates. There are 16 objects in the catalog coincident with known radio sources. None of the sources with spectra shows the high velocities (>500 km/s) characteristic of young, ejecta-dominated SNRs like Cas A; instead, most if not all appear to be middle-aged SNRs. The general properties of the SNRs, size distribution and spectral characteristics, resemble those in other nearby spiral galaxies, notably M33, M83, and NGC6946, where similar samples exist. However, the spectroscopically observed [N II]:Hα ratios appear to be significantly higher than in any of these other galaxies. Although we have explored various ideas to explain the high ratios in M51, none of the explanations appears to be satisfactory.

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arXiv2020

First Co-spatial Comparison of Stellar, Neutral-, and Ionized-gas Metallicities in a metal-rich galaxy: M83

We carry out a comparative analysis of the metallicities from the stellar, neutral-gas, and ionized-gas components in the metal-rich spiral galaxy M83. We analyze spectroscopic observations taken with the Hubble Space Telescope (HST), the Large Binocular Telescope (LBT) and the Very Large Telescope (VLT). We detect a clear depletion of the HI gas, as observed from the HI column densities in the nuclear region of this spiral galaxy. We find column densities of log[$N$(HI) cm$^{-2}$] $<$ 20.0 at galactocentric distances of $<$ 0.18 kpc, in contrast to column densities of log[$N$(HI) cm$^{-2}$] $\sim$ 21.0 in the galactic disk, a trend observed in other nearby spiral galaxies. We measure a metallicity gradient of $-$0.03 $\pm$ 0.01 dex kpc$^{-1}$ for the ionized gas, comparable to the metallicity gradient of a local benchmark of 49 nearby star-forming galaxies of $-$0.026 $\pm$ 0.002 dex kpc$^{-1}$. Our co-spatial metallicity comparison of the multi-phase gas and stellar populations shows excellent agreement outside of the nucleus of the galaxy hinting at a scenario where the mixing of newly synthesized metals from the most massive stars in the star clusters takes longer than their lifetimes ($\sim$10 Myr). Finally, our work shows that caution must be taken when studying the metallicity gradient of the neutral-gas component in star-forming galaxies, since this can be strongly biased, as these environments can be dominated by molecular gas. In these regions the typical metallicity tracers can provide inaccurate abundances as they may trace both the neutral- and molecular-gas components.

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arXiv2020

Imagery and UV Spectroscopy of the LMC Supernova Remnant N103B Using HST

We present HST/WFC3 multiband imagery of N103B, the remnant of a Type Ia supernova in the Large Magellanic Cloud, as well as HST/COS ultraviolet spectroscopy of the brightest radiatively shocked region. The images show a wide range of morphology and relative emission-line intensities, from smooth Balmer-line dominated collisionless shocks due to the primary blast wave, to clumpy radiative shock filaments due to secondary shocks in density enhancements. The COS data show strong FUV line emission despite a moderately high extinction along this line of sight. We use the COS data with previous optical spectra to constrain the shock conditions and refine the abundance analysis, finding abundances typical of the local interstellar medium within the uncertainties. Under an assumption that the material being shocked was shed from the pre-supernova system, this finding places constraints on any significant enrichment in that material, and thus on the non-degenerate star in what was presumably a single-degenerate Type Ia supernova.

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arXiv2020

The Supernova Remnant Population of NGC6946 as Observed in [Fe II] 1.644 $μ$m with HST

NGC6946 is a high star formation rate face-on spiral galaxy that has hosted ten supernovae since 1917. Not surprisingly, a large number of supernova remnants and candidates have been identified either as optical nebulae with high [S II]:H$α$ line ratios (147) or as compact non-thermal radio sources (35). However, there are only seven overlaps between these two samples. Here, we apply [Fe II] 1.644 $μ$m emission as a new diagnostic to search for supernova remnants in an attempt to resolve this discrepancy. [Fe II] is expected to be relatively strong in the radiative shocks of supernova remnants and almost absent in HII regions. It is less susceptible to the effects of absorption along the line of sight than the optical lines normally used to identify remnants. Using data from the WFC3 camera on HST}, we identify 132 [Fe II] emission nebulae in NGC6946 as likely supernova remnants. Of these, 54 align with previously known optical supernova remnants. The remaining 78 objects are new; of these 44 are visible in new HST imagery in H$α$ and [S II]. This brings the total number of supernova remnant candidates (from optical and/or IR data) in NGC6946 to 225. A total of 14 coincidences with radio supernova remnant candidates (out of 30 in our search area) are found in this expanded list. The identification of so many new remnant candidates validates the use of [Fe II] imagery for finding remnants, and suggests that previous remnant searches in other galaxies may be far from complete.

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