arXiv subjects

Mark R. Morris

Publications and source records attributed to Mark R. Morris.

At least 73 records · Page 4Linked to original sources

arXiv2021

Modeling Turbulence in Galactic Centers

Turbulence is a prevalent phenomenon in the interstellar medium, and in particular, the environment at the centers of galaxies. For example, detailed observations of the Milky Way's Central Molecular Zone (CMZ) revealed that it has a complex and turbulent structure. Turbulence on galactic scales is often modeled using star formation and feedback. However, these effects do not appear to be sufficient for explaining the high-velocity dispersion observed in the CMZ, indicating that additional gas-stirring processes are likely to be operating. Here we introduce a proof-of-concept method to drive turbulence in gas that orbits under the influence of a galactic potential. Instead of relying on a particular physical mechanism, we have adopted a Fourier forcing module and have applied it using a Smoothed Particle Hydrodynamics code. To test our method, we performed simulations of a simplistic model of the CMZ. Our turbulence injection method is capable of balancing the self-gravity of the gas, which allows us to run the simulations for long timescales and thereby follow the evolution of the CMZ. Our results show that turbulence induces a flocculent spiral pattern in our model, analogous to that found in galactic-scale simulations. Furthermore, we find that our turbulence injection method induces inward migration of gas, a result consistent with previous numerical simulations. We submit that this injection method is a promising new tool to simulate turbulence in galactic centers.

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arXiv2021

The effects of turbulence on galactic nuclear gas rings

The gas dynamics in the inner few kiloparsecs of barred galaxies often results in configurations that give rise to nuclear gas rings. However, the generic dynamical description of the formation of galactic nuclear rings does not take into account the effects of thermal pressure and turbulence. Here we perform 3D hydrodynamic simulations of gas in a galactic barred potential out to a radius of $2$~kpc. We include self-gravity and a large-scale turbulence-driving module. We study how the formation of gaseous nuclear rings is affected by changing the bar pattern speed and the strength of the turbulence, and compare the results to simulations with a relatively high sound speed. We use two values for the bar pattern speed that have been discussed in the literature for our Milky Way Galaxy (40 and 63 km s$^{-1}$ kpc$^{-1}$). Our results show that turbulence produces broader and smaller nuclear rings, and enhances gas migration towards the inner few pc of the galaxy, compared to simulations without turbulence.

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arXiv2021

A Supernova-driven, Magnetically-collimated Outflow as the Origin of the Galactic Center Radio Bubbles

A pair of non-thermal radio bubbles recently discovered in the inner few hundred parsecs of the Galactic center bears a close spatial association with elongated, thermal X-ray features called the X-ray chimneys. While their morphology, position, and orientation vividly point to an outflow from the Galactic center, the physical processes responsible for the outflow remain to be understood. We use three-dimensional magnetohydrodynamic simulations to test the hypothesis that the radio bubbles/X-ray chimneys are the manifestation of an energetic outflow driven by multiple core-collapsed supernovae in the nuclear stellar disk, where numerous massive stars are known to be present. Our simulations are run with different combinations of two main parameters, the supernova birth rate and the strength of a global magnetic field being vertically oriented with respect to the disk. The simulation results show that a hot gas outflow can naturally form and acquire a vertically elongated shape due to collimation by the magnetic pressure. In particular, the simulation with an initial magnetic field strength of 80 $μ$G and a supernova rate of 1 $kyr^{-1}$ can well reproduce the observed morphology, internal energy and X-ray luminosity of the bubbles after an evolutionary time of 330 kyr. On the other hand, a magnetic field strength of 200 $μ$G gives rise to an overly elongated outflow that is inconsistent with the observed bubbles. The simulations also reveal that, inside the bubbles, mutual collisions between the shock waves of individual supernovae produce dense filaments of locally amplified magnetic field. Such filaments may account for a fraction of the synchrotron-emitting radio filaments known to exist in the Galactic center.

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arXiv2021

Upper Limit on Brackett-gamma Emission from the Immediate Accretion Flow onto the Galactic Black Hole

We present the first observational constraint on the Br-gamma recombination line emission associated with the supermassive black hole at the center of our Galaxy, known as Sgr A*. By combining 13 years of data with the Adaptive Optics fed integral field spectrograph OSIRIS at the W. M. Keck Observatory obtained as part of the Galactic Center Orbits Initiative, we extract the near-infrared spectrum within ~0.2'' of the black hole and we derive an upper limit on the Br-gamma flux. The aperture was set to match the size of the disk-like structure that was recently reported based on millimeter-wave ALMA observations of the hydrogen recombination line, H30-alpha. Our stringent upper limit is at least a factor of 80 (and up to a factor of 245) below what would be expected from the ALMA measurements and strongly constrains possible interpretation of emission from this highly under-luminous supermassive black hole.

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arXiv2020

A population of compact radio variables and transients in the radio bright zone at the Galactic center observed with the Jansky Very Large Array

Using JVLA data obtained from high-resolution observations at 5.5 GHz at multiple epochs in 2014 and 2019, we have detected a population of radio variables and transients in the radio bright zone at the Galactic center. With observations covering a sky area of 180 arcmin$^2$ at an angular resolution of 0.4 arcsec, we report new detections of 110 Galactic center compact radio (GCCR) sources with a size of $<1$ arcsec. The flux densities of GCCRs exceed 70 $μ$Jy, with at least 10$σ$ significance. Among these sources, 82 are variable or transient and 28 are non-variable. About 10\% of them are expected to be extragalactic background sources. We discuss the possible astrophysical nature of the detected sources. As compared to the Galactic disk (GD) population of normal pulsars (NPs) and millisecond pulsars (MSPs), a majority (80\%) of the GCCRs appears to fall within the high flux-density tail of the pulsar distribution, as extrapolated from a sample of NPs in the Galactic disk. However, MSPs extrapolated from the GD population are too weak to have contributed significantly to the GCCR population that have been detected. We also cross-correlated the GCCRs with X-ray sources in Chandra X-ray catalogs and found that 42 GCCRs have candidate X-ray counterparts. Most of the GCCRs having X-ray counterparts are likely to be associated with unresolved or slightly resolved radio jets launched from X-ray binaries with a compact object, either a black hole or a neutron star.

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arXiv2020

Can supernova shells feed supermassive black holes in galactic nuclei?

We simulate shells created by supernovae expanding into the interstellar medium (ISM) of the nuclear region of a galaxy, and analyze how the shell evolution is influenced by the supernova (SN) position relative to the galactic center, by the interstellar matter (ISM) density, and by the combined gravitational pull of the nuclear star cluster (NSC) and supermassive black hole (SMBH).We adopted simplified hydrodynamical simulations using the infinitesimally thin layer approximation in 3D (code RING) and determined whether and where the shell expansion may bring new gas into the inner parsec around the SMBH. The simulations show that supernovae occurring within a conical region around the rotational axis of the galaxy can feed the central accretion disk surrounding the SMBH. For ambient densities between 10$^3$ and 10$^5$ cm$^{-3}$, the average mass deposited into the central parsec by individual supernovae varies between 10 to 1000 solar masses depending on the ambient density and the spatial distribution of supernova events. Supernova occurring in the aftermath of a starburst event near a galactic center can supply two to three orders of magnitude more mass into the central parsec, depending on the magnitude of the starburst. The deposited mass typically encounters and joins an accretion disk. The fate of that mass is then divided between the growth of the SMBH and an energetically driven outflow from the disk.

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arXiv2020

A Population of Compact Radio Sources at the Galactic Center

The radio bright zone (RBZ) at the Galactic center has been observed with the JVLA in the A, B and C array configurations at 5.5 and 9 GHz. With a procedure for high-dynamic range imaging developed on CASA, we constructed deep images a resolution up to 0.2", achieving rms noises of a few $μ$Jy/beam. From the high-resolution and high-dynamics range images at 5.5 and 9 GHz, a population of compact radio sources ranging from a few mJy to a few tens $μ$Jy in flux density is revealed. The steep-spectrum radio sources in RBZ are likely the candidates of high-energy objects that are associated with neutron stars and/or stellar mass black holes at the Galactic center. We report new results of the Cannonball and Galactic center transient (GCT).

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arXiv2020

HAWC+ Far-Infrared Observations of the Magnetic Field Geometry in M51 and NGC 891

SOFIA HAWC+ polarimetry at $154~\micron$ is reported for the face-on galaxy M51 and the edge-on galaxy NGC 891. For M51, the polarization vectors generally follow the spiral pattern defined by the molecular gas distribution, the far-infrared (FIR) intensity contours, and other tracers of star formation. The fractional polarization is much lower in the FIR-bright central regions than in the outer regions, and we rule out loss of grain alignment and variations in magnetic field strength as causes. When compared with existing synchrotron observations, which sample different regions with different weighting, we find the net position angles are strongly correlated, the fractional polarizations are moderately correlated, but the polarized intensities are uncorrelated. We argue that the low fractional polarization in the central regions must be due to significant numbers of highly turbulent segments across the beam and along lines of sight in the beam in the central 3 kpc of M51. For NGC 891, the FIR polarization vectors within an intensity contour of 1500 $\rm{MJy~sr^{-1}}$ are oriented very close to the plane of the galaxy. The FIR polarimetry is probably sampling the magnetic field geometry in NGC 891 much deeper into the disk than is possible with NIR polarimetry and radio synchrotron measurements. In some locations in NGC 891 the FIR polarization is very low, suggesting we are preferentially viewing the magnetic field mostly along the line of sight, down the length of embedded spiral arms. There is tentative evidence for a vertical field in the polarized emission off the plane of the disk.

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arXiv2020

Galactic Center IRS13E: Colliding Stellar Winds or an Intermediate Mass Black Hole?

A small cluster of massive stars residing in the Galactic center, collectively known as IRS13E, is of special interest due to its close proximity to Sgr A* and the possibility that an embedded intermediate-mass black hole (IMBH) binds its member stars. It has been suggested that colliding winds from two member stars, both classified as Wolf-Rayet type, are responsible for the observed X-ray, infrared and radio emission from IRS13E. We have conducted an in-depth study of the X-ray spatial, temporal and spectral properties of IRS13E, based on 5.6 Ms of ultra-deep Chandra observations obtained over 20 years. These X-ray observations show no significant evidence for source variability. We have also explored the kinematics of the cluster members, using Keck near-infrared imaging and spectroscopic data on a 14-yr baseline that considerably improve the accuracy of stars' proper motions. The observations are interpreted using 3-dimensional hydrodynamical simulations of colliding winds tailored to match the physical conditions of IRS13E, leading us to conclude that the observed X-ray spectrum and morphology can be well explained by the colliding wind scenario, in the meantime offering no support for the presence of a putative IMBH. An IMBH more massive than a few $10^3{\rm~M_\odot}$ is also strongly disfavored by the stellar kinematics.

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arXiv2020

SOFIA/FORCAST Galactic Center Legacy Survey: Overview

The Galactic Center contains some of the most extreme conditions for star formation in our Galaxy as well as many other phenomena that are unique to this region. Given our relative proximity to the Galactic Center, we are able to study details of physical processes to a level that is simply not yet possible for more distant galaxies, yielding an otherwise inaccessible view of the nuclear region of a galaxy. We recently carried out a targeted imaging survey of mid-infrared bright portions of the Galactic Center at 25 and 37 $μ$m using the FORCAST instrument on SOFIA. This survey was one of the inaugural Legacy Programs from SOFIA cycle 7, observing a total area of 403 arcmin$^2$ (2180 pc$^2$), including the Sgr A, B, and C complexes. Here we present an overview of the survey strategy, observations, and data reduction as an accompaniment to the initial public release of the survey data. We discuss interesting regions and features within the data including extended features near the circumnuclear disk, structures in the Arched Filaments and Sickle H II regions, and signs of embedded star formation in Sgr B2 and Sgr C. We also feature a handful of less well studied mid-infrared sources located between Sgr A and Sgr C that could be sites of relatively isolated star formation activity. Last, we discuss plans for subsequent publications and future data releases from the survey.

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arXiv2020

Carbon- and Oxygen-rich Asymptotic Giant Branch (AGB) stars in the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey

Detections of SiO masers from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey more tightly define the region where Oxygen-rich (O) Asymptotic Giant Branch (AGB) stars reside in multiple infrared (IR) color-color diagrams. Using MSX and 2MASS data along with radio spectra from the BAaDE survey we find that three main populations were observed in the BAaDE survey: O-rich AGB stars of which about $73\%$ host SiO masers, Carbon-rich (C) AGB stars which do not host these masers, and a small contaminating set of possible Young Stellar Objects (YSOs). The distinction between YSOs and AGB stars can be drawn using only MSX data, specifically the $[D]-[E]$ color, while the shorter wavelengths provided by 2MASS are necessary to divide potential C- and O-rich AGB stars. Divisions similar to these have been seen in multiple earlier IR-studies, but BAaDE currently provides a sample of $\sim$ 15000 sources which far exceeds previous studies in sample size, and therefore provides much more distinct divisions. With these IR distinctions in place, we discuss the sources which are exceptions in either their molecular detections or IR colors, as well as the distribution of the three populations in Galactic coordinates.

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arXiv2020

A population of dust-enshrouded objects orbiting the Galactic black hole

The central 0.1 parsecs of the Milky Way host a supermassive black hole identified with the position of the radio and infrared source Sagittarius A*, a cluster of young, massive stars (the S stars) and various gaseous features. Recently, two unusual objects have been found to be closely orbiting Sagittarius A*: the so-called G sources, G1 and G2. These objects are unresolved (having a size of the order of 100 astronomical units, except at periapse, where the tidal interaction with the black hole stretches them along the orbit) and they show both thermal dust emission and line emission from ionized gas. G1 and G2 have generated attention because they appear to be tidally interacting with the supermassive Galactic black hole, possibly enhancing its accretion activity. No broad consensus has yet been reached concerning their nature: the G objects show the characteristics of gas and dust clouds but display the dynamical properties of stellar-mass objects. Here we report observations of four additional G objects, all lying within 0.04 parsecs of the black hole and forming a class that is probably unique to this environment. The widely varying orbits derived for the six G objects demonstrate that they were commonly but separately formed.

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arXiv2019

Chandra Spectral and Timing Analysis of Sgr A*'s Brightest X-ray Flares

We analyze the two brightest Chandra X-ray flares detected from Sagittarius A*, with peak luminosities more than 600 x and 245 x greater than the quiescent X-ray emission. The brightest flare has a distinctive double-peaked morphology --- it lasts 5.7 ksec ($\sim 2$ hours), with a rapid rise time of 1500 sec and a decay time of 2500 sec. The second flare lasts 3.4 ksec, with rise and decay times of 1700 sec and 1400 sec. These luminous flares are significantly harder than quiescence: the first has a power law spectral index $Γ= 2.06\pm 0.14$ and the second has $Γ= 2.03\pm 0.27$, compared to $Γ= 3.0\pm0.2$ for the quiescent accretion flow. These spectral indices (as well as the flare hardness ratios) are consistent with previously-detected Sgr A* flares, suggesting that bright and faint flares arise from similar physical processes. Leveraging the brightest flare's long duration and high signal-to-noise, we search for intraflare variability and detect excess X-ray power at a frequency of $ν\approx 3$ mHz, but show that it is an instrumental artifact and not of astrophysical origin. We find no other evidence (at the 95% confidence level) for periodic or quasi-periodic variability in either flares' time series. We also search for non-periodic excess power but do not find compelling evidence in the power spectrum. Bright flares like these remain our most promising avenue for identifying Sgr A*'s short timescale variability in the X-ray, which may probe the characteristic size scale for the X-ray emission region.

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arXiv2019

6.7 GHz CH3OH absorption towards the N3 Galactic Center point-source

We present evidence of 6.7 GHz methanol (CH3OH)~and 4.8 GHz formaldehyde (H2CO) absorption towards the Galactic Center (GC) point-source `N3'. Both absorption features are unresolved and spatially aligned with N3. The 6.7 GHz CH3OH contains a single velocity component (centered at ~10 km/s) while the 4.8 GHz H2CO shows two velocity components (centered at ~-3 and +8 km/s). We find that the velocity of these absorption components are similar to that of emission lines from other molecules (e.g., SiO and H3CN) detected toward this compact-source (-13 to +25 km/s; `N3 cloud'). The detection of these absorption features is a firm indication that some of the molecular gas in the N3 molecular cloud is on the near-side of the continuum source. Analysis of the CH3OH absorption kinematics shows a relatively large velocity dispersion (3.8 km/s) for the size scale of this feature (<0.1'', <0.01 pc at the GC; Ludovici et al. 2016), when compared with other similar size GC clouds in the Larson linewidth-size relationship. Further, this linewidth is closer to velocity dispersion measurements for size scales of 1.3 pc, which is roughly the width of the N3 cloud (25''; 1.0 pc). We argue that this relatively broad linewidth, over a small cross-sectional area, is due to turbulence through the depth of the cloud, where the cloud has a presumed line-of-sight thickness of ~1 pc.

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arXiv2019

A VLA Polarimetric Study of the Galactic Center Radio Arc: Characterizing Polarization, Rotation Measure, and Magnetic Field Properties

The Radio Arc is one of the brightest systems of non-thermal filaments (NTFs) in the Galactic Center, located near several prominent HII regions (Sickle and Pistol) and the Quintuplet stellar cluster. We present observations of the Arc NTFs using the S-, C-, and X-bands of the Very Large Array interferometer. Our images of total intensity reveal large-scale helical features that surround the Arc NTFs, very narrow sub-filamentation, and compact sources along the NTFs. The distribution of polarized intensity is confined to a relatively small area along the NTFs. There are elongated polarized structures that appear to lack total intensity counterparts. We detect a range of rotation measure values from -1000 to -5800 rad m$\rm^{-2}$, likely caused by external Faraday rotation along the line of sight. After correcting for Faraday rotation, the intrinsic magnetic field orientation is found to generally trace the extent of the NTFs. However, the intrinsic magnetic field in several regions of the Arc NTFs shows an ordered pattern that is rotated with respect to the extent of the NTFs. We suggest this changing pattern may be caused by an additional magnetized source along the line of sight, so that we observe two field systems superposed in our observations. We suggest that the large scale helical segments near the Radio Arc could be components of such a source causing these changes in intrinsic magnetic field, and some variations in the polarization and rotation measure values along the NTFs.

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arXiv2019

2 mm GISMO Observations of the Galactic Center. II. A Nonthermal Filament in the Radio Arc and Compact Sources

We have used the Goddard IRAM 2-Millimeter Observer (GISMO) with the 30 m IRAM telescope to carry out a 2 mm survey of the Galaxy's central molecular zone (CMZ). These observations detect thermal emission from cold ISM dust, thermal free-free emission from ionized gas, and nonthermal synchrotron emission from relatively flat-spectrum sources. Archival data sets spanning $3.6 μ$m to 90 cm are used to distinguish different emission mechanisms. After the thermal emission of dust is modeled and subtracted, the remaining 2 mm emission is dominated by free-free emission, with the exception of the brightest nonthermal filament (NTF) that runs though the middle of the bundle of filaments known as the Radio Arc. This is the shortest wavelength at which any NTF has been detected. The GISMO observations clearly trace this NTF over a length of ~0.2$^\circ$, with a mean 2 mm spectral index which is steeper than at longer wavelengths. The 2 mm to 6 cm (or 20 cm) spectral index steepens from $α\approx -0.2$ to $-0.7$ as a function distance from the Sickle H II region, suggesting that this region is directly related to the NTF. A number of unresolved (at $21''$) 2 mm sources are found nearby. One appears to be thermal dust emission from a molecular cloud that is associated with an enigmatic radio point source whose connection to the Radio Arc is still debated. The morphology and colors at shorter IR wavelengths indicate other 2 mm unresolved sources are likely to be compact H II regions.

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arXiv2019

2 mm GISMO Observations of the Galactic Center. I. Dust Emission

The Central Molecular Zone (CMZ), covering the inner ~1$^\circ$ of the Galactic plane has been mapped at 2 mm using the GISMO bolometric camera on the 30 m IRAM telescope. The $21''$ resolution maps show abundant emission from cold molecular clouds, from star forming regions, and from one of the Galactic center nonthermal filaments. In this work we use the Herschel Hi-GAL data to model the dust emission across the Galactic center. We find that a single-temperature fit can describe the 160 -- 500 $μ$m emission for most lines of sight, if the long-wavelength dust emissivity scales as $λ^{-β}$ with $β\approx 2.25$. This dust model is extrapolated to predict the 2 mm dust emission. Subtraction of the model from the GISMO data provides a clearer look at the 2 mm emission of star-forming regions and the brightest nonthermal filament.

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arXiv2019

The Bulge Asymmetries and Dynamical Evolution (BAaDE) SiO Maser Survey at 86 GHz with ALMA

We report on the first 1,432 sources observed using the Atacama Large Millimeter/submillimeter Array (ALMA), from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey, which aims to obtain tens of thousands of line-of-sight velocities from SiO masers in Asymptotic Giant Branch (AGB) stars in the Milky Way. A 71% detection rate of 86 GHz SiO masers is obtained from the infrared color-selected sample, and increases to 80% when considering the likely oxygen-rich stars using Midcourse Space Experiment (MSX) colors isolated in a region where [D]-[E] <= 1.38. Based on Galactic distributions, the presence of extended CS emission, and likely kinematic associations, the population of sources with [D]-[E] > 1.38 probably consists of young stellar objects, or alternatively, planetary nebulae. For the SiO detections we examined whether individual SiO transitions provide comparable stellar line-of-sight velocities, and found that any SiO transition is suitable for determining a stellar AGB line-of-sight velocity. Finally, we discuss the relative SiO detection rates and line strengths in the context of current pumping models.

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