Magnetic fields in the multi-phase interstellar medium ▸ Susan Clark (Stanford)

Susan Clark, an assistant professor at Stanford, explores the complex interplay between the interstellar medium (ISM) and magnetic fields in the Milky Way. She highlights the use of various observational tracers, such as gas and dust emissions, to unravel the physical properties of the ISM. Clark's research utilizes advanced techniques like wavelet scattering transforms to analyze the morphology of neutral hydrogen emissions, revealing a strong correlation between these structures and the magnetic field orientation. Her findings challenge previous assumptions about velocity effects and emphasize the significance of cold, dense regions in shaping ISM structures.

• Susan Clark discusses the importance of observational constraints on the turbulent magnetic interstellar medium (ISM) of the Milky Way.
• The focus is on using observational tracers and combining measurements to understand the physical properties of the ISM.
• Key observational tracers include gas and dust emission, gas absorption, starlight polarization, and polarized dust emission.

"I want to use my time here today to talk about one of the greatest Laboratories we have for observing a turbulent medium in the universe that's the ISM, the interstellar medium of our own Milky Way galaxy."

• Susan Clark emphasizes the significance of the ISM as a laboratory for studying turbulence in the universe.

• The structure of the ISM can be analyzed through hyperspectral data and resolved emission structures.
• Observations of the 21 cm neutral atomic hydrogen line are crucial for understanding the ISM.
• Techniques include machine vision algorithms to quantify the orientation of linear structures and comparing them with magnetic field observations.

"One of the things that we have access to when we observe the gas and dust emission in our own Galaxy is structure in the resolved emission or hyperspectral data that we observe, and this structure is encoding physical information."

• Observations of gas and dust emissions provide valuable structural data to decode physical information about the ISM.

• Linear structures in H1 emission are well-aligned with the magnetic field orientation in the dusty neutral medium.
• The orientation of H1 structures can be used to infer magnetic field tangling and dust emission properties.
• The alignment is quantitatively verified through comparisons of H1 structures and dust polarization measurements.

"The orientation of these linear neutral structures in this H1 emission are extremely well aligned with the magnetic field that we can measure in either the dust polarization or the polarized dust emission."

• The alignment between H1 emission structures and magnetic fields is a key observation in understanding ISM dynamics.

• A mapping technique is proposed to relate H1 distribution to magnetic ISM structure by using velocity orientation space.
• This mapping does not introduce physical models but relies on observed emission data.
• The mapping aims to provide a morphology-based distribution of the H1-based magnetic field.

"The idea is basically just to take your data and map them into velocity orientation space."

• The mapping technique is a novel approach to understand the magnetic structure of the ISM using H1 data.

• H1-based q and u maps are compared to actual measurements of polarized thermal dust emission.
• This comparison helps in validating the morphology-based understanding of the ISM.

"You can come up with something analogous to the q and u Strokes parameters of linear polarization but here defined purely by the geometry of the neutral hydrogen emission."

• The development of H1-based q and u maps is a significant step in correlating ISM morphology with magnetic field measurements.

• The study involves comparing the geometry of H1 with actual measurements of dust polarization to understand the influence of the magnetic field.
• The polarization fraction and Stokes parameters are used to compare geometric structures and real measurements.
• The correlation between the H1 base map and measured polarization fraction indicates that variations are due to geometric tangling of the magnetic field rather than dust grain alignment.

"This gives you sort of a null hypothesis with which to compare your actual measurements of the Dust polarization to understand what's attributable purely to this geometric structure of what we think is very correlated with the magnetic field."

• The null hypothesis helps differentiate between geometric influences and actual magnetic field effects on dust polarization.

"It's the depolarization due to the geometrical tangling of the magnetic field within your beam or along the line of sight that must be causing that large-scale correlation between these two maps."

• The geometric tangling of the magnetic field is responsible for the observed correlation in polarization maps.

• The study reproduces certain properties of dust polarization, such as the excess of the parity even component over the B mode component.
• The correlation between dust total intensity and parity even components is driven by magnetic field alignment along anisotropic dust density structures.

"We reproduce this correlation between the dust total intensity in that parody even component because that is driven by a preferential alignment of the magnetic field along anisotropic dust density structures."

• The alignment of the magnetic field with dust density structures influences dust polarization properties.

"You can even show that if you have enough small misalignments with the same handedness, you can predict or produce nonzero parity odd quantities in the polarization as well."

• Small misalignments in magnetic field alignment can lead to nonzero parity odd quantities in polarization.

• Cold gas density structures are identified as the main drivers of magnetic alignment, rather than turbulent velocity fields.
• There was a hypothesis that magnetic structures were due to turbulent velocity fields, but this has been disproven by observational data.

"There was an alternative idea that maybe the magnetically aligned structures were due to Velocity CICS, but that is a testable prediction and it's not born out in the data."

• The hypothesis of turbulent velocity fields causing magnetic alignment is not supported by data.

"What all of these different ways of probing the data support is that what's predominantly structuring the small-scale Channel map emission are preferentially cold denser regions of the H1 gas."

• Cold, dense regions of H1 gas are responsible for structuring small-scale channel map emissions and magnetic alignment.

• A new technique, wavelet scattering transform, is used to analyze the morphology of H1 emission without predefined structures like filaments.
• This technique allows for a flexible analysis of scale and orientation interactions in H1 emission images.

"We have turned to a technique called The W scattering transform, which is very simple and flexible, essentially convolving an image with predefined wavelet kernels."

• The wavelet scattering transform provides a flexible approach to analyzing the morphology of H1 emissions.

"For us, we like this because it allows us to explore the scale and orientation interactions of structure in your image."

• The technique helps in exploring interactions of scale and orientation in H1 emission images, enhancing understanding of gas phase structures.

• Wavelet coefficients provide a flexible and interpretable way to describe the geometry of images.
• Different configurations of wavelet coefficients can highlight various structural features in images, such as orthogonal or parallel orientations.
• The manipulation of these coefficients can be used to synthesize images and explore the effects of different structural emphases.

"It gives us a set of coefficients that very flexibly describe the geometry of anything in an image and in an interpretable way."

• Wavelet coefficients are a versatile tool for visually interpreting image geometry.

"You can make synthetic images that give you an intuitive sense of what's happening in the image space when you crank up one coefficient or another."

• Adjusting wavelet coefficients allows for the creation of synthetic images to understand the impact of different structural elements.

• The structure of H1 emission is highly correlated with the CNM mass fraction.
• The correlation aligns with predictions about filamentary cold structures and their magnetic field alignment.
• The s parallel coefficient at small scales correlates positively with the CNM mass fraction, while the perpendicular component is anti-correlated.

"The structure of the H1 emission is extremely correlated with this CNN Mass fraction in ways that we would have predicted based on our understanding of these filamentary cold structures."

• The observed correlation between H1 emission and CNM mass fraction supports existing theories about cold filamentary structures.

"The s parallel coefficient at small scales is nicely correlated with the CNN Mass fraction, and the perpendicular component is actually anti-correlated."

• Different wavelet coefficients exhibit distinct correlations with the CNM mass fraction, indicating complex structural relationships.

• A model of the ISM suggests a correlation between dust polarization fraction and CNM mass fraction.
• The polarization fraction is uncorrelated with the total H1 column, suggesting distinct structural components.
• A hierarchical model is used to quantify geometric depolarization and magnetic field dispersion.

"The polarization fraction of the dust in the fuse High Galactic latitude ism is correlated with the CNM Mass fraction and uncorrelated with the total H1 column."

• Dust polarization fraction shows a specific correlation pattern with CNM, indicating unique ISM structural properties.

"We can constrain the geometric depolarization from the dispersion of the magnetic field in the rest of the column by fitting this cartoon model to our measurements."

• A theoretical model helps quantify the magnetic field's impact on dust polarization through geometric depolarization.

• The findings have implications for understanding the three-dimensional geometry of the magnetic field in the ISM.
• Differences in path lengths between CNM and other components suggest varying magnetic field structures.
• Collaboration with numerical simulations can further illuminate the phase-dependent magnetic field behavior.

"I'm excited to think with people here about some of the implications of this and some of the different questions we can ask, especially about how this might be telling us about the three-dimensional geometry of the magnetic field."

• The study opens avenues for exploring magnetic field geometry in the ISM through collaboration and further research.

"The path length between the CNM and the rest of the column is quite different, and so I would love to talk to people doing detailed work with multiphase numerical simulations about what we can learn from these data."

• Understanding path length differences can provide insights into the magnetic field's phase-dependent structure.

• Observations of the ISM contain rich morphological information that can reveal the structure of magnetic fields and phase structures.
• New constraints on magnetic field disorder between ISM phases have been developed.
• Ongoing research and collaboration will continue to advance the understanding of ISM magnetic structures.

"Observations of the ism have a wealth of morphological information if we can just be clever about how to get it out."

• The ISM's rich data offers opportunities for uncovering detailed structural and magnetic information.

"We've recently put some of this together to have a new constraint on the magnetic field disorder between phases of the neutral ism."

• Recent work has led to new insights into the magnetic field disorder across different ISM phases.

• The integrated intensity weighted orientation of H1 morphology is correlated with dust polarization.
• Dust polarization performs a line of sight integral, revealing insights into the orientation of magnetic fields.

"It's the integrated intensity weighted orientation of the H1 morphology as a function of that line of sight velocity, and that is what is extremely well correlated with the dust polarization."

• This quote highlights the correlation between H1 morphology and dust polarization, which is important for understanding magnetic field orientations.

• Dust polarization samples the molecular medium, while H1 does not.
• The magnetic field inside the molecular medium is more chaotic due to self-gravity compared to the CNM envelopes.

"The dust polarization is also going to sample the molecular medium, but your H1 is not."

• This quote emphasizes the distinction between dust polarization and H1 in sampling different media, pointing to the complexity in the molecular medium.

• Correlation between H1 column and dust polarization changes at higher column densities.
• Anti-correlation observed between total H1 column and dust polarization fraction in higher column densities.

"If you look at this correlation at higher column densities, you actually see the same anti-correlation that you see between the total H1 column and the dust polarization fraction."

• This quote indicates that the relationship between H1 and dust polarization varies with column density, suggesting different dynamics at play.

• Observations show a tight statistical correlation between CNM filamentary structures and local magnetic field orientation.
• At higher column densities, the preferred alignment between density structures and magnetic field orientation is lost.

"There is a very tight statistical correlation in the observations between the CNM filamentary elongated in isotropic structure and the local magnetic field orientation."

• The quote highlights the observed correlation between CNM structures and magnetic fields, which is crucial for understanding magnetic field influences.

• Simulations sometimes show H1 structures perpendicular to magnetic fields, contrary to observations.
• This discrepancy raises questions about the accuracy of simulations in replicating observed data.

"In that simulation, we find H1 could be code neutral media could be perpendicular to magnetic field, not just the very high column density one."

• This quote points to discrepancies in simulations, suggesting a need to refine models to better match observations.

• Correlation between synchrotron polarization and other multiphase ISM tracers provides insights into synchrotron emission origins.
• Morphological correspondence observed between magnetic field orientation and cold gas structures.

"We see sometimes this morphological correspondence between the Planck measured magnetic field orientation between the cold gas structures in the H1 Channel map and between synchrotron measurements of the magnetic field orientation."

• This quote illustrates the observed alignment between different tracers, which helps in understanding the magnetic field and emission sources.

• Filaments predominantly align with magnetic fields in diffuse media.
• Possible formation of filaments due to isobaric thermal instabilities with magnetic fields.

"If these filaments are for instance formed as a consequence of isobaric thermal instabilities with magnetic fields, do you have any insights on this?"

• This quote suggests a potential mechanism for filament formation, linking it to thermal instabilities and magnetic fields.