arXiv subjects

Enrique Lopez-Rodriguez

Publications and source records attributed to Enrique Lopez-Rodriguez.

At least 55 records · Page 3Linked to original sources

arXiv2023

An X-rays-to-radio investigation of the nuclear polarization from the radio-galaxy Centaurus A

Centaurus A is one of the closest radio-galaxies to Earth. Its proximity allowed us to extensively study its active galactic nucleus but the core emission mechanism remains elusive because of local strong dust and gas obscuration. The capability of polarimetry to shave-off contaminating emission has been exploited without success in the near-infrared by previous studies but the very recent measurement of the 2 - 8 keV polarization by the Imaging X-ray Polarimetry Explorer (IXPE) brought the question back to the fore. To determine what is the prevalent photon generation mechanism to the multi-wavelength emission from the core of Centaurus A, we retrieved from the archives the panchromatic polarization measurements of the central compact component. We built the total and polarized flux spectral energy distributions of the core and demonstrated that synchrotron self-Compton models nicely fit the polarized flux from the radio to the X-ray band. The linear polarization of the synchrotron continuum is perpendicular to the jet radio axis from the optical to the radio band, and parallel to it at higher energies. The observed smooth rotation of the polarization angle in the ultraviolet band is attributed to synchrotron emission from regions that are getting closer to the particle acceleration site, where the orientation of the jet's magnetic fields become perpendicular to the jet axis. This phenomenon support the shock acceleration mechanism for particle acceleration in Centaurus A, in line with IXPE observations of several high-synchrotron peak blazars.

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arXiv2023

The Pandora project. I: the impact of radiation and cosmic rays on baryonic and dark matter properties of dwarf galaxies

Enshrouded in several well-known controversies, dwarf galaxies have been extensively studied to learn about the underlying cosmology, notwithstanding that physical processes regulating their properties are poorly understood. To shed light on these processes, we introduce the Pandora suite of 17 high-resolution (3.5 parsec half-cell side) dwarf galaxy formation cosmological simulations. Commencing with thermo-turbulent star formation and mechanical supernova feedback, we gradually increase the complexity of physics incorporated leading to full-physics models combining magnetism, on-the-fly radiative transfer and the corresponding stellar photoheating, and SN-accelerated cosmic rays. We investigate combinations of these processes, comparing them with observations to constrain what are the main mechanisms determining dwarf galaxy properties. We find hydrodynamical `SN feedback-only' simulations struggle to produce realistic dwarf galaxies, leading either to overquenched or too centrally concentrated, dispersion dominated systems when compared to observed field dwarfs. Accounting for radiation with cosmic rays results in extended and rotationally-supported systems. Spatially `distributed' feedback leads to realistic stellar and HI masses as well as kinematics. Furthermore, resolved kinematic maps of our full-physics models predict kinematically distinct clumps and kinematic misalignments of stars, HI and HII after star formation events. Episodic star formation combined with its associated feedback induces more core-like dark matter central profiles, which our `SN feedback-only' models struggle to achieve. Our results demonstrate the complexity of physical processes required to capture realistic dwarf galaxy properties, making tangible predictions for integral field unit surveys, radio synchrotron emission, and for galaxy and multi-phase interstellar medium properties that JWST will probe.

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arXiv2023

Forgotten treasures in the HST/FOC UV imaging polarimetric archives of active galactic nuclei. I. Pipeline and benchmarking against NGC~1068 and exploring IC~5063

Over its 13 years of operation (1990 -- 2002), the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST) observed 26 individual active galactic nuclei (AGNs) in ultraviolet (UV) imaging polarimetry. However, not all of the observations have been reduced and analyzed or set within a standardized framework. We plan to reduce and analyze the AGN observations that have been neglected in the FOC archives using a consistent, novel, and open-access reduction pipeline of our own. We then extend the method to the full AGN sample, thus leading to potential discoveries in the near future. We developed a new pipeline in Python that will be able to reduce all the FOC observations in imaging polarimetry in a homogeneous way. Most of the previously published reduced observations are dispersed throughout the literature, with the range of different analyses and approaches making it difficult to fully interpret the FOC AGN sample. By standardizing the method, we have enabled a coherent comparison among the different observational sets. In this first paper of a series exploring the full HST/FOC AGN sample, we present an exhaustively detailed account of how to properly reduce the observational data. Current progress in data-analysis is implemented in and has provided state-of-the-art UV polarimetric maps. We compare our new maps to the benchmark AGN case of NGC~1068 and successfully reproduce the main results previously published, while pushing the polarimetric exploration of this AGN futher, thanks to a finer resolution and a higher signal-to-noise ratio (S/N) than previously reported. We also present, for the first time, an optical polarimetric map of the radio-loud AGN IC~5063 and we examine the complex interactions between the AGN outflows and the surrounding interstellar medium (ISM).

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arXiv2023

The magnetic fields of starburst galaxies. I. Identification and characterization of the thermal polarization in the galactic disk and outflow

Far-infrared polarized emission by means of magnetically aligned dust grains is an excellent tracer of the magnetic fields (B-fields) in the cold phase of the galactic outflows in starburst galaxies. We present a comprehensive study of the B-fields in three nearby ($3.5$-$17.2$ Mpc) starbursts (M82, NGC 253, and NGC 2146) at $5$ pc-$1.5$ kpc resolutions using publicly available $53$-$890$ $μ$m imaging polarimetric observations with SOFIA/HAWC+, JCMT/POL-2, and ALMA. We find that the polarized spectral energy distributions (SEDs) of the full galaxies are dominated by the polarized SEDs of the outflows with dust temperatures of $T_{\rm{d,outflow}}^{\rm{PI}}\sim45$ K and emissive index of $β_{\rm{outflow}}^{\rm{PI}}\sim2.3$. The disks are characterized by low $T_{\rm{d,disk}}^{\rm{PI}}=[24,31]$ K and $β_{\rm{disk}}^{\rm{PI}}\sim1$. We show that disk- and outflow-dominated galaxies can be better distinguished by using polarized SEDs instead of total SEDs. We compute the $53$-$850$ $μ$m polarization spectrum of the disk and outflow and find that dust models of the diffuse ISM can reproduce the fairly constant polarization spectrum of the disk, $\langle P_{\rm{disk}} \rangle=1.2\pm0.5$%. The dust models of heterogenous clouds and two temperature components are required to explain the polarization spectrum of the outflow ($2$-$4$% at $53$ $μ$m, $\sim1$% at $850$ $μ$m, and a minimum within $89$-$154$ $μ$m). We conclude that the polarized dust grains in the outflow arise from a dust population with higher dust temperature and emissivities than those from the total flux. The B-fields of the outflows have maximum extensions within $89$-$214$ $μ$m reaching heights of $\sim4$ kpc, and flatter polarized fluxes than total fluxes. The extension of the B-field permeating the circumgalactic medium increases with increasing the star formation rate.

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arXiv2023

Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- V: First results on the magnetic field orientation of galaxies

We present the analysis of the magnetic field ($B$-field) structure of galaxies measured with far-infrared (FIR) and radio (3 and 6 cm) polarimetric observations. We use the first data release of the Survey on extragALactic magnetiSm with SOFIA (SALSA) of 14 nearby ($<20$ Mpc) galaxies with resolved (5 arcsec-18 arcsec; $90$ pc--$1$ kpc) imaging polarimetric observations using HAWC+/SOFIA from $53$ to $214$ \um. We compute the magnetic pitch angle ($Ψ_{B}$) profiles as a function of the galactrocentric radius. We introduce a new magnetic alignment parameter ($ζ$) to estimate the disordered-to-ordered $B$-field ratio in spiral $B$-fields. We find FIR and radio wavelengths to not generally trace the same $B$-field morphology in galaxies. The $Ψ_{B}$ profiles tend to be more ordered with galactocentric radius in radio ($ζ_{\rm{6cm}} = 0.93\pm0.03$) than in FIR ($ζ_{\rm{154μm}} = 0.84\pm0.14$). For spiral galaxies, FIR $B$-fields are $2-75$\% more turbulent than the radio $B$-fields. For starburst galaxies, we find that FIR polarization is a better tracer of the $B$-fields along the galactic outflows than radio polarization. Our results suggest that the $B$-fields associated with dense, dusty, turbulent star-forming regions, those traced at FIR, are less ordered than warmer, less-dense regions, those traced at radio, of the interstellar medium. The FIR $B$-fields seem to be more sensitive to the activity of the star-forming regions and the morphology of the molecular clouds within a vertical height of few hundred pc in the disk of spiral galaxies than the radio $B$-fields.

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arXiv2023

The structure of magnetic fields in spiral galaxies: a radio and far-infrared polarimetric analysis

We propose and apply a method to quantify the morphology of the large-scale ordered magnetic fields (B-fields) in galaxies. This method is adapted from the analysis of Event Horizon Telescope polarization data. We compute a linear decomposition of the azimuthal modes of the polarization field in radial galactocentric bins. We apply this approach to five low-inclination spiral galaxies with both far-infrared (FIR: 154 $μ$m) dust polarimetric observations taken from the Survey of ExtragALactic magnetiSm with SOFIA (SALSA) and radio (6 cm) synchrotron polarization observations. We find that the main contribution to the B-field structure of these spiral galaxies comes from the $m=2$ and $m=0$ modes at FIR wavelengths and the $m=2$ mode at radio wavelengths. The $m=2$ mode has a spiral structure and is directly related to the magnetic pitch angle, while $m=0$ has a constant B-field orientation. The FIR data tend to have a higher relative contribution from other modes than the radio data. The extreme case is NGC 6946: all modes contribute similarly in the FIR, while $m=2$ still dominates in the radio. The average magnetic pitch angle in the FIR data is smaller and has greater angular dispersion than in the radio, indicating that the B-fields in the disk midplane traced by FIR dust polarization are more tightly wound and more chaotic than the B-field structure in the radio, which probes a larger volume. We argue that our approach is more flexible and model-independent than standard techniques, while still producing consistent results where directly comparable.

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arXiv2023

The origin of dust polarization in the Orion Bar

The linear polarization of thermal dust emission provides a powerful tool to probe interstellar and circumstellar magnetic fields, because aspherical grains tend to align themselves with magnetic field lines. While the Radiative Alignment Torque (RAT) mechanism provides a theoretical framework to this phenomenon, some aspects of this alignment mechanism still need to be quantitatively tested. One such aspect is the possibility that the reference alignment direction changes from the magnetic field ("B-RAT") to the radiation field k-vector ("k-RAT") in areas of strong radiation fields. We investigate this transition toward the Orion Bar PDR, using multi-wavelength SOFIA HAWC+ dust polarization observations. The polarization angle maps show that the radiation field direction is on average not the preferred grain alignment axis. We constrain the grain sizes for which the transition from B-RAT to k-RAT occur in the Orion Bar (grains > 0.1 μm toward the most irradiated locations), and explore the radiatively driven rotational disruption that may take place in the high-radiation environment of the Bar for large grains. While the grains susceptible to rotational disruption should be in supra-thermal rotation and aligned with the magnetic field, k-RAT aligned grains would rotate at thermal velocities. We find that the grain size at which the alignment shifts from B-RAT to k-RAT corresponds to grains too large to survive the rotational disruption. Therefore, we expect a large fraction of grains to be aligned at supra-thermal rotation with the magnetic field, and potentially be subject to rotational disruption depending on their tensile strength.

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arXiv2023

On the origin of radio-loudness in active galactic nuclei using far-infrared polarimetric observations

The dichotomy between radio-loud (RL) and radio-quiet (RQ) active galactic nuclei (AGN) is thought to be intrinsically related to radio jet production. This difference may be explained by the presence of a strong magnetic field (B-field) that enhances, or is the cause of, the accretion activity and the jet power. Here, we report the first evidence of an intrinsic difference in the dust polarized emission cores of four RL and five RQ obscured AGN using 89 $μ$m polarization with HAWC+/SOFIA. We find that the thermal polarized emission increases with the nuclear radio-loudness, $R_{20} = L_{\rm 5GHz}/ L_{\rm 20μm}$. The dust emission cores of RL AGN are measured to be polarized, $\sim5-11$%, while RQ AGN are unpolarized, $<1$%. For RQ AGN, our results are consistent with the observed region being filled with an unmagnetized or highly turbulent, disk and/or expanding outflow at scales of $5-130$ pc from the AGN. For RL AGN, the measured $89$ $μ$m polarization arises primarily from magnetically aligned dust grains associated with a $5-130$ pc-scale dusty obscuring structure with a toroidal B-field orientation highly offset, $65\pm22^{\circ}$, with respect to the jet axis. Our results indicate that the size and strength of the B-fields surrounding the AGN are intrinsically related to the strength of the jet power -- the stronger the jet power is, the larger and stronger the toroidal B-field is. The detection of a $\le130$ pc-scale ordered toroidal B-field suggests that a) the infalling gas that fuels RL AGN is magnetized, b) there is a magnetohydrodynamic wind that collimates the jet, and/or c) the jet is able to magnetize its surroundings.

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arXiv2022

Dissecting the active galactic nucleus in Circinus -- III. VLT/FORS2 polarimetry confirms dusty cone illuminated by a tilted accretion disc

We present polarimetric maps of the Circinus galaxy nucleus in the $BVRI$ bands, obtained with VLT/FORS2. Circinus is the closest Seyfert 2 galaxy and harbours an archetypal obscured active galactic nucleus (AGN). Recent high angular resolution imaging revealed that a major fraction of its mid-infrared (MIR) emission is coming from the polar region. Previously, we demonstrated that these observations are consistent with a model of a compact dusty disc and a hyperboloid shell, resembling a hollow cone on larger scales. Here we focus on the AGN core, up to 40 pc from the central engine, and compare the observations to the radiative transfer models. Polarization maps reveal a conical structure, coinciding with the ionization cone. The wavelength-dependence of the polarization degree indicates that scattering on dust grains is producing polarization. The observed polarization degree ($\approx1-3\%$) is lower than predicted by the models; however, this is only a lower limit, since stellar emission dominates the total flux in the optical. The observed polarization angle ($\approx30$ degrees) is reproduced by the model of a dusty disc with a hollow cone that is illuminated by a tilted anisotropic central source. An accretion disc aligned with the ionization cone axis, and alternative dust geometries, such as a paraboloid shell, or a torus enveloped by ambient dust, are inconsistent with the data. We conclude that the optical polarimetric imaging supports earlier evidence for the presence of dust in the polar region, tentatively associated with dusty outflows.

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arXiv2022

SOFIA observations of 30 Doradus: II -- Magnetic fields and large scale gas kinematics

The heart of the Large Magellanic Cloud, 30 Doradus, is a complex region with a clear core-halo structure. Feedback from the stellar cluster R$\,$136 has been shown to be the main source of energy creating multiple pc-scale expanding-shells in the outer region, and carving a nebula core in the proximity of the ionization source. We present the morphology and strength of the magnetic fields (B-fields) of 30 Doradus inferred from the far-infrared polarimetric observations by SOFIA/HAWC+ at 89, 154, and 214$\,μ$m. The B-field morphology is complex, showing bending structures around R$\,$136. In addition, we use high spectral and angular resolution [\textsc{CII}] observations from SOFIA/GREAT and CO(2-1) from APEX. The kinematic structure of the region correlates with the B-field morphology and shows evidence of multiple expanding shells. Our B-field strength maps, estimated using the Davis-Chandrasekhar-Fermi method and structure-function, show variations across the cloud within a maximum of 600, 450, and 350$\,μ$G at 89, 154, and 214$\,μ$m, respectively. We estimated that the majority of the 30 Doradus clouds are sub-critical and sub-Alfvénic. The probability distribution function of the gas density shows that the turbulence is mainly compressively driven, while the plasma beta parameter indicates supersonic turbulence. We show that the B-field is sufficient to hold the cloud structure integrity under feedback from R$\,$136. We suggest that supersonic compressive turbulence enables the local gravitational collapse and triggers a new generation of stars to form. The velocity gradient technique (VGT) using [\textsc{CII}] and CO(2-1) is likely to confirm these results.

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arXiv2022

Extragalactic magnetism with SOFIA (SALSA Legacy Program). VI. The magnetic fields in the multi-phase interstellar medium of the Antennae galaxies

Mergers are thought to be a fundamental channel for galaxy growth, perturbing the gas dynamics and the magnetic fields (B-fields) in the interstellar medium (ISM). However, the mechanisms that amplify and dissipate B-fields during a merger remain unclear. We characterize the morphology of the ordered B-fields in the multi-phase ISM of the closest merger of two spiral galaxies, the Antennae galaxies. We compare the inferred B-fields using $154~μ$m thermal dust and $11$ cm radio synchrotron emission polarimetric observations. We find that the $154~μ$m B-fields are more ordered across the Antennae galaxies than the $11$ cm B-fields. The turbulent-to-ordered $154~μ$m B-field increases at the galaxy cores and star-forming regions. The relic spiral arm has an ordered spiral $154~μ$m B-field, while the $11$ cm B-field is radial. The $154~μ$m B-field may be dominated by turbulent dynamos with high $^{12}$CO(1-0) velocity dispersion driven by star-forming regions, while the $11$ cm B-field is cospatial with high HI velocity dispersion driven by galaxy interaction. This result shows the dissociation between the warm gas mainly disturbed by the merger, and the dense gas still following the dynamics of the relic spiral arm. We find a $\sim8.9$ kpc scale ordered B-field connecting the two galaxies. The base of the tidal tail is cospatial with the HI and $^{12}$CO(1-0) emission and has compressed and/or sheared $154~μ$m and $11$ cm B-fields driven by the merger. We suggest that amplify B-fields, with respect to the rest of the system and other spiral galaxies, may be supporting the gas flow between both galaxies and the tidal tail.

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arXiv2022

Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- IV: Program overview and first results on the polarization fraction

We present the first data release of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program) with a set of 14 nearby ($<20$ Mpc) galaxies with resolved imaging polarimetric observations using HAWC+ from $53$ to $214$ $μ$m at a resolution of $5-18$" ($90$ pc $-$ $1$ kpc). We introduce the definitions and background on extragalactic magnetism, and present the scientific motivation and sample selection of the program. Here, we focus on the general trends in the emissive polarization fraction. Far-infrared polarimetric observations trace the thermal polarized emission of magnetically aligned dust grains across the galaxy disks with polarization fractions of $P=0-15$% in the cold, $T_{\rm d} = [19,48]$ K, and dense, $\log_{10}(N_{\rm HI+H_{2}}) = [19.96,22.91]$, interstellar medium. The spiral galaxies show a median $\langle P_{154μm} \rangle = 3.3\pm0.9 $% across the disks. We report the first polarized spectrum of starburst galaxies showing a minimum within $89-154$ $μ$m. The falling $53-154$ $μ$m polarized spectrum may be due to a decrease in the dust grain alignment efficiency produced by variations in dust temperatures along the line-of-sight in the galactic outflow. We find that the starburst galaxies and the star-forming regions within normal galaxies have the lowest polarization fractions. We find that 50% (7 out of 14) of the galaxies require a broken power-law in the $P-N_{HI+H_{2}}$ and $P-T_{d}$ relations with three different trends. Group 1 has a relative increase of anisotropic random B-fields produced by compression or shear of B-fields in the galactic outflows, starburst rings, and inner-bar of galaxies; and Groups 2 and 3 have a relative increase of isotropic random B-fields driven by star-forming regions in the spiral arms, and/or an increase of dust grain alignment efficiency caused by shock-driven regions or evolutionary stages of a galaxy.

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arXiv2022

The Role of Magnetic Fields in the Stability and Fragmentation of Filamentary Molecular Clouds: Two Case Studies at OMC-3 and OMC-4

We present the stability analysis of two regions, OMC-3 and OMC-4, in the massive and long molecular cloud complex of Orion A. We obtained $214~μ$m HAWC+/SOFIA polarization data, and we make use of archival data for the column density and C$^{18}$O (1-0) emission line. We find clear depolarization in both observed regions and that the polarization fraction is anti-correlated with the column density and the polarization-angle dispersion function. We find that the filamentary cloud and dense clumps in OMC-3 are magnetically supercritical and strongly subvirial. This region should be in the gravitational collapse phase and is consistent with many young stellar objects (YSOs) forming in the region. Our histogram of relative orientations (HROs) analysis shows that the magnetic field is dynamically sub-dominant in the dense gas structures of OMC-3. We present the first polarization map of OMC-4. We find that the observed region is generally magnetically subcritical except for an elongated dense core, which could be a result of projection effect of a filamentary structure aligned close to the line-of-sight. The relative large velocity dispersion and the unusual positive shape parameters at high column densities in the HROs analysis suggest that our viewing angle may be close to axes of filamentary substructures in OMC-4. The dominating strong magnetic field in OMC-4 is unfavorable for star formation and is consistent with much fewer YSOs than in OMC-3.

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arXiv2022

Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- III: First data release and on-the-fly polarization mapping characterization

We describe the data processing of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program). This first data release presents 33% (51.34h out of 155.7h, including overheads) of the total awarded time taken from January 2020 to December 2021. Our observations were performed using the newly implemented on-the-fly mapping (OTFMAP) technique in the polarimetric mode. We present the pipeline steps to obtain homogeneously reduced high-level data products of polarimetric maps of galaxies for use in scientific analysis. Our approach has a general design and can be applied to sources smaller than the field-of-view of the HAWC+ array in any given band. We estimate that the OTFMAP polarimetric mode offers a reduction of observing overheads by a factor 2.34, and an improvement in sensitivity by a factor 1.80 when compared to previously obtained polarimetric observations using the chopping and nodding mode. The OTFMAP is a significant optimization of the polarimetric mode of HAWC+ as it ultimately reduces the cost of operations of SOFIA/HAWC+ by increasing the science collected per hour of observation up to an overall factor of 2.49. The OTFMAP polarimetric mode is the standard observing strategy of SALSA. The results and quantitative analysis of this first data release are presented in Papers IV and V of the series.

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arXiv2021

Mapping the magnetic field in the Taurus/B211 filamentary cloud with SOFIA HAWC+ and comparing with simulation

Optical and infrared polarization mapping and recent Planck observations of the filamentary cloud L1495 in Taurus show that the large-scale magnetic field is approximately perpendicular to the long axis of the cloud. We use the HAWC+ polarimeter on SOFIA to probe the complex magnetic field in the B211 part of the cloud. Our results reveal a dispersion of polarization angles of $36^\circ$, about five times that measured on a larger scale by Planck. Applying the Davis-Chandrasekhar-Fermi (DCF) method with velocity information obtained from IRAM 30m C$^{18}$O(1-0) observations, we find two distinct sub-regions with magnetic field strengths differing by more than a factor 3. The quieter sub-region is magnetically critical and sub-Alfvénic; the field is comparable to the average field measured in molecular clumps based on Zeeman observations. The more chaotic, super-Alfvénic sub-region shows at least three velocity components, indicating interaction among multiple substructures. Its field is much less than the average Zeeman field in molecular clumps, suggesting that the DCF value of the field there may be an underestimate. Numerical simulation of filamentary cloud formation shows that filamentary substructures can strongly perturb the magnetic field. DCF and true field values in the simulation are compared. Pre-stellar cores are observed in B211 and are seen in our simulation. The appendices give a derivation of the standard DCF method that allows for a dispersion in polarization angles that is not small, present an alternate derivation of the structure function version of the DCF method, and treat fragmentation of filaments.

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arXiv2021

Extragalactic magnetism with SOFIA (Legacy Program) -- II: A magnetically-driven flow in the starburst ring of NGC 1097

Galactic bars are frequent in disk galaxies and they may support the transfer of matter towards the central engine of active nuclei. The barred galaxy NGC 1097 has magnetic forces controlling the gas flow at several kpc scales, which suggest that magnetic fields (B-fields) are dynamically important along the bar and nuclear ring. However, the effect of the B-field on the gas flows in the central kpc scale has not been characterized. Using thermal polarized emission at $89$ $μ$m with HAWC+/SOFIA, here, we measure that the polarized flux is spatially located at the contact regions of the outer-bar with the starburst ring. The linear polarization decomposition analysis shows that the $89$ $μ$m and radio ($3.5$ and $6.2$ cm) polarization traces two different modes, $m$, of the B-field: a constant B-field orientation and dominated by $m=0$ at $89$ $μ$m, and a spiral B-field dominated by $m=2$ at radio. We show that the B-field at 89 $μ$m is concentrated in the warmest region of a shock driven by the galactic-bar dynamics in the contact regions between the outer-bar with the starburst ring. Radio polarization traces a superposition of the spiral B-field outside and within the starburst ring. According to Faraday rotation measures between $3.5$ and $6.2$ cm, the radial component of the B-field along the contact regions points toward the galaxy's center on both sides. We conclude that gas streams outside and within the starburst ring follow the B-field, which feeds the black hole with matter from the host galaxy.

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arXiv2021

Extragalactic Magnetism with SOFIA (Legacy Program) -- I: The magnetic field in the multi-phase interstellar medium of M51

The recent availability of high-resolution far-infrared (FIR) polarization observations of galaxies using HAWC+/SOFIA has facilitated studies of extragalactic magnetic fields in the cold and dense molecular disks.We investigate if any significant structural differences are detectable in the kpc-scale magnetic field of the grand design face-on spiral galaxy M51 when traced within the diffuse (radio) and the dense and cold (FIR) interstellar medium (ISM). Our analysis reveals a complex scenario where radio and FIR polarization observations do not necessarily trace the same magnetic field structure. We find that the magnetic field in the arms is wrapped tighter at 154um than at 3 and 6 cm; statistically significant lower values for the magnetic pitch angle are measured at FIR in the outskirts (R > 7 kpc) of the galaxy. This difference is not detected in the interarm region. We find strong correlations of the polarization fraction and total intensity at FIR and radio with the gas column density and 12CO(1-0) velocity dispersion. We conclude that the arms show a relative increase of small-scale turbulent B-fields at regions with increasing column density and dispersion velocities of the molecular gas. No correlations are found with HI neutral gas. The star formation rate shows a clear correlation with the radio polarized intensity, which is not found in FIR, pointing to a small-scale dynamo-driven B-field amplification scenario. This work shows that multi-wavelength polarization observations are key to disentangling the interlocked relation between star formation, magnetic fields, and gas kinematics in the multi-phase ISM.

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arXiv2021

SOFIA observations of 30 Doradus: I -- Far-Infrared dust polarization and implications for grain alignment and disruption by radiative torques

Located in the Large Magellanic cloud and mostly irradiated by a massive-star cluster R$\,$136, 30 Doradus is an ideal target to test the leading theory of the grain alignment and rotational disruption by RAdiative Torques (RATs). Here, we use publicly available polarized thermal dust emission observations of 30 Doradus at 89, 154, and 214$\,μ$m using SOFIA/HAWC+. We analyse the variation of the dust polarization degree ($p$) with the total emission intensity ($I$), the dust temperature ($T_{\rm d}$), and the gas column density ($N_{\rm H}$) constructed from ${\it Herschel}$ data. The 30 Doradus complex is divided into two main regions relative to R$\,$136, namely North and South. In the North, we find that the polarization degree first decreases and then increases before decreasing again when the dust temperature increases toward the irradiating cluster R$\,$136. The first depolarization likely arises from the decrease of grain alignment efficiency toward the dense medium due to the attenuation of the interstellar radiation field and the increase of the gas density. The second trend (the increase of $p$ with $T_{\rm d}$) is consistent with the RAT alignment theory. The final trend (the decrease of $p$ with $T_{\rm d}$) is consistent with the RAT alignment theory only when the grain rotational disruption by RATs is taken into account. In the South, we find that the polarization degree is nearly independent of the dust temperature, while the grain alignment efficiency is higher around the peak of the gas column density and decreases toward the radiation source. The latter feature is also consistent with the prediction of the rotational disruption by RATs.

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