The magnetic fields of the dusty nuclei and molecular outflows of Arp 220

TL;DR

The study maps magnetic fields in the dusty nuclei and fast molecular outflows of the nearby merger Arp 220 using ALMA polarimetry of 870 μm dust emission and CO(3-2) line polarization. It reports the first detection of CO(3-2) polarization in an outflow via the Goldreich-Kylafis effect and reveals distinct B-field geometries: a spiral field in the East nucleus and a field aligned with the West outflow, plus a highly polarized dusty bridge linking the nuclei. Magnetic-field strengths in the outflows are inferred to be in the milligauss range, with estimates spanning equipartition and Davis-Chandrasekhar-Fermi methods, suggesting amplification by compression and turbulence and a role in transferring metals and cosmic rays into the circumgalactic medium. These results underscore the importance of strong, organized magnetic fields in extreme starbursts and establish GK polarization as a powerful diagnostic for magnetic fields in outflows of distant galaxies.

Abstract

Galaxy mergers trigger starburst activity and galactic outflows that enrich the circumgalactic medium, profoundly impacting galaxy evolution. These phenomena are intrinsically linked to the physical conditions of the medium, which is permeated by magnetic (B) fields affecting its transport and dynamics. Here, we spatially resolve, $0.24$" (96 pc), the B-fields in the dusty and molecular outflows of Arp 220, the closest ($78$ Mpc) Ultra-Luminous Infrared Galaxy hosting two interacting nuclei, denoted as East and West. We perform ALMA $870~μ$m dust continuum polarization and CO(3-2) emission line polarization, and report the first detection of CO(3-2) emission line polarization through the Goldreich-Kylafis effect in an outflow. Dust polarization shows that Arp 220 E has a spiral-like B-field on the disk with a linear polarization fraction of $0.4\pm0.1$% that may produce the detected circular polarization passing through foreground aligned dust grains. Arp 220 W reveals a B-field parallel to the red- and blueshifted outflows in both the dust and emission line polarization maps. The outflows show a dust polarization of $0.2$%, while the CO(3-2) emission line polarization is $1-2$% at $4-6σ$ significance across independent velocity channels. A highly polarized ($3-5$%) dusty bridge has a B-field orientation of $\sim110^{\circ}$ connecting both nuclei. Mean B-field strengths of $1.6$ mG and $8$ mG for the blue- and redshifted outflows, respectively, are estimated. These strong B-fields are attributed to amplification by compression in nuclear clouds and supernova remnants. This amplified B-field is likely sustained by the turbulent kinetic energy in the outflow and may be critical in directing the transport of metals and cosmic rays into the circumgalactic medium.

Figures (6)

• Figure 1: The dusty and CO(3-2) molecular B-field orientations in Arp 220. The disks of Arp 220 E and Arp 220 W observed at 33 GHz radio wavelengths BM2015 are shown with the overlaid of our measured B-field orientations. These B-field orientations are derived from the linear polarization of the $870\,\mu$m dust continuum (grey lines; see Section \ref{['sec:DustPol']} and Fig. \ref{['fig:fig2']}) and the CO(3-2) molecular outflow (red and blue lines; see Section \ref{['sec:COPol']} and Fig. \ref{['fig:fig3']}) observed with ALMA. For reference, the figure also indicates a spiral B-field (orange line), the bipolar molecular outflow direction in Arp 220 W (blue and red arrows), the beam size ($0.24\arcsec\times0.16\arcsec$; $96\times60$ pc$^{2}$ oriented at $-31^{\circ}$) of our ALMA continuum observations (red ellipse), the central position of both cores (black crosses), and a physical scale of 200 pc.
• Figure 2: The dust continuum polarization and B-field orientation in the central $1\times1$ kpc$^{2}$ of the merger galaxy Arp220 at $345.8$ GHz ($870\,\mu$m). Top left: The panel shows the total intensity (I) in colorscale and white contours, and the polarized intensity (PI) in green dashed contours. The contours for I increase as $2^{n}\sigma_{\rm{I}}$, where $n=3,4,5,\dots$, and the contours for PI increase as $n\sigma_{\rm{PI}}$, where $n=4,5,6,\dots$. Top right: This panel displays the continuum total intensity image (as in the top-left panel), with the length of the black lines representing the polarization fraction (PF). A legend for a $1$% polarization is provided in the bottom right of the panel. The integrated total intensity, polarized intensity, polarization fraction, and B-field orientation are shown for the East (Arp 220 E, cyan) and West (Arp 220 W, orange) cores within the apertures (grey ellipses) of $0.5\arcsec\times0.6\arcsec$ ($200\times240$ pc$^{2}$) oriented at $25^{\circ}$ and $-25^{\circ}$, respectively. In both top panels, the B-field orientation (black lines) is shown at the Nyquist sampling for polarization intensities $\geq 4 \sigma_{PI}$. The synthesized beam (red ellipse) is $0.24\arcsec\times0.16\arcsec$ ($96\times60$ pc$^{2}$), oriented at $-31^{\circ}$. Bottom left: Histogram of the polarization fraction in $0.2$% bins. Bottom right: Histogram of the B-field orientation in $5^{\circ}$ bins. For both histograms, only polarization measurements with $\geq 4\sigma_{\rm{PI}}$ are included. The polarization measurements for the East (grey) and West (orange) cores are shown separately.
• Figure 3: The polarized CO(3-2) molecular outflows. Top-left: the integrated CO(3-2) spectrum of the East (top) and West (bottom) cores, measured within the same aperture as shown in Fig. \ref{['fig:fig2']}. The plot indicates the rest-frame CO(3-2) line (black dashed line) and the velocity ranges used to define the blueshifted (blue shaded region) and redshifted (red shaded region) outflows. Top-right: Fast molecular redshifted (red contours) and blueshifted (blue contours) outflows are overlaid on the total intensity and B-field orientation, derived from dust polarization data shown in Fig. \ref{['fig:fig2']}. Bottom-left: The integrated total intensity (top), Stokes Q (middle), and Stokes U (bottom) components for the redshifted outflow. Gray vertical lines denote the velocity channels that have Nyquist sampled statistically significant polarization measurements (Fig. \ref{['App:fig2']}). Bottom-right: Line emission polarization measurements are rotated by $90^{\circ}$ to display the B-field orientation of the redshifted and blueshifted outflows over the total dust continuum. The individual Nyquist sampled polarization measurements per channel that satisfy $PI/\sigma_{PI}\ge4$ are shown (Fig. \ref{['App:fig3']} shows each channel). The legend provides key properties for both the blue and redshifted outflows, including the range of polarization, the median position angle of polarization, the scatter of the position angle within the outflow, and the number of independent Nyquist-sampled polarization measurements.
• Figure 4: Left panel: A colour image of Stokes $V$ overlaid with Stokes $I$ contours. Contours are at levels of 1, 5, 20, 40, 80 and 160 mJy beam$^{-1}$. Center and Right panels: The fraction (in percentage) of the Stokes $Q$ (green), $U$ (red), $V$ (blue) flux with respect to the Stokes $I$ flux for Arp 220 E and Arp 220 W. The data are shown for two different HA intervals: 0.0h to 2.0h and 2.0h to 3.4h. The linear polarization fraction (black) is also displayed. The two horizontal dashed lines mark the 0.0 and 0.1% polarization levels.
• Figure 5: Detected CO(3--2) polarization maps per channel in the outflows within the central $1\times1$ kpc$^{2}$ region of Arp 220. Each panel shows the polarization vectors (black lines) satisfying $PI/\sigma_{PI} \ge4$ per channel overlaid on the polarized intensity (colormap) and total intensity of the blueshifted (blue contours) and redshifted (red contours) outflows. The velocity, range of polarization fraction, and polarization angles are noted at the top of each panel. Dust continuum image contours, as shown in Figure \ref{['fig:fig2']}, are also presented. Black crosses mark the central positions of Arp 220 E and Arp 220 W.