Focus on New Results from SOFIA

We report the detection of linearly polarized emission at 53 and 89 μm from the radio-loud active galactic nucleus (AGN) Cygnus A using High-resolution Airborne Wideband Camera-plus (HAWC+) on board the Stratospheric Observatory For Infrared Astronomy (SOFIA). We measure a highly polarized core of 11 ± 3% and 9 ± 2% with a position angle (PA) of polarization of 43° ± 8° and 39° ± 7° at 53 and 89 μm, respectively. We find (1) a synchrotron-dominated core with a flat spectrum (+0.21 ± 0.05) and a turnover at 543 ± 120 μm, which implies synchrotron emission is insignificant in the infrared (IR), and (2) a 2–500 μm bump peaking at ∼40 μm described by a blackbody component with color temperature of 107 ± 9 K. The polarized spectral energy distribution (SED) has the same shape as the IR bump of the total flux SED. We observe a change in the PA of polarization of ∼20° from 2 to 89 μm, which suggests a change of polarization mechanisms. The ultraviolet, optical, and near-IR (NIR) polarization has been convincingly attributed to scattering by polar dust, consistent with the usual torus scenario, though this scattered component can only be directly observed from the core in the NIR. By contrast, the gradual rotation by ∼20° toward the far-IR (FIR), and the near-perfect match between the total and polarized IR bumps, indicate that dust emission from aligned dust grains becomes dominant at 10–100 μm, with a large polarization of 10% at a nearly constant PA. This result suggests that a coherent dusty and magnetic field structure dominates the 10–100 μm emission around the AGN.

The Becklin–Neugebauer (BN) object in Orion has a large proper motion and radial velocity with respect to the gas and other stars in the region where it is presumed to have formed. Multiple dynamical interaction scenarios have been proposed to explain this motion. In one case BN is thought to have interacted with stars in the Trapezium cluster, while in another it is thought to have interacted with source I while deeply embedded in molecular gas. If there is dense gas that has been retained in close proximity to BN, it may be evidence that the latter scenario is favored. We observed BN at high spectral resolution in three windows near 6 μm using the Echelon-Cross-Echelle Spectrograph on board the Stratospheric Observatory for Infrared Astronomy targeting the ν₂ vibrational band of H₂O. Absorption from only three transitions of H₂O is detected, and through kinematic analysis is associated with cool, dense foreground gas, not BN itself. We find no evidence for H₂O absorption or emission at the systemic velocity of BN.

We report spatially resolved [C ii] λ158 μm observations of HE 0433-1028, which is the first detection of a nearby luminous active galactic nucleus (AGN; redshift 0.0355) with the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) on board the airborne Stratospheric Observatory For Infrared Astronomy (SOFIA). We compare the spatially resolved star formation tracers [C ii], as provided by our SOFIA observations, and Hα from the Multi Unit Spectroscopic Explorer (MUSE) optical integral-field spectroscopy. We find that the [C ii] emission is mainly matching the extended star formation as traced by the extinction-corrected Hα line emission but some additional flux is present. While a larger sample is needed to statistically confirm our findings and investigate possible dependencies on AGN luminosity and star formation rate, our study underlines the necessity of collecting a spatially resolved optical–far-infrared data set for nearby AGNs, and shows that it is technically feasible to collect such data sets with FIFI-LS on board SOFIA.

We present multi-wavelength data on the globally infalling molecular cloud/protostellar cluster BYF 73. These include new far-infrared (FIR) spectral line and continuum data from the Stratospheric Observatory for Infrared Astronomy's (SOFIA's) Far Infrared Field-Imaging Line Spectrometer (FIFI-LS), mid-infrared (MIR) observations with the Thermal-Region Camera Spectrograph (T-ReCS) on Gemini-South, and 3 mm continuum data from the Australia Telescope Compact Array (ATCA), plus archival data from Spitzer/Infrared Array Camera (IRAC), and Herschel/Photodetecting Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE). The FIFI-LS spectroscopy in [O i]$\lambda 63\,\mu {\rm{m}}$, [O iii]$\lambda 88\,\mu {\rm{m}}$, [O i]$\lambda 145\,\mu {\rm{m}}$, and [C ii]$\lambda 158\,\mu {\rm{m}}$ highlights different gas environments in and between the dense molecular cloud and H ii region. The photo dissociation region (PDR) between the cloud and H ii region is best traced by [O i]$\lambda 145\,\mu {\rm{m}}$ and may have density >10¹⁰ m⁻³, but the observed $\lambda 145\,\mu {\rm{m}}/\lambda 63\,\mu {\rm{m}}$ and λ63 μm/λ158 μm line ratios in the densest gas are well outside model values. The H ii region is well-traced by [C ii], with the λ158 μm/λ145 μm line ratio, indicating a density of 10^8.5 m⁻³ and a relatively weak ionizing radiation field, 1.5 ≲ log(G/G₀) ≲ 2. The T-ReCS data reveal eight protostellar objects in the cloud, of which six appear deeply embedded (A_V > 30^m or more) near the cloud's center. MIR 2 has the most massive core at ∼240 M${}_{\odot }$, more massive than all the others combined by up to tenfold, with no obvious gas outflow, negligible cooling line emission, and ∼3%–8% of its 4.7 × 10³ L${}_{\odot }$ luminosity originating from the release of gravitational potential energy. MIR 2's dynamical age may be as little as 7000 years. This fact, and the cloud's total embedded stellar mass being far less than its gas mass, confirm BYF 73's relatively early stage of evolution.

The current paradigm of Galactic Center (GC) gas motions and star formation envisions sequential star formation in streams of gas as they pass near the supermassive black hole Sgr A*. This is based on the relative positions of dense molecular clouds, the very young star-forming region Sgr B2, the much older region Sgr C, and the several Myr old Arches and Quintuplet Clusters. Because Sgr B1 is found with Sgr B2 in a common envelope of molecular gas and far-infrared emission, the two sources are thought to be physically related, even though there are indicators of a significantly greater age for Sgr B1. To clarify the status of Sgr B1, we have mapped it with the FIFI-LS spectrometer on the Stratospheric Observatory for Infrared Astronomy in the far-infrared lines of [O iii] 52 and 88 μm. From the ratios of these lines and lines measured with the Spitzer Infrared Spectrograph, we find that there are at least eight separate sub-regions that must contain the stars that excite the gas. We infer spectral energy distributions (SEDs) of the ionizing sources from models and find they are in agreement only with SEDs of late O stars augmented at the highest frequencies with interstellar X-rays from fast shocks. We suggest that although the gas, from its velocity structure, must be part of the very young Sgr B2 complex, the stars that are ionizing the gas were not formed there but are the remnants of a previous generation of star formation in the GC.

We have performed a 5–8 μm spectral line survey of the hot molecular core associated with the massive protostar AFGL 2591, using the Echelon-Cross-Echelle Spectrograph (EXES) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). We have supplemented these data with a ground-based study in the atmospheric M band around 4.5 μm using the iSHELL instrument on the Infrared Telescope Facility (IRTF), and the full N-band window from 8 to 13 μm using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the IRTF. Here we present the first detection of rovibrational transitions of CS in this source. The absorption lines are centered on average around −10 km s⁻¹ and the line widths of CS compare well with the hot component of ¹³CO (around 10 km s⁻¹). Temperatures for CS, hot ¹³CO, and ¹²CO v = 1–2 agree well and are around 700 K. We derive a CS abundance of 8 × 10⁻³ and 2 × 10⁻⁶ with respect to CO and H₂, respectively. This enhanced CS abundance with respect to the surrounding cloud (1 × 10⁻⁸) may reflect sublimation of H₂S ice followed by gas-phase reactions to form CS. Transitions are in local thermodynamic equilibrium and we derive a density of >10⁷ cm⁻³, which corresponds to an absorbing region of <0.04''. EXES observations of CS are likely to probe deeply into the hot core, to the base of the outflow. Submillimeter and infrared observations trace different components of the hot core as revealed by the difference in systemic velocities, line widths, and temperatures, as well as the CS abundance.

Sulfur has been observed to be severely depleted in dense clouds leading to uncertainty in the molecules that contain it and the chemistry behind their evolution. Here, we aim to shed light on the sulfur chemistry in young stellar objects (YSOs) by using high-resolution infrared spectroscopy of absorption by the ν₃ rovibrational band of SO₂ obtained with the Echelon-Cross-Echelle Spectrograph on the Stratospheric Observatory for Infrared Astronomy. Using local thermodynamic equilibrium models we derive physical parameters for the SO₂ gas in the massive YSO MonR2 IRS3. This yields a SO₂/H abundance lower limit of 5.6 ± 0.5 × 10⁻⁷, or >4% of the cosmic sulfur budget, and an intrinsic line width (Doppler parameter) of b < 3.20 km s⁻¹. The small line widths and high temperature (T_ex = 234 ± 15 K) locate the gas in a relatively quiescent region near the YSO, presumably in the hot core where ices have evaporated. This sublimation unlocks a volatile sulfur reservoir (e.g., sulfur allotropes as detected abundantly in comet 67P/Churyumov–Gerasimenko), which is followed by SO₂ formation by warm, dense gas-phase chemistry. The narrowness of the lines makes formation of SO₂ from sulfur sputtered off grains in shocks less likely toward MonR2 IRS3.

We present a [C ii] 158 μm map of the entire M51 (including M51b) grand design spiral galaxy observed with the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) instrument on board the Stratospheric Observatory For Infrared Astronomy (SOFIA). We compare the [C ii] emission with the total far-infrared (TIR) intensity and star formation rate (SFR) surface density maps (derived using Hα and 24 μm emission) to study the relationship between [C ii] and the star formation activity in a variety of environments within M51 on scales of 16'' corresponding to ∼660 pc. We find that [C ii] and the SFR surface density are well correlated in the central, spiral arm, and inter-arm regions. The correlation is in good agreement with that found for a larger sample of nearby galaxies at kpc scales. We find that the SFR, and [C ii] and TIR luminosities in M51, are dominated by the extended emission in M51's disk. The companion galaxy M51b, however, shows a deficit of [C ii] emission compared with the TIR emission and SFR surface density, with [C ii] emission detected only in the SW part of this galaxy. The [C ii] deficit is associated with an enhanced dust temperature in this galaxy. We interpret the faint [C ii] emission in M51b to be a result of suppressed star formation in this galaxy, while the bright mid- and far-infrared emission, which drive the TIR and SFR values, are powered by other mechanisms. A similar but less-pronounced effect is seen at the location of the black hole in M51's center. The observed [C ii] deficit in M51b suggests that this galaxy is a valuable laboratory to study the origin of the apparent [C ii] deficit observed in ultra-luminous galaxies.

We present far-infrared polarimetry observations of M82 at 53 and 154 μm and NGC 253 at 89 μm, which were taken with High-resolution Airborne Wideband Camera-plus (HAWC+) in polarimetry mode on the Stratospheric Observatory for Infrared Astronomy. The polarization of M82 at 53 μm clearly shows a magnetic field geometry perpendicular to the disk in the hot dust emission. For M82 the polarization at 154 μm shows a combination of field geometry perpendicular to the disk in the nuclear region, but closer to parallel to the disk away from the nucleus. The fractional polarization at 53 μm (154 μm) ranges from 7% (3%) off nucleus to 0.5% (0.3%) near the nucleus. A simple interpretation of the observations of M82 invokes a massive polar outflow, dragging the field along, from a region ∼700 pc in diameter that has entrained some of the gas and dust, creating a vertical field geometry seen mostly in the hotter (53 μm) dust emission. This outflow sits within a larger disk with a more typical planar geometry that more strongly contributes to the cooler (154 μm) dust emission. For NGC 253, the polarization at 89 μm is dominated by a planar geometry in the tilted disk, with weak indication of a vertical geometry above and below the plane from the nucleus. The polarization observations of NGC 253 at 53 μm were of a insufficient signal-to-noise ratio for a detailed analysis.

We present mid-infrared Stratospheric Observatory for Infrared Astronomy (SOFIA)/Echelon Cross Echelle Spectrograph (EXES) spectroscopy of Europa, seeking direct evidence of the presence of water vapor arising from plumes venting from the surface. We place quantitatively useful upper limits on the strength of water vibrational-rotational emission lines. Conversion to water mass limits is dependent on the rotational temperature of the vapor. For low rotational temperature, the limits lie below the inferred water mass from previous Hubble Space Telescope (HST) plume observations. For higher temperatures, the limits are comparable. We also present coordinated HST transit observations obtained close in time to the SOFIA observations. There is evidence for a feature close to the location of the previously seen feature north of the crater Pwyll in one of the HST images, although it was not observable by EXES given its location. We conclude that if a water plume had been active at the time of the SOFIA observation, with the strength implied by previous HST observations, then under the right Earth atmospheric and geometric conditions, the plume could have been detected by EXES; however, no infrared water vibrational-rotational emission was detected.

We present the detection at 89 μm (observed frame) of the Herschel-selected gravitationally lensed starburst galaxy HATLAS J1429-0028 (also known as G15v2.19) in 15 minutes with the High-resolution Airborne Wideband Camera-plus (HAWC+) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectacular lensing system consists of an edge-on foreground disk galaxy at z = 0.22 and a nearly complete Einstein ring of an intrinsic ultra-luminous infrared (IR) galaxy at z = 1.03. Is this high IR luminosity powered by pure star formation (SF) or also an active galactic nucleus (AGN)? Previous nebular line diagnostics indicate that it is star formation dominated. We perform a 27-band multiwavelength spectral energy distribution (SED) modeling including the new SOFIA/HAWC+ data to constrain the fractional AGN contribution to the total IR luminosity. The AGN fraction in the IR turns out to be negligible. In addition, J1429-0028 serves as a testbed for comparing SED results from different models/templates and SED codes (magphys, sed3fit, and cigale). We stress that star formation history is the dominant source of uncertainty in the derived stellar mass (as high as a factor of ∼10) even in the case of extensive photometric coverage. Furthermore, the detection of a source at z ∼ 1 with SOFIA/HAWC+ demonstrates the potential of utilizing this facility for distant galaxy studies including the decomposition of SF/AGN components, which cannot be accomplished with other current facilities.

The orientation of the magnetic field (B field) in the filamentary dark cloud GF 9 was traced from the periphery of the cloud into the L1082C dense core that contains the low-mass, low-luminosity Class 0 young stellar object (YSO) GF 9-2 (IRAS 20503+6006). This was done using SOFIA HAWC+ dust thermal emission polarimetry (TEP) at 216 μm in combination with Mimir near-infrared background starlight polarimetry (BSP) conducted in the H band (1.6 μm) and K band (2.2 μm). These observations were augmented with published I-band (0.77 μm) BSP and Planck 850 μm TEP to probe B-field orientations with offset from the YSO in a range spanning 6000 au to 3 pc. No strong B-field orientation change with offset was found, indicating remarkable uniformity of the B-field from the cloud edge to the YSO environs. This finding disagrees with weak-field models of cloud core and YSO formation. The continuity of inferred B-field orientations for both TEP and BSP probes is strong evidence that both are sampling a common B field that uniformly threads the cloud, core, and YSO region. Bayesian analysis of Gaia DR2 stars matched to the Mimir BSP stars finds a distance to GF 9 of 270 ± 10 pc. No strong wavelength dependence of B-field orientation angle was found, contrary to previous claims.

We detect widespread [C ii] 157.7 μm emission from the inner 5 kpc of the active galaxy NGC 4258 with the SOFIA integral field spectrometer FIFI-LS. The emission is found to be associated with warm H₂, distributed along and beyond the end of the southern jet, in a zone known to contain shock-excited optical filaments. It is also associated with soft X-ray hotspots, which are the counterparts of the "anomalous radio arms" of NGC 4258, and a 1 kpc long filament on the minor axis of the galaxy that contains young star clusters. Palomar CWI Hα integral field spectroscopy shows that the filament exhibits non-circular motions within NGC 4258. Many of the [C ii] profiles are very broad, with the greatest line width, 455 km s⁻¹, observed at the position of the southern jet bow-shock. Abnormally high ratios of L([C ii])/L(FIR) and L([C ii])/L(PAH 7.7 μm) are found along and beyond the southern jet and in the X-ray hotspots. These are the same regions that exhibit unusually large intrinsic [C ii] line widths. This suggests that the [C ii] traces warm molecular gas in shocks and turbulence associated with the jet. We estimate that as much as 40% (3.8 × 10³⁹ erg s⁻¹) of the total [C ii] luminosity from the inner 5 kpc of NGC 4258 arises in shocks and turbulence (<1% bolometric luminosity from the active nucleus), the rest being consistent with [C ii] excitation associated with star formation. We propose that the highly inclined jet is colliding with, and being deflected around, dense irregularities in a thick disk, leading to significant energy dissipation over a wide area of the galaxy.