Field-Imaging Far-Infrared Line Spectrometer Research Articles

Electron Density Distribution in H ii Regions in IC 10

We present the [O III] λ52 μm map of the dwarf galaxy IC 10 obtained with the Field-Imaging Far-Infrared Line Spectrometer on board the Stratospheric Observatory for Infrared Astronomy. We combine the [O III] λ52 μm map with Herschel and Spitzer observations to estimate the electron density distribution of the brightest H ii regions of IC 10. We find that the line ratio [O III] λ88 μm/[O III] λ52 μm gives electron density (n e) values (n e [O III]) that cover a broad range, while the n e values obtained using the line ratio [S III] λ33 μm/[S III] λ18 μm (n e [S III]) are all similar within the uncertainties. n e [O III] is similar to n e [S III] for the M1, M2, and A1 regions, and it is higher than n e [S III] for the two regions, A2 and M1b, which are the brightest in the 24 μm continuum emission. These results suggest that for these regions, the two ions, O++ and S++, trace two different ionized gas components and that the properties of the ionized gas component traced by the O++ ion are more sensitive to the local physical conditions. In fact, while the gas layer traced by [S III] does not keep track of the characteristics of the radiation field, the n e [O III] correlates with the star formation rate, the dust temperature, and the 24 μm. Therefore, n e [O III] is an indicator of the evolutionary stage of the H ii region and the radiation field, with higher n e [O III] found in younger star-forming regions and in more energetic environments.

CO Excitation in High-z Main-sequence Analogues: Resolved CO(4−3)/CO(3−2) Line Ratios in DYNAMO Galaxies

The spectral line energy distribution of carbon monoxide contains information about the physical conditions of the star-forming molecular hydrogen gas; however, the relation to local radiation field properties is poorly constrained. Using ∼1–2 kpc scale Atacama Large Millimeter Array observations of CO(3−2) and CO(4−3), we characterize the CO(4−3)/CO(3−2) line ratios of local analogues of main-sequence galaxies at z ∼ 1–2, drawn from the DYnamics of Newly Assembled Massive Objects (DYNAMO) sample. We measure CO(4−3)/CO(3−2) across the disk of each galaxy and find a median line ratio of R 43 = 0.54 for the sample. This is higher than literature estimates of local star-forming galaxies and is consistent with multiple lines of evidence that indicate DYNAMO galaxies, despite residing in the local universe, resemble main-sequence galaxies at z ∼ 1–2. Comparing with existing lower-resolution CO(1−0) observations, we find R 41 and R 31 values in the range ∼0.2–0.3 and ∼0.4–0.8, respectively. We combine our kiloparsec-scale resolved line ratio measurements with Hubble Space Telescope observations of Hα to investigate the relation to the star formation rate surface density and compare this relation to expectations from models. We find increasing CO(4−3)/CO(3−2) with increasing star formation rate surface density; however, models overpredict the line ratios across the range of star formation rate surface densities we probe, in particular at the lower range. Finally, Stratospheric Observatory for Infrared Astronomy observations with the High-resolution Airborne Wideband Camera Plus and Field-Imaging Far-Infrared Line Spectrometer reveal low dust temperatures and no deficit of [Cii] emission with respect to the total infrared luminosity.

C ii] Map of the Molecular Ring and Arms of the Spiral Galaxy NGC 7331*

Abstract We present the [C ii] 157.7 μm map of galaxy NGC 7331 obtained with the Field-Imaging Far-Infrared Line Spectrometer on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). This map extends an existent Herschel/Photodetector Array Camera and Spectrometer observation of the central strip of the galaxy to encompass the entire molecular ring and much of the disk, including multiple spiral arms with intense far-IR emission. We also present Herschel archival data of the [N ii] 205 μm line, which covers a substantial part of the [C ii] SOFIA observations and allows us to estimate the neutral fraction of the [C ii] emission along the ring and disk of the galaxy. We find that the neutral fraction rises with the distance from the center. In addition, by tracing the azimuthal variation of the neutral fraction, we are able to see how our observing perspective affects this measurement. The high inclination of NGC 7331 allows us to glimpse the internal walls of the molecular ring. There, young bright stars emit UV radiation causing more [C ii] emission to be produced in the ionized gas. On the outer walls, opaque dust shrouds the rest of the ring, making the neutral medium the dominant source of [C ii] emission. Through spatial analysis comparing the [C ii] emission to tracers of gas heating, we are able to investigate how the photoelectric heating efficiency varies throughout NGC 7331 and extend global measurements of the [C ii] deficit to local environments. Since the origin of [C ii] emission has typically been studied in face-on galaxies, our results shed a new light on the interpretation of [C ii] emission, especially when studying distant galaxies with unknown inclination.

Characterizing spatial variations of PAH emission in the reflection nebula NGC 1333

ABSTRACT Infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 µm, attributed to polycyclic aromatic hydrocarbons (PAHs), show variations in relative intensity, shape, and peak position. These variations depend on the physical conditions of the photodissociation region (PDR) in which strong PAH emission arises but their relationship has yet to be fully quantified. We aim to better calibrate the response of PAH species to their environment using observations with matching apertures and spatial resolution. We present observations from the Field-Imaging Far-Infrared Line Spectrometer onboard the Stratospheric Observatory for Infrared Astronomy of the gas cooling lines [O i] 63, 146 µm and [C ii] 158 µm in the reflection nebula NGC 1333 and use archival dust continuum observations from the Photodetector Array Camera and Spectrometer (PACS) onboard Herschel. We employ PDR modelling to derive the physical conditions and compare these with the characteristics of the PAH emission as observed with the Infrared Spectrometer onboard Spitzer. We find distinct spatial characteristics for the various PAH spectral components. We conclude that the ionic bands (6.2, 7.7, 8.6, and 11.0) and the 7–9 µm emission are due to multiple PAH subpopulations and that the plateaus are distinct from the features perched on top. The 6–9 µm PAH emission exhibits a significant change in behaviour between the irradiated PDR and diffuse outskirts, confirming these bands arise from multiple PAH subpopulations with different underlying molecular properties. We find multiple promising relationships between PAH ratios and the far-ultraviolet radiation field strength but no clear correlations with the PAH ionization parameter.

Upgrading the Field-Imaging Far-Infrared Line Spectrometer for the Stratospheric Observatory for Infrared Astronomy with kinetic inductance detectors: enabling large sample (extragalactic) surveys

We present the initial design, performance improvements, and science opportunities for an upgrade to the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS). FIFI-LS efficiently measures fine structure cooling lines, delivering critical constraints of the interstellar medium and star-forming environments. The Stratospheric Observatory for Infrared Astronomy (SOFIA) provides the only far-infrared (FIR) observational capability in the world, making FIFI-LS a workhorse for FIR lines, combining optimal spectral resolution and a wide velocity range. Its continuous coverage of 51 to 203 μm makes FIFI-LS a versatile tool to investigate a multitude of diagnostic lines within our galaxy and in extragalactic environments. The sensitivity and field of view (FOV) of FIFI-LS are limited by its 90s-era photoconductor arrays. These limits can be overcome by upgrading the instrument using the latest developments in kinetic inductance detectors (KIDs). KIDs provide sensitivity gains in excess of 1.4 and allow larger arrays, enabling an increase in pixel count by an order of magnitude. This increase allows a wider FOV and instantaneous velocity coverage. The upgrade provides gains in point source observation speed by a factor >2 and in mapping speed by a factor >3.5, enabled by the improved sensitivity and pixel count. This upgrade has been proposed to NASA in response to the 2018 SOFIA Next Generation Instrumentation call.

Infrared observations of the flaring maser source G358.93−0.03

Context. Class II methanol masers are signposts of massive young stellar objects (MYSOs). Recent evidence shows that flares of these masers are driven by MYSO accretion bursts. Thus, maser monitoring can be used to identify such bursts which are hard to discover otherwise. Infrared observations reveal burst-induced changes in the spectral energy distribution (first and foremost a luminosity increase), which provide valuable information on a very intense phase of high-mass star formation. Aims. In mid-January 2019, flaring of the 6.7 GHz CH3OH maser (hereafter maser) of the MYSO G358.93-0.03 (hereafter G358) was reported. The international maser community initiated an extensive observational campaign which revealed extraordinary maser activity and yielded the detection of numerous new masering transitions. Interferometric imaging with the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array resolved the maser emitting core of the star forming region and proved the association of the masers with the brightest continuum source (MM1), which hosts a hot molecular core. These observations, however, failed to detect a significant rise in the (sub)millimeter dust continuum emission. Therefore, we performed near-infrared (NIR) and far-infrared (FIR) observations to prove or disprove whether the CH3OH flare was driven by an accretion burst. Methods. NIR imaging with the Gamma-Ray Burst Optical/Near-infrared Detector has been acquired and integral-field spectroscopy with the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) was carried out on two occasions to detect possible counterparts to the (sub)millimeter sources and compare their photometry to archival measurements. The comparison of pre-burst and burst spectral energy distributions is of crucial importance to judge whether a substantial luminosity increase, caused by an accretion burst, is present and if it triggered the maser flare. Radiative transfer modeling of the spectral energy distribution (SED) of the dust continuum emission at multiple epochs provides valuable information on the bursting MYSO. Results. The FIR fluxes of MM1 measured with FIFI-LS exceed those from Herschel significantly, which clearly confirms the presence of an accretion burst. The second epoch data, taken about 16 months later, still show increased fluxes. Our radiative transfer modeling yielded major burst parameters and suggests that the MYSO features a circumstellar disk which might be transient. From the pre-burst, burst, and post-burst SEDs, conclusions on heating and cooling time-scales could be drawn. Circumstances of the burst-induced maser relocation have been explored. Conclusions. The verification of the accretion burst from G358 is another confirmation that Class II methanol maser flares represent an alert for such events. Thus, monitoring of these masers greatly enhances the chances of identifying MYSOs during periods of intense growth. The few events known to date already indicate that there is a broad range in burst strength and duration as well as environmental characteristics. The G358 event is the shortest and least luminous accretion burst known to date. According to models, bursts of this kind occur most often.

Spectral and Spatial Characterization and Calibration of FIFI-LS — The Field Imaging Spectrometer on SOFIA

The field-imaging far-infrared line spectrometer (FIFI-LS) is a science instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA). FIFI-LS allows simultaneous observations in two spectral channels. The “blue” channel is sensitive from 51[Formula: see text][Formula: see text]m to 125[Formula: see text][Formula: see text]m and the “red” channel from 115[Formula: see text][Formula: see text]m to 203[Formula: see text][Formula: see text]m. The instantaneous spectral coverage is 1000–3000[Formula: see text]km/s in the blue and 800–2500[Formula: see text]km/s in the red channel with a spectral resolution between 150[Formula: see text]km/s and 600[Formula: see text]km/s. Each spectral channel observes a field of five by five spatial pixels on the sky. The pixel size in the blue channel is 6.14 by 6.25 square arc seconds and it is 12.2 by 12.5 square arc seconds in the red channel. FIFI-LS has been operating on SOFIA since 2014. It is available to the astronomical community as a facility science instrument. We present the results of the spectral and spatial characterization of the instrument based on laboratory measurements. This includes the measured spectral resolution and examples of the line spread function in the spectral domain. In the spatial domain, a model of the instrument’s point spread function (PSF) and the description of a second pass ghost are presented. We also provide an overview of the procedures used to measure the instrument’s field of view geometry and spectral calibration. The spectral calibration yields an accuracy of 15–60[Formula: see text]km/s depending on wavelength.

FIFI-LS: The Field-Imaging Far-Infrared Line Spectrometer on SOFIA

We describe the design of the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS), operated as a Facility-Class instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). FIFI-LS is an imaging spectrometer for medium resolution spectroscopy. Since being commissioned in 2014, it has performed over 50 SOFIA commissioning and science flights. After operating as a principal investigator instrument in 2014 and early 2015, it was accepted as a Facility Science Instrument in 2015. In addition to the description of the design, we report on the in-flight performance and the concept of operation. We also provide an overview of the science opportunities with FIFI-LS and describe how FIFI-LS observations complement and complete observations with the PACS instrument on the Herschel observatory.

The Close AGN Reference Survey (CARS): SOFIA Detects Spatially Resolved [C ii] Emission in the Luminous AGN HE 0433-1028∗

Abstract 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.

Development and Manufacture of a Linear 16-Pixel FIR Array—The PACS Module

The high- and low-stressed photodetector array camera and spectrometer (PACS) modules (linear 16-pixel arrays) are the basic constituents of the 16 × 25 pixel far-infrared (57–220 μm) PACS cameras aboard the Herschel satellite. The essentials for the stress application and the pixel stacking technique originate from detectors aboard the Infrared Space Observatory (ISO, 1995–1998) and for the field imaging far infrared line spectrometer (FIFI-LS) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). To meet the mass budget of the cameras and provide the required high tensile strength, the only usable material for the module body turned out to be an aluminum alloy (AL 7075 T651). Especially challenging was the tight thermal budget for the PACS modules. Compared to FIFI-LS, in the case of the PACS module the heat dissipation from the front end electronics (FEE) and harness substrates at about 4 K to the module body containing the pixel stack at about 2 K had to be reduced drastically. A genuine suspension concept and packaging for the respective substrates was developed and successfully applied, allowing reliable electrical wiring from the harness over the FEE on to the pixels. The FEE with the cryogenic readout electronics (CRE) is contributed by the Interuniversity Microelectronics Centre (IMEC).