The Ames Background Interstellar Medium Spectral Catalog

The Catalog

The Ames Background Interstellar Medium catalog is a collection of off-order extracted, low-resolution, 5-15 μm Spitzer-IRS spectra. To avoid the crowded Milky Way, the spectra probe regions outside a one-degree exclusion zone around the galactic plane. Observations consist of IRS-staring observations with both SL1 and SL2 data and either having a ramp time of 60 or 240 seconds to ensure enough exposure to detect the weak emission associated with the background ISM. The total number of observations is 3294; with 2813 and 481 having a 60- and 240-seconds ramp time, respectively.


Positions of the Spitzer-IRS staring observations in the catalogue overlain on a GAIA all-sky color image. Some well-known constellations are indicated as well. Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal. Left: All staring observations.

The Spectra

The CUPID software tool at version 2.0 was used to generate Basic Calibrated Data (BCD), taking into account different pointings—traced by the CLNUMPOS FITS header keyword—in a single observation. BCDs were created both with and without using the pipeline’s default dark subtraction. BCDs for a total of 4,090 positions were obtained, where those having multiple pointings and those only containing peak-up data were taken into account. Spectra were extracted from the BCDs using the The CUbe Builder for IRS Spectra Maps (CUBISM) tool. CUBISM was modified to allow automation of the process. Spectra were extracted using a 24x2 window in the off-order position and saved to disk using the IPAC table format.


Slit layout. The top four narrow rectangles resemble the long slit of the low-resolution (SL) Infrared Spectrograph (IRS) module onboard Spitzer projected onto the sky. Each slit depicts the position of the target (yellow star) in the slit when employing a nodding strategy. When the target is, in either nod position, on the left half of the slit its light is diffracted by the grating in first order (SL1). Subsequently, when the source is on the right half the light is diffracted in second order (SL2). As a bonus, the SL2 configuration also produces a spectral segment covering 7.3 ≤ λ ≤ 8.7 µm (SL3). Note that the 22" part of the slit shaded black is not used. Simultaneously, when the target is observed in one order the background is observed in the other. The on-sky geometry shown at the bottom reveals a spatial separation of 158" between the SL1 and SL2 observations of the background and a 3".6x38" (2x21 pixels) overlapping aperture.

Complementary Data

The spectroscopic data are augmented with statistical representations for E(B-V), IRAS100 μm emission, and the dust temperature as retrieved from the Galactic Dust Reddening and Extinction service at IPAC. This information is for a region covering 5 degrees on the sky. Here, the values at the reference pixel (RefPixel) are used. In addition, WISE 12 micron measurements are included, where the 430 8,000x8,000-pixel 'cleaned' tiles covering the entire sky were obtained and the 12 μm flux and standard error are computed over a 3-pixel circular aperture. These data can been downloaded separately in IPAC table format using the link below and also includes parameters derived directly from the spectra by Boersma+ (under review).

download complementary data [2.45 MB]


Boersma, C., Bregman, J.D., Allamandola, L.J., Temi, P., Marakagkoudakis, A., 2024, "The Background Interstellar Medium as Observed from Off-Order Low-Resolution Spitzer-IRS Spectra", The Astrophysical Journal, under review

Christiaan Boersma 2022-2024