DOI: 10.5281/zenodo.17244849
title: Project AMIGA

description: Project AMIGA Database and Coadded Spectra

This dataset contains measurements of absorption components and coadded COS spectra toward background sources used to study the circumgalactic medium (CGM) of Andromeda (M31) (Lehner et al. 2020, 2025).

Absorption Component Data

The data are provided in enhanced character-separated value (ECSV) format, which preserves data types and includes metadata headers. The file can be read using:```pythonfrom astropy.table import Tabledata = Table.read('amiga_data.ecsv')

The data are organized by target sightline, with each row representing an individual velocity component for a specific ion. The table is sorted by impact parameter, target name, component number, and ion.

Column Definitions



name: Target name

piname: PI name of the original HST program

ra, dec: Right Ascension and Declination (J2000, degrees)

gl, gb: Galactic longitude and latitude (degrees)

lms, bms: Magellanic Stream coordinate system longitude and latitude (degrees, following Nidever et al. 2008 definition)

dang: Angular distance from M31 center (degrees)

rho: Projected impact parameter from M31 center (kpc), assuming a distance of M31 d_M31 = 752 kpc

x_m31, y_m31: Cartesian coordinates relative to M31 center (kpc), where East is positive x and North is positive y (assuming d_M31 = 752 kpc; M31 center at α = 10.684686°, δ = 41.268703°, J2000)

ion: Ion species

vl1, vl2: Lower and upper velocity limits of the component (km/s, LSR frame)

vc: Average velocity of the component (km/s, LSR frame)

evc: Uncertainty (1 sigma) on the average velocity (km/s)

ncol: Logarithmic column density, log N [cm^-2]

encol1, encol2: Upper and lower uncertainties (1 sigma) on logarithmic column density (dex)

flg_col: Column density flag (0 = detection, -1 = upper limit, -2 = lower limit)

compno: Component number for each target (incremented sequentially for a given target)

flg_ms: Magellanic Stream flag (0 = not associated with MS, -1 = MS component)

flg_m31: M31 thick disk flag (0 = M31 CGM if flg_ms=0, -1 = M31 thick disk)


Important Notes

This file contains measurements, including some updated ones, see Appendix C in Lehner et al. (2025).

Coadded COS Spectra

File Structure

The tarball archive_project_amiga.tar.gz unpacks to a Project_AMIGA/ directory containing coadded spectra files. Each target has separate files for the G130M and G160M gratings:



Format: {target_name}_spec-{grating}.gz

Example: PG0003+158_spec-G130M.gz, PG0003+158_spec-G160M.gz


Coaddition Methodology

The COS spectra were coadded using the methodology described in Lehner et al. (2020), which addresses wavelength misalignments between exposures that can reach ±40 km/s and vary with wavelength. Key steps:



Strong ISM and IGM absorption lines are identified in each exposure

Each line is cross-correlated using ~3 Å regions to determine wavelength-dependent shifts

Shifts are applied to align absorption features across exposures

Absolute wavelength calibration is established by comparing UV absorption line centroids to H I 21 cm emission data (GBT survey, LAB, or GASS)

Aligned spectra are combined by summing counts at each pixel and converting back to flux using the average flux/count ratio

Flux errors are estimated from Poisson noise


Spectrum Format

Each gzipped ASCII file contains three columns: 



Column 1: Wavelength (Angstroms)

Column 2: Flux (1e-14 erg/cm2/s/Angstroms)

Column 3: Flux error (1e-14 erg/cm2/s/Angstroms)


The G130M grating covers approximately 1150-1450 Å, and the G160M grating covers approximately 1400-1800 Å.

Version History

See Zenodo version history for details on changes between releases.

Citation

If you use these data, please cite: Lehner et al (2020, 2025): 



Lehner et al. (2020), ApJ, 900, 9

Lehner et al. (2025), ApJ 997, 183


Contact

Nicolas Lehner (nlehner@nd.edu)