Galaxy Line Emission & Absorption Modeling

Citation: Andra Stroe and Victor-Nicolae Savu 2021 AJ 161 158

gleam is a Python package for fitting Gaussian models to emission and absorption lines in large samples of 1D galaxy spectra. gleam is tailored to work well without much human interaction on optical and infrared spectra in a wide range of instrument setups and signal-to-noise regimes. gleam will create a fits table with Gaussian line measurements, including central wavelength, width, height and amplitude, as well as estimates for the continuum under the line and the line flux, luminosity, equivalent width and velocity width. gleam will also, optionally, make plots of the spectrum with fitted lines overlaid.

• Process large numbers of sources in batch mode.
• Jointly fit lines located close together, report upper limits and identify lines without spectral coverage.
• A single configuration file for an entire project, which you can use to customize the fitting for specific telescope/instrument configurations or even single sources.
• Human-readable YAML configuration file, in which units can be specified.
• Plots of an entire spectrum with the line fits, as well as plots for each individual line fitted.
• Well integrated with Astropy, which enables the use of units and fits tables.
• Uses LMFIT to perform the fitting and report errors on fit parameters.
• Simple installation with pip.

gleam fits lines in 1D spectra using redshift information from a metadata file and several other parameters from a central configuration file.

To run gleam, the following are needed:

• A set of 1D spectra (optical convention, λ versus F_λ), in fits table format
• A set of metadata files, in fits table or ASCII format, where details of each source are listed
• A configuration file, in YAML format, to specify line lists and fitting constraints
• A line catalog, in fits table format
• (Optional) A sky absorption/emission catalog, in fits table format

Details on the input files can be found further down.

The outputs of gleam include:

• A fits table with line measurements for each source
• (Optional) Plots of the spectrum with overplotted line fits and upper limits.

To run the gleam using the defaults, you can type in the terminal:

gleam

gleam has a number of optional command line arguments. For details type:

gleam --help

An example dataset is contained within the git repository. To download it, either use the download button or in the terminal:

wget https://github.com/multiwavelength/gleam/raw/main/example.tar.gz

The input spectra should be in fits format, ideally with units in the headers. They should contain 3 columns: the observed wavelength, the flux and the error, as follow:

Note: gleam assumes that the spectrum in given in the optical convention, i.e. wavelength (λ) versus flux density (F_λ). If the spectral axis is given in frequencies (ν), or if the y axis is given in λF_λ, F_ν, or νF_ν, gleam will not produce the intended results.

In order to identify source across the spectra and the metadata files, a naming convention needs to be followed:

• spec1d.Sample.Setup.Pointing.SourceNumber.fits

The metadata file contains information about individual sources in the project, such as the setup and pointing they were observed with, the source number to identify them and their redshift. The metadata file is used to pull information about each source. You can have a single metadata file or multiple ones, as long as sources are unique between them.

The metadata file can be in fits format or ASCII format (with commented header), but should contain the following columns:

Column descriptions:

• Setup: the telescope, instrument or mode the source was observed with (must match with setup in the spectrum name)
• Pointing: usually multiple pointings or configurations are observed (must match with pointing in the spectrum name),
• SourceNumber: a unique identifier for the source, within a single setup+pointing combination (must match with source number in the spectrum name),
• Sample: parent sample for the source, e.g. if part of a single galaxy cluster or a famous field (must match with sample in the spectrum name),
• Redshift: redshift for the source, ideally correct to within 0.0001.

Metadata files must start with "meta.":

• meta.Sample.Setup.Pointing.fits
• meta.Sample.Setup.Pointing.dat
• meta.fits

The configuration file enables the user to customize the fitting at 4 levels: use the gleam defaults as much as possible, use a global set of parameters for an entire project, specify a number of telescope/instrument specific overrides or even specify overrides for individual sources.

A minimal working configuration example:

globals:
  line_table: line_lists/Main_optical_lines.fits
  resolution: 4.4 Angstrom

To report luminosities based on the fitted models, gleam uses Hubble constant, Omega0 and the CMB temperature. While these parameters are reasonably accurate and up to date, your project may require slightly different values. You can use the cosmology section to override one or more of these parameters. The same cosmology is going to be used consistently across all the spectra within a project.

Here is another full example of a configuration file demonstrating how you could define custom cosmological parameters for your project (the values here happen correspond to the defaults):

globals:
  line_table: line_lists/Main_optical_lines.fits
  resolution: 4.4 Angstrom

cosmology:
  H0: 75 km / (Mpc s)
  Om0: 0.3
  Tcmb0: 2.725 K

The configuration has a few more fully customizable parameters, related to model fitting, line selection and sky absorption masking.

With gleam, you can analyze large numbers of spectra in a uniform manner, even with data taken in different conditions, with different instruments on different telescopes and for a wide variety of sources. To make it easy to capture the specifics of each spectrum, gleam offers you the possibility to specify parameters at three different levels.

1. The global level (globals) allows you to override the default configuration for all the spectra. This is the most coarse level of customization while the next levels provide more fine-grained overrides.
2. The setup level offers a way to apply configuration overrides to groups of spectra (named setups). Each spectrum belongs to exactly one setup. What setups mean is entirely up to you. In general, you would use this level to capture differences between telescopes or instruments, such as the spectral resolution. The configuration parameters specified at this level supersede the the global configuration and the built-in defaults. Note that the setup name needs to match that in the corresponding sources.
3. The per-source level (named sources) allows you to customize the parameters for each and every source. While this can be very helpful to account for some particularly troublesome cases, it should be used sporadically both due to the associated typing burden as well as in the spirit of keeping the results comparable. The naming convention of the source should be in line with the input spectrum file, without the 'spec1d' and '.fits'. For example:
   • Example source within sources: "Sample.Setup.Pointing.SourceNumber"

The full structure of the configuration file is:

globals:
  <global overrides> ...
setups:
  <setup name>:
    <per-setup overrides> ...
  <setup name>:
    <per-setup overrides> ...  
sources:
  <source locator>:
    <per-source overrides> ...
cosmology:
  H0: 75 km / (Mpc s)
  Om0: 0.3
  Tcmb0: 2.725 K

The parameters for each spectrum will be computed by stacking the applicable overrides on top of the default in order: first the global overrides, then the applicable per-setup overrides (if any) and finally the applicable per-source overrides.

Here are the parameters that can be overridden at either the global, setup or source level.

There are two parameters that you can use to control whether the fitting should ignore portions of the spectrum where sky bands may not have been reliably subtracted.

First is the path to a catalog which defines the wavelength intervals of sky bands. The file it points to must be in the fits file format. See below for exact details of how to create this file.

sky: line_lists/Sky_bands.fits

The second is a flag that enables or disables the masking of all the sky bands in the spectrum.

mask_sky: True

By default (i.e. if no sky or mask_sky overrides are applied to a source), there is no sky masking and the entire spectrum is used. In order for masking to take place, sky must be set appropriately and mask_sky must be set to True.

Note that the two overrides don't need to be specified at the same level. For example, you might want to specify sky at the global level and then just set mask_sky to True for individual sources (or setups) for which the sky subtraction is inadequate.

By default, it will fit all lines listed in the line table. Otherwise, only the lines specified under "lines" will be fitted. One needs to use the same names for the lines as in the line table.

line_table: line_lists/Main_optical_lines.fits # line catalog
lines: # names of line to select from line table
  - Hb 
  - OIII4
  - OIII5  
  - OII
  - Ha
  - NII1
  - SII1
  - SII2

There are a number of parameters that each affect the way the line models are fit to the data. To distinguish them visually in the configuration file, they are grouped under the fitting field. However, they can be individually overridden at any of the three levels (global, per-setup and per-source).

fitting:
  # Signal to noise limit for a detection
  SN_limit: 2
  # Tolerance for considering emission lines part of a group and thus fitting them together
  tolerance: 26.0 Angstrom
  # Range around either side of the Gaussian center to be probed
  w: 3.0 Angstrom
  # Wavelength range used to mask lines
  mask_width: 20.0 Angstrom
  # Range used for selecting continuum left and right of   the source
  cont_width: 70.0 Angstrom
  # Constraints on the center of each gaussian. The options are:
  # - free: (default) the center can be anywhere within the fitting range
  # - constrained: the center must be within a distance of `w` from the expected
  #   position specified in the `line_table`
  # - fixed: the center is fixed to the expected position of the corresponding
  #   line as specified in the `line_table`
  center: constrained

The line catalog contains a list of lines to draw from when fitting. It should be in the fits format (preferably with units) and contain columns for the line name, wavelength and (optionally) the LaTeX representation of the line name (which is only used when plotting).

A subset of the lines can be specified in the configuration file, otherwise the entire list of lines from the catalog will be used for fitting.

In cases where the sky correction is not done perfectly, your data may still be affected by sky absorption or emission. You can specify a list of bands (in observed wavelength units) to avoid. These bands will be masked and disregarded for fitting and treated as if no spectral coverage is available. (i.e. no upper limits will be reported). Masking of the sky can be turned on and off in the config file.

The sky catalog must be in the fits format (preferably with units) and contain the following columns:

For each of the sources in your sample, gleam will produce a table with all of the line fits and upper limits (if possible with units derived from the input data). Each line fitted is represented in a separate row, with all the corresponding line fit details contained in different column. The table contains information from the expected wavelength of the line and the redshift of the source, to emission line fit parameters, line fluxes and equivalent widths.

All of the output files will start with "linefits" and follow the naming convention described above.

• Line fits table: "linefits.Sample.Setup.Pointing.SourceNumber.fits"

An example of the header of such a table and a description of the columns can be found below. Since there are many column, they are listed here in multiple groups.

A description of each column:

• line: Name of the line, from the input line table.
• wavelength: Rest wavelength of the line.
• latex: Name of the line in Latex format.
• z: Redshift of the source, from the input metadata file.
• zline, zline_err: Redshift derived from this particular line and its error.
• zoffset, zoffset_err: Offset between the systemic source redshift and this line.
• cont, cont_err: Continuum estimation around the line and its error.
• wl, wl_err: Central wavelength of the line and its error, estimated from the Gaussian fit.
• height, height_err: Height of the line and its error, estimated from the Gaussian fit.
• sigma, sigma_err: Sigma of the line and its error, estimated from the Gaussian fit.
• amplitude, amplitude_err: Amplitude of the line and its error, estimated from the Gaussian fit.
• flux, flux_err: Flux of the line and its error.
• luminosity, luminosity_err: Luminosity of the line and its error.
• EWrest, EWrest_err: Rest-frame equivalent width of the line and its error.
• FWHM, FWHM_err: Deconvolved full-width-at-half-maximum width, in wavelength units and its error.
• v, v_err: Deconvolved full-width-at-half-maximum velocity width, in wavelength units and its error.
• detected: True if line is detected, False if non-detection.
• covered: True if line is covered by the spectrum. False is coverage is missing at the location of the lines, i.e. fits or upper limits are not possible.

If the input spectrum has units, the line parameters will also be reported with units. When the spectral line is not covered by the spectrum, fit values and errors are omitted. If a line is not detected, gleam only reports an upper limit in the amplitude column and omits all other parameters. The FWHM and the velocity are only reported if the line is spectrally resolved.

If plotting is enabled, gleam produces two types of figures: a figure showing the entire spectrum with zoom-ins on the emission line fits. The second type of plots are focused on each line fit. Areas masked by sky are shaded gray for clarity.

• Spectrum plot with the fitted lines overlaid: "linefits.Sample.Setup.Pointing.SourceNumber.png"
• Spectrum plot zooming in on Halpha and NII1: "linefits.Sample.Setup.Pointing.SourceNumber.Ha.NII1.png"

NOTE: Plotting high quality figures makes gleam very slow (a factor of at least 15 slower than without it). Matplotlib with Latex has some memory leak issues, which can cause gleam to slowly consume all the memory. I recommend avoiding batch processing more than 500 sources when also creating plots.

• Python ^3.8
• pip ^20.0
• (Optional, but recommended) Latex, when plotting.

The recommended way to get gleam is from the Python Package Index (PyPI). The package is named astro-gleam on PyPI:

pip install astro-gleam

In rare cases, when you need to install a version that is not published on PyPI, you can install gleam directly from the source repository:

pip install git+https://github.com/multiwavelength/gleam

You can learn about what options are available when installing from source by reading the official documentation.

If you get an error that contains the following message, it means that your Linux system is missing the C libraries for your version of python.

fatal error: Python.h: No such file or directory
      4 | #include "Python.h"
        |          ^~~~~~~~~~
  compilation terminated.

To fix the issue, you need to install the dev package for your version of python. Different Linux distributions have slight differences in naming, but here is what the command would look like on a Debian-based distribution (e.g. Ubuntu, Mint) for python 3.10:

sudo apt install libpython3.10-dev

After this, the command to install gleam should work.

If you use gleam in your published projects or as a dependency in your code, please include a citation to the companion paper in the Astronomical Journal, as well as a citation to the repository through Zenodo:

Citation: Andra Stroe and Victor-Nicolae Savu 2021 AJ 161 158