BOSA

New-generation dust emission templates for star-forming galaxies

Boquien and Salim

What is BOSA?

BOSA is a software suite written in Python 3, developed to facilitate the distribution and usage of IR templates presented in Boquien & Salim (2021).

What can BOSA do?

With BOSA a user can generate a set of IR templates parameterized on IR luminosity or on IR luminosity and specific SFR (sSFR). Furthermore, a user can calculate the total IR luminosity and the total (obscured+unobscured) SFR by providing between one and four IR flux points in JWST, WISE, Spitzer or Herschel filters, at 0 < z < 4.

What is the basis for templates in BOSA?

The templates (and relations) have been developed from an extensive set of IR SEDs with wavelength coverage from 12 to 500 microns, coming from the WISE and Herschel-ATLAS surveys. The observed IR SEDs have been fit with Draine et al. (2014) models in order to obtain dust parameters and continuous IR spectra. The templates have then been obtained by a fitting linear function (in log monochromatic luminosity) at each spectral wavelength.

Why use BOSA templates and relations?

1. The BOSA 21 templates are based on a large sample (2584) of normal star-forming (SF) galaxies, and as such they avoid the biases that may be present in shallow IRAS or ISO samples that picked particularly luminous galaxies.

2. Type 1 and 2 AGN have thoroughly been removed from the sample to reduce non-stellar contamination.

3. The simultaneous dependence of the templates on two parameters (L_TIR and sSFR, see image below) increases the utility at a range of redshifts and leads to more accurate results at any redshift.

4. The templates have no artificial discontinuities in either wavelength or luminosity. As a matter of fact, the use of functional relations entirely abandons the discreteness of the templates.

5. Obtaining the IR luminosities and SFRs based on single or multiple fluxes from JWST, WISE, Spitzer or Herschel is quite straightforward using the tools provided in the BOSA software suite.

Some other things you may want to know

Does the software given here correspond to what was used in the paper? Currently (v2021.6) it does. Any future modifications will be documented on this page.

Given that these templates have been developed using low-z galaxies, are they appropriate to be used at higher redshifts? BOSA templates can optionally depends on the sSFR, which should capture the IR spectrum “evolution”. The templates are based on a large dataset containing galaxies that can serve as higher redshift analogs, while mostly not being merger-driven starbursts. Redshift-dependent templates derived from stacking of high-redshift galaxies are available from other authors, but they offer less galaxy to galaxy flexibility.

How do I take advantage of the sSFR- dependent estimation of IR luminosity (or total SFR) when knowing sSFR requires knowing SFR in the first place, which is what I am after with my IR data? This can be achieved iteratively. In the first pass one can assign all galaxies some average sSFR typical for that redshift or stellar mass (or both). The sSFR can be refined in the second pass based on the IR SFR obtained in the first pass. Our tests indicate that two passes are sufficient. Stellar masses should, of course, be known.

How is it that even a single flux point can give a result? This is made possible by the fact that the templates are parameterized on an extensive quantify (IR luminosity or total SFR) as opposed to an intensive quantity such as a color. When the redshift is known, only one template can fit that single flux point, so the result is unique. When the user supplies multiple flux points we calculate L_TIR (and IR SFR) based on each point independently, which can serve as an important consistency check, or reveal, e.g., an AGN contribution affecting some wavelengths.

You provide IR luminosity (and total SFR) estimates based on sSFR-dependent relations only when those are based on a single flux point, but not for estimates from multiple bands. Why is that? We have determined in testing that when more than one flux point is present (assuming they are not very close in wavelength), the IR color information essentially confers the same benefits as the knowledge of sSFR, so the latter is redundant.

What is the idea behind providing SFR in addition to L_TIR. Is this just the obscured SFR that you obtained from L_TIR with some conversion factor? No, this is the estimate of the total (obscured plus unobscured) SFR and we derive it by parametrizing the templates directly on the total SFR (instead of on L_TIR). We have good estimates of the total SFR because we perform the SED fitting with UV, optical and IR constraints, allowing, importantly for free attenuation curves. Furthermore, our sSFR-dependent estimates of the total SFR will properly take into account the dust heating from older stellar populations.

How can I estimate the errors on the derived parameters? We do not provide errors since we are not actually fitting to the templates, but rather using functional relationships (see the paper for details). One can estimate the errors in a Monte Carlo fashion, by independently perturbing the flux points and redoing the estimation.

Other questions or comments? Please consult the paper or send us an email.