MANUAL

3_FSPS_manual

User Manual:

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Manual for FSPS v3.1

July 15, 2018

1. Overview

The collection of fortran routines contained in this package allows the user to compute simple stellar pop-

ulations (SSPs) for a variety of IMFs and metallicities, and for a variety of assumptions regarding the

morphology of the horizontal branch, the blue straggler population, the post–AGB phase, and the location

in the HR diagram of the TP-AGB phase. A variety of simple and ﬂexible prescriptions for attenuation

by dust are also included, as are dust emission models based on the Draine & Li 2007 dust models. From

these SSPs the user may then generate composite stellar populations (CSPs) for a variety of star formation

histories (SFHs) and dust attenuation prescriptions. Outputs include the spectra and magnitudes of the

SSPs and CSPs at arbitrary redshift. In addition to these fortran routines a collection of IDL routines are

provided that allow easy manipulation of the output.

The user is strongly encouraged to read Conroy et al. 2009 for an overview of stellar population synthesis

(SPS) and for details regarding this collection of routines. As of v2.0, this code has been extensively calibrated

against a suite of observational data (for details see Conroy & Gunn 2010). The code package is a in essence

a highly ﬂexible SPS code and has therefore come to be called FSPS.

The rest of this manual is organized as follows. In §1 we brieﬂy discuss the overall philosophy of the code

structure and highlight the main features. In §2 we present the routines, how they are used, and provide the

user with a basic program that demonstrates the use of the routines. §3 discusses the IDL routines provided

to read and manipulate the outputs from the fortran package. §4 discusses a variety of questions (ok, only

one) the user may have.

For a description of revisions since the initial release of the code, see the ﬁle REVISION HISTORY in

the doc directory. Installation instructions are also provided in that directory.

1.1 Downloading the code

As of v2.5, FSPS is now distributed via github: github.com/cconroy20/fsps (the googlecode SVN repos-

itory is no longer supported). The code can be checked out with standard github commands. Emails will

be sent to the mailing list whenever new versions become available (this is one reason why it is essential for

users of this code to be on the mailing list).

1.2 Environment variables

As mentioned on the webpage, you will need to set an environment variable called SPS HOME that points

to the root SPS directory (i.e. the directory that contains the src,OUTPUTS, etc. directories)

1.3 Inclusion of TP-AGB spectra

The TP-AGB empirical spectral library does not extend past the rest-frame K-band and so in previous

versions the integrated spectra were not reliable beyond λ≈2.4µm. Up to v2.2 the empirical TP-AGB

spectra were extrapolated with the BaSeL library for models at the same Teﬀ and the lowest surface gravities

available in the BaSeL library. As of v2.3, the TP-AGB spectra are extrapolated blueward with simpler linear

slopes (as advocated in Lancon & Wood 2002). The spectra of the carbon stars are now extrapolated redward

with the Aringer et al. 2009 synthetic carbon star spectral library. The spectra of the oxygen-rich TP-AGB

stars are now extrapolated with the latest version of the PHOENIX stellar spectral library (the BT-SETTL

library).

1.4 Units

This code produces two types of ﬁles. The ﬁrst are spectral ﬁles (*.spec). The spectra are in units of

L⊙/Hz, i.e. they are fν. Integrating over frequency generates the bolometric luminosity of the spectrum.

Wavelengths are in angstroms in vacuum, and the wavelength array for the stellar libraries can be found in

the ﬁles BaSeL3.1/basel.lambda and MILES/miles.lambda. The full wavelength array is also now printed

in the ﬁrst line in the *.spec ﬁles. The second type of ﬁle contains magnitudes in a variety of ﬁlters. The

magnitude zero point (i.e., AB or Vega) is set by the variable compute vega mags described below. Both of

these output ﬁles contain information on the age, mass, bolometric luminosity, and star formation rate. All

of these quantities are in the log (base-10), with units of years, M⊙,L⊙, and M⊙yr−1, respectively.

1.5 Redshift Eﬀects

The code allows the user to specify a redshift (see Section 3.1.2 below). As of v2.4, specifying a non-zero

redshift aﬀects both the computed magnitudes and spectra. There are two qualitatively diﬀerent options for

redshifting. See the parameter redshift colors below for more details. As of v2.5 the returned magnitudes

include both the relevant (1 + z) factors and the distance modulus. Also as of v2.5, IGM absorption via

Madau (1995) can be turned on and will attenuate the spectrum (and mags). This feature is enabled only

for non-zero redshifts.

1.6 Filters

The current release contains 105 ﬁlters. The ﬁlter names can be found in the ﬁle FILTER LIST in the data

directory. The actual ﬁlter deﬁnitions are in the ﬁle allfilters.dat in the same directory. The transmission

proﬁles in this ﬁle include atmospheric absorption and are in units of relative response per photon (as opposed

for example to relative response per unit power). The ﬁlters are normalized internally within the code. The

output magnitudes are listed in the ﬁlter order displayed in the FILTER LIST ﬁle. The user is cautioned to

use whenever possible the exact ﬁlter transmission curve appropriate for the data being considered. There

is, for example, no such thing as THE B−band ﬁlter. Diﬀerences of a few hundredths of a magnitude are

common between diﬀerent deﬁnitions of a given ﬁlter such as B.

1.7 Computation of magnitudes

The AB magnitude through a ﬁlter b,mb, is deﬁned according to the following formuale:

hfνib=RRb

γfνd ln ν

R(ν/νb)βRb

γd ln ν(1)

mb=−2.5log10(hfνib)−48.60 (2)

where fνis the spectrum and Rb

γis the relative response per photon of the ﬁlter. The factor (ν/νb)βin

the demoninator of Equation 1 may surprise those used to working with optical photometry. Indeed, for

UV, optical, and near-IR photometry, one usually adopts β= 0 (e.g., for the GALEX, SDSS, and 2MASS

surveys). However, IR photometry typically assumes a diﬀerent calibration. For example, the IRAS,Spitzer

IRAC, and Herschel PACS and SPIRE magnitudes are frequently quoted assuming β= 1 while the Spitzer

MIPS ﬁlters adopt β= 2 (i.e., a 104K blackbody). The parameter νbis the central wavelength of the ﬁlter.

The motivation underlying this convention is that for sources with an intrinsic spectral slope β, the

bandpass-convolved ﬂux quoted at frequency νbwill be precisely the ﬂux at that frequency. For example,

observing a 104K blackbody through the MIPS ﬁlters with the above calibration will return a ﬂux density at

the central frequency of the ﬁlter that is precisely the true ﬂux at νb. In the end, this is merely a convention,

but one that must be handled carefully when comparing data to models. See the routine sps setup.f90 for

implementation.

1.8 Interpretation of the ∆Land ∆Tparameters

As described below, the user may modify the bolometric luminosity and eﬀective temperature of the TP-

AGB phase by applying overall shifts in log(Lbol) and log(Teﬀ ) via the parameters ∆Land ∆T. In previous

versions, these parameters were with respect to the default Padova model calculations circa 2008. As of v2.0,

these parameters represent shifts with respect to the best-ﬁt values found in Conroy & Gunn 2010. In other

words, leaving these values set to 0.0 means that the user adopts the calibrations described in Section 3.1.3

in Conroy & Gunn 2010.

As of v2.5 these default settings of these parameters have been redeﬁned in order to agree with the LFs

of AGB stars in the LMC; see Villaume et al. 2014 for details.

1.9 Dust absorption models

The philosophy of the primary dust absorption model closely follows the two component model of Charlot

& Fall (2000). In the general case there is dust associated with young stars (where the deﬁntion of young

is deﬁned by the parameter dust tesc) with an optical depth dust1 and a power-law attenuation curve

with index dust1 index. There is a second dust component aﬀecting all stars equally and is implemented

as a uniform screen. This component has an optical depth dust2 and an attenuation curve speciﬁed by

the parameter dust type. Currently supported attenuation models include a power-law curve with index

dust index, a Milky-Way-like curve (following CCM89 but with a variable UV bump strength), a Calzetti

et al. (2000) curve, models from Witt & Gordon (2000) and the parameterized models from Kriek & Conroy

(2013). Note that one can specify an eﬀective uniform screen (as commonly employed when implementing

the Calzetti curve, although technically one should not think of of the Calzetti law as representing a uniform

screen) by setting dust1= 0.0.

1.10 Dust emission model

A new feature in FSPS as of v2.3 is the option of including a model for dust emission (see the parameter

add dust emission below). Since there is currently no publication discussing this aspect of FSPS, a few

words regarding the details and implementation of the dust emission model are in order.

We have adopted the dust emission model of Draine & Li 2007 (DL07), which is a silicate-graphite-PAH

grain model. The model produces dust emission spectra from 1 −104µm as a function of the interstellar

radiation ﬁeld, U, expressed in units of the Milky Way radiation ﬁeld. DL07 advocate constructing spec-

tra for entire galaxies by summing up emission spectra over a range of radiation ﬁeld strengths, P(U)dU

approximated by a delta function at Umin and a power-law component from Umin < U ≤Umax. As sug-

gested by DL07, we have ﬁxed Umax = 106and the power-law slope to be α= 2.0. We therefore have:

P(U)dU = (1 −γ)δ(U−Umin) + γAU−2, where (1 −γ) is the fraction of dust mass exposed to starlight

intensity Umin and Ais a normalization constant. The DL07 model thus has three parameters, Umin,γ, and

qPAH, the latter parameter being the PAH fraction. These three parameters are contained in the parameter

set and are called duste umin,duste gamma,duste qpah. The user can deﬁne each of these parameters in

FSPS.

1.11 Circumstellar dust

Circumstellar dust around AGB stars is an option that can be included if the switch add agb dust model

is turned on, which it is by default. The circumstellar dust models are a grid of models derived from the

radiative transfer code DUSTY, see Villaume et al. 2015 for details.

1.12 AGN dust

Dust emission associated with an AGN torus is an option that can be included if the switch add agn dust

is turned on, which it is by default. In order for AGN dust to have an eﬀect one must also set the pset

parameter fagn to a non-zero value; by default this is set to 0.0. The AGN dust model is from Nenkova

et al. 2008. Note that the models available within FSPS are a very small subset of the grids provided by

Nenkova et al.; i.e., aside from the overall amplitude, the only other free parameter is the V−band optical

depth of individual clouds.

1.13 Nebular emission

Nebular continuum and line emission is included within FSPS based on Cloudy tables provided by Nell

Byler. The details of these grids are presented in Byler et al. (2017). This feature is on by default, and

the relevant switch is add neb emission. One can also selectively turn oﬀ the continuum emission, choose

between Cloudy tables that do or do not include dust, and can vary the gas-phase metallicity and ionization

parameter.

1.14 Bursts of star formation

At present the user is able to specify a single burst time and burst strength. In the future it will be possible

to specify multiple bursts. Note that while the bursts are added into the spectrophotometric outputs, they

are not included in the output SFR columns. It is up to the user to add in these components by hand to the

resulting eﬀective SFR.

1.15 Output quantities

The basic executable routines available in the public distribution of FSPS outputs two ﬁles containing

magnitudes and spectra. FSPS can also output two other ﬁles: color magnitude diagrams in all available

ﬁlters and spectral indices in the Lick system with several additional indices. In order to output these ﬁles

the user must modify the syntax calling compsp (speciﬁcally the write compsp variable).

1.16 BPASS models

As of v3.1, the BPASS stellar population models are now included in FSPS. These are binary stellar evolution

models (v2.2; Eldridge, Stanway et al. 2017) that are made available as SSP SEDs for a variety of ages,

metallicities, and IMFs. For this reason the BPASS models are handled diﬀerently than the other isochrone

models in FSPS. Speciﬁcally, because the models are tabulated directly as SSPs, the user cannot modify

the spectral library nor the adopted IMF. The BPASS models we have made available within FSPS are the

“-bin-imf135all 100” models, i.e., they assume a Salpeter IMF with an upper mass cutoﬀ of 100M⊙. We

have not included the very lowest metallicity model and have cut oﬀ the age grid at 15.8 Gyr.

1.17 How to use this code, generally speaking

The code provided in this package is optimally designed to be integrated into a larger fortran program. The

code reads all of the libraries into memory and then utilizes various routines to compute SSPs and CSPs.

The beneﬁt of the structure of this package is that it allows the user to produce very large numbers of models

relatively quickly (e.g., ∼105models in 30 minutes on a 2.66 GHz Intel processor). It also allows one to

easily integrate SPS into particular science tasks.

For those more interested in generating quick results, rather than using the more ﬂexible aspects of the

code, we also provide a routine that generates results after prompting the user for input. This program is

called autosps.exe. Note that all outputs are placed in the OUTPUTS directory, included in the tarball, by

default.

There is a well-maintained set of Python interfaces to FSPS, originally created by Dan Foreman-Mackey

and now maintained by Ben Johnson, available here: https://github.com/dfm/python-fsps.

2. Details of Fortran Routines

2.1 The sps vars.f90 Module: Common Variables, Parameters, and Structure Set-up

It is recommended that the user read through the sps vars.f90 module. This module must be called at

the beginning of every program that uses the routines described below. It sets up two types of variables:

common and parameters. Both parameters and common variables can be seen by all routines and thus

do not have to be explicitly passed during a call to a routine. The diﬀerence between the two is that the

parameters are deﬁned once in sps vars.f90 and thereafter cannot be changed. The common variables, by

contrast, can be changed by various routines (see §3.1.1). Be careful when changing common variables! The

variables in this module are well-documented. Most parameters should not be changed.

The sps vars.f90 module also sets up structures (for those who are not familiar with fortran structures,

they are similar in many ways to structures in IDL). There are two main structures that are used extensively

in many routines. One of these is deﬁned to conveniently handle the output of the compsp.f90 routine;

it should not be of much concern for most users. The second structure deﬁnes the parameter set. It is

suﬃciently important to warrant a more detailed discussion (see §3.1.2).

2.1.1 Deﬁning the isochrones and stellar libraries

At the top of the sps vars.f90 module there are lines that start with #define. These lines set switches that

tell the code to compile certain portions of the module depending on which switches are set. The switches

are binary, with ‘1’=on and ‘0’=oﬀ. So for example in the standard release of sps vars.f90 you will see:

#define BASEL 1

#define MILES 0

#define PADOVA 1

#define MIST 0

#define PARSEC 0

#define BASTI 0

#define GENEVA 0

which tells the code to use the BaSeL stellar library and the Padova isochrones. It is very important that

you ‘make clean’ every time you change one of these switches.

2.1.2 Common variables & parameters

We brieﬂy describe several of the most important and most used common variables and parameters. These

parameters are deﬁned in sps vars.f90.

•add agb dust model Parameter to turn on/oﬀ the AGB circumstellar dust emission model presented

in Villaume et al. (2014). By default this is function is turned on as of v2.5.

•add dust emission Parameter to turn on/oﬀ the dust emission model of Draine & Li (2007).

•add agn dust Parameter to turn on/oﬀ the AGN dust emission model of Nenkova et al. (2008).

•add igm absorption Parameter to turn on/oﬀ IGM absorption according to Madau (1995). By default

this option is turned oﬀ.

•add neb emission Parameter to turn on/oﬀ the nebular emission model (both continuum and line

emission), based on Cloudy models from Nell Byler. By default this option is turned on.

•add neb continuum Parameter to turn on/oﬀ the nebular continuum. By default this option is turned

on. add neb emission must be turned on for this option to have any eﬀect.

•add stellar remnants Parameter to turn on/oﬀ the inclusion of stellar remnants in the calculation

of stellar masses.

•compute vega mags A switch that sets the zero points of the magnitude system:

– 0: AB system

– 1: Vega system

•dust type Common variable deﬁning the attenuation curve for the diﬀuse dust component:

– 0: power–law; see variable dust index below.

– 1: Milky Way extinction law parameterized by Cardelli et al. 1989, with variable UV bump

strength; see variables mwr and uvb below.

– 2: Calzetti et al. 2000 attenuation curve. Note that if this option is set then the dust attenuation

is applied to all starlight equally (not split by age), and therefore the only relevant parameter is

dust2 (deﬁned below), which sets the overall normalization (you must set dust1= 0.0 for this to

work correctly).

– 3: allows the user to access a variety of attenuation curve models from Witt & Gordon 2000. See

the parameters wgp1,wgp2, and wgp3 in §3.1.2. In this case the parameters dust1 and dust2

have no eﬀect because the WG00 models specify the full attenuation curve.

– 4: Kriek & Conroy (2013) attenuation curve. In this model the slope of the curve, set by the

variable dust index, is linked to the strength of the UV bump.

•imf type Common variable deﬁning the IMF type:

– 0: Salpeter 1955

– 1: Chabrier 2003

– 2: Kroupa 2001

– 3: van Dokkum 2008

– 4: Dave 2008

– 5: tabulated piece-wise power-law IMF, speciﬁed in imf.dat ﬁle located in the data directory

(or speciﬁed via the parameter imf filename; see below).

•redshift colors

– 0: Magnitudes are computed at a ﬁxed redshift speciﬁed in the parameter set (see below)

– 1: Magnitudes are computed at a redshift that corresponds to the age of the output SSP/CSP

(assuming a redshift–age relation appropriate for a WMAP5 cosmology). This switch is useful if

the user wants to compute the evolution in observed colors of a SSP/CSP.

•smooth velocity Switch to smooth the spectrum in velocity space. If oﬀ, the spectrum is smoothed

in wavelength space. The degree of smoothing is determined by the variable sigma smooth in the

parameter set (see below).

•smooth lsf Parameter to smooth the SSPs by a tabulated instrumental LSF provided in data/lsf.dat.

The assumed units are km/s (sigma, not FWHM). Turned oﬀ by default.

•tpagb norm type Parameter to choose the normalization of the TP-AGB stars (only applies to the

Padova isochrones):

– 0: Default normalization of the Padova isochrones

– 1: Normalization from Conroy & Gunn (2010)

– 2: Normalization from Villaume et al. (2014)

•time res incr Parameter setting the factor by which the resolution of the default isochrone age array

is increased. Default value is 2, which is suﬃcient for most purposes.

•vactoair flag Parameter to write wavelengths in air rather than vacuum if set to 1. The default is

to write wavelengths in vacuum.

•verbose Parameter that controls output to screen. If set to 1 a lot of output will be printed to screen,

if set to 0 the programs will be silent.

2.1.3 The Parameter Set

The parameter set is a structure deﬁned in the sps vars.f90 module. It must be deﬁned at the beginning

of every program for the various routines to properly work. This structure acts as the primary interface

between what the user would like to compute and the various subroutines that actually do the work. Simply

deﬁning the structure at the beginning of the program will set all structure elements to their default values

(see the example program simple.f90). The elements of the structure, and the default values, are described

below. The parameters are organized by topic, e.g., star formation history, IMF, dust, and stellar evolution

parameters.

Basic Parameters

•zred Redshift. If this value is non-zero and if redshift colors= 0, the magnitudes will be computed

for the spectrum placed at redshift zred.Default value is 0.0.

•zmet Metallicity. The metallicity is speciﬁed as an integer ranging between 1 and 22 for the Padova

isochrones and between 1 and 10 for the BaSTI isochrones. A lookup table for the actual metallicities

corresponding to the integer values is provided at the end of this manual. Note that Z⊙= 0.0190.

Default value is 1.

•imf1 Logarithmic slope of the IMF over the range 0.08 < M < 0.5M⊙. Only used if imf type= 2.

Default value is 1.3.

•imf2 Logarithmic slope of the IMF over the range 0.5< M < 1.0M⊙. Only used if imf type= 2.

Default value is 2.3.

•imf3 Logarithmic slope of the IMF over the range 1.0< M < 100 M⊙. Only used if imf type= 2.

Default value is 2.3.

•vdmc IMF parameter deﬁned in van Dokkum 2008. Only used if imf type= 3. Default value is 0.08.

•mdave IMF parameter deﬁned in Dave 2008. Only used if imf type= 4. Default value is 0.5.

•evtype Only include isochrone points at a particular evolutionary phase speciﬁed in the isochrone

table (see data/ev phases.tex for phase options). Only available with the BaSTI isochrone option.

All phases used when set to -1. Default value is -1.

•masscut Only include masses above masscut in the synthesis. Default value is 150.0.

•imf filename Filename of the tabulated IMF ﬁle located in the data directory. If unset, the default

imf.dat is assumed. Only applies if imf type= 5.

•mag compute Integer array of length nbands. Allows the user to choose which magnitudes are computed

from the ﬁlter list. 1=yes, 0=no. All magnitudes are still printed to the mag ﬁle, but if the particular

band has a value of 0 in this array, the output value will be 99. Default values are 1 for all bands.

•sigma smooth Broadening of the spectrum, in units of km s−1(if smooth velocity= 1) or ˚

A, (if

smooth velocity= 0). This speciﬁes the width of the Gaussian in terms of σ,not FWHM. Default

value is 0.0.

•min wave smooth Minimum wavelength, in ˚

A, that will be smoothed if sigma smooth>0. Default

value is 1000 ˚

•max wave smooth Maximum wavelength, in ˚

A, that will be smoothed if sigma smooth>0. Default

value is 10,000 ˚

•gas logu Value of the gas ionization parameter, U; relevant only for the nebular emission model Default

value is -2.0.

•gas logz Value of the gas metallicity; relevant only for the nebular emission model Default value is

0.0.

•igm factor Factor by which to multiply the default IGM absorption optical depth Default value is

1.0.

SFH Parameters

•sfh deﬁnes the type of star formation history, normalized such that one solar mass of stars is formed

over the full SFH. Default value is 0.

– 0: SSP

– 1: A six parameter SFH (tau model plus a constant component and a burst), with parameters

tau,const,sf start,sf trunc,tburst, and fburst (see below).

– 2: Tabulated SFH deﬁned in a ﬁle called sfh.dat that must reside in the data directory (or a

ﬁle speciﬁed via the parameter sfh filename; see below). The ﬁle must contain three columns.

The ﬁrst column is time since the Big Bang in Gyr, the second is the SFR in units of solar masses

per year, the third is the absolute metallicity. An example is provided in the data directory. The

time grid in this ﬁle can be arbitrary (so long as the units are correct), but it is up to the user

to ensure that the tabulated sfh is well-sampled so that the outputs are stable. Obviously, highly

oscillatory data require dense sampling.

– 3: Reserved for special use of the tabulated SFH option. Allows the user to read in the tabu-

lated SFHs directly into the necessary arrays, bypassing the need to create sfh.dat ﬁles. Email

cconroy@cfa.harvard.edu if you would like further instructions for how to use this option.

– 4: This is the same as option 1 except that the tau-model component is replaced with a delayed

tau model of the form t e−t/τ .

– 5: Delayed tau model with a transition at a time sf trunc to a linearly decreasing SFH with the

slope speciﬁed by sf slope. See Simha et al. 2014 for details.

•tau Deﬁnes e-folding time for the SFH, in Gyr. Only used if sfh= 1 or 4. The range is 0.1< τ <

102.Default value is 1.0.

•const Deﬁnes the constant component of the SFH. This quantity is deﬁned as the fraction of mass

formed in a constant mode of SF; the range is therefore 0 ≤C≤1. Only used if sfh= 1 or 4. Default

value is 0.0.

•sf start Start time of the SFH, in Gyr. Default value is 0.0.

•sf trunc Truncation time of the SFH, in Gyr. If set to 0.0, there is no trunction. Default value is 0.0.

•tage If set to a non-zero value, the compsp routine will compute the spectra and magnitudes only at

this age, and will therefore only output one age result. The units are Gyr. (The default is for compsp

to compute and return results from t≈0 to the maximum age in the isochrones). Default value is 0.0.

•fburst Deﬁnes the fraction of mass formed in an instantaneous burst of star formation. Only used if

sfh= 1 or 4. Default value is 0.0.

•tburst Deﬁnes the age of the Universe when the burst occurs, in Gyr. If tburst>tage then there is

no burst. Only used if sfh= 1 or 4. Default value is 11.0.

•sf slope For sfh= 5, this is the slope of the SFR after time sf trunc.Default value is 0.0.

•sfh filename Filename of the tabulated SFH ﬁle located in the data directory. If unset, the default

sfh.dat is assumed. Only applies if sfh= 2.

Dust Parameters

•dust tesc Stars younger than dust tesc are attenuated by both dust1 and dust2, while stars older

are attenuated by dust2 only. Units are log(yrs). Default value is 7.0.

•dust1 Dust parameter describing the attenuation of young stellar light, i.e. where t≤dust tesc (for

details, see Conroy et al. 2009a). Speciﬁcally, it is the opacity at 5500˚

A. Default value is 0.0.

•dust2 Dust parameter describing the attenuation of old stellar light, i.e. where t >dust tesc (for

details, see Conroy et al. 2009a). Speciﬁcally, it is the opacity at 5500˚

A. Default value is 0.0.

•frac nodust Fraction of starlight that is not attenuated by the diﬀuse dust component (i.e. that is

not aﬀected by dust2). Default value is 0.0.

•frac obrun Fraction of starlight that is not attenuated by the birth cloud dust component (i.e. that

is not aﬀected by dust1). Default value is 0.0.

•dust index Power–law index of the diﬀuse dust attenuation curve. Only used when dust type=0 or

4. Default value is -0.7.

•dust1 index Power–law index of the birth cloud dust attenuation curve. Used for all dust types.

Default value is -1.0.

•mwr The ratio of total to selective absorption which characterizes the MW extinction curve: R≡

AV/E(B−V). Only used when dust type=1. Default value is 3.1.

•uvb Parameter characterizing the strength of the 2175˚

A extinction feature with respect to the standard

Cardelli et al. determination for the MW. Only used when dust type=1. Default value is 1.0.

•wgp1 Integer specifying the optical depth in the Witt & Gordon 2000 (WG00) models. Values range

from 1 −18, corresponding to optical depths of

0.25,0.50,0.75,1.00,1.50,2.00,2.50,3.00,3.50,4.00,4.50,5.00,5.50,6.00,7.00,8.00,9.00,10.0. Note that

these optical depths are deﬁned diﬀerently from the optical depths deﬁned by the parameters dust1

and dust2. See WG00 for details.

•wgp2 Integer specifying the type of large–scale geometry and extinction curve. Values range from

1−6, corresponding to MW+dusty, MW+shell, MW+cloudy, SMC+dusty, SMC+shell, SMC+cloudy.

MW= Milky Way extinction; SMC= Small Magellanic Cloud extinction. Dusty, shell, and cloudy

specify the geometry and are described in WG00.

•wgp3 Integer specifying the local geometry for the WG00 dust models. A value of 1 corresponds to

a homogeneous distribution, and a value of 2 corresponds to a clumpy distribution. See WG00 for

details.

•duste gamma Parameter of the Draine & Li (2007) dust emission model. Speciﬁes the relative con-

tribution of dust heated at a radiation ﬁeld strength of Umin and dust heated at Umin < U ≤Umax.

Allowable range is 0.0−1.0. Default value is 0.01.

•duste umin Parameter of the Draine & Li (2007) dust emission model. Speciﬁes the minimum radiation

ﬁeld strength in units of the MW value. Valid range is between 0.1 and 25.0. Default value is 1.0.

•duste qpah Parameter of the Draine & Li (2007) dust emission model. Speciﬁes the grain size distri-

bution through the fraction of grain mass in PAHs. This parameter has units of % and a valid range

of 0.0−10.0. Default value is 3.5.

•fagn This parameter is the luminosity of the AGN expressed as a fraction of the stellar bolometric

luminosity. Default value is 0.0.

•agn tau V−band optical depth of individual clouds in the AGN model of Nenkova et al. 2008. Default

value is 10.0.

Isochrone Parameters

•redgb Modify weight given to the RGB. Only available with BaSTI isochrone set. Default value is 1.0.

•dell Shift in log(Lbol) of the TP-AGB isochrones. Note that the meaning of this parameter and the

one below has changed to reﬂect the updated calibrations presented in Conroy & Gunn 2009. That is,

these parameters now refer to a modiﬁcation about the calibrations presented in that paper. Default

value is 0.0.

•delt Shift in log(Teﬀ ) of the TP-AGB isochrones. Default value is 0.0.

•fcstar Fraction of stars that the Padova isochrones identify as Carbon stars that FSPS assigns to a

Carbon star spectrum. Set this to 0.0 if for example the users wishes to turn all Carbon stars into

regular M-type stars. Valid range is 0.0−1.0. Default value is 1.0.

•sbss Speciﬁc frequency of blue straggler stars. See Conroy et al. 2009a for details and a plausible

range. Default value is 0.0.

•fbhb Fraction of horizontal branch stars that are blue. The blue HB stars are uniformly spread in

log(Teﬀ ) to 104K. See Conroy et al. 2009a for details and a plausible range. Default value is 0.0.

•pagb Weight given to the post–AGB phase. A value of 0.0 turns oﬀ post–AGB stars; a value of 1.0

implies that the Vassiliadis & Wood 1994 tracks are implemented as–is. Default value is 1.0.

2.2 Description of Fortran Routines

We now discuss the purpose and syntax of the routines in this package. §3.2.1 discusses the example routines

provided in FSPS that demonstrate how many of these routines are used. Routines included in the src

directory that are not discussed below are meant to only be accessed through other routines.

•COMPSP(write compsp,nzin,outfile,mass ssp,lbol ssp,spec ssp,pset,ocompsp)

This routine takes as input mass ssp,lbol ssp, and spec ssp, which are the outputs of the routine

ssp gen.f90. The user must also provide the parameter set pset and ﬁlename for output in outfile, if

output is desired (a blank string may be speciﬁed if no output is desired). There are four possible outputs,

speciﬁed by write compsp: No output (0), output magnitudes (1), output spectra (2), output magnitudes

and spectra (3), output spectral indices (4), output color-magnitude diagrams (5). The outputs are written

to ﬁles with the output ﬁlename with “.mags”, “.spec”, “.indx”, or “.cmd” appended.

A variety of output from this routine is saved in the ocompsp variable, which must be a structure as

deﬁned in sps vars.f90. Again, see the example routine below.

The nzin parameter sets the number of metallicity points passed. For standard, single metallicity cal-

culations, set this value to one and simply pass the outputs from ssp gen.f90. However, if one wants to

compute spectra for an evolving metallicity (i.e. when computing a tabulated SFH, the metallicity history

may be speciﬁed), one needs to set this parameter to the number of metallicity elements available (in the

default release this is 22). One then must take care to pass the metallicity–dependent ssp gen.f90 outputs.

Currently the code only allows the speciﬁcation of a metallicity history when passing tabulated SFH.

•GETMAGS(zred,spec,mags)

The input is the redshift zred and spectrum spec. The output is an array of magnitudes mags for the

redshifted spectrum.

•GETINDX(lambda,spec,indices)

This routine computes the spectral indices for the input spectrum spec with corresponding wavelength

array lambda, returning the indices in an array indices. The indices array must be deﬁned with an array

length speciﬁed by the variable nindsps, which speciﬁes the number of indices. The indices are deﬁned in

the ﬁle allindices.dat in the data directory.

•PZ CONVOL(yield,zave,spec pz,lbol pz,mass pz)

This routine convolves the full array of metallicity-dependent SSPs with a closed-box metallicity distri-

bution function (MDF). The yield is the only input. Outputs include the average metallicity (zave) and

the spectra, lbol, and mass integrated over the MDF. The full metallicity-dependent SSPs must have been

previously set up. An example of how to use this routine is provided in lesssimple.f90.

•SFHSTAT(pos,model,ssfr6,ssfr7,ssfr8,ave age)

This routine returns basic statistics for a given star formation history. The inputs are the parameter

set (pos) and a single element output from the compsp routine (model). The outputs are the speciﬁc SFR

(SSFR), averaged over 106, 107, and 108yrs (ssfr6, ssfr7, ssfr8), and the mass-weighted average stellar

age (ave age). An example of how to use this routine is provided in simple.f90.

•SPS SETUP(zin)

This routine must be called at least once before running any routines. It reads in all of the isochrones

and spectral libraries and stores them in a common block. If the user requires only one metallicity, then

that metallicity can be speciﬁed as zin. The metallicity must be speciﬁed as an integer corresponding to

the look-up table at the end of this manual. If the user wishes to read in all metallicities, then zin should

be set to -1.

•SSP GEN(pset,mass ssp,lbol ssp,spec ssp)

This routine takes as input the parameter set, pset, and outputs the time-dependent mass, mass ssp,

bolometric luminosity, lbol ssp, and spectrum, spec ssp, of an SSP deﬁned by the parameter set. Each

of these input variables must be properly deﬁned at the beginning of the main routine. See the example

routines for guidance.

•SMOOTHSPEC(lambda,spec,sigma,minl,maxl)

This routine broadens the input spectrum spec with corresponding wavelength array lambda by a velocity

dispersion sigma measured in km/s. Note that the velocity broadening is only approximate in that we are

ignoring the variation in dλwith λwithin each integration step.

•WRITE ISOCHRONE(outfile,zz)

This routine writes the isochrones for all ages at a single metallicity to a ﬁle with name outfile. The

metallicity of the isochrone is speciﬁed via the variable zz in the internal integer metallicity units (see the

Table at the end of this manual). At each age the isochrone is written for all of the magnitudes speciﬁed in

the FILTER LIST ﬁle.

2.2.1. Example Routines

The code package contains several routines that highlight many of the features of FSPS. The routine

simple.f90 demonstrates the basic syntax required to generate simple models. In addition, there is a less

simple routine located in the src directory, called lesssimple.f90, that highlights some more advanced

features of the code.

3. Description of IDL Routines

•res = read indx(file)

This function takes as input an index ﬁle and reads it into a simple IDL structure. The index ﬁle must be

created by the user (unlike the .spec and .mags ﬁles, which are automatically created in the compsp routine).

The format is the age followed by all of the indices deﬁned in the ﬁle INDEX LIST.

•res = read mags(file)

This function takes as input the magnitude ﬁle (*.mags) produced by compsp.f90. The output is an

IDL structure with elements including the some of the magnitudes listed in FILTER LIST and computed

by compsp.f90. Note that not all magnitudes computed and listed in the *.mags ﬁle are contained in the

resulting structure. This routine can be trivially modiﬁed to include other/all of the magnitudes computed.

•res = read spec(file)

This function takes as input the spectra ﬁle (*.spec) produced by compsp.f90. The output is an IDL

structure with elements including the time-dependent spectrum computed by compsp.f90.

•res = read cmd(file)

This function takes as input the CMD ﬁle (*.cmd) produced by compsp.f90. The output is an IDL

structure with elements including the time-dependent spectrum computed by compsp.f90.

•res = read indx(file)

This function takes as input the index ﬁle (*.indx) produced by compsp.f90. The output is an IDL

structure with elements including the time-dependent spectrum computed by compsp.f90.

•res = read fsps(file)

This function takes as input any ﬁle produced by compsp.f90. The function determines which type of

ﬁle was passed based on the ﬁlename (*.spec, *.mags, etc).

4. How do I....

4.1 add additional ﬁlters?

Adding additional ﬁlters is straightforward. There are three things the user must do: 1) modify the nbands

parameter in the sps vars.f90 routine; 2) add the ﬁlter to the allfilters.dat ﬁle located in the data

directory. Follow the format: there must be a line starting with a # sign, followed by two columns, the

ﬁrst being the wavelength in angstroms, the second being the total throughput. The ﬁlter can be of any

resolution, and need not be properly normalized. 3) The user would be wise to add details of the ﬁlter to the

FILTER LIST ﬁle located in the data directory, although this is not, strictly speaking, necessary for proper

functioning of the code.

We would appreciate it if the user would email us if they add a ﬁlter so that we can include this ﬁlter in

later releases (thereby saving others the trouble of adding ﬁlters).

5. Acknowledgements

FSPS has been in active development since 2009 and has been developed and tested by a core group of

users over the years including Martin White, Ben Johnson, Nell Byler, Alexa Villaume, Joel Leja, and Demitri

Muna. Special thanks in particular to Ben Johnson who has re-written entire routines and has extensively

tested the outputs. I would also like to thank the many researchers who have made publicly available their

codes and/or models, which made it possible to include many of the features in FSPS, including B. Aringer,

Bruce Draine, Gary Ferland, Karl Gordon, Zeljko Ivezic, Ariane Lancon, Maia Nenkova, Linda Smith, the

Padova, BaSTI, and Geneva stellar evolution groups, and the teams responsible for the MILES and BaSeL

spectral libraries including Bob Kurucz, whose stellar atmosphere and spectral libraries underpine the BaSeL

library and the C3K spectral library under development.

The development of FSPS has been supported by Packard and Sloan foundation fellowships, NASA grants

NNX14AR86G and NNX15AK14G and NSF grant AST-1524161.

Table 1 Lookup table of metallicity values depends on the isochrone set being used.

zmet Z[log(Z/Z⊙)] Z[log(Z/Z⊙)] log(Z/Z⊙)Z[log(Z/Z⊙)] Z[log(Z/Z⊙)] Z

Padova BaSTI MIST Geneva PARSEC BPASS

1 0.0002 (-1.98) 0.0003 (-1.82) -2.50 0.0010 (-1.30) 0.0001 (-2.18) 0.0001

2 0.0003 (-1.80) 0.0006 (-1.52) -2.00 0.0040 (-0.70) 0.0002(-1.88) 0.0010

3 0.0004 (-1.68) 0.0010 (-1.30) -1.75 0.0080 (-0.40) 0.0005 (-1.48) 0.0020

4 0.0005 (-1.58) 0.0020 (-1.00) -1.50 0.0200 (+0.00) 0.0010 (-1.18) 0.0030

5 0.0006 (-1.50) 0.0040 (-0.70) -1.25 0.0400 (+0.30) 0.0020 (-0.88) 0.0040

6 0.0008 (-1.38) 0.0080 (-0.40) -1.00 0.0040 (-0.58) 0.0060

7 0.0010 (-1.28) 0.0100 (-0.30) -0.75 0.0060 (-0.40) 0.0080

8 0.0012 (-1.20) 0.0200 (+0.00) -0.50 0.0080 (-0.28) 0.0100

9 0.0016 (-1.07) 0.0300 (+0.18) -0.25 0.0100 (-0.18) 0.0140

10 0.0020 (-0.98) 0.0400 (+0.30) +0.00 0.0140 (-0.04) 0.0200

11 0.0025 (-0.89) +0.25 0.0170 (+0.05) 0.0300

12 0.0031 (-0.79) +0.50 0.0200 (+0.12) 0.0400

13 0.0039 (-0.69) 0.0300 (+0.30)

14 0.0049 (-0.59) 0.0400 (+0.42)

15 0.0061 (-0.49) 0.0600 (+0.60)

16 0.0077 (-0.39)

17 0.0096 (-0.30)

18 0.0120 (-0.20)

19 0.0150 (-0.10)

20 0.0190 (+0.00)

21 0.0240 (+0.10)

22 0.0300 (+0.20)

Note. — Z⊙= 0.0190 for Padova, Z⊙= 0.0200 for BaSTI and Geneva, Z⊙= 0.0142 for MIST, Z⊙= 0.0152 for PARSEC,

and Z⊙= 0.020 for BPASS.

Table 2 Emission lines included in FSPS

923.148 Ly 923 H 1 923.156A

926.249 Ly 926 H 1 926.231A

930.751 Ly 930 H 1 930.754A

937.814 Ly 937 H 1 937.809A

949.742 Ly-δ949 H 1 949.749A

972.517 Ly-γ972 H 1 972.543A

1025.728 Ly-β1025 H 1 1025.73A

1215.6701 Ly-α1216 H 1 1215.68A

1640.42 He ii 1640 He 2 1640.00A

1661.241 O iii] 1661 O 3 1661.00A

1666.15 O iii] 1666 O 3 1666.00A

1812.205 [Ne iii] 1815 Ne 3 1815.00A

1854.716 [Al iii] 1855 Al 3 1855.00A

1862.7895 [Al iii] 1863 Al 3 1863.00A

1906.68 [C iii]C 3 1907.00A

1908.73 [C iii]C 3 1910.00A

2142.3 N ii] 2141 N 2 2141.00A

2321.664 [O iii] 2321 O 3 2321.00A

2324.21 C ii] 2326 C 2 2324.00A

2325.4 C ii] 2326 C 2 2325.00A

2326.11 C ii] 2326 C 2 2327.00A

2327.64 C ii] 2326 C 2 2328.00A

2328.83 C ii] 2326 C 2 2329.00A

2471.088 [O ii] 2471 O II 2471.00A

2661.146 [Al ii] 2660 Al 2 2660.00A

2669.951 [Al ii] 2670 Al 2 2670.00A

2796.352 Mg ii 2800 Mg 2 2795.53A

2803.53 Mg ii 2800 Mg 2 2802.71A

3109.98 [Ar iii] 3110 Ar 3 3109.00A

3343.5 [Ne iii] 3343 Ne 3 3343.00A

3722.75 [S iii] 3723 S 3 3722.00A

3727.1 [O ii] 3726 O II 3726.00A

3729.86 [O ii] 3729 O II 3729.00A

3798.987 H 3798 H 1 3797.92A

3836.485 H 3835 H 1 3835.40A

3869.86 [Ne iii] 3870 Ne 3 3869.00A

3889.75 He i3889 He 1 3888.63A

3890.166 H 3889 H 1 3889.07A

3968.59 [Ne iii] 3968 Ne 3 3968.00A

3971.198 H 3970 H 1 3970.09A

4069.75 [S ii] 4070 S II 4070.00A

4077.5 [S ii] 4078 S II 4078.00A

4102.892 H-δ4102 H 1 4101.76A

4341.692 H-γ4340 H 1 4340.49A

4364.435 [O iii] 4364 TOTL 4363.00A

4472.735 He i4472 He 1 4471.47A

4622.864 [C i] 4621 C 1 4621.00A

4725.47 [Ne iv] 4720 Ne 4 4720.00A

4862.71 H-β4861 H 1 4861.36A

4960.295 [O iii] 4960 O 3 4959.00A

5008.24 [O iii] 5007 O 3 5007.00A

5193.27 [Ar iii] 5193 Ar 3 5192.00A

5201.705 [N i] 5200 N 1 5200.00A

Table 3 Emission lines included in FSPS (cont.)

5519.242 [Cl iii] 5518 Cl 3 5518.00A

5539.411 [Cl iii] 5538 Cl 3 5538.00A

5578.89 [O i] 5578 O 1 5577.00A

5756.19 [N ii] 5756 N 2 5755.00A

5877.249 He i5877 He 1 5875.61A

6302.046 [O i] 6302 O 1 6300.00A

6313.81 [S iii] 6314 S 3 6312.00A

6365.535 [O i] 6365 O 1 6363.00A

6549.86 [N ii] 6549 N 2 6548.00A

6564.6 H-α6563 H 1 6562.85A

6585.27 [N ii] 6585 N 2 6584.00A

6679.995 He i6680 He 1 6678.15A

6718.294 [S ii] 6717 S II 6716.00A

6732.673 [S ii] 6732 S II 6731.00A

7067.138 He i7065 He 1 7065.18A

7137.77 [Ar iii] 7138 Ar 3 7135.00A

7321.94 [O ii] 7323 O II 7323.00A

7332.21 [O ii] 7332 O II 7332.00A

7334.17 [Ar iv] 7330 Ar 4 7331.00A

7753.19 [Ar iii] 7753 Ar 3 7751.00A

8581.06 [Cl ii] 8579 Cl 2 8579.00A

8729.53 [C i] 8727 C 1 8727.00A

9017.8 Pa 9015 H 1 9014.92A

9071.1 [S iii] 9071 S 3 9069.00A

9126.1 [Cl ii] 9124 Cl 2 9124.00A

9232.2 Pa 9229 H 1 9229.03A

9533.2 [S iii] 9533 S 3 9532.00A

9548.8 Pa 9546 H 1 9545.99A

9852.96 [C i] 9850 TOTL 9850.00A

10052.6 Pa-δ10050 H 1 1.00494m

10323.32 [S ii] 10331 S 2 1.03300m

10832.057 He i10829 He 1 1.08299m

10833.306 He i10833 He 1 1.08303m

10941.17 Pa-γ10939 H 1 1.09381m

12570.21 [Fe ii] 1.26µmFe 2 1.25668m

12821.578 Pa-β12819 H 1 1.28181m

17366.885 Br 17363 H 1 1.73621m

18179.2 Br 18175 H 1 1.81741m

18756.4 Pa-α18752 H 1 1.87511m

19450.89 Br-δ19447 H 1 1.94456m

21661.178 Br-γ21657 H 1 2.16553m

26258.71 Br-β26254 H 1 2.62515m

30392.02 Pf 30386 H 1 3.03837m

32969.8 Pf-δ32964 H 1 3.29609m

37405.76 Pf-γ37398 H 1 3.73953m

40522.79 Br-α40515 H 1 4.05116m

46537.8 Pf-β46529 H 1 4.65250m

51286.5 Hu-δ51277 H 1 5.12725m

59082.2 Hu-γ59071 H 1 5.90659m

69852.74 [Ar ii] 7µmAr 2 6.98000m

74599.0 Pf-α74585 H 1 7.45781m

75024.4 Hu-β75011 H 1 7.50043m

89913.8 [Ar iii] 9µmAr 3 9.00000m

Table 4 Emission lines included in FSPS (cont.)

105105.0 [S iv] 10.5µmS 4 10.5100m

123719.12 Hu-α12.4µmH 1 12.3685m

128135.48 [Ne ii] 12.8µmNe 2 12.8100m

143678.0 [Cl ii] 14.4µmCl 2 14.4000m

155551.0 [Ne iii] 15.5µmNe 3 15.5500m

187130.0 [S iii] 18.7µmS 3 18.6700m

218302.0 [Ar iii] 22µmAr 3 21.8300m

328709.0 [P ii] 32µmP 2 32.8700m

334800.0 [S iii] 33.5µmS 3 33.4700m

348140.0 [Si ii] 35µmSi 2 34.8140m

360135.0 [Ne iii] 36µmNe 3 36.0140m

518145.0 [O iii] 52µmO 3 51.8000m

573300.0 [N iii] 57µmN 3 57.2100m

606420.0 [P ii] 60µmP 2 60.6400m

631852.0 [O i] 63µmO 1 63.1700m

883564.0 [O iii] 88µmO 3 88.3300m

1218000.0 [N ii] 122µmN 2 121.700m

1455350.0 [O i] 145µmO 1 145.530m

1576429.62 [C ii] 157.7µmC 2 157.600m

2053000.0 [N ii] 205µmN 2 205.400m

3703700.0 [C i] 369µmC 1 369.700m

6097000.0 [C i] 610µmC 1 609.200m

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