Stellar metallicity and nebular metallicity in the model

[mengtaotang]

Hi Jacopo,

I have an issue about metallicity in the BEAGLE model. I have generated a set of mock spectra to investigate how nebular spectral properties (e.g., O32 ratio) change with stellar metallicity. To create the mock spectra, I assume constant SFH with max_stellar_age = 10 Myr, and vary stellar metallicity from log(Z/Z_sun) = -2.2 to 0.0. I also fix the nebular gas parameters (log U, Z_ISM, xi_d, etc.). The parameter file is listed below. While I expect the nebular spectral properties to change with stellar metallicity (e.g., O32 ratio should be larger at lower metallicities as low-Z models predict harder ionizing spectra), the line ratio indicated from mock spectra seems does not change with different stellar metallicity. Moreover, if I look the mock spectra with different Z_star, the spectra look same at all as shown in attached figure 1 (spectra are normalized to the flux at lambda = 910 A, with some offset in order to show different spectra). The mock spectra FITS file is also attached.

I also looked at the pure stellar spectra (i.e., generated using the same stellar parameters described above but without nebular spectra added on the top). As shown in attached figure 2 (spectra are normalized to the flux at lambda = 910 A, no nebular spectra added), now I can see the difference among ionizing spectra (lambda < 912 A) with different stellar metallicities.

It seems to me that the stellar+nebular spectra are the same even with different stellar metallicities. I am wondering if it is because I fixed the nebular metallicity Z_ISM? As for me, I would expect the nebular spectra to be different as the input stellar spectra have different metallicities. Does BEAGLE allow varying both stellar and nebular metallicities separately?

Thank you! Mengtao

model_spec.fits.zip

[main]

# This is a stripped down version of the BEAGLE parameter file for 
# with checking the correct installation of BEAGLE using Docker.
# This file will be used for producing artificial photometry using
# an analytic star formation history.
# Verbose mode: 0 ---> no info
#               1 ----> print only errors
#               2 ---> print errors and warnings
#               3 ----> print errors, warnings and info 
VERBOSE = 1

# Seed for the random number generator
SEED = 994189

#****************************************************************
#******************** COSMOLOGICAL PARAMETERS *******************
#****************************************************************
#
# If left blank, default parameter values will be used.

HUBBLE_0 = 

OMEGA_MATTER =

OMEGA_LAMBDA =

#****************************************************************
# ******************* TEMPLATES  *****************************
#****************************************************************

#TEMPLATES = $BEAGLE_TEMPLATES/cb15/cb2013_s2_chab_hr_xmiless_ssp 
TEMPLATES NEBULAR = $BEAGLE_TEMPLATES/ineb_Jan16_C100/cb2013_n2_mup100_N015_O01_deplO70_C100_Jan16
EMISSION LINES CONFIGURATION = $BEAGLE_TEMPLATES/ineb_Jan16_C100/cb2013_n2_mup100_N015_O01_deplO70_C100_Jan16_line_wavelengths_PHOTOMETRY.dat

SHRINK TEMPLATES WL RANGE = 200 10000
REBIN TEMPLATES = 10

#****************************************************************
# ******************* SPECTROSCOPY ***************************
#****************************************************************

SPECTRAL INDICES CONFIGURATION = $BEAGLE_FILTERS/model_spec.dat

#SPECTRAL INDICES CATALOGUE = $BEAGLE_DATA/

FIT SPECTRUM = F

#****************************************************************
# ******************* PHOTOMETRY *****************************
#****************************************************************

#FILTERS THROUGHPUTS = $BEAGLE_FILTERS/

#FILTERS CONFIGURATION = $BEAGLE_FILTERS/

#PHOTOMETRIC CATALOGUE = $BEAGLE_DATA/

#PRIORS CATALOGUE = $BEAGLE_DATA/

FIT PHOTOMETRY = F

#****************************************************************
#******************** DUST ATTENUATION AND IGM ABSORPTION *******
#****************************************************************
#
IGM ABSORPTION = Inoue

# File containing set of radiativa transfer (RT) calculations of dust attenuation of starlight
#RT CALCULATIONS = Tuffs_bulge_NEW-28-11.dat Tuffs_thick_NEW-28-11.dat Tuffs_thin_NEW-28-11.dat
#RT COMPONENTS NAMES = bulge thick_disc thin_disc

# **************************************************
# ************** SF BIN #1 **************************
# **************************************************

SF_PARAMETER  = name:sfh_type                 mock:type:fixed  char_value:constant
#options for analytic SFHs are 'ssp' (for a burst), 'constant', 'exponential', 'delayed', 'rising'

# In log(yr)
#SF_PARAMETER  = name:tau                      type:fitted  order_priority:1  prior:distribution:uniform  prior:range:[7.,10.5]

# in log(Z/Z_sun)
SF_PARAMETER  = name:metallicity              mock:type:gridded  mock:grid:range:[-2.2, 0.0]  mock:grid:step:0.1

# In log(M/M_sun)
SF_PARAMETER  = name:mass                     mock:type:random  mock:distribution:uniform  mock:range:[5.0, 12.0]

# **************************************************
# **************  OTHER SF PARAMETERS **************************
# **************************************************

# In log(M_sun yr^-1)
#SF_PARAMETER  = name:sfr                mock:type:fixed  value:0.0

# In log(yr^-1)
#SF_PARAMETER  = name:specific_sfr                type:fitted  order_priority:1  prior:distribution:uniform  prior:range:[-14.,-7.]

# In log(yr)
#SF_PARAMETER  = name:current_sfr_timescale        mock:type:fixed  value:7.0

# In log(yr)
SF_PARAMETER  = name:max_stellar_age          mock:type:fixed  value:7.0

#****************************************************************
# ******************* PRINTED OUTPUT  *****************************
#****************************************************************

RESULTS DIRECTORY = $BEAGLE_RESULTS/model_spec/

MOCK CATALOGUE NAME = model_spec.fits

PRINT PHOTOMETRY = F
PRINT SPECTRUM = T
PRINT SPECTRAL INDICES = T
PRINT SF AND ChE HISTORIES = F

#****************************************************************
# ******************* PARAMETERS HANDLING  *****************************
#****************************************************************

PDF SAMPLER FILE = $BEAGLE_PARAM_DIR/MCMC_example.param

PARAMETER  = name:redshift      mock:type:fixed  value:0.0

PARAMETER  = name:nebular_logU  mock:type:fixed  value:-2.5

PARAMETER  = name:nebular_xi    mock:type:fixed  value:0.3

PARAMETER  = name:nebular_Z     mock:type:fixed  value:-0.7

PARAMETER  = name:attenuation_type  type:fixed  char_value:CF00   
# values can be: CF00, Calzetti, CCWW13_universal, CCWW13_full

PARAMETER  = name:tauV_eff      mock:type:fixed  value:0.0

PARAMETER  = name:mu            mock:type:fixed  value:0.4

[jacopo-chevallard]

It seems to me that the stellar+nebular spectra are the same even with different stellar metallicities. I am wondering if it is because I fixed the nebular metallicity Z_ISM? As for me, I would expect the nebular spectra to be different as the input stellar spectra have different metallicities. Does BEAGLE allow varying both stellar and nebular metallicities separately?

Hi Mengtao, this is indeed an important point to keep in mind: in Beagle, for self-consistency, we do not allow the metallicity of young stars (< 10 Myr) and of the gas to be different, while we do allow the metallicity of older stars (> 10 Myr) to be different form the gas metallicity. So if you set nebular_Z Beagle will set the metallicity of young stars to be equal to nebular_Z.

The reason (@scharlot correct me if I'm wrong) is that we expect young stars (those emitting ionizing photons) to have formed from the same gas that they then ionize, and so using different chemical compositions can be hardly justified from a physical point of view. A potential limitation of this approach (which is often highlighted by Chuck Steidel and his group) is that gas and stars are sensitive to different metal species: gas cooling is mainly driven by C, O, N, S, while stellar opacities are mostly sensitive to Fe. Ideally, one would use a self-consistent model in which the gas-phase abundance of C, O, N, S can be varied independently from the stellar Fe abundance, and this is what our models allow us to do through the dust depletion factor. It is reasonable to think, however, that for some types of galaxies dust depletion is not enough to account for differences in the gas-phase and stellar abundances, for instance because of short star formation timescales which produce alpha-enhanced chemical mixtures.

Let me know if this is clear enough !

[mengtaotang]

Hi Jacopo,

Thank you for explaining these!

Mengtao