Input Files¶
MCSED is designed to model the SEDs of a set of galaxies with a
common set of photometric and spectroscopic data. Since MCSED can
accept a wide range of constraints, with dozens of possible filter
combinations, emission line fluxes, and/or absorption line spectra
indices, the format of the input file has some flexibility. But the
basic file structure is simple: the data are entered in a simple,
space-delimited ascii file, with the first line of the file labeling the
file’s columns. The rest of the file gives the measured flux densities
and/or emission-line fluxes and/or absorption line spectral indices for
each object, one object per line.
Required Columns¶
The input data file has three required columns, which must have these
exact labels: Field, ID, and z. In other words, an
object’s identification consists in two parts: a string which contains
the name of the field in which the object is found, and a unique integer
ID which is specific to that field. If both field and ID are
unnecessary, one can simply enter a placeholder for one of the entries.
Redshifts must be specified for every source.
The remaining columns in the input file should be pairs of numbers
representing photometric flux densities (and their \(1\,\sigma\)
errors), emission line fluxes (and their errors), and/or absorption line
indices (and their errors). Since the quoted uncertainties for
photometric observations often do not include systematic and/or external
errors, MCSED also allows the user to specify a minimum fractional
uncertainty for any type of observation. The defaults for the minimum
errors can be found in config.py, and by default are set to
phot_floor_error = 0.05 (for photometric errors),
emline_floor_error = 0.05 (for errors in emission-line fluxes), and
absindx_floor_error = 0.05 (for errors in absorption line indices).
These defaults can be changed by editing the above parameters in
config.py.
Photometry¶
Using Skelton et al. (2014)¶
MCSED was originally written to analyze galaxies in the five CANDELS
fields (AEGIS, COSMOS, GOODS-N, GOODS-S, and UDS), hence there are
special commands built into the program to handle the PSF-matched
photometry from Skelton et al. (2014). If the user’s sources are in the
Skelton catalog, the objects can be specified by their field (i.e.,
aegis, cosmos, goodsn, goodss, or uds) and the
unique Skelton ID number. This links the input line directly to the
object’s photometry in the files provided by Skelton et al. (2014).
Momcheva et al. (2016) provide grism redshifts (and emission line
fluxes) for all Skelton photometry. Users with Skelton sources are
encouraged to use the grism redshifts for the redshift column.
Additional photometry not included in the Skelton catalogs can be specified in the input file in the same way as general photometry as discussed in General Case.
General Case¶
If the input objects are not associated with the Skelton et al. (2014) catalog
(identified via the Field and ID columns described above), or if users
wish to supplement this catalog with additional photometry, the input file must
include additional columns. Photometric measurements should be given as flux
densities with \(1\,\sigma\) uncertainties associated with each
measurement (null value \(=-99\)).
The columns containing these data in the input file should be labeled
f_filter_name and e_filter_name, where filter_name is the
name of a .res file in the FILTERS directory. (In other words,
columns named f_hst_acsF606W and e_hst_acsF606W should refer to
the flux densities (not magnitudes!) and uncertainties taken through the
filter defined in FILTERS/hst_acsF606W.res.) Following Skelton
et al. (2014), the units for flux density are scaled to an AB magnitude
of 25, so \(1.00 = 3.63 \times 10^{-30}\) ergs cm\(^{-2}\) s\(^{-1}\) Hz\(^{-1}\) (e.g., if the user’s flux densities are in \(\mu\)Jy, the values must be multiplied by \(10^{0.4(25-23.9)} \approx 2.754\)).
Emission Lines¶
MCSED can include emission line fluxes in the likelihood function.
To do this, the user first specifies the line’s name (keyword Name),
rest-frame wavelength (in Angstroms), and relative weight in the
config.py emission-line dictionary. A weight of 1.0 means the line
contributes just as much weight to the likelihood function as a
photometric data point; a weight of 0.0 implies that the line is
ignored. The user then provides the objects’ emission line strengths and
\(1\,\sigma\) error bars by entering the data in the input file and labeling
the columns as Name_FLUX and Name_ERR, where Name is the
line’s keyword listed in the emline_list_dict dictionary
defined in config.py. The emission line fluxes and
errors must be specified in units of \(10^{-17}\) ergs
cm\(^{-2}\) s\(^{-1}\), unless a different multiplication
factor to the base unit of ergs cm\(^{-2}\) s\(^{-1}\) is
specified by the keyword emline_factor in config.py. The
emission lines currently included in config.py are given below.
Additional lines can be added by expanding the emline_list_dict in
config.py.
| Line | Name | Wavelength (Å) | Weight |
|---|---|---|---|
| H\(\beta\) | Hb |
4861 | 1.0 |
| H\(\alpha\) | Ha |
6563 | 1.0 |
| [O III] | OIII |
5007 | 0.5 |
| [O II] | OII |
3727 | 0.5 |
| [N II] | NII |
6583 | 0.5 |
Currently, MCSED cannot fit blended emission lines.
Absorption Line Indices¶
Absorption line indices can also be used in MCSED’s likelihood
function. These measurements are input in a similar way as additional
photometry or emission line fluxes are included. In the input file, the
columns containing an absorption line index and its uncertainty are
labeled as Name_INDX and Name_Err, where Name is the line’s
keyword, as listed in the absorption_index_dict dictionary
defined in config.py. The indices that are pre-defined in MCSED are
listed in the table below. As one can see from the table,
the indices are defined via their wavelength ranges, the units they are
quoted in, and a relative weight similar to that defined for the
emission lines.
| Index Band (Å) | Blue Continuum (Å) | Red Continuum (Å) | ||||||
|---|---|---|---|---|---|---|---|---|
| Name | Weight | Blue | Red | Blue | Red | Blue | Red | Units¹ |
| Lick_CN1 | 1.0 | 4142.125 | 4177.125 | 4080.125 | 4117.625 | 4244.125 | 4284.125 | 1 |
| Lick_CN2 | 1.0 | 4142.125 | 4177.125 | 4083.875 | 4096.375 | 4244.125 | 4284.125 | 1 |
| Lick_Ca4227 | 1.0 | 4222.250 | 4234.750 | 4211.000 | 4219.750 | 4241.000 | 4251.000 | 0 |
| Lick_G4300 | 1.0 | 4281.375 | 4316.375 | 4266.375 | 4282.625 | 4318.875 | 4335.125 | 0 |
| Lick_Fe4383 | 1.0 | 4369.125 | 4420.375 | 4359.125 | 4370.375 | 4442.875 | 4455.375 | 0 |
| Lick_Ca4455 | 1.0 | 4452.125 | 4474.625 | 4445.875 | 4454.625 | 4477.125 | 4492.125 | 0 |
| Lick_Fe4531 | 1.0 | 4514.250 | 4559.250 | 4504.250 | 4514.250 | 4560.500 | 4579.250 | 0 |
| Lick_Fe4668 | 1.0 | 4634.000 | 4720.250 | 4611.500 | 4630.250 | 4742.750 | 4756.500 | 0 |
| Lick_Hb | 1.0 | 4847.875 | 4876.625 | 4827.875 | 4847.875 | 4876.625 | 4891.625 | 0 |
| Lick_Fe5015 | 1.0 | 4977.750 | 5054.000 | 4946.500 | 4977.750 | 5054.000 | 5065.250 | 0 |
| Lick_Mg1 | 1.0 | 5069.125 | 5134.125 | 4895.125 | 4957.625 | 5301.125 | 5366.125 | 1 |
| Lick_Mg2 | 1.0 | 5154.125 | 5196.625 | 4895.125 | 4957.625 | 5301.125 | 5366.125 | 1 |
| Lick_Mgb | 1.0 | 5160.125 | 5192.625 | 5142.625 | 5161.375 | 5191.375 | 5206.375 | 0 |
| Lick_Fe5270 | 1.0 | 5245.650 | 5285.650 | 5233.150 | 5248.150 | 5285.650 | 5318.150 | 0 |
| Lick_Fe5335 | 1.0 | 5312.125 | 5352.125 | 5304.625 | 5315.875 | 5353.375 | 5363.375 | 0 |
| Lick_Fe5406 | 1.0 | 5387.500 | 5415.000 | 5376.250 | 5387.500 | 5415.000 | 5425.000 | 0 |
| Lick_Fe5709 | 1.0 | 5696.625 | 5720.375 | 5672.875 | 5696.625 | 5722.875 | 5736.625 | 0 |
| Lick_Fe5782 | 1.0 | 5776.625 | 5796.625 | 5765.375 | 5775.375 | 5797.875 | 5811.625 | 0 |
| Lick_NaD | 1.0 | 5876.875 | 5909.375 | 5860.625 | 5875.625 | 5922.125 | 5948.125 | 0 |
| Lick_TiO1 | 1.0 | 5936.625 | 5994.125 | 5816.625 | 5849.125 | 6038.625 | 6103.625 | 1 |
| Lick_TiO2 | 1.0 | 6189.625 | 6272.125 | 6066.625 | 6141.625 | 6372.625 | 6415.125 | 1 |
| Lick_Hd_A | 1.0 | 4083.500 | 4122.250 | 4041.600 | 4079.750 | 4128.500 | 4161.000 | 0 |
| Lick_Hg_A | 1.0 | 4319.750 | 4363.500 | 4283.500 | 4319.750 | 4367.250 | 4419.750 | 0 |
| Lick_Hd_F | 1.0 | 4091.000 | 4112.250 | 4057.250 | 4088.500 | 4114.750 | 4137.250 | 0 |
| Lick_Hg_F | 1.0 | 4331.250 | 4352.250 | 4283.500 | 4319.750 | 4354.750 | 4384.750 | 0 |
| D4000 | 1.0 | …… | …… | 3750.000 | 3950.000 | 4050.000 | 4250.000 | 2 |
| ¹Unit codes: 0 = Å; 1 = mag; 2 = ratio | ||||||||
These definitions come from Bruzual (1983) and Worthey et al. (1994); they are calculated by finding the average value of \(F_{\lambda}\) within the blue and red continuum bands, interpolating a line through these values to estimate the continuum, \(F_C\), and then computing equivalent width via
Important Note: absorption line indices are defined for a specific
spectral resolution. MCSED makes no attempt to match this
resolution: it uses the SSP spectra as is. The user should consider this
carefully before deciding on the utility of this feature.