Lightcone Photometry¶
The LightSAGE flow's photometry stage (python -m visage.sed.photometry) gives every galaxy in a lightcone the magnitudes an observer would measure, built forward from its own SAGE26 star-formation history. The settings used are recorded in the file's PhotometryHeader attributes.
Stellar light¶
| Step | How |
|---|---|
| Stellar populations | FSPS (Conroy, Gunn & White 2009; Conroy & Gunn 2010) with MIST isochrones and MILES spectra, Chabrier (2003) IMF, as SAGE26 assumes. |
| Star-formation history | SAGE26 files the stars formed between snapshots k and k+1 in bin k, as the mass that survives recycling. Each bin is turned back into the mass formed, ÷ (1 − R) with the run's RecycleFraction, and spread at a constant rate over its own time span by integrating the populations over age (as TAO does: Shamshiri et al. 2015). |
| Youngest stars | The last 100 Myr come from the galaxy's current SFR (SfrDisk + SfrBulge), not the last bin spread evenly over its 0.3–0.6 Gyr. |
| Metallicity | One present-day value per galaxy, MetalsStellarMass / StellarMass; the spectrum is interpolated in log Z between population nodes. SAGE26 doesn't keep a metallicity history. |
Observed magnitudes¶
| Step | How |
|---|---|
| Redshifting | The filter is shifted onto the rest-frame spectrum at each galaxy's own observed redshift (K-correction), photon-counting AB magnitudes. |
| Distance | DL = (1 + zobs) DC(zcos), so peculiar velocities count (Davis et al. 2011). |
| Absolute magnitudes | Rest frame, at 10 pc. |
Dust (--dust-model charlot_fall)¶
How much dust. Dusty-SAGE's dust model (Triani et al. 2020), solved for each galaxy's steady state from its SAGE26 cold gas, metals, H₂ and SFR. It is solved for the dust-to-metal ratio μ: gas inflow, outflows and locking into new stars move dust and metals together, so they cancel.
0 = Y (δc − μ) − η mswept(Z) NSN Z μ + tdep Z (fH₂ / τacc) μ (1 − 2μ) / (1 − μ)
| Term | Source |
|---|---|
Stars: new metals Y (the run's Yield), a share δc = 0.2 condensed into dust |
Dusty-SAGE, after Popping et al. (2017) |
| Supernova destruction: η = 0.1 of the swept mass, mswept = 1535 (Z/0.02 + 0.039)−0.289 M☉ per SN; SNe per mass formed from the Chabrier IMF (8–40 M☉) | Asano et al. (2013) |
| Grain growth in molecular clouds, τacc = 20 Myr × 0.02/Z, stopping at μ = ½ | Asano et al. (2013) |
Metal-poor gas ends near μ = δc and metal-rich gas near ½, the critical-metallicity behaviour of Asano et al. (2013). Dust in hot gas and the CGM is sputtered (Tsai & Mathews 1995) and its column is far below the disk's, so it isn't included.
Where the dust is. The molecular gas follows the stars (Leroy et al. 2008). The atomic gas sits in its own exponential disk sized by the HI size–mass relation (Wang et al. 2016). The optical depth in front of the starlight is the mean hydrogen column inside the stellar half-light radius, with the Milky Way's AV/NH (Bohlin, Savage & Drake 1978; RV = 3.1) scaled by the galaxy's dust per hydrogen over the Milky Way's 0.010 (Draine & Li 2007).
How it dims the light (Charlot & Fall 2000):
- Diffuse ISM: dust mixed with the stars in a slab seen at the galaxy's inclination (its spin against the line of sight), τλ ∝ λ−0.7.
- Birth clouds: stars younger than 10 Myr are also behind their birth cloud, of optical depth τISM(1 − μCF)/μCF with μCF = 0.3, τλ ∝ λ−1.3 (da Cunha, Charlot & Elbaz 2008; Charlot & Fall used λ−0.7 for both).
Where the absorbed light goes (--dust-emission). Each galaxy re-emits exactly the starlight its dust absorbed (energy balance, as in da Cunha et al. 2008), with the Draine & Li (2007) dust-emission spectrum FSPS uses (its default Umin = 1, γ = 0.01, qPAH = 3.5 %). LDust is the total.
Disk and bulge (--disk-bulge)¶
disk_mag_* and bulge_mag_* are built from each component's own star-formation history and current SFR, through the same dust. SAGE26's SFHMassBulge holds only the stars formed in bulge starbursts: the disk stars that mergers and disk instabilities move into the bulge stay in SFHMassDisk. So the share of the disk's history that makes the bulge's total equal its BulgeMass is moved into the bulge, taking those stars to have the disk's age mix.
Emission lines (--lines)¶
[OII] 3727, Hβ, [OIII] 5007, Hα, [NII] 6584 and [SII] 6716+6731: LineLum_* (erg/s, through the dust), LineLumIntr_* (no dust), LineLumAGN_* (the AGN's share, no dust) and LineFlux_* (erg/s/cm², observed).
HII regions. Lines from the star-forming nebular models of Gutkin, Charlot & Bruzual (2016): Bruzual & Charlot stellar populations through Cloudy, for a Chabrier IMF and constant star formation over 100 Myr, given per unit SFR. They are the companions of the AGN models below and are coupled to each galaxy the same way (Hirschmann et al. 2017). Each galaxy's lines are its current SFR times the grid at:
- the cold gas's metallicity;
- the ionisation parameter of SDSS galaxies with its specific SFR, metallicity and density (Kashino & Inoue 2019, eq. 5), with O/H from the solar 12 + log O/H = 8.69 (Asplund et al. 2009);
- the electron density from their eq. 3 (within the grid's 100–1000 cm⁻³);
- dust-to-metal ratio 0.3 and solar C/O, the models' fiducial values.
The lines pass through the birth cloud and the diffuse ISM. The FSPS spectra used for the magnitudes include FSPS's own nebular lines and continuum (Byler et al. 2017), for each metallicity node at a main-sequence galaxy's ionisation parameter.
AGN. SAGE26's black holes accrete in quasar mode (QuasarModeBHaccretionMass over the snapshot step dT, which is in h⁻¹ Myr) and radio mode (RadioModeBHaccretionMass over the same step, or in older outputs its power Heating = ηṀc², η = 0.1). SAGE26's black holes start at zero mass, so a dwarf's first merger or disk instability makes one of a tiny mass all at once; its Eddington ratio then reflects only the step's length (the card notes such a black hole), and its luminosity is negligible. Their luminosity is ηṀc² above 1 % of Eddington and falls as ṁ² below it, where the flow becomes radiatively inefficient (Narayan & Yi 1995; Merloni & Heinz 2008). The narrow-line region uses the photoionisation models of Feltre, Charlot & Gutkin (2016), coupled to each galaxy as Hirschmann et al. (2017) do: an ionising spectrum Sν ∝ ν−1.7, gas clumps of nH = 10³ cm⁻³, dust-to-metal ratio 0.3, the cold gas's metallicity, and the ionisation parameter at the Strömgren radius from the AGN's ionising photons and the filling factor of the cold gas within 1 kpc. The lines pass through half the disk's dust, as a screen. LAGN, EddingtonRatio, LogU_HII and LogU_AGN are written too.
Mock images¶
Lightcone Mode's Create mock image (visage/sed/mock_image.py) draws the
sky around a galaxy as a survey would record it. Each galaxy is an exponential
disk (DiskRadius as its scale length) plus a de Vaucouleurs bulge
(BulgeRadius as its half-light radius), at its angular size (radius over the
angular-diameter distance). The disk is flattened by its inclination, with
axis ratio q = (cos² i + q₀² sin² i)½, q₀ = 0.2 (Hubble
1926; Holmberg 1958), and turned to the position angle of its spin on the sky.
Disk and bulge fluxes come from their observed-frame magnitudes (or the total,
shared by bulge-to-total stellar mass, when a cone has no disk/bulge
photometry). The image is blurred by a Gaussian of the survey's median seeing
on its pixel grid; at survey depth, sky noise is added so that a point source
at the quoted limit has the quoted signal-to-noise. Colour images are asinh
composites (Lupton et al. 2004).
| Survey | Bands | Pixel | Seeing (FWHM) | Depth | Source |
|---|---|---|---|---|---|
| SDSS | g, r, i | 0.396″ | 1.4″ | 95 % completeness for point sources: g 22.2, r 22.2, i 21.3 (taken as 5σ) | Gunn et al. 2006; Abazajian et al. 2009 |
| DES DR2 (DECam) | g, r, z | 0.263″ | 1.11″, 0.95″, 0.83″ | 10σ in a 1.95″ aperture: g 24.7, r 24.4, z 23.1 | Abbott et al. 2021 |
| Pan-STARRS1 3π | g, r, i | 0.25″ | 1.31″, 1.19″, 1.11″ | 5σ point source: g 23.3, r 23.2, i 23.1 | Chambers et al. 2016 |
| Rubin LSST, 10 years | g, r, i | 0.2″ | 0.7″ | 5σ point source: g 27.4, r 27.5, i 26.8 | Ivezić et al. 2019 |
| 2MASS | J, H, Ks | 1″ (Atlas) | 2.5″ | 10σ point source: J 15.8, H 15.1, Ks 14.3 Vega (+0.91, +1.39, +1.85 to AB) | Skrutskie et al. 2006; Blanton & Roweis 2007 |
| WISE | W1, W2, W3 | 1.375″ (Atlas) | 6.1″, 6.4″, 6.5″ | 5σ point source: 0.08, 0.11, 1 mJy | Wright et al. 2010 |
| GALEX AIS | FUV, NUV | 1.5″ | 4.2″, 5.3″ | 5σ: FUV 19.9, NUV 20.8 | Morrissey et al. 2007 |
| JWST NIRCam | F115W, F200W, F444W | 0.031″ | 0.040″, 0.066″, 0.145″ | noiseless (depth depends on the programme) | JWST User Documentation |
The PNG is the colour composite (north up, east left); the FITS file holds one extension per band in nanomaggies, with a TAN WCS centred on the chosen galaxy. Only the cone's own galaxies are drawn: there are no stars, and no galaxies beyond the cone's redshift range.
Not included (yet)¶
- IGM absorption (irrelevant below z ≈ 2).
- The AGN's own continuum in the magnitudes (its lines aren't in the broad bands either).
- Line emission from old (post-AGB) stars, which powers many LINER-like spectra in quiescent galaxies.
References¶
- Abazajian K. N. et al. 2009, ApJS 182, 543
- Abbott T. M. C. et al. 2021, ApJS 255, 20
- Asano R. S. et al. 2013, Earth, Planets and Space 65, 213
- Asplund M. et al. 2009, ARA&A 47, 481
- Blanton M. R., Roweis S. 2007, AJ 133, 734
- Bohlin R. C., Savage B. D., Drake J. F. 1978, ApJ 224, 132
- Byler N. et al. 2017, ApJ 840, 44
- Chabrier G. 2003, PASP 115, 763
- Chambers K. C. et al. 2016, arXiv:1612.05560
- Charlot S., Fall S. M. 2000, ApJ 539, 718
- Conroy C., Gunn J. E., White M. 2009, ApJ 699, 486; Conroy C., Gunn J. E. 2010, ApJ 712, 833
- da Cunha E., Charlot S., Elbaz D. 2008, MNRAS 388, 1595
- Davis T. M. et al. 2011, ApJ 741, 67
- Draine B. T., Li A. 2007, ApJ 657, 810
- Feltre A., Charlot S., Gutkin J. 2016, MNRAS 456, 3354
- Gunn J. E. et al. 2006, AJ 131, 2332
- Gutkin J., Charlot S., Bruzual G. 2016, MNRAS 462, 1757
- Hirschmann M. et al. 2017, MNRAS 472, 2468
- Holmberg E. 1958, Medd. Lunds Astron. Obs. Ser. II 136, 1
- Hubble E. P. 1926, ApJ 64, 321
- Ivezić Ž. et al. 2019, ApJ 873, 111
- Kashino D., Inoue A. K. 2019, MNRAS 486, 1053
- Leroy A. K. et al. 2008, AJ 136, 2782
- Lupton R. et al. 2004, PASP 116, 133
- Merloni A., Heinz S. 2008, MNRAS 388, 1011
- Morrissey P. et al. 2007, ApJS 173, 682
- Narayan R., Yi I. 1995, ApJ 452, 710
- Popping G., Somerville R. S., Galametz M. 2017, MNRAS 471, 3152
- Shamshiri S. et al. 2015, MNRAS 451, 2681
- Skrutskie M. F. et al. 2006, AJ 131, 1163
- Triani D. P. et al. 2020, MNRAS 493, 2490
- Tsai J. C., Mathews W. G. 1995, ApJ 448, 84
- Wang J. et al. 2016, MNRAS 460, 2143
- Wright E. L. et al. 2010, AJ 140, 1868