HD Galaxy Components

Every part of an HD Mode galaxy is built from the galaxy's own row in the SAGE26 output. These tables list each component, the SAGE26 fields it uses (HDF5 names, as written in model_N.hdf5), and how they set it.

• Masses are converted to M☉ and radii to kpc on reading.
• Metallicities are in solar units, with Z☉ = 0.02.
• Optional fields: if one is missing from a catalogue, the component it drives is simply left out.
• Randomness: random choices, such as arm class, are seeded by GalaxyIndex, so a galaxy always looks the same.

Shape and size

Component	SAGE26 fields	How
Disk or elliptical	BulgeMass / StellarMass (B/T), ColdGas, Regime	Elliptical if B/T > 0.75, or if B/T > 0.5 in a hot-regime halo with a cold-gas fraction under 2%. Otherwise a disk.
Size	StellarMass, DiskRadius, BulgeRadius, snapshot redshift, FFBRegime	The Shen et al. (2003) size–mass relations set the size. SAGE's own radius nudges it as (R_SAGE / R_relation)^¼, by at most ±30%. Sizes shrink with redshift, as (1+z)^−0.75 for disks and (1+z)^−1 for ellipticals (van der Wel et al. 2014). An FFB galaxy is 0.6× more compact.
Disk thickness	StellarMass, VelDisp, Vmax, redshift	Low-mass disks are thicker, by M*^−0.15. Thickness grows by (1 + 0.5z) and with (σ/Vmax)⁶.
Elliptical shape	StellarMass, Vmax, VelDisp	More massive ellipticals are rounder and have more extended (cD-like) envelopes. Fast rotators (high Vmax/σ) are flatter.
Classical bulge	MergerBulgeMass, MergerBulgeRadius	Round, at the merger bulge's radius. Its light scales with the merger-bulge mass relative to the disk.
Pseudo-bulge	InstabilityBulgeMass, InstabilityBulgeRadius, VelDisp, Vmax	Compact, at the instability bulge's radius, flatter in rotation-dominated (low σ/Vmax) disks. It holds whatever instability mass doesn't go into the bar.
Bar and peanut	InstabilityBulgeMass, ColdGas	Barred if the instability bulge is more than 6% of M*. At most 8% of M* goes into the bar, whose half-length grows with that fraction (0.9–1.8 disk scale lengths) and which is thinner the stronger it is. Gas-poor disks buckle it into a stronger boxy/peanut bulge.
Old stellar colour	MetalsStellarMass, MetalsBulgeMass	The disk and bulge are each tinted by their own stars' metallicity: metal-rich is redder.
Stellar populations by age	SFHMassDisk, SFHMassBulge (with SAGE's SaveFullSFH 1)	The disk's and the bulge's histories are summed as populations of known age: each population's colour and light per unit mass follow its age (younger: bluer, brighter), with metallicity still tinting them. A recently quenched (post-starburst) galaxy turns white-blue with no HII regions, a rejuvenated bulge turns blue, and 0.1–1 Gyr-old star clusters spread through the disk. Without a saved history, colours come from metallicity alone.
Brightness	StellarMass, BulgeMass	Brightness ∝ M*^0.35, using disk mass for disks. Every member of a group shares one exposure, so relative brightness follows mass.

Spiral structure

Component	SAGE26 fields	How
Arm class: grand-design, multi-arm, partial or flocculent	InstabilityBulgeMass, StellarMass, ColdGas	Drawn from probabilities set by the galaxy. Barred and massive disks are mostly grand-design or multi-arm. Gas-rich or low-mass disks lean flocculent.
Pitch angle	B/T, Vmax	Bigger bulges and faster rotation (more shear) wind the arms tighter.
Flocculence	ColdGas / (ColdGas + StellarMass), StellarMass	Gas-rich, low-mass disks are patchier within their arm class.
Clumpy disk	redshift, ColdGas, VelDisp, Vmax, FFBRegime	Gas-rich, turbulent disks at z ≳ 1 are clumpy instead of spiral: a Toomre-like score from redshift, gas fraction and σ/Vmax decides it, not chance. FFB galaxies always are. Clumps (4–12) are drawn diffuse and blend into the disk.

Star formation

Component	SAGE26 fields	How
Young (blue) light	SfrDisk, disk mass	Scales with the disk's specific SFR as (sSFR / 10⁻¹⁰)^0.7. FFB galaxies are bluer still.
Young star clusters	SfrDisk	Their number scales as SFR^0.5 (400–20,000). They follow the young-light map.
HII regions	H2gas, SfrDisk, DiskRadius, ColdGas, SfrDiskZ	Star-forming complexes along the arms, in number following the H₂ reservoir: small crisp knots in a faint nebula. Each complex is richer the denser the star formation (SFR surface density) and bigger the more gas-rich the disk. Each complex's colour runs from pink (metal-poor gas) to orange (metal-rich): the galaxy's star-forming gas metallicity sets which dominates (dwarfs pinker, massive disks oranger), an abundance gradient makes the inner disk oranger and the outer disk pinker (about −0.1 dex per effective radius; Sánchez et al. 2014), and each region scatters about that.
Nuclear ring	SfrBulge, SfrDisk	A faint, diffuse ring of clusters and dust appears when the bulge forms more than 8% of all stars (with SfrBulge > 0.05 M☉/yr). It sits just inside the bar ends.
Gas-disk truncation (satellites)	Type, TimeOfInfall, Mvir, infallMvir	Satellites lose their outer gas disk the longer since infall and the more of their subhalo they have lost. Star formation stops beyond the cut.

Dust

Component	SAGE26 fields	How
Disk dust	H2gas, H1gas, MetalsColdGas, ColdGas	Face-on optical depth ∝ (Σ_H₂ + 0.4 Σ_HI) × gas metallicity. It follows the arms and the H₂, and reddens what it dims. Thin, streaky dark lanes run along the arms' inner edges, darker the larger that dust column (on a log scale), so gas-poor disks have almost none.
Elliptical dust lane	H2gas	A nuclear lane in ellipticals that hold molecular gas.

Nucleus

Component	SAGE26 fields	How
Nucleus	BlackHoleMass, BulgeMass	Brightness and compactness grow with black-hole mass (relative to the bulge, for ellipticals).
AGN point source	QuasarModeBHaccretionMass	Quasar-mode accretion this snapshot adds a bright, bluish point.

Merger debris and the intracluster light

Component	SAGE26 fields	How
Shells (ellipticals) or tidal tails (disks)	TimeOfLastMajorMerger, redshift	A major merger leaves debris that fades over 4 Gyr at z = 0, and faster at high z (4 Gyr / (1+z), as dynamical times shorten).
Stellar stream	TimeOfLastMinorMerger, redshift	A minor merger leaves a stream, fading over 2.5 Gyr / (1+z).
Intracluster light (ICL)	IntraClusterStars, MetalsIntraClusterStars, Rvir	A faint envelope and glow reaching into the halo (scale 0.12 Rvir), with scattered stripped stars. Its brightness follows the ICS mass, so ICL-rich clusters and groups show it and others barely do. Its colour follows the ICS metallicity.

Halo gas (the Gas button)

Component	SAGE26 fields	How
CGM and cold streams	CGMgas, MetalsCGMgas, mdot_stream, redshift, neighbouring haloes	Cold-regime haloes. Three curling streams plus filaments out to Rvir, brighter with log CGMgas (or the stream rate), tinted by metallicity. The streams follow the cosmic web: two come in from opposite sides along the direction of the biggest halo within 4 Mpc/h (the filament the galaxy sits in), the third from the next biggest, each brighter the bigger its halo. At high z they are narrower, brighter and reach in to the galaxy (×(1 + 0.15z), width / (1 + 0.35z)).
Cold streams into a hot halo	mdot_stream, HotGas	Hot haloes still fed by cold streams draw them by mdot_stream, aimed and scaled with redshift as above.
Hot halo / intracluster medium	HotGas, Mvir, Concentration, MetalsHotGas, Rvir, redshift	A textured atmosphere to Rvir. Its opacity rises with log HotGas, with the hot gas's share of the halo's baryons, and with redshift (denser haloes). Its core is 1.5 Rvir / c, so concentrated haloes are peakier. Its colour follows the virial temperature (Mvir, hotter at high z): magenta in small haloes, violet in groups, ice-blue in rich clusters, tinted by metallicity.
X-ray cavities	Heating, r_heat, Regime, HotGas	Bubbles at the radio-heating radius r_heat, along the jet axis, in hot haloes with radio heating.
Radio jets	Heating, BlackHoleMass, r_heat, Rvir	Jets and lobes along the spin axis, brighter with radio-mode heating and black-hole mass, reaching about the radius the AGN heats (r_heat, or 8% of Rvir).
Galactic winds	OutflowRate, SfrDisk, SfrBulge, DiskRadius, MassLoading, Vmax, EjectedMass	A hollow bipolar cone along the spin axis, in Hα red (as M82's wind), with a few thin streamers running outward through it. It is brighter with the outflow rate and with the SFR surface density (so compact high-z disks drive stronger winds). It reaches further in deeper potentials (Vmax) and the more of the halo gas has been ejected, and fades out by three times that reach. MassLoading opens it wider (20–60°, on a log scale). It is drawn in 3D, so it looks right from any angle.
Extended HI disk	H1gas, H2gas, Type, TimeOfInfall	A faint, clumpy, pale-blue gas disk well past the stars, sized by the HI size–mass relation (Wang et al. 2016). It is clumpier where more of the cold gas is molecular, and warped in satellites, more so the longer since infall.

Environment (groups, pairs, satellites)

Component	SAGE26 fields	How
Group members	CentralGalaxyIndex, Type, Pos*	Every member of the FOF group is drawn at its own position, magnified to its normal-view size (at most 5×). A galaxy alone in its halo is drawn at true size.
Spin and orientation	Spinx, Spiny, Spinz	Each disk's plane is set by its spin vector.
Tidal bridges and tails	Pos*, StellarMass, Vel*, Spin*	Group members within 100 kpc of each other with mass ratio above 1:10 raise tidal features. These are stronger the closer and more massive the companion, and weaker (×0.6) for retrograde encounters.
Merger about to happen	mergeType	A galaxy SAGE merges before the next snapshot interacts with the nearest more massive galaxy, at a strength set by the merger type.
Jellyfish (ram-pressure) tail	Vel*, Pos*, HotGas (host), Rvir (host), StellarMass, ColdGas, DiskRadius	The strength is ρ_ICM v² against the disk's restoring force (Σ* Σ_gas). It is set so a few per cent of satellites in hot haloes have a tail, as observed jellyfish are a few per cent of cluster galaxies (e.g. Poggianti et al. 2016): on SAGE26 at z = 0, 7% any tail, 2% a strong one. The tail is faint gas in the galaxy's own HII colour with blue star-forming knots, trailing the motion. It is brightened to undo the group's shared exposure, and ram pressure also strips up to half the satellite's CGM.
Tidal stripping (satellites)	Mvir, infallMvir	A satellite that has lost more than 65% of its infall subhalo mass sheds stars into tidal streams.

Fields not used for visuals

Concentration, ICS_accrete and infallStellarMass are read but don't change the picture. Cooling was used only by an early offline prototype.