HD Mode¶
HD replaces the selected galaxy's splat with a detailed 3D model built from its SAGE properties, ray-marched on the GPU every frame. In the selection card's Galaxy context HD draws just that galaxy; in Halo + group context it draws its whole pair, group or cluster. Nothing else is drawn: no other galaxies or background.
Select a galaxy (double-click it), then press HD. The button sits next to Isolate (🔍) at the top of the right panel, and also on the selection card. Press HD again, or Esc, to go back to the normal view, framed as Go frames it.
What is drawn¶
Each galaxy's model follows from its own catalogue row:
| Component | From |
|---|---|
| Disk or elliptical, size and brightness | Stellar mass, bulge-to-total ratio, disk and bulge radii |
| Spiral arms (several arm classes), bars, peanut bulges | Disk mass, instability bulge, Vmax |
| Classical and pseudo-bulges | Merger vs. instability bulge masses |
| Young star clusters and HII regions | Disk SFR, cold and H₂ gas, gas metallicity |
| Dust | Cold-gas metals, seen edge-on as lanes |
| Shells, tails and streams | Merger history (merger bulge, mergeType) |
| Intracluster light | ICS mass |
| AGN nucleus | Black hole mass and radio heating |
| Disk thickness and clumpiness | Velocity dispersion over rotation, redshift, FFB regime |
Sizes shrink with redshift as in the size–mass relation, and disks at high redshift are thicker and clumpier.
If the run saved its star-formation histories (SAGE's SaveFullSFH 1), colours also follow the stars' ages: young disks are bluer, recently quenched galaxies white-blue, and a rejuvenated bulge stands out from an older disk. Without them, colours follow metallicity only.
The full list of components, the SAGE26 fields each one uses and how, is in HD Galaxy Components.
Groups and clusters¶
In Halo + group context, HD draws every member of the FOF group at its own position and spin:
- Members are drawn at the size they have in the normal view, so the group keeps its layout. The halo gas keeps its true size.
- Tidal features: close pairs (within 100 kpc, mass ratio above 1:10) and galaxies that SAGE merges before the next snapshot raise tidal bridges and tails. These are stronger for prograde encounters.
- Satellites in hot haloes can show ram-pressure ("jellyfish") tails and tidal stripping.
In Galaxy context HD draws the selected galaxy alone, at its true size and its own exposure. Its group still shapes it: a satellite keeps its jellyfish tail, and a close companion still raises tidal features, though the companion isn't drawn.
A big group or cluster has its 100 most massive members drawn as models (the 4 most massive in full detail) and the rest as soft glows. Go on a glow (from the card's member list, a double-click or the browsing keys) builds that galaxy on its own and adds it to the scene. The galaxy you selected is always a model. The card tags each member HD, glow or HD extra.
Inside HD, double-click picks among the galaxies HD draws; under the cursor, the most massive one wins. Go on the card flies to that galaxy without leaving HD.
The group on the card¶
The card lists every member of the selected galaxy's group (scroll for the rest). In Halo + group context it also describes the group:
- Type: a Cluster (host halo from 10¹⁴ M☉) or Group (10¹³–10¹⁴ M☉) by its halo mass (e.g. Kravtsov & Borgani 2012; Lovisari et al. 2021); in smaller haloes a Small group, Pair or Isolated galaxy by how many members have at least 10⁹ M☉ of stars, so the name doesn't change with the resolution cut.
- the host halo's Mvir and Rvir, the members (and how many have M* ≥ 10⁹ M☉), their velocity dispersion along one axis, total stellar mass, the gap in stellar mass between the two most massive (and in a lightcone with photometry, the r-band gap Δm₁₂, a fossil group from 2 mag; Jones et al. 2003), the share quenched (sSFR < 10⁻¹¹ yr⁻¹; Weinmann et al. 2006), the intracluster light and the hot gas.
In a lightcone¶
HD works in Lightcone Mode with each galaxy as an observer sees it:
- Its own redshift, star-formation history and spin, and its group (the cone repeats the simulation box, so a group is the copies of its members around this copy of its central).
- Seen from the observer: HD frames the galaxy along the line of sight from the cone's apex, so it is at the inclination its photometry used.
- Its photometry's dust: with each galaxy's own dust (the Charlot & Fall model of the photometry), HD dims and reddens it with the same face-on optical depth and attenuation curve as its magnitudes. Photometry made with one dust screen for every galaxy, or without dust, leaves HD its own dust.
- Its colours: the galaxy's overall colour is matched to its observed g, r and i magnitudes, on HD's colour scale (fitted on 300 cone galaxies) and on the colours stars can have: emission lines in the broadband magnitudes are drawn as its HII regions, not as a tint.
The card shows its redshift, distance, apparent and absolute r magnitude, rest-frame g − r and A_V at its inclination; All properties adds the sky position, dust mass, dust-to-gas ratio, face-on optical depth, emission-line fluxes, AGN and dust luminosities and every magnitude. Galaxies are named by SAGE's GalaxyIndex.
The Photometry tab's mock images can draw galaxies from their HD models (see Lightcone Mode → Mock images).
Components¶
In Galaxy context, the card's ? (Components) button, next to the nearest-galaxy button, labels the parts of the HD galaxy, with leader lines to each: the central black hole, bulge or pseudo-bulge, bar, nuclear ring, spiral arms, star-forming regions, dust lanes, merger shells, tails and streams, the intracluster light and, with Gas on, the wind, radio jet, HI disk, cold stream and hot halo. Only what the galaxy has, from its SAGE26 properties, is labelled, and the labels follow the galaxy as you turn it. Press it again to hide them. (It isn't offered in Halo + group context.)
Gas¶
The card's Gas button adds the halo gas: every member of the group gets its own, out to its own Rvir. It is off by default; with it off, nothing but the galaxies (and their starlight) is drawn.
| Gas | From |
|---|---|
| CGM with cold streams along the cosmic web | CGMgas (cold regime), mdot_stream |
| Hot halo / intracluster medium, coloured by temperature, with X-ray cavities | HotGas, Mvir, concentration, metals, radio heating radius |
| Galactic winds | OutflowRate, ejected gas |
| Extended HI disk | H1gas |
| Radio jets | Heating |
Gas is a shader switch: turning it on or off doesn't rebuild anything.
Building¶
The CPU side of a build runs in background worker processes. A card shows the progress, with a Cancel button (or press Esc). A build takes a few seconds; a rich cluster takes longer.
To keep it quick:
- the worker processes are started, and the shaders compiled, a few seconds after ViSAGE opens;
- recent builds are kept in memory (a tenth of the machine's RAM, between 0.5 and 3 GB), so turning HD off and on again, or going back to a group you just viewed, is immediate;
- a moment after you select a galaxy, its HD is built quietly in the background, so pressing HD is usually immediate;
- while HD is on, the same galaxy is built at the previous and next snapshots, so stepping through time in HD doesn't wait for a build.
Changing snapshot rebuilds HD for the same galaxy (found again by its GalaxyIndex), keeping your view of it, once the slider settles: you can step a galaxy through time in HD. If it isn't in the new snapshot (not formed yet, or merged), HD switches off and says so. Playback, and changing model or box, leave HD.
GPU requirement¶
HD needs a GPU. If OpenGL is running on the CPU (llvmpipe, Apple Software Renderer, a Remote Desktop session, a VM without graphics drivers), the HD buttons are greyed out, and hovering over them says why. Set VISAGE_HD=1 to turn HD on anyway; expect seconds per frame.