The ACS Fornax Cluster Survey. II. The Central Brightness Profiles of Early-Type Galaxies: A Characteristic Radius on Nuclear Scales and the Transition from Central Luminosity Deficit to Excess
Bibliographic record
Abstract
We analyze brightness profiles for 143 early-type galaxies in the Virgo and Fornax Clusters, observed with the Advanced Camera for Surveys on the Hubble Space Telescope . Sérsic models are found to provide accurate representations of the global profiles with a notable exception: the observed profiles deviate systematically inside a characteristic "break" radius of R b ≈ 0.02 −0.01 +0.025 R e , where R e is the effective radius of the galaxy. The sense of the deviation is such that bright galaxies ( M B ≲ − 20) typically show central light deficits with respect to the inward extrapolation of the Sérsic model, while the great majority of low- and intermediate-luminosity galaxies (-19.5 ≲ M B ≲ − 15) show central light excesses ; galaxies of intermediate luminosities (-20≲ M B ≲ − 19.5) are generally well fitted by Sérsic models over all radii. We show that the slope, γ', of the central surface brightness profiles, when measured at fixed fractions of R e , varies smoothly as a function of galaxy luminosity in a manner that depends sensitively on the choice of measurement radius. We find no evidence for a core/power-law dichotomy, and show that a recent claim of strong bimodality in γ ' is likely an artifact of the biased galaxy selection function used in that study. To provide a more robust characterization of the inner regions of galaxies, we introduce a parameter Δ 0.02 = log ( g / S ) —where g and S are the integrated luminosities inside 0.02 R e of the observed profile and of the inward extrapolation of the outer Sérsic model—to describe the central luminosity deficit (Δ 0.02 < 0) or excess (Δ 0.02 > 0). We find that Δ 0.02 varies smoothly over the range of ≈720 in luminosity spanned by the sample galaxies, with again no evidence for a dichotomy. We argue that the central light excesses in M B ≳ − 19 galaxies may be the analogs of the dense central cores predicted by some numerical simulations to form via gas inflows.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.004 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.004 | 0.003 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.011 | 0.009 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".