[Paper Review] Downsizing among disk galaxies and the role of the environment

Giuseppe Gavazzi|ArXiv.org|Jan 19, 2009

Astronomy and Astrophysical Research3 citations

TL;DR

This paper demonstrates that galaxy mass is the primary driver of downsizing in disk galaxies, with six independent scaling relations—ranging from star formation rate to structural parameters—showing strong mass dependence. Environmental effects, while accelerating gas depletion and quenching, play a minor role in shaping the fundamental structural and evolutionary trends.

ABSTRACT

The study of PopI and PopII indicators in galaxies has a profound impact on our understanding of galaxy evolution. Their present (z=0) ratio suggests that the star formation history of galaxies was primarily dictated by their global mass. Since 1989 Luis Carrasco and I spent most of our sleepless nights gathering H_alpha and near infrared surface photometry of galaxies in the local Universe and focused most of our scientific career on these two indicators trying to convince the community that the mass was the key parameter to their evolution. We were unsuccessful, until in 2004 the Sloan team rediscovered this phenomenon and named it "downsizing"

Motivation & Objective

• To investigate whether galaxy mass is the dominant parameter governing the evolution of disk galaxies, challenging the influence of environmental and merger-driven processes.
• To test the robustness of downsizing—where massive galaxies form stars earlier and more intensely—across multiple independent observables.
• To assess the role of environment by comparing HI-deficient (perturbed) and HI-normal (unperturbed) galaxies in the same mass sequence.
• To determine whether structural and star formation properties of disk galaxies scale primarily with mass, independent of environment or AGN activity.

Proposed method

• Used a sample of 868 disk galaxies from the Goldmine Database, selected based on H-band magnitude, HI mass, and morphological type (Sa or later).
• Classified galaxies into HI-normal (HI_def ≤ 0.3) and HI-deficient (HI_def > 0.4) to probe environmental effects.
• Measured dynamical mass via H-band luminosity using the relation logM_dyn = logL_H + 0.7, assuming constant M/L in the NIR.
• Analyzed six scaling relations: HI mass, current SFR per unit mass, B-H color, NIR effective surface brightness, light concentration index, and R-band nuclearity.
• Used Hα equivalent width as a proxy for current star formation efficiency (birth rate parameter b).
• Applied SED fitting and τ-models to infer star formation histories, assuming a single free parameter τ that decreases with increasing mass.

Experimental results

Research questions

• RQ1 Do multiple independent observables in disk galaxies scale with galaxy mass, supporting the downsizing paradigm?
• RQ2 To what extent does environmental quenching (via HI deficiency) alter the observed scaling relations between mass and structural/star formation properties?
• RQ3 Is the observed downsizing pattern in disk galaxies consistent with a mass-regulated star formation history, independent of AGN feedback or merger activity?
• RQ4 How do the structural and photometric properties of disk galaxies—such as surface brightness and concentration—depend on mass versus environment?

Key findings

• Six independent scaling laws—covering star formation, color, surface brightness, concentration, and nuclear features—show strong, consistent correlations with dynamical mass, supporting mass-driven downsizing.
• The B-H color index spans 4 magnitudes across the mass range, indicating a genuine change in stellar population age from dwarfs to massive spirals, not driven by metallicity alone.
• HI-deficient galaxies show a larger scatter in the B-H and surface brightness relations, but the overall mass trends remain robust, indicating environment has minor influence on the mass-driven scaling.
• The concentration index and R-band nuclearity both increase non-linearly with mass, indicating that central bulge-like structures are more prominent in massive disk galaxies.
• The SED-fitting-derived τ parameter (star formation timescale) is inversely proportional to mass, implying massive galaxies formed stars earlier and more intensely, naturally explaining downsizing.
• Environmental effects (via HI deficiency) primarily affect gas content and current SFR, but do not significantly alter the fundamental mass-dependent structural and photometric trends.