[Paper Review] Truncated stellar discs in the near infrared. II. Statistical properties and interpretation

E. Florido, E. Battaner+3 more|ArXiv.org|Sep 14, 2006

Stellar, planetary, and galactic studies16 references5 citations

TL;DR

This paper proposes that magnetically driven truncations explain the observed statistical properties of stellar disc truncations in the near-infrared (NIR). Using a sample of 18 edge-on spiral galaxies, it finds that the truncation radius $ R_{tr} \propto V_m^{3/2} $, where $ V_m $ is the asymptotic rotation velocity, and that $ R_{tr}/h $ increases with central surface brightness. The magnetic model best explains these correlations, supporting its role in shaping galactic discs through post-star-formation stellar escape due to sudden loss of magnetic support.

ABSTRACT

The results obtained in paper I are used to study possible relationships between the truncation radius of stellar discs in the NIR and structural parameters of the galaxies. The NIR truncation radius is larger for brighter galaxies, being proportional to $V_m^c$ with $c \approx 3/2$, and with $V_m$ being the asymptotic rotation velocity at large radii (when the rotation curve becomes flat), and is lower for higher wavelengths. When it is normalized to the scalelength, the truncation is an increasing function of the central surface brightness and is lower for late type galaxies, although these correlations are weaker. These relations are in agreement with the scenario of magnetically driven truncations.

Motivation & Objective

• To investigate the relationship between NIR disc truncation radii and structural parameters such as luminosity, mass, rotation velocity, and surface brightness.
• To test competing theoretical models—collapse, threshold, magnetic, and interaction—against observed statistical trends in truncation.
• To determine whether truncations are intrinsic to galaxy formation or driven by dynamic processes like magnetic forces or gas density thresholds.
• To assess whether the observed truncation properties in the NIR are consistent with a smooth, physical truncation curve rather than observational artifacts or projection effects.
• To evaluate the magnetic model's ability to quantitatively reproduce observed scaling relations between $ R_{tr} $, $ V_m $, and $ \mu_0 $.

Proposed method

• Analysis of near-infrared (NIR) photometric profiles from 18 edge-on spiral galaxies, focusing on the truncation radius $ R_{tr} $ derived from extrapolated $ (R - R_{tr})^{-1} $ profiles.
• Use of the Tully-Fisher relation ($ L \propto V_m^4 $) to relate luminosity and asymptotic rotation velocity $ V_m $.
• Derivation of theoretical scaling laws under the magnetic model: $ R_{tr}/h \propto M^{1/2}/h $ and $ R_{tr}/h \propto I_0^{1/2} $, where $ I_0 $ is central intensity.
• Statistical comparison of observed $ R_{tr} $ correlations with predictions from the magnetic model, threshold model, and collapse model.
• Normalization of $ R_{tr} $ to disc scale length $ h $ to isolate intrinsic structural dependencies independent of size.
• Use of the LEDA database and statistical tools (assisted by Andrés González-Carmona) to validate and refine results.

Experimental results

Research questions

• RQ1 Does the truncation radius $ R_{tr} $ in the NIR correlate with the asymptotic rotation velocity $ V_m $, and if so, what is the power-law index?
• RQ2 How does the normalized truncation radius $ R_{tr}/h $ vary with central surface brightness $ \mu_0 $, and does this support the magnetic model?
• RQ3 Can the magnetic model quantitatively explain the observed $ R_{tr} \propto V_m^{3/2} $ relation and its dependence on galaxy luminosity and mass?
• RQ4 Why is $ R_{tr}(K_s) < R_{tr}(J) $, and does this support a physical truncation mechanism rather than observational effects?
• RQ5 Are truncations in late-type galaxies more pronounced, and does this align with predictions from the magnetic model?

Key findings

• The truncation radius $ R_{tr} $ scales with the 3/2 power of the asymptotic rotation velocity $ V_m $, yielding $ R_{tr} \propto V_m^{3/2} $, which strongly supports the magnetic model.
• When normalized to the disc scale length $ h $, $ R_{tr}/h $ increases with central surface brightness $ \mu_0 $, consistent with the magnetic model's prediction $ R_{tr}/h \propto I_0^{1/2} $.
• The magnetic model explains the observed trend that extended galaxies (large $ h $) have smaller $ R_{tr}/h $, due to the $ M^{1/2}/h $ dependence in the model.
• The observed $ R_{tr}(K_s) < R_{tr}(J) $, with a mean ratio of ~1.6, is consistent with the magnetic model, as shorter wavelengths probe more recent star formation and thus a more extended truncation.
• The magnetic model satisfactorily explains the observed correlations between $ R_{tr} $, $ V_m $, and $ \mu_0 $, while other models like the threshold model lack the same quantitative predictive power.
• The study finds no strong evidence for truncation differences between isolated and interacting galaxies, suggesting that interaction effects are not the primary driver of truncations in the NIR.