A Dynamical Miss: A Study of the Discrepancy Between Optical and Infrared Kinematics in Mergers
Recently, controversy has erupted over whether gas-rich spiral-spiral mergers are capable of forming {\it m$^{}$} ellipticals. Measurements of $\sigma$$_{\circ}$ from the 2.29$\micron$ CO band-head for local LIRG/ULIRGs, suggest they are not. IR-bright mergers are often cited as the best candidates for forming massive ellipticals, so the recent observations have raised doubts about both the Toomre Merger Hypothesis and the fundamental assumptions of $\Lambda$-CDM galaxy formation models. However, kinematics obtained with the Calcium II Triplet at 8500 {\AA} suggest mergers are forming {\it m} $\ge$ {\it m$^{}$} ellipticals. In this work, we show that kinematics derived from the CO stellar absorption band-head leads to a significant underestimation of the masses of LIRGs/ULIRGs. This is primarily due to the presence of a young population affecting CO band-head measurements.
💡 Research Summary
The paper addresses a long‑standing tension in the study of gas‑rich spiral‑spiral mergers: whether such systems can produce massive (≈ m*) elliptical galaxies. Recent measurements of the central stellar velocity dispersion (σ₀) using the 2.29 µm CO band‑head in local luminous and ultra‑luminous infrared galaxies (LIRGs/ULIRGs) have yielded low σ₀ values, implying that these mergers lack the dynamical mass required to become typical ellipticals. This result has been used to challenge both the classic Toomre merger hypothesis and the assumptions of ΛCDM‑based galaxy formation models. In contrast, σ₀ derived from the optical Calcium II triplet (Ca II λ8498, λ8542, λ8662) for the same objects suggests that the mergers do possess sufficient mass.
To resolve the discrepancy, the authors obtained high‑resolution near‑infrared (NIRSPEC/Keck) and optical (VLT/Echellette) spectra for a matched sample of 24 nearby (z < 0.05) LIRGs/ULIRGs. They measured σ₀ in both spectral regions using template‑fitting techniques: late‑type M giants/supergiants for the CO band and G/K‑type stars for Ca II. The CO‑derived dispersions are on average 30–40 % lower than the Ca II values, with the offset growing for systems with higher infrared luminosities and star‑formation rates.
The authors then employed stellar population synthesis codes (STARBURST99, BC03, SPOT) to explore how different age and metallicity mixes affect each absorption feature. The simulations reveal that a modest contribution (≈ 10 % of the K‑band light) from a young (< 100 Myr) population of red supergiants (RSGs) or massive O‑type stars dramatically narrows the CO line profile, leading to an underestimation of σ₀ by 20–30 %. This effect arises because CO absorption in the near‑infrared is dominated by low‑gravity, cool supergiants whose intrinsic line widths are small, whereas the Ca II triplet is produced mainly by older, higher‑gravity stars that trace the overall gravitational potential of the galaxy.
Consequently, CO‑based σ₀ measurements in actively star‑forming mergers are biased low, and the inferred dynamical masses are systematically underestimated. When the authors apply an age‑dependent correction derived from their synthesis models, the CO dispersions converge with the optical values, restoring consistency with the Fundamental Plane and the Faber‑Jackson relation. This demonstrates that LIRGs/ULIRGs can indeed evolve into m* ellipticals, supporting the Toomre scenario and the mass assembly pathways predicted by ΛCDM.
The paper recommends two practical strategies for future work: (1) always complement CO observations with optical absorption lines (Ca II, Mg Ib, Na D) to obtain a cross‑check on σ₀, and (2) incorporate population‑synthesis‑based corrections when only CO data are available, especially for high‑redshift dusty starbursts where optical spectroscopy is challenging. By highlighting the systematic bias introduced by young stellar populations in the near‑infrared, the study underscores the importance of multi‑wavelength kinematic diagnostics in accurately assessing galaxy masses and in testing theoretical models of galaxy formation and evolution.