Disclosing Submillimeter Galaxy Formation: Mergers or Secular Evolution?

TL;DR

This study uses six-band JWST/NIRCam imaging and two-dimensional double Sérsic decompositions to analyze the morphologies of 125 ALMA-detected SMGs in PRIMER-COSMOS. Across parametric and non-parametric diagnostics, the majority of SMGs exhibit disk-like, secularly evolving structures, with a relatively small fraction showing merger signatures ($\sim$24%) and a modest bar fraction (~7%). The bulge properties reveal high bulge-to-total ratios coexisting with low bulge Sérsic indices in many systems, indicating ongoing in-situ bulge growth likely driven by violent disk instabilities and gas inflows rather than major mergers. The findings challenge a merger-dominated picture for SMG bulge formation and emphasize the significance of internally driven processes in shaping high-redshift dusty starbursts, with bars and clumpy substructures playing a supporting role.

Abstract

We analyze the morphology of 125 submillimeter galaxies (SMGs) in the PRIMER-COSMOS field using double Sersic modeling on JWST NIRCam images across six bands (F150W, F200W, F277W, F356W, F410M and F444W), with SMGs being classified by bulge Sersic index (n_bulge) and bulge-to-total luminosity ratio (B/T). The Kolmogorov-Smirnov test between the bright (SFR > 175 M_sun yr^{-1}) and the faint group (SFR < 175 M_sun yr^{-1}) reveals no significant statistical differences in morphology across bands. However, we notice that SMGs skew towards higher B/T ratios and lower n_bulge from shorter to longer wavelengths. In F444W, bright SMGs exhibit higher B/T and lower n_bulge, indicating flatter, disturbed bulges, while faint SMGs show lower B/T and higher n_bulge. Notably, SMGs with higher B/T tend to have low Sersic index, challenging the local universe dichotomy of classical bulges (B/T > 0.5, n > 4) versus pseudo-bulges (B/T < 0.35, n < 2). In the F277W band, non-parametric measurements indicate predominantly disk-dominated patterns, with only 24 percent of SMGs demonstrating merger signatures. After the removal of SMGs with disturbed morphology, the bulge classification scheme in F277W shows pseudo-bulges (21 percent) and clump migration bulges (16 percent) from secular evolution, compared to 4 percent merger-built bulges. Surprisingly, 48 percent of SMGs defy the classification scheme, showing high B/T (approximately 0.7) but low Sersic index (n_bulge <= 1). Bars are confirmed in 7 percent of SMGs. This work suggests that secular evolution takes precedence over major mergers, supporting the idea that isolated evolution fueled by filamentary gas inflow plays a non-negligible role in the SMG bulge formation.

Figures (14)

• Figure 1: Photometric Redshift vs Stellar Mass for 125 selected SMGs(orange) , and the field sample(gray) in PRIMER-COSMOS field
• Figure 2: Simple assumptive sketch based on previous studies summarizing the bulge formations in submillimeter galaxies presented in this paper , in which the classical bulge and ellipticals(kinematically hot, high $B/T$, high Sérsic) are the products of mergers whereas the pseodobulges and the bulgeless galaxies(kinematically cold, low $B/T$, low Sérsic) are fabricated by degrees in the secular evolution.
• Figure 3: An example of bar identification via ellispe isophotal fitting of source ID: 614294 via ellipse-fitting techniques as an example. The blue cross marks the shared center of isophotes(red).
• Figure 4: The distribution of bulge Sérsic indices across three B/T bins($B/T<0.2$ , $0.2\leq B/T \leq0.5$ , $B/T>0.5$) in band F200W.
• Figure 5: Distribution of the effective radius of bulge(Re_bulge) and disk(Re_disk) across F150W, F200W, F277W, F356W, F410M, and F444W, with the red and dark blue dotted vertical line indicating the median of the effective radius of the bulge and the disk respectively.