No evidence for small disk-like bulges in a sample of late-type spirals

Abstract

Context. About 20% of low-redshift galaxies are late-type spirals with a small or no bulge component. Although they are the simplest disk galaxies in terms of structure and dynamics, the role of the different physical processes driving their formation and evolution is not yet fully understood. Aims. We investigated whether small bulges of late-type spirals follow the same scaling relations traced by ellipticals and large bulges and if they are disk-like or classical bulges. Methods. We derived the photometric and kinematic properties of 9 nearby late-type spirals. To this aim, we analyzed the surfacebrightness distribution from the i-band images of the Sloan Digital Sky Survey and obtained the structural parameters of the galaxies from a two-dimensional photometric decomposition. We found a bulge component in 7 galaxies of the sample, while the remaining 2 resulted to be pure disk galaxies. We measured the line-of-sight stellar velocity distribution within the bulge effective radius from the long-slit spectra taken with high spectral resolution at the Telescopio Nazionale Galileo. We used the photometric and kinematic properties of the sample bulges to study their location in the Fundamental Plane, Kormendy, and Faber-Jackson relations defined for ellipticals and large bulges. Results. We found that our sample bulges satisfy some of the photometric and kinematic prescriptions for being considered disk-like bulges such as small sizes and masses with nearly exponential light profiles, small bulge-to-total luminosity ratios, low stellar velocity dispersions, and ongoing star formation. However, each of them follows the same scaling relations of ellipticals, massive bulges, and compact early-type galaxies so they cannot be classified as disk-like systems. Conclusions. We found a single population of galaxy spheroids that follow the same scaling relations, where the mass seems to lead to a smooth transition in the photometric and kinematic properties from less massive bulges to more massive bulges and ellipticals.

On the observational diagnostics to separate classical and disk-like bulges

“…The position of our sample bulges in the FPR, FJR, and KR confirmed the recent findings of Costantin et al (2017) on scaling relations for elliptical galaxies and bulges. They claimed that there is a single population of galaxy spheroids that follow the same FPR (Fig.…”

“…to separate classical from disk-like bulges. We found that all our bulges are consistent with the magnitude trend highlighted by Nigoche-Netro et al (2008) and Costantin et al (2017), discriminating between less and more massive bulges. As a consequence, using the KR to separate bulge types results in classifying less massive bulges as disk like.…”

“…7; diagnostics (III-2)C) and FJR ( Fig. 8; diagnostics (II-4)D), using the best-fitting relations provided by Costantin et al (2017) log(re) = 0.99 log(σe) + 0.24 µe − 6.46 (9) and log(σe) = −0.152(±0.003)Mi − 1.07(±0.07) ,…”

“…The fact that only three out of eight bulges in our sample (NGC 3245,NGC 3998,and NGC 5687) show a central velocity dispersion σ re/10 > 130 km s −1 does not rule out the possibility of label other sample bulges as classical. Costantin et al (2017) investigated a sample of small bulges (σe 50 km s −1 ) of late-type spirals and found they follow the same scaling relations of ellipticals, massive bulges, and compact early-type galaxies so they cannot be classified as disk-like systems.…”

The Carnegie-Irvine Galaxy Survey. IX. Classification of Bulge Types and Statistical Properties of Pseudo Bulges

“…Intrinsic shapes of bulges have been extensively investigated by Méndez-Abreu et al (2008, Costantin et al (2017Costantin et al ( , 2018, and de Lorenzo-Cáceres et al (2019). Without constraints from stellar kinematics, however, it is extremely difficult to measure the intrinsic shapes of individual bulges.…”