The continuous rise of bulges out of galactic disks

Abstract

Context. A key subject in extragalactic astronomy concerns the chronology and driving mechanisms of bulge formation in late-type galaxies (LTGs). The standard scenario distinguishes between classical bulges and pseudo-bulges (CBs and PBs, respectively), the first thought to form monolithically prior to disks and the second gradually out of disks. These two bulge formation routes obviously yield antipodal predictions on the bulge age and bulge-to-disk age contrast, both expected to be high (low) in CBs (PBs). Aims. Our main goal is to explore whether bulges in present-day LTGs segregate into two evolutionary distinct classes, as expected from the standard scenario. Other questions motivating this study center on evolutionary relations between LTG bulges and their hosting disks, and the occurrence of accretion-powered nuclear activity as a function of bulge stellar mass M and stellar surface density Σ . Methods. In this study we have combined three techniques -surface photometry, spectral modeling of integral field spectroscopy data and suppression of stellar populations younger than an adjustable age cutoff with the code REMOVEYOUNG (RY) -toward a systematic analysis of the physical and evolutionary properties (e.g., M , Σ and mass-weighted stellar age t M and metallicity Z M , respectively) of a representative sample of 135 nearby (≤130 Mpc) LTGs from the CALIFA survey that cover a range between 10 8.9 M and 10 11.5 M in total stellar mass M ,T . In particular, the analysis here revolves around <δµ 9G >, a new distance-and formally extinction-independent measure of the contribution by stellar populations of age ≥9 Gyr to the mean r-band surface brightness of the bulge. We argue that <δµ 9G > offers a handy semi-empirical tracer of the physical and evolutionary properties of LTG bulges and a promising means for their characterization. Results. The essential insight from this study is that LTG bulges form over three dex in M and more than one dex in Σ a tight continuous sequence of increasing <δµ 9G > with increasing M , Σ , t M and Z M . Along this continuum of physical and evolutionary properties, our sample spans a range of ∼4 mag in <δµ 9G >: high-<δµ 9G > bulges are the oldest, densest and most massive ones ( t M ∼11.7 Gyr, Σ > 10 9 M kpc −2 , M ≥ 10 10 M ), whereas the opposite is the case for low-<δµ 9G > bulges ( t M ∼7 Gyr) that generally reside in low-mass LTGs. Furthermore, we find that the bulge-to-disk age and metallicity contrast, as well as the bulge-todisk mass ratio, show a positive trend with M ,T , raising from, respectively, ∼0 Gyr, ∼0 dex and 0.25 to ∼3 Gyr, ∼0.3 dex and 0.67 across the mass range covered by our sample. Whereas gas excitation in lower-mass ( 10 9.7 M ) bulges is invariably dominated by star formation (SF), LINER-and Seyfert-specific emission-line ratios were exclusively documented in high-mass ( 10 10 M ), high-Σ ( 10 9 M kpc −2 ) bulges. This is in agreement with previous work and consistent with the notion that the Eddington ratio or the black hole-to-bulge mass rati...

Self-consistent population spectral synthesis with FADO

“…Cardoso et al (2016) presented a pilot study on this issue by simulating an AGN power-law component that is embedded within a Lyc-leaking ( f esc = 1) galaxy of 10 Gyr in age. This model set-up approximates the situation in many ETG LINER nuclei (Papaderos et al 2013;Gomes et al 2016b) or high-M bulges (Breda & Papaderos 2018). This study revealed that the AGN would generally evade detection with purely stellar PS codes if its contribution to the monochromatic luminosity of the SED continuum at 4020 Å were lower than ∼26%.…”

“…e.g. Breda & Papaderos 2018). Indeed, the sudden increase of EW(Hα) at an age ∼10 8 yr illustrated in this figure is due to photoionisation by post-AGN stars (e.g.…”

Stellar age gradients and inside-out star formation quenching in galaxy bulges

“…Thus, a <δµ 9G >≈0 mag characterizes bulges that have completed their make-up earlier than 9 Gyr ago (z 1.34), whereas a <δµ 9G > of, say, -2.5 mag translates into a contribution of 90% from stars younger than 9 Gyr. Breda & Papaderos (2018) showed that <δµ 9G > tightly correlates with physical and evolutionary properties of LTG bulges (e.g., stellar age, surface density and mass) and proposed it as a convenient means for their classification. The most massive, dense and old bulges fall onto the iC class (<δµ 9G > ≥ -0.5 mag) whereas bulges classified as iA (<δµ 9G > ≤ -2.5 mag) are the youngest and reside in the least massive LTGs.…”

Structural analysis of massive galaxies using HST deep imaging at z < 0.5

“…As demonstrated in e.g. Gadotti (2008) (see also Méndez-Abreu et al 2008;Breda & Papaderos 2018), disregarding a bar could contribute to considerable uncertainties in the derived bulge parameters and an overestimation of the bulge-to-total (B/T ) ratio.…”