We estimate an empirical lower limit for the fraction of cosmic star formation that is triggered by minor mergers in the local Universe. Splitting the star formation budget by galaxy morphology, we ...find that early-type galaxies (ETGs) host ∼14 per cent of the budget, while Sb/Sc galaxies host the bulk (∼53 per cent) of the local star formation activity. Recent work indicates that star formation in nearby ETGs is driven by minor mergers, implying that at least ∼14 per cent of local star formation is triggered by this process. A more accurate estimate can be derived by noting that an infalling satellite likely induces a larger starburst in a galaxy of 'later' morphological type, both due to higher availability of gas in the accreting galaxy and also because a bigger bulge better stabilizes the disc against star formation. This enables us to use the star formation in ETGs to estimate a lower limit for the fraction of star formation in late-type galaxies (LTGs) that is minor-merger-driven. Using a subsample of ETGs that is mass- and environment-matched to the LTGs (implying a similar infalling satellite population), we estimate this limit to be ∼24 per cent. Thus, a lower limit for the fraction of cosmic star formation that is induced by minor mergers is ∼35 per cent 14 per cent (ETGs) + 0.24 × 86 per cent (LTGs). The observed positive correlation between black hole and galaxy mass further implies that a similar fraction of black hole accretion may also be triggered by minor mergers. Detailed studies of minor-merger remnants are therefore essential, to quantify the role of this important process in driving stellar mass and black hole growth in the local Universe.
Abstract While active galactic nuclei (AGN) are considered to be key drivers of the evolution of massive galaxies, their potentially significant role in the dwarf-galaxy regime (M* < 109 M⊙) remains ...largely unexplored. We combine optical and infrared data, from the Hyper Suprime-Cam (HSC) and the Wide-field Infrared Explorer, respectively, to explore the properties of ∼800 AGN in dwarfs at low redshift (z < 0.3). Infrared-selected AGN fractions are ∼10–30 per cent in dwarfs, which, for reasonable duty cycles, indicates a high black hole (BH)-occupation fraction. Visual inspection of the deep HSC images indicates that the merger fraction in dwarf AGN (∼6 per cent) shows no excess compared to a control sample of non-AGN, suggesting that the AGN-triggering processes are secular in nature. Energetic arguments indicate that, in both dwarfs and massive galaxies, bolometric AGN luminosities (LAGN) are significantly greater than supernova luminosities (LSN). LAGN/LSN is, in fact, higher in dwarfs, with predictions from simulations suggesting that this ratio only increases with redshift. Together with the potentially high BH-occupation fraction, this suggests that if AGN feedback is an important driver of massive-galaxy evolution, the same is likely to be true in the dwarf regime, contrary to our classical thinking.
We use the Sloan Digital Sky Survey Stripe 82 to empirically quantify the stellar-mass and black hole growth triggered by minor mergers in local spiral (disc) galaxies. Since major mergers destroy ...discs and create spheroids, morphologically disturbed spirals are likely remnants of minor mergers. Disturbed spirals exhibit enhanced specific star formation rates (SSFRs), the enhancement increasing in galaxies of ‘later’ morphological type (which have more gas and smaller bulges). By combining the SSFR enhancements with the fraction of time spirals spend in this ‘enhanced’ mode, we estimate that ∼40 per cent of the star formation in local spirals is directly triggered by minor mergers. The disturbed spirals also exhibit higher nuclear-accretion rates, implying that minor mergers enhance the growth rate of the central black hole. However, the specific accretion rate shows a lower enhancement than that in the SSFR, suggesting that the coupling between stellar-mass and black hole growth is weak in minor-merger-driven episodes. Given the significant fraction of star formation that is triggered by minor mergers, this weaker coupling may contribute to the large intrinsic scatter observed in the stellar versus black hole mass relation in spirals. Combining our results with the star formation in early-type galaxies – which is minor-merger-driven and accounts for ∼14 per cent of the star formation budget – suggests that around half of the star formation activity in the local Universe is triggered by the minor-merger process.
We study nearby galaxies in close pairs to study the key factors affecting star formation and active galactic nuclei (AGN) activity triggered during galaxy interactions. Close pairs are selected from ...the Sloan Digital Sky Survey assuming a projected separation of <30 kpc and recessional velocity difference <500 km s−1. Near-ultraviolet fluxes from GALEX are used to estimate specific star formation rates (SSFRs). We find a factor of ∼5.3 increase in SSFR for low-mass (108-1011 M) close pair galaxies and a factor of ∼2.1 increase in SSFR for high-mass (1011-1013 M) close pairs compared to the general galaxy population. Considering galaxies of all masses, we find a factor of ∼1.8 enhancement in SSFR for close pairs in field environments compared to non-pairs, with no significant increase for pairs in group and cluster environments. A modest decrease of a factor of ∼1.4 is found in the Seyfert fraction in close pair galaxies when compared to isolated galaxies, which suggests that mergers may not trigger AGN activity at the close pair stage or may trigger a different class of AGN. This becomes a factor of ∼4.2 decrease when we restrict our analysis to high-mass close pairs in group or cluster environments.
We use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in low-redshift galaxies. We show that the green valley between the blue cloud of star-forming galaxies and the red sequence ...of quiescent galaxies in the colour-mass diagram is not a single transitional state through which most blue galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear that only a small population of blue early-type galaxies move rapidly across the green valley after the morphologies are transformed from disc to spheroid and star formation is quenched rapidly. In contrast, the majority of blue star-forming galaxies have significant discs, and they retain their late-type morphologies as their star formation rates decline very slowly. We summarize a range of observations that lead to these conclusions, including UV-optical colours and halo masses, which both show a striking dependence on morphological type. We interpret these results in terms of the evolution of cosmic gas supply and gas reservoirs. We conclude that late-type galaxies are consistent with a scenario where the cosmic supply of gas is shut off, perhaps at a critical halo mass, followed by a slow exhaustion of the remaining gas over several Gyr, driven by secular and/or environmental processes. In contrast, early-type galaxies require a scenario where the gas supply and gas reservoir are destroyed virtually instantaneously, with rapid quenching accompanied by a morphological transformation from disc to spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases transforms galaxies from disc to elliptical morphology, and mergers could play a role in inducing black hole accretion and possibly active galactic nuclei feedback.
We present the data release for Galaxy Zoo 2 (GZ2), a citizen science project with more than 16 million morphological classifications of 304 122 galaxies drawn from the Sloan Digital Sky Survey ...(SDSS). Morphology is a powerful probe for quantifying a galaxy's dynamical history; however, automatic classifications of morphology (either by computer analysis of images or by using other physical parameters as proxies) still have drawbacks when compared to visual inspection. The large number of images available in current surveys makes visual inspection of each galaxy impractical for individual astronomers. GZ2 uses classifications from volunteer citizen scientists to measure morphologies for all galaxies in the DR7 Legacy survey with m
r
> 17, in addition to deeper images from SDSS Stripe 82. While the original GZ2 project identified galaxies as early-types, late-types or mergers, GZ2 measures finer morphological features. These include bars, bulges and the shapes of edge-on disks, as well as quantifying the relative strengths of galactic bulges and spiral arms. This paper presents the full public data release for the project, including measures of accuracy and bias. The majority ( 90 per cent) of GZ2 classifications agree with those made by professional astronomers, especially for morphological T-types, strong bars and arm curvature. Both the raw and reduced data products can be obtained in electronic format at http://data.galaxyzoo.org.
We demonstrate that the large scatter in the ultraviolet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation (RSF) in these ...objects can be reproduced by minor mergers in the standard Λ cold dark matter (ΛCDM) cosmology. Numerical simulations of mergers with mass ratios ≤1:4, with reasonable assumptions for the ages, metallicities and dust properties of the merger progenitors, produce good agreement with the observed UV colours of the early-type population, if the infalling satellites are assumed to have (cold) gas fractions ≥20 per cent. Early-types that satisfy (NUV−r) ≲ 3.8 are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ∼1.5 Gyr, while those that satisfy 3.8 < (NUV−r) < 5.5 are consistent with either recent mergers with mass ratios ≤1:6 or mergers with higher mass ratios that occurred more than ∼1.5 Gyr in the past. We demonstrate that the early-type colour–magnitude relations and colour distributions, in both the UV and optical spectral ranges, are consistent with the expected frequency of minor merging activity in the standard ΛCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level RSF observed in early-type galaxies in the nearby Universe.
Abstract
Based on the recent advancements in numerical simulations of galaxy formation, we anticipate the achievement of realistic models of galaxies in the near future. Morphology is the most basic ...and fundamental property of galaxies, yet observations and simulations still use different methods to determine galaxy morphology, making it difficult to compare them. We hereby perform a test on the recent
NewHorizon
simulation, which has spatial and mass resolutions that are remarkably high for a large-volume simulation, to resolve the situation. We generate mock images for the simulated galaxies using SKIRT, which calculates complex radiative transfer processes in each galaxy. We measure morphological and kinematic indicators using photometric and spectroscopic methods following observers’ techniques. We also measure the kinematic disk-to-total ratios using the Gaussian mixture model and assume that they represent the true structural composition of galaxies. We found that spectroscopic indicators such as
V
/
σ
and
λ
R
closely trace the kinematic disk-to-total ratios. In contrast, photometric disk-to-total ratios based on the radial profile fitting method often fail to recover the true kinematic structure of galaxies, especially small ones. We provide translating equations between various morphological indicators.
In this work we present IRAM 30-m telescope observations of a sample of bulge-dominated galaxies with large dust lanes, which have had a recent minor merger. We find these galaxies are very gas rich, ...with H2 masses between 4 × 108 and 2 × 1010 M⊙. We use these molecular gas masses, combined with atomic gas masses from an accompanying paper, to calculate gas-to-dust and gas-to-stellar-mass ratios. The gas-to-dust ratios of our sample objects vary widely (between ≈50 and 750), suggesting many objects have low gas-phase metallicities, and thus that the gas has been accreted through a recent merger with a lower mass companion. We calculate the implied minor companion masses and gas fractions, finding a median predicted stellar mass ratio of ≈40:1. The minor companion likely had masses between ≈107 and 1010 M⊙. The implied merger mass ratios are consistent with the expectation for low-redshift gas-rich mergers from simulations. We then go on to present evidence that (no matter which star formation rate indicator is used) our sample objects have very low star formation efficiencies (star formation rate per unit gas mass), lower even than the early-type galaxies from ATLAS3D which already show a suppression. This suggests that minor mergers can actually suppress star formation activity. We discuss mechanisms that could cause such a suppression, include dynamical effects induced by the minor merger.
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