AGN in dwarf galaxies: frequency, triggering processes and the plausibility of AGN feedback

© 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical SocietyWhile 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 ∗ < 10 9 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 (L AGN) are significantly greater than supernova luminosities (L SN). L AGN/L SN 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.Peer reviewedFinal Published versio

On the Key Processes that Drive Galaxy Evolution: the Role of Galaxy Mergers, Accretion, Local Environment and Feedback in Shaping the Present-Day Universe

The study of galaxy evolution is a fundamental discipline in modern astrophysics, dealing with how and why galaxies of all types evolve over time. The diversity of present-day galaxies is a reflection of the processes through which these populations were assembled and offers insights into how these processes influence and regulate their mass assembly over the lifetime of the Universe. The currently favoured hierarchical paradigm of structure formation hypothesises that much of a galaxy’s evolution must be driven by mergers. It is therefore important to understand the role of the merger process in shaping the galaxy populations in today’s Universe. Together with data from large observational surveys, statistical studies of galaxy evolution rely on comparison to simulations, which can be used to make realistic survey-scale predictions. Together these two approaches can offer powerful insights into the processes that drive galaxy evolution over cosmic time. I have used the Horizon-AGN simulation to study the effect of galaxy mergers on the stellar populations and central super-massive black holes of galaxies over cosmic time. I have shown that, while mergers can enhance star formation and black-hole growth significantly in the low redshift Universe, these enhancements are small at high redshift when the cosmic SFH peaks. This is because galaxies are already gas-rich at early epochs and mergers are not able to increase gas densities in the central regions of the galaxy. As a result, mergers are directly responsible for creating only around 30 per cent of the stellar mass and black-hole mass found and in today’s galaxies and that mergers never dominate the budget (e.g. ~35 and ~20 per cent of star formation at z~3 and z~1 respectively are a result of mergers). Notwithstanding their relatively minor role in driving stellar and BH mass growth, mergers are important drivers of morphological change, with major and minor mergers accounting for essentially all (95 per cent) of the morphological change experienced by massive present-day spheroids over their lifetime. However, at a given stellar mass, the average merger histories of discs and spheroids do not differ strongly enough to explain the survival of discs to the present day. Instead, their survival is largely due to a preponderance of prograde and gas rich mergers. Prograde mergers trigger milder morphological transformation than retrograde mergers - the average change due to retrograde mergers is around twice that due to their prograde counterparts at ɀ ~ 0 and remnant morphology also depends strongly on the gas fraction of a merger, with gas-rich mergers routinely re-growing discs. My results also emphasise the important role of minor mergers, which dominate the stellar mass and black-hole growth budget after ɀ = 1 and are a potentially important reservoir of cold gas which plays a role in the rejuvenation and survival of discs. I have also investigated the biases that this morphological evolution produces in observational studies of galaxy populations. In particular, I have shown that ‘progenitor bias’ i.e. the bias produced by using only early-type galaxies to define the progenitor population of today’s early-types, is a significant problem at all but the lowest redshifts and an important considerations for large, deep observational surveys (JWST, LSST etc.). For example while early-types attain their final morphology at relatively early epochs – by ɀ ~ 1, around 60 per cent of today’s early-types have had their last significant merger, progenitor bias is severe at all but the lowest redshifts. At ɀ ~ 0.6, less than 50 per cent of the stellar mass in today’s early-types is actually in progenitors with early-type morphology, while, at the peak epoch of cosmic of star-formation (ɀ ~ 2), studying only early-types misses almost all (80 per cent) of the stellar mass that eventually ends up in local early-type systems. I have explored the significance and formation mechanisms of low-surface-brightness galaxies (LSBGs). For M ͙ > 108Mʘ, LSBGs contribute 50 per cent of the local number density and exist in significant numbers across all environments. Their progenitors have stronger, burstier star formation at high redshift which causes stronger supernova feedback. This feedback flattens the gas-density profiles (but does not remove the gas reservoirs). This, in turn, gives rise to flatter stellar profiles, which are more susceptible to environmental processes and galaxy interactions, which produce today’s LSBG populations by driving the steady removal of cold gas and gradually increasing galaxy effective radii over time. The ability of these populations to elucidate key questions in the field of galaxy evolution and significantly alter our current paradigm is becoming increasingly clear, especially with the advent of new deep surveys. Finally, I have implemented a new unsupervised machine learning technique (UML) on images from the Hyper-Suprime-Cam Subaru-Strategic-Program Ultra-Deep survey. The algorithm autonomously reduces galaxy populations down to a small number of ‘morphological clusters’, populated by galaxies with similar morphologies, which are then benchmarked using visual inspection. The morphological classifications reproduce known trends in key galaxy properties as a function of morphological type (e.g. stellar mass functions and colours). This study demonstrates the power of UML in performing accurate morphological analysis, which will become indispensable in the forthcoming era of deep-wide surveys

Investigating the role of interactions and mergers in driving the star-forming properties of dwarf galaxies in field and group environments

Recent advancements in astronomical instruments including JWST, Rubin Observatory, and Euclid, in conjunction with increasingly sophisticated cosmological simulations, are now opening up opportunities for statistical studies in low-mass and low-surface-brightness regimes, enabling investigations of faint dwarf galaxies at distances far beyond the Local Volume. These galaxies are critical for understanding the stellar mass assembly of high-mass galaxies and represent essential laboratories for investigating various physical processes play fundamental roles the growth and evolution of all galaxies. In this study, we use the NEWHORIZON cosmological simulation to examine how local environment influences the mass assembly and structural evolution of approximately 1000 field and group dwarfs. Our results demonstrate that in the dwarf regime, unlike in the high-mass regime, mergers are not the dominant mechanism for ex-situ mass assembly. Instead, fly-bys and interactions within the tidal field play a more substantial role in the stellar mass budget, as these interactions become more efficient at driving star formation enhancements in lower mass haloes. Significantly, we also find that both mergers and fly-bys or other environmental interactions produce similar morphological signatures in dwarf galaxies. Consequently, dwarfs display morphological disturbances consistent with them being mergers or merger remnants at a similar rate to high-mass galaxies, but the majority of these disturbances are, in fact, the result of other kinds of interaction (in contrast to high-mass galaxies, for which almost all disturbances arise from mergers). I will discuss the implications of these findings for future studies in the dwarf regime as new instruments begin to obtain more robust, statistical samples beyond the local Universe and examine whether these observations may allow us to differentiate between these two processes

A flat trend of star-formation rate with X-ray luminosity of galaxies hosting AGN in the SCUBA-2 Cosmology Legacy Survey

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.Feedback processes from active galactic nuclei (AGN) are thought to play a crucial role in regulating star formation in massive galaxies. Previous studies using Herschel have resulted in conflicting conclusions as to whether star formation is quenched, enhanced, or not affected by AGN feedback. We use new deep 850 μm observations from the SCUBA-2 Cosmology Legacy Survey (S2CLS) to investigate star formation in a sample of X-ray selected AGN, probing galaxies up to L 0.5-7keV = 10 46 erg s -1. Here, we present the results of our analysis on a sample of 1957 galaxies at 1 < z < 3, using both S2CLS and ancilliary data at seven additional wavelengths (24-500 μm) from Herschel and Spitzer. We perform a stacking analysis, binning our sample by redshift and X-ray luminosity. By fitting analytical spectral energy distributions (SEDs) to decompose contributions from cold and warm dust, we estimate star formation rates (SFRs) for each 'average' source. We find that the average AGN in our sample resides in a star-forming host galaxy, with SFRs ranging from 80 to 600 M ⊙ yr -1. Within each redshift bin, we see no trend of SFR with X-ray luminosity, instead finding a flat distribution of SFR across ∼3 orders of magnitude of AGN luminosity. By studying instantaneous X-ray luminosities and SFRs, we find no evidence that AGN activity affects star formation in host galaxies.Peer reviewedFinal Accepted Versio

How the spectral energy distribution and galaxy morphology constrain each other, with application to morphological selection using galaxy colours

© 2022 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.We introduce an empirical methodology to study how the spectral energy distribution (SED) and galaxy morphology constrain each other and implement this on 8000 galaxies from the HST CANDELS survey in the GOODS-South field. We show that the SED does constrain morphology and present a method that quantifies the strength of the link between these two quantities. Two galaxies with very similar SEDs are around three times more likely to also be morphologically similar, with SED constraining morphology most strongly for relatively massive red ellipticals. We apply our methodology to explore likely upper bounds on the efficacy of morphological selection using colour. We show that, under reasonable assumptions, colour selection is relatively ineffective at separating homogeneous morphologies. Even with the use of up to six colours for morphological selection, the average purity in the resultant morphological classes is only around 60 per cent. While the results can be improved by using the whole SED, the gains are not significant, with purity values remaining around 70 per cent or below.Peer reviewedFinal Published versio

Relaxed blue ellipticals: accretion-driven stellar growth is a key evolutionary channel for low mass elliptical galaxies

How elliptical galaxies form is a key question in observational cosmology. While the formation of massive ellipticals is strongly linked to mergers, the low mass (Mstar < 10^9.5 MSun) regime remains less well explored. In particular, studying elliptical populations when they are blue, and therefore rapidly building stellar mass, offers strong constraints on their formation. Here, we study 108 blue, low-mass ellipticals (which have a median stellar mass of 10^8.7 MSun) at z < 0.3 in the COSMOS field. Visual inspection of extremely deep optical HSC images indicates that less than 3 per cent of these systems have visible tidal features, a factor of 2 less than the incidence of tidal features in a control sample of galaxies with the same distribution of stellar mass and redshift. This suggests that the star formation activity in these objects is not driven by mergers or interactions but by secular gas accretion. We combine accurate physical parameters from the COSMOS2020 catalog, with measurements of local density and the locations of galaxies in the cosmic web, to show that our blue ellipticals reside in low-density environments, further away from nodes and large-scale filaments than other galaxies. At similar stellar masses and environments, blue ellipticals outnumber their normal (red) counterparts by a factor of 2. Thus, these systems are likely progenitors of not only normal ellipticals at similar stellar mass but, given their high star formation rates, also of ellipticals at higher stellar masses. Secular gas accretion, therefore, likely plays a significant (and possibly dominant) role in the stellar assembly of elliptical galaxies in the low mass regime.Comment: Published in MNRA

The morphological mix of dwarf galaxies in the nearby Universe

© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We use a complete, unbiased sample of 257 dwarf (10 8 M < M < 10 9.5 M) galaxies at z < 0.08, in the COSMOS field, to study the morphological mix of the dwarf population in low-density environments. Visual inspection of extremely deep optical images and their unsharp-masked counterparts reveals three principal dwarf morphological classes. 43 per cent and 45 per cent of dwarfs exhibit the traditional ‘early-type’ (elliptical/S0) and ‘late-type’ (spiral) morphologies, respectively. However, 10 per cent populate a ‘featureless’ class, that lacks both the central light concentration seen in early-types and any spiral structure – this class is missing in the massive-galaxy regime. 14 per cent, 27 per cent, and 19 per cent of early-type, late-type, and featureless dwarfs respectively show evidence for interactions, which drive around 20 per cent of the overall star formation activity in the dwarf population. Compared to their massive counterparts, dwarf early-types show a much lower incidence of interactions, are significantly less concentrated and share similar rest-frame colours as dwarf late-types. This suggests that the formation histories of dwarf and massive early-types are different, with dwarf early-types being shaped less by interactions and more by secular processes. The lack of large groups or clusters in COSMOS at z < 0.08, and the fact that our dwarf morphological classes show similar local density, suggests that featureless dwarfs in low-density environments are created via internal baryonic feedback, rather than by environmental processes. Finally, while interacting dwarfs can be identified using the asymmetry parameter, it is challenging to cleanly separate early and late-type dwarfs using traditional morphological parameters, such as ‘CAS’, M 20, and the Gini coefficient (unlike in the massive-galaxy regime).Peer reviewe

Why do extremely massive disc galaxies exist today?

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 4, June 2020, Pages 5568–5575, https://doi.org/10.1093/mnras/staa970. ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios. Since mergers are the principal drivers of disc-to-spheroid morphological transformation, and the most massive galaxies typically have the richest merger histories, it is surprising that discs exist at all at the highest stellar masses (e.g. beyond the knee of the mass function). Using Horizon-AGN, a cosmological hydro-dynamical simulation, we show that extremely massive (M*> 10^11.4 MSun) discs are created via two channels. In the primary channel (accounting for ~70% of these systems and ~8% of massive galaxies) the most recent, significant merger (stellar mass ratio > 1:10) between a massive spheroid and a gas-rich satellite `spins up' the spheroid by creating a new rotational stellar component, leaving a massive disc as the remnant. In the secondary channel (accounting for ~30% of these systems and ~3% of massive galaxies), a system maintains a disc throughout its lifetime, due to an anomalously quiet merger history. Not unexpectedly, the fraction of massive discs is larger at higher redshift, due to the Universe being more gas-rich. The morphological mix of galaxies at the highest stellar masses is, therefore, a strong function of the gas fraction of the Universe. Finally, these massive discs have similar black-hole masses and accretion rates to massive spheroids, providing a natural explanation for why a minority of powerful AGN are surprisingly found in disc galaxies.Peer reviewedFinal Published versio

A physically motivated framework for measuring the mass and redshift dependence of galaxy pair fractions across cosmic time

Low mass galaxy pair fractions are under-studied across cosmic time. In the era of JWST, Roman, and Rubin, a self-consistent framework is needed to select both low and high mass galaxy pairs to connect observed pair fractions to cosmological merger rates across all mass scales and redshifts. We use the Illustris TNG100 simulation to identify physically associated pairs between $z=0-4.2$. Our sample includes low mass ($\rm 10^8<M_*<5\times10^9\,M_{\odot}$) and high mass ($\rm 5\times10^9<M_*<10^{11}\,M_\odot$) isolated subhalo pairs, with stellar masses from abundance matching. The low mass pair fraction, i.e. the fraction of galaxies in pairs, increases from $z=0-2.5$, while the high mass pair fraction peaks at $z=0$ and is constant or slightly decreasing at $z>1$. At $z=0$ the low mass major (1:4 mass ratio) pair fraction is 4$\times$ lower than high mass pairs, consistent with findings for cosmological merger rates. Our results indicate that pair fractions can faithfully reproduce trends in merger rates if galaxy pairs are selected appropriately. Specifically, static pair separation limits applied equivalently to all galaxy pairs do not recover the evolution of low and high mass pair fractions. Instead, we advocate for separation limits that vary with the mass and redshift of the system, such as separation limits scaled by the virial radius of the host halo ($r_{\mathrm{sep}}< 1 R_{\rm vir}$). Finally, we place isolated mass-analogs of Local Group galaxy pairs (i.e., MW--M31, MW--LMC, LMC--SMC) in a cosmological context, showing that isolated analogs of LMC--SMC-mass pairs, and low separation ($<50$kpc) MW--LMC-mass pairs, are $2-3\times$ more common at $z\gtrsim2-3$.Comment: 17 pages, 5 figured, submitted to ApJ, comments welcom