arXiv subjects

Alister W. Graham

Publications and source records attributed to Alister W. Graham.

At least 19 recordsLinked to original sources

arXiv2026

Rubin J122659.4+090236: An Extremely Low Surface Brightness Galaxy Candidate Discovered in the Rubin LSST Early Data Preview 2

We report the serendipitous discovery of an exceptionally low surface brightness galaxy (LSBG) candidate, Rubin J122659.4+090236, in Rubin Observatory imaging of the interacting NGC 4410 system, identified in the Cosmic Treasure Chest public release. 2D Sérsic modelling of the Rubin g, r, and i images reveals a nearly round system with a shallow profile (n ~ 0.4), an effective radius of R_e ~ 6'', and central surface brightnesses of $μ_{0,g}=27.52\pm0.04$, $μ_{0,r}=27.62\pm0.07$, and $μ_{0,i}=27.04\pm0.08$ mag arcsec$^{-2}$. EAZY photo-z fitting favours an intermediate-z solution at z~0.3, while a low-redshift solution at z~0.028, consistent with the NGC 4410 system, is also permitted by a restricted EAZY fit over 0<z<0.1 without imposing a redshift prior. These alternatives imply substantially different physical interpretations, ranging from a diffuse dwarf-like system to an exceptionally extended background LSBG. This discovery demonstrates Rubin's sensitivity to extremely diffuse galaxies and highlights the potential of the LSST survey to uncover large samples of such elusive systems across wide areas, enabling systematic studies of the LSBG population and its role in galaxy evolution.

astro-ph.GA↗

arXiv2026

$R_{\rm e}$, or not $R_{\rm e}$: Developing $R_5\equiv R_{-2}$ as a scale radius for galaxy sizes, masses, and mass-to-light ratios

The effective half-light radius $R_{\rm e}$ marks an arbitrary 50-per-cent light boundary, and scaling relations involving such {\it effective} radii and their associated surface brightnesses, $μ_{\rm e}$, systematically (and undesirably) vary as the percentage changes. Here, the projected radius $R_5\equiv R_{-2}$, where the logarithmic slope of the surface-brightness and intensity profile equals $5.00\,\text{mag\,dex}^{-1}$ and $-2$, respectively, and where the luminosity contributed per logarithmic radial interval is maximal, is developed as an alternative. It can be measured non-parametrically or with a parametrized fit. For the Sérsic $R^{1/n}$ family, the exact relation $R_5=(2n/b_n)^n\,R_{\rm e}$ is derived, with $R_5/R_{\rm e}\rightarrow{\rm e}^{1/6}\approx1.181$ as $n\rightarrow\infty$. Reparameterizing the (now $b_n$-free) $R^{1/n}$ model in terms of the observable pair $(R_5,μ_5)$ removes the non-linear $R_{\rm e}$--$n$ coupling, and because the local slope is $5\,\text{mag\,dex}^{-1}$ at $R_5$, correlated measurement errors in $R_5$ and $μ_5$ largely cancel when deriving the inferred total magnitude. Additionally, an exact single-integral identity is provided to relate any projected light fraction to the fraction within a sphere of the same radius. The directly observable $R_5$ is shown to be connected, through a weakly $n$-dependent factor to the anisotropy-insensitive intrinsic radius $r_{-3}$, yielding a refined $n$-dependent Wolf-type mass estimator $M_{-3}$ and spatial mass-to-light ratio $(M_{\rm dyn}/L)_{-3}$. Specifically, $M_{-3}\approx4\,G^{-1}\langleσ_{\rm los}^2\rangle\,R_{-2}\approx4.72\,G^{-1}\langleσ_{\rm los}^2\rangle\,R_{\rm e}$. Past half-light substitutions in dynamical mass estimators introduce systematic Sérsic-dependent offsets of 12--18 per~cent in enclosed mass and offsets spanning $>20$ per~cent in the mass-to-light ratio.

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arXiv2026

X-ray and optical decline of the intermediate mass black hole HLX-1

HLX-1 is a prominent intermediate-mass black hole (IMBH) candidate, historically exhibiting recurrent X-ray outbursts with spectral state transitions analogous to those observed in stellar-mass black holes. Here, we present new Hubble Space Telescope, Chandra, and Swift observations from 2018-2022 to characterise the late-time flux decline. HLX-1 has persisted in a low X-ray luminosity state (L_X ~ a few x 10^{39} erg/s) since the end of its last outburst in 2017. We observe a significant decoupling between the X-ray and optical/UV emission: while the X-rays have faded by at least two orders of magnitude from peak outburst luminosity (in 2010) to the current low state, the optical/UV flux has declined much more slowly over the same time. This results in an X-ray/optical luminosity ratio inconsistent with X-ray reprocessing in a standard accretion disk, as this would require an unphysical reprocessing fraction >100% at late times. Instead, we find that the optical/UV evolution is well-fitted by a cooling, expanding photosphere (T ~ 30,000 K), similar to the late-stage evolution seen in tidal disruption events (TDEs). The redder component of the optical emission is instead consistent with the old stellar population of a massive star cluster (IMBH host). The pre-2017 X-ray bursting phase is consistent with simulations of disk instabilities in TDE evolution: this strengthens the scenario of HLX-1 as an IMBH TDE. Furthermore, our observations resolve the morphology and flux of the mysterious far-UV emitter, seen in projection next to HLX-1, into a ring-like star-forming structure. We re-assess the possibility that HLX-1 and its host star cluster are physically associated with this starburst dwarf, perhaps via a high-speed collision.

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arXiv2026

Galaxy morphology dependent (black hole mass)-(velocity dispersion) relations: implications for gravitational wave forecasts and cosmological simulations

The correlation between black hole mass, $M_{\rm bh}$, and stellar velocity dispersion, $σ_0$, is revisited using 137 galaxies with quantitative bar strengths and enhanced morphological awareness. Interpreted within the `Triangal' evolutionary framework, gas-rich and gas-poor assembly pathways emerge in the $M_{\rm bh}$--$σ_0$ diagram. To quantify these scaling relations, a versatile Bayesian hierarchical regression code, dubbed the Symmetric COvariance Population Estimator (SCOPE), is introduced. Unlike conditional estimators, SCOPE derives the intrinsic population covariance, natively accommodating asymmetric measurement errors while guaranteeing directional invariance between axes. Primeval, dust-poor S0 galaxies (including dwarf early-type galaxies with $R_{\rm e,gal}\approx1$~kpc) follow a shallow relation ($M_{\rm bh}\proptoσ_0^{2.5\text{--}3.1}$). Explained via the virial theorem, this flattening reframes expectations for intermediate-mass black holes. In contrast, tracing the `Disc Down-sizing' sequence - where dry mergers erase discs - yields a steep relation for massive elliptical and ellicular galaxies ($M_{\rm bh}\proptoσ_0^{7.8\pm1.4}$). The historical practice of applying a single, monolithic scaling relation across all morphological types averages over different formation histories, potentially skewing AGN virial $f$-factor calibrations and systematically under-predicting the ultra-massive black holes needed to generate the nanohertz gravitational wave background. Furthermore, strongly barred, dust-poor S0 galaxies appear offset to higher $σ_0$, while this dynamical signature is lost in the complexities of spiral galaxies. Ultimately, these morphology-dependent relations provide physically-motivated benchmarks for cosmological simulations and a framework for disentangling regimes driven by AGN feedback from those driven by collisionless mergers.

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arXiv2026

The LISA Astrophysics MBHcatalogues Project: A comparison of predictions of simulated massive black hole binaries

In the hierarchical paradigm of galaxy formation, central massive black holes (MBHs) are expected to coalesce after the merger of their host galaxies. One of the main goals of the Laser Interferometer Space Antenna (LISA) is to constrain the origin and growth of MBHs through their merger rates and mass distribution. Predicting MBH merger rates requires not only tracing their statistical population from large to small physical scales (kpc to sub-pc) but also modelling their formation, accretion, dynamics, mergers, and their galactic physical processes across cosmic time. This project is the result of a large collaborative effort undertaken by the LISA Astrophysics Working Group, bringing together its collective expertise on MBH formation, evolution, and modelling, to build a comprehensive understanding of MBH merger rates across cosmic time. The project compares various theoretical predictions of MBH merger rates, quantifies the spread, and evaluates the global astrophysical uncertainties of the LISA event rates. To build a unique and complete view, our work is based on about 20 semi-analytical models and cosmological simulations from the literature, all employing distinct approaches to modelling MBH and galaxy physics. To compute the merger rates, we also incorporate delays arising from the dynamical phase of MBH hardening to coalescence. We present the expected LISA merger rates given current galaxy formation models and discuss how the merger rate depends on model assumptions, such as the seeding model and the resolution of cosmological simulations.

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arXiv2026

Clash of the Trident and Tuning Fork: insights from bar and spiral strength in the (massive black hole)-stellar mass diagrams, and the `Triangal' galaxy evolution schema

The `Triangal' galaxy evolution schema is used to assess whether the Tuning Fork (bar strength) or the van~den~Bergh Trident and ATLAS$^{3D}$ Comb (spiral strength) offer greater evolutionary insight. A new catalogue of quantitative bar strengths (measured by the bar-to-total luminosity ratio, $P$), refined galaxy morphologies, and dust bin classifications is presented. It contains 137 galaxies with spheroid stellar masses, obtained from multi-component decompositions, and directly measured black hole masses, $M_{\rm bh}$. By placing these galaxies within the $M_{\rm bh}$-($M_{\rm\star,sph}$, $M_{\rm\star,gal}$) parameter space, an evolutionary reference frame reflecting integrated growth is established. Galaxies with varying bar strengths, and double bars, are observed to not occupy preferred locations, highlighting that bars are products of secular evolution-and can be transient or recurrent phenomena-and that they track neither hierarchical mass assembly nor galaxy speciation. In contrast, three physically distinct formation channels for S0/a galaxies are identified: (primeval S0)-to-S transitions; faded spiral galaxies; and, most commonly, wet-major-merger-built dust-rich S0 galaxies (on the `green mountain'). Galaxies with particularly strong spirals appear on the right-hand side of the spiral galaxy distribution. Furthermore, a `Dust Attrition/Retention' sequence places S0 (and compact massive ES,b) galaxies with dusty nuclear discs between the dust-poor and dust-rich S0 galaxies, and a `Disc Down-sizing' sequence is revealed, in which E galaxies with dusty nuclear discs-potentially formed through `damp' mergers-bridge the ES,e (ellicular) galaxies with intermediate-scale stellar discs and the dust-poor pure E galaxies. Extensive historical context is provided, and, finally, suspected biases in precision cosmology stemming from neglected precision galaxy morphology are discussed.

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arXiv2026

statmorph-lsst: Quantifying and correcting morphological biases in galaxy surveys

Quantitative morphology provides a key probe of galaxy evolution across cosmic time and environments. However, these metrics can be biased by changes in imaging quality - resolution and depth - either across the survey area or the sample. To prepare for the upcoming Rubin LSST data, we investigate this bias for all metrics measured by statmorph and single-component Sérsic fitting with Galfit. We find that geometrical measurements (ellipticity, axis ratio, Petrosian radius, and effective radius) are robust within 10% at most depths and resolutions. Light concentration measurements ($C$, Gini, $M_{20}$) systematically decrease with resolution, leading low-mass or high-redshift bulge-dominated sources to appear indistinguishable from disks. Sérsic index $n$, while unbiased, suffers from a 20-40% uncertainty due to degeneracies in the Sérsic fit. Disturbance measurements ($A$, $A_S$, $D$) depend on signal-to-noise and are thus affected by noise and surface-brightness dimming. We quantify this dependence for each parameter, offer empirical correction functions, and show that the evolution in $C$ observed in JWST galaxies can be explained purely by observational biases. We propose two new measurements - isophotal asymmetry $A_X$ and substructure $St$ - that aim to resolve some of these biases. Finally, we provide a Python package statmorph-lsst implementing these changes and a full dataset that enables tests of custom functions (see text for links).

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arXiv2025

$R_{\rm e}$. II. Understanding the IC 3475 galaxy type, including ultra-diffuse galaxy, structural scaling relations

It is explained why relatively gas-poor ultra-diffuse galaxies (UDGs), a subset of IC 3475 galaxy types, do not have unexpectedly large sizes but large sizes that are in line with expectations from the curved size-luminosity relation defined by brighter early-type galaxies (ETGs). These UDGs extend the faint end of the (absolute magnitude, $\mathfrak{M}$)-log(Sérsic index, $n$) and $\mathfrak{M}$-(central surface brightness, $μ_{\rm 0}$) relations defined by all ETGs, leading to the large effective half-light radii, $R_{\rm e}$, in these UDGs. It is detailed how the scatter in $μ_{\rm 0}$, at a given $\mathfrak{M}$, relates to variations in the galaxies' values of $n$ and effective surface brightness, $μ_{\rm e}$. These variations map into changes in $R_{\rm e}$ and produce the scatter about the $\mathfrak{M}$-$R_{\rm e}$ relation at fixed $\mathfrak{M}$. Similarly, the scatter in $\mathfrak{M}$, at fixed $μ_{\rm 0}$ and $n$, can be mapped into changes in $R_{\rm e}$. The suggestion that there may be two types of relatively gas-poor UDGs appears ill-founded, arising from the scatter about the $\mathfrak{M}$-$μ_{\rm 0}$ relation. The increased scatter about the faint end of the $\mathfrak{M}$-$R_{\rm e}$ relation and the smaller scatter about $\mathfrak{M}$-(isophotal radii, $R_{\rm iso}$) relations are explained. Artificial and potentially misleading size-luminosity relations for UDGs are also addressed. Finally, expected trends with dynamical mass, and evolutionary pathways towards relatively gas-rich galaxies, are briefly discussed. Hopefully, the understanding presented here will prove helpful for interpreting the many low surface brightness galaxies that the Large Synoptic Survey Telescope will detect.

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arXiv2025

Supermassive black hole mass measurement in the spiral galaxy NGC 4736 Using JWST/NIRSpec stellar kinematics

We present accurate mass measurements of the central supermassive black hole (SMBH) in NGC 4736 (M 94).\ We used the ``gold-standard" stellar absorption features (CO band heads) at $\sim$2.3 ${\rm μm}$, as opposed to gas emission lines, to trace the dynamics in the nuclear region, easily resolving the SMBH's sphere of influence. The analysis uses observations made with the integral field unit of the Near-Infrared Spectrograph (NIRSpec) on the {\it James Webb} Space Telescope and a surface brightness profile derived from {\it Hubble} Space Telescope archival images. We used Jeans anisotropic models within a Bayesian framework, and comprehensive Markov chain Monte Carlo optimization, to determine the best-fit black hole mass, orbital anisotropy, mass-to-light ratio, and nucleus kinematical inclination. We obtained a SMBH mass $M_{\rm BH}=(1.60\pm0.16)\times10^7$ M$_\odot$ (1$σ$ random error), which is consistent with the $M_{\rm BH}$-$σ$ and $M_{\rm BH}$-$M_\star$ relations. This is the first dynamical measurement of a $M_{\rm BH}$ in NGC 4736 based on the stellar kinematics observed with NIRSpec. We thus settle a longstanding inconsistency between estimates based on nuclear emission-line tracers and the $M_{\rm BH}$-$σ$ relation. Our analysis shows that NIRSpec can detect SMBHs with $M_{\rm BH,min}\approx 5\times10^6$ M$_\odot$ in galaxies within 5 Mpc and $σ\approx100$ km s$^{-1}$

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arXiv2025

Dot to dot: high-$z$ little red dots in $M_{\rm bh}$-$M_{\rm \star}$ diagrams with galaxy-morphology-specific scaling relations

The high redshift 'little red dots' (LRDs) detected with the James Webb Space Telescope are considered to be the cores of emerging galaxies that host active galactic nuclei (AGN). For the first time, we compare LRDs with local compact stellar systems and an array of galaxy-morphology-dependent stellar mass-black hole mass scaling relations in the $M_{\rm bh}$-$M_{\star}$ diagrams. When considering the 2023-2024 masses for LRDs, they are not equivalent to nuclear star clusters (NSCs), with the latter having higher $M_{\rm bh}/M_{\star}$ ratios. However, the least massive LRDs exhibit similar $M_{\rm bh}$ and $M_{\rm \star,gal}$ values as ultracompact dwarf (UCD) galaxies, believed to be the cores of stripped/threshed galaxies. We show that the LRDs span the $M_{\rm bh}$-$M_{\rm \star,gal}$ diagram from UCD galaxies to primaeval lenticular galaxies. In contrast, local spiral galaxies and the subset of major-merger-built early-type galaxies define $M_{\rm bh}$-$M_{\star,gal}$ relations that are offset to higher stellar masses. Based on the emerging 2025 masses for LRDs, they may yet have similarities with NSCs, UCD galaxies, and green peas. Irrespective of this developing situation, we additionally observe that low-redshift galaxies with AGN align with the quasi-quadratic or steeper black hole scaling relations defined by local disc galaxies with directly measured black hole masses. This highlights the benefits of considering a galaxy's morphology - which reflects its accretion and merger history - to understand the coevolution of galaxies and their black holes. Future studies of spatially-resolved galaxies with secure masses at intermediate-to-high redshift hold the promise of detecting the emergence and evolution of the galaxy-morphology-dependent $M_{\rm bh}$-$M_{\star}$ relations.

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arXiv2025

Multi-wavelength study of a hyperluminous X-ray source near NGC 6099: a strong IMBH candidate

We report on the intriguing properties of a variable X-ray source projected at the outskirts of the elliptical galaxy NGC 6099 ($d \approx 139$ Mpc). If truly located near NGC 6099, this is a hyperluminous X-ray source that reached an X-ray luminosity $L_{X} \approx $ a few times $10^{42}$ erg s$^{-1}$ in 2012 February (XMM-Newton data), about 50 to 100 times brighter than in 2009 May (Chandra) and 2023 August (XMM-Newton). The X-ray spectrum was soft at all three epochs, with a thermal component at $kT \approx 0.2$ keV and a power-law photon index $>3$. Such properties make it a strong candidate for an intermediate mass black hole (IMBH). We also discovered a point-like, blue optical counterpart ($m_{g,{Vega}}\approx24.7$~mag, $M_{g,{Vega}}\approx-11.2$~mag), from images taken by the Canada-France-Hawaii Telescope, and later confirmed with Hubble Space Telescope observations. The optical continuum can be modeled as stellar emission from a compact star cluster or an X-ray-irradiated accretion disk, consistent with the IMBH scenario. We discuss alternative explanations for the nature of this system. A possible scenario is tidal stripping of an orbiting star, with repeated X-ray outbursts every few years. An alternative possibility is that the thermal X-ray emission seen in 2009 was from shocked gas in the self-intersecting tidal stream during the rising phase of a tidal disruption event, while the 2012 and 2023 emissions were from the fully-formed accretion disk.

astro-ph.HE↗

arXiv2025

Elliptical (E) and ellicular (ES) galaxies in the M_bh-M_*,sph diagram, and a merger-driven explanation for the origin of ES galaxies, anti-truncated stellar discs in lenticular galaxies, and the Sersicification of E galaxy light profiles

In a recent series of papers, supermassive black holes were used to discern pathways in galaxy evolution. By considering the black holes' coupling with their host galaxy's bulge/spheroid, the progression of mass within each component has shed light on the chronological sequence of galaxy speciation. Offsets between the galaxy-morphology-dependent M_bh-M_*,sph scaling relations trace a pattern of 'punctuated equilibrium' arising from merger-driven transitions between galaxy types, such as from spirals to dust-rich lenticulars and further to `ellicular' and elliptical galaxies. This study delves deeper into the distinction between the ellicular galaxies - characterised by their intermediate-scale discs - and elliptical galaxies. Along the way, it is shown how some anti-truncated large-scale discs in lenticular galaxies can arise from the coexistence of a steep intermediate-scale disc and a relatively shallow large-scale disc. This observation undermines application of the popular exponential-disc plus Sérsic-bulge model for lenticular galaxies and suggests some past bulge mass measurements have been overestimated. Furthermore, it is discussed how merger-driven disc-heating and blending likely leads to the spheroidalisation of discs and the conglomeration of multiple discs leads to the (high n) Sersicification of light profiles. The ellicular and elliptical galaxy distribution in the $M_{\rm bh}$-$M_{\rm\star,sph}$ diagram is explored relative to major-merger-built lenticular galaxies and brightest cluster galaxies. The (super-)quadratic $M_{\rm bh}$-$M_{\rm\star}$ relations, presented herein, for merger-built systems should aid studies of massive black hole collisions and the gravitational wave background. Finally, connections to dwarf compact elliptical and ultra-compact dwarf galaxies, with their 100-1000 times higher $M_{\rm bh}/M_{\rm\star,sph}$ ratios, are presented.

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arXiv2024

Identification of Intermediate-mass Black Hole Candidates Among a Sample of Sd Galaxies

We analyzed images of every northern hemisphere Sd galaxy listed in the Third Reference Catalogue of Bright Galaxies (RC3) with a relatively face-on inclination ($θ\leq30°$). Specifically, we measured the spiral arms' winding angle, $ϕ$, in 85 galaxies. We applied a novel black hole mass planar scaling relation involving the rotational velocities (from the literature) and pitch angles of each galaxy to predict central black hole masses. This yielded 23 galaxies, each having at least a 50% chance of hosting a central intermediate-mass black hole (IMBH), $10^2 $1.22% of bright galaxies ($B_{\rm T}\lesssim15.5$ mag) in the local Universe host an IMBH (i.e., the "occupation fraction"), which implies a number density $>$$4.96\times10^{-6}$ Mpc$^{-3}$ for central IMBHs. Finally, we observe that Sd galaxies exhibit an unexpected diversity of properties that resemble the general population of spiral galaxies, albeit with an enhanced signature of the eponymous prototypical traits (i.e., low masses, loosely wound spiral arms, and smaller rotational velocities).

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arXiv2024

Repainting the colour-mass diagrams by unearthing the green mountain: dust-rich S0 galaxies in the colour-(galaxy stellar mass) diagram, and the colour-(black hole mass) relations for dust-poor versus dust-rich galaxies

Lenticular galaxies are notoriously misclassified as elliptical galaxies and, as such, a (disc inclination)-dependent correction for dust is often not applied to the magnitudes of dusty lenticular galaxies. This results in overly red galaxy colours, impacting their distribution in the colour-magnitude diagram. It is revealed how this has led to an underpopulation of the `green valley' by hiding a `green mountain' of massive dust-rich lenticular galaxies - known to be built from gas-rich major mergers - within the `red sequence' of colour-(stellar mass) diagrams. Correcting for dust, a `green mountain' appears at $M_{\rm *,gal}\sim10^{11}$ M$_\odot$, along with signs of an extension to lower masses producing a `green range' or `green ridge' on the green side of the `red sequence' and `blue cloud.' The `red sequence' is shown to be comprised of two components: a red plateau defined by elliptical galaxies with a near-constant colour and by lower-mass dust-poor lenticular galaxies, which are mostly a primordial population but may include faded/transformed spiral galaxies. The quasi-triangular-shaped galaxy evolution sequence, previously called the `Triangal', is revealed in the galaxy colour-(stellar mass) diagram. It tracks the speciation of galaxies and their associated migration through the diagram. The connection of the `Triangal' to previous galaxy morphology sequences (Fork, Trident, Comb) is also shown herein. Finally, the colour-(black hole mass) diagram is revisited, revealing how the dust correction generates a blue-green sequence for the spiral $and$ dust-rich lenticular galaxies that is offset from a green-red sequence defined by the dust-poor lenticular and elliptical galaxies.

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arXiv2024

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $μ$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $μ$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

astro-ph.IM↗

arXiv2023

Specific star formation rates in the $M_{\rm bh}$-$M_{\rm ,sph}$ diagram and the evolutionary pathways of galaxies across the sSFR-$M_{\rm }$ diagram

It has been suggested that the bulge-to-total stellar mass ratio or feedback from black holes (BHs), traced by the BH-to-(total stellar) mass ratio, might establish a galaxy's specific star formation rate (sSFR). We reveal that a galaxy's morphology -- reflecting its formation history, particularly accretions and mergers -- is a far better determinant of the sSFR. Consequently, we suggest that galaxy formation models which regulate the sSFR primarily through BH feedback prescriptions or bulge-regulated disc fragmentation consider acquisitions and mergers which establish the galaxy morphology. We additionally make several new observations regarding current ($z\sim0$) star-formation rates. (i) Galaxies with little to no star formation have bulges with an extensive range of stellar masses; bulge mass does not dictate presence/absence on the `star-forming main sequence'. (ii) The (wet merger)-built, dust-rich S0 galaxies are the `green valley' bridging population between elliptical galaxies on the `red sequence' and spiral galaxies on the blue star-forming main sequence. (iii) The dust-poor S0 galaxies are not on the star-forming main sequence nor in the `green valley'. Instead, they wait in the field for gas accretion and/or minor mergers to transform them into spiral galaxies. Mid-infrared sample selection can miss these (primordial) low dust-content and low stellar-luminosity S0 galaxies. Finally, the appearance of the quasi-triangular-shaped galaxy-assembly sequence, previously dubbed the Triangal, which tracks the morphological evolution of galaxies, is revealed in the sSFR-(stellar mass) diagram.

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arXiv2023

Massive Black Hole Binaries as LISA Precursors in the Roman High Latitude Time Domain Survey

With its capacity to observe $\sim 10^{5-6}$ faint active galactic nuclei (AGN) out to redshift $z\approx 6$, Roman is poised to reveal a population of $10^{4-6}\, {\rm M_\odot}$ black holes during an epoch of vigorous galaxy assembly. By measuring the light curves of a subset of these AGN and looking for periodicity, Roman can identify several hundred massive black hole binaries (MBHBs) with 5-12 day orbital periods, which emit copious gravitational radiation and will inevitably merge on timescales of $10^{3-5}$ years. During the last few months of their merger, such binaries are observable with the Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave mission set to launch in the mid-2030s. Roman can thus find LISA precursors, provide uniquely robust constraints on the LISA source population, help identify the host galaxies of LISA mergers, and unlock the potential of multi-messenger astrophysics with massive black hole binaries.

astro-ph.HE↗

arXiv2023

Resequencing the Hubble sequence and the quadratic (black hole mass)-(spheroid stellar mass) relation for elliptical galaxies

One of the most protracted problems in astronomy has been understanding the evolution of galaxy morphology. Much discussion has surrounded how lenticular galaxies may form a bridging population between elliptical and spiral galaxies. However, with recourse to a galaxy's central black hole mass, accretion-built spiral galaxies have emerged as the bridging population between low-mass lenticular galaxies and the dusty merger-built lenticular galaxies contiguous with elliptical galaxies and `brightest cluster galaxies' in the black hole/galaxy mass diagram. Spiral galaxies, including the Milky Way, appear built from gas accretion and minor mergers onto what were initially lenticular galaxies. These connections are expressed as a new morphology sequence, dubbed the `Triangal', which subsumes elements of the Hubble sequence and the van den Bergh trident and reveals the bridging nature of the often overlooked ellicular galaxies. Furthermore, a quadratic black hole/galaxy mass relation is found to describe ordinary elliptical galaxies. The relation is roughly parallel to the quadratic-like relations observed for the central spheroidal component of spiral galaxies, dust-rich lenticular galaxies, and old dust-poor lenticular galaxies. The brightest cluster galaxies are offset according to expectations from an additional major merger. The findings have implications for feedback from active galactic nuclei, mapping morphology into simulations, and predicting gravitational wave signals from colliding supermassive black holes. A new galaxy speciation model is presented. It disfavours the `monolithic collapse' scenario for spiral, dusty lenticular, and elliptical galaxies. It reveals substantial orbital angular momentum in the Universe's first galaxies and unites dwarf and ordinary `early-type' galaxies.

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