arXiv subjects

Michelle E. Cluver

Publications and source records attributed to Michelle E. Cluver.

At least 19 recordsLinked to original sources

arXiv2026

The Galactic Squeeze: How Aggregate and Highly Dynamical Environments Shape Star Formation in the Local Universe

We investigate how galaxy evolution varies with environment in the nearby Universe by comparing an ``average'' reference volume in the Southern Galactic Pole (SGP) dataset from \citet{VanKempen2024} to the Nexus region, a dynamically assembling superstructure centred on the Abell~4038 galaxy cluster. We quantify environmental effects using the quenched fraction ($f_{\mathrm{Q}}$) and the specific star formation rate ($\mathrm{sSFR}$) for the star-forming population, measured as functions of stellar mass and group-scale halo mass from \citet{VanKempen2026}. We decouple the stellar--halo mass dependence, demonstrating that $f_{\mathrm{Q}}$ increases with stellar mass in both field and group environments, while group galaxies show an additional dependence on halo mass. The Nexus exhibits systematic differences relative to the SGP baseline, consistent with increased heterogeneity in accretion histories and pre-processing within a forming superstructure. For star-forming galaxies, the mean $\log(\mathrm{sSFR})$ declines strongly with stellar mass and shows additional environment-linked suppression in group-scale halos. Within the Nexus, splitting the sample into three projected radial zones around Abell~4038 shows that environmental regulation is not spatially uniform, driven largely by variations in the sampled halo mass function. Finally, a projected phase-space (PPS) analysis of Abell~4038 links quenching to orbital history within the cluster, though this trend is strongly mass-dependent: low-mass galaxies ($\log(M_{\mathrm{stellar}}) < 10$) show no significant change in $f_{\mathrm{Q}}$. These results demonstrate that the drivers of galaxy evolution depend jointly on stellar mass, local group halo mass, and location within the surrounding large-scale structure, motivating future large-scale, multi-wavelength cosmic web surveys.

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arXiv2026

Group Therapy for Halos: Advancing Halo Mass Estimation for Galaxy Groups

Accurate estimation of dark matter halo masses for galaxy groups is central to studies of galaxy evolution and for leveraging group catalogues as cosmological probes. We present a calibration and evaluation of two complementary halo mass estimators: a dynamical estimator based on the virial theorem, and an empirical relation between the sum of the stellar masses of the three most massive group galaxies and the halo mass (SHMR). Using state-of-the-art semi-analytic models (SHARK, SAGE, and GAEA) to generate mock light-cone catalogues, we quantify the accuracy, uncertainty, and model dependence of each method. The calibrated virial theorem achieves negligible systematic bias (mean $Δ$ = -0.01 dex) and low scatter (mean $σ$ = 0.20 dex) with no sensitivity to baryonic physics. The calibrated SHMR yields the highest precision (mean $Δ$ = 0.02 dex, mean $σ$ = 0.14 dex) but shows greater model dependence due to sensitivity to baryonic physics across the models. We demonstrate applications to observational catalogues, including the empirical halo mass function and mapping quenched fractions in the stellar mass-halo mass plane. We provide guidance: the virial theorem is recommended for GAMA-like surveys (i < 19.2) at z < 0.1 where minimal model dependence is required, while the SHMR is optimal for high-precision halo mass estimates across diverse catalogues with limits of z < 0.3. These calibrated estimators will aid upcoming wide-area spectroscopic surveys in probing the connection between galaxies and their host dark matter halos.

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arXiv2025

Starburst galaxies in the Hydra I cluster

We present a new catalog of 196 galaxies of the nearby Hydra I cluster out to $\sim$1.75$\rm r_{200}$, consisting of broad u,g,r,i,z along with narrowband H$α$ measurements. These deep optical images were obtained with the DECam camera (CTIO) and reach down to a surface brightness limit of $μ( 3σ;10''\times10'')$=26.9 mag $\rm arcsec^2$ in the g band. We also report the HI properties for 89 cluster members detected with MeerKAT. A color magnitude diagram (CMD) shows a bimodal distribution typical of a cluster population, more evolved than those found in isolation. We combine optical H$α$ and WISE infrared data to compare the star formation history at two distinct timescales. Differences in the star forming activity depicted by both populations manifest as starburst in 24 found members. Of these, 18 starburst galaxies have neutral gas measurements, and show disturbed HI disks that suggest an environmentally-triggered boost in star formation within the last 10$^7$ yrs. Processes such as ram pressure stripping or tidal interactions may underlie their enhanced star-forming activity and asymmetric disks. Since Hydra's dynamical history is unclear, we examine the spatial and velocity distribution of the sample. We reveal a possible link between the large scale structure feeding the Hydra I cluster and the heightened star-forming activity of the starburst galaxies. This feeding pattern matches the few substructure that has been identified in Hydra in previous works, and may explain its origin. Our results portray a picture of a cluster with an evolved nature, plus a population of new infalling galaxies that manifest the impact of their first contact with the cluster environment through star formation, color, morphology and gas content transformations.

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arXiv2025

The central spectra of massive star-forming galaxies

We have examined the nuclear spectra of very massive star-forming galaxies at $z \sim 0$ to understand how they differ from other galaxies with comparable masses, which are typically passive. We selected a sample of 126 nearby massive star-forming galaxies ($<100~{\rm Mpc}$, $10^{11.3}~\rm{M_\odot} \leq M_{\rm stellar} \leq 10^{11.7}~\rm{M_\odot}$, $1 ~{\rm M_\odot~yr^{-1}}< {\rm SFR} <13 ~{\rm M_\odot~yr^{-1}}$) from the 2MRS-Bright WXSC catalogue. LEDA morphologies indicate at least 63\% of our galaxies are spirals, while visual inspection of Dark Energy Survey images reveals 75\% of our galaxies to be spirals with the remainder being lenticular. Of our sample 59 have archival nuclear spectra, which we have modelled and subsequently measured emission lines ([NII]$\rm{λ6583}$, H$α\rm{λ6563}$, [OIII]$\rm{λ5008}$, and H$β\rm{λ4863}$), classifying galaxies as star-forming, LINERS, or AGNs. Using a BPT diagram we find $83 \pm 6$ \% of our galaxies, with sufficient signal-to-noise to measure all 4 emission lines, to be LINERs. Using the [NII]$\rm{λ6583}$/H$α\rm{λ6563}$ emission line ratio alone we find that $79 \pm 6$ \% of the galaxies (46 galaxies) with archival spectra are LINERs, whereas just $\sim 30\%$ of the overall massive galaxy population are LINERs (Belfiore et al. 2016). Our sample can be considered a local analogue of the Ogle et al. (2016, 2019) sample of $z \sim 0.22$ massive star-forming galaxies in terms of selection criteria, and we find 64\% of their galaxies are LINERs using SDSS spectra. The high frequency of LINER emission in these massive star-forming galaxies indicates that LINER emission in massive galaxies may be linked to the presence of gas that fuels star formation.

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arXiv2023

Measurement of the evolving galaxy luminosity and mass function using clustering-based redshift inference

We develop a framework for using clustering-based redshift inference (cluster-$z$) to measure the evolving galaxy luminosity function (GLF) and galaxy stellar mass function (GSMF) using WISE W1 ($3.4μm$) mid-infrared photometry and positions. We use multiple reference sets from the Galaxy And Mass Assembly (GAMA) survey, Sloan Digital Sky Survey (SDSS) and Baryon Oscillation Spectroscopic Survey (BOSS). Combining the resulting cluster-$z$s allows us to enlarge the study area, and by accounting for the specific properties of each reference set, making best use of each reference set to produce the best overall result. Thus we are able to measure the GLF and GSMF over $\sim 7500\, \mathrm{deg}^2 $ of the Northern Galactic Cap (NGC) up to $z<0.6$. Our method can easily be adapted for new studies with fainter magnitudes, which pose difficulties for the derivation of photo-$z$s. The measurement of the GSMF is currently limited by the models for k-corrections and mass-to-light ratios, rather than more complicated effects tied to the evolution of the differential galaxy bias. With better statistics in future surveys this technique is a strong candidate for studies with new emerging data from, e.g. the Vera C. Rubin Observatory, the Euclid mission or the Nancy Grace Roman Space Telescope.

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arXiv2022

Dark matter halos and scaling relations of extremely massive spiral galaxies from extended HI rotation curves

We present new and archival atomic hydrogen (\hi) observations of \galnum\ of the most massive spiral galaxies in the local Universe ($M_\star>10^{11} \, \mathrm{M}_\odot$). From 3D kinematic modeling of the datacubes, we derive extended \hi\ rotation curves, and from these, we estimate masses of the dark matter halos and specific angular momenta of the discs. We confirm that massive spiral galaxies lie at the upper ends of the Tully-Fisher relation (mass vs velocity, $M \propto V^{4}$) and Fall relation (specific angular momentum vs mass, $j \propto M^{0.6}$), in both stellar and baryonic forms, with no significant deviations from single power laws. We study the connections between baryons and dark matter through the stellar (and baryon)-to-halo ratios of mass $f_\mathrm{M} \equiv M_\star/M_\mathrm{h}$ and specific angular momentum $f_\mathrm{j} \equiv j_\star/j_\mathrm{h}$ and $f_\mathrm{j,bar} \equiv j_\mathrm{bar}/j_\mathrm{h}$. Combining our sample with others from the literature for less massive disc-dominated galaxies, we find that $f_\mathrm{M}$ rises monotonically with $M_\star$ and $M_\mathrm{h}$ (instead of the inverted-U shaped $f_\mathrm{M}$ for spheroid-dominated galaxies), while $f_\mathrm{j}$ and $f_\mathrm{j,bar}$ are essentially constant near unity over four decades in mass. Our results indicate that disc galaxies constitute a self-similar population of objects closely linked to the self-similarity of their dark halos. This picture is reminiscent of early analytical models of galaxy formation wherein discs grow by relatively smooth and gradual inflow, isolated from disruptive events such as major mergers and strong AGN feedback, in contrast to the more chaotic growth of spheroids.

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arXiv2022

The stellar populations of quiescent ultra-diffuse galaxies from optical to mid-infrared spectral energy distribution fitting

We use spectral energy distribution (SED) fitting to place constraints on the stellar population properties of 29 quiescent ultra-diffuse galaxies (UDGs) across different environments. We use the fully Bayesian routine PROSPECTOR coupled with archival data in the optical, near, and mid-infrared from Spitzer and WISE under the assumption of an exponentially declining star formation history. We recover the stellar mass, age, metallicity, dust content, star formation time scales and photometric redshifts (photo-zs) of the UDGs studied. Using the mid-infrared data, we probe the existence of dust in UDGs. Although its presence cannot be confirmed, we find that the inclusion of small amounts of dust in the models brings the stellar populations closer to those reported with spectroscopy. Additionally, we fit the redshifts of all galaxies. We find a high accuracy in recovering photo-zs compared to spectroscopy, allowing us to provide new photo-z estimates for three field UDGs with unknown distances. We find evidence of a stellar population dependence on the environment, with quiescent field UDGs being systematically younger than their cluster counterparts. Lastly, we find that all UDGs lie below the mass--metallicity relation for normal dwarf galaxies. Particularly, the globular cluster (GC)-poor UDGs are consistently more metal-rich than GC-rich ones, suggesting that GC-poor UDGs may be puffed-up dwarfs, while most GC-rich UDGs are better explained by a failed galaxy scenario. As a byproduct, we show that two galaxies in our sample, NGC 1052-DF2 and NGC 1052-DF4, share equivalent stellar population properties, with ages consistent with 8 Gyr. This finding supports formation scenarios where the galaxies were formed together.

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arXiv2022

Decoding the star forming properties of gas-rich galaxy pairs

We extend the analysis of Bok et al. (2020) in which the HI content of isolated galaxies from the AMIGA sample and selected paired galaxies from ALFALFA were examined as a potential driver of galaxy location on the WISE mid-infrared SFR-Mstar sequence. By further characterizing the isolated and pair galaxy samples, i.e. in terms of optical galaxy morphology, a more detailed and quantitative description of local galaxy environment by way of the local number density (eta) and tidal strength (Q) parameters, star formation efficiency (SFE(HI)), and HI integrated profile asymmetries, we present plausible pathways for the broadening of the pair sample HI deficiency distribution towards both high and low deficiencies compared to the narrower isolated galaxy sample distribution (i.e. sigma_pairs = 0.34 versus sigma_AMIGA = 0.28). We associate the gas-rich tail of the pair deficiency distribution with the highest Q values, large profile asymmetries, and low SFEs. From this we infer that merger activity is enhancing gas supplies, as well as disrupting the efficiency of star formation, via strong gravitational torques. The gas-poor wing of the deficiency distribution appears to be populated with galaxies in denser environments (with larger eta values on average), more akin to groups. Despite our gas-rich selection criterion, there is a small population of early-type galaxies in the pair sample, which primarily fall in the positive deficiency wing of the distribution. These results suggest that a combination of a denser galaxy environment, early-type morphology, and higher stellar mass, is contributing to the broadening of the deficiency distribution towards larger deficiencies.

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arXiv2022

Galaxy And Mass Assembly (GAMA): Data Release 4 and the z < 0.1 total and z < 0.08 morphological galaxy stellar mass functions

In Galaxy And Mass Assembly Data Release 4 (GAMA DR4), we make available our full spectroscopic redshift sample. This includes 248682 galaxy spectra, and, in combination with earlier surveys, results in 330542 redshifts across five sky regions covering ~250deg^2. The redshift density, is the highest available over such a sustained area, has exceptionally high completeness (95 per cent to r_KIDS=19.65mag), and is well suited for the study of galaxy mergers, galaxy groups, and the low redshift (z<0.25) galaxy population. DR4 includes 32 value-added tables or Data Management Units (DMUs) that provide a number of measured and derived data products including GALEX, ESO KiDS, ESO VIKING, WISE and Herschel Space Observatory imaging. Within this release, we provide visual morphologies for 15330 galaxies to z<0.08, photometric redshift estimates for all 18million objects to r_KIDS~25mag, and stellar velocity dispersions for 111830 galaxies. We conclude by deriving the total galaxy stellar mass function (GSMF) and its sub-division by morphological class (elliptical, compact-bulge and disc, diffuse-bulge and disc, and disc only). This extends our previous measurement of the total GSMF down to 10^6.75 M_sol h^-2_70 and we find a total stellar mass density of rho_*=(2.97+/-0.04)x10^8 M_sol h_70 Mpc^-3 or Omega_*=(2.17+/-0.03)x10^-3 h^-1_70. We conclude that at z<0.1, the Universe has converted 4.9+/-0.1 per cent of the baryonic mass implied by Big Bang Nucleosynthesis into stars that are gravitationally bound within the galaxy population.

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arXiv2021

Galaxy And Mass Assembly (GAMA): $\mathbf{z \sim 0}$ Galaxy Luminosity Function down to $\mathbf{L \sim 10^{6}~L_\odot}$ via Clustering Based Redshift Inference

In this study we present a new experimental design using clustering-based redshift inference to measure the evolving galaxy luminosity function (GLF) spanning 5.5 decades from $L \sim 10^{11.5}$ to $ 10^6 ~ \mathrm{L}_\odot$. We use data from the Galaxy And Mass Assembly (GAMA) survey and the Kilo-Degree Survey (KiDS). We derive redshift distributions in bins of apparent magnitude to the limits of the GAMA-KiDS photometric catalogue: $m_r \lesssim 23$; more than a decade in luminosity beyond the limits of the GAMA spectroscopic redshift sample via clustering-based redshift inference. This technique uses spatial cross-correlation statistics for a reference set with known redshifts (in our case, the main GAMA sample) to derive the redshift distribution for the target ensemble. For the calibration of the redshift distribution we use a simple parametrisation with an adaptive normalisation factor over the interval $0.005 < z < 0.48$ to derive the clustering redshift results. We find that the GLF has a relatively constant power-law slope $α\approx -1.2$ for $-17 \lesssim M_r \lesssim -13$, and then appears to steepen sharply for $-13 \lesssim M_r \lesssim -10$. This upturn appears to be where Globular Clusters (GCs) take over to dominate the source counts as a function of luminosity. Thus we have mapped the GLF across the full range of the $z \sim 0$ field galaxy population from the most luminous galaxies down to the GC scale.

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arXiv2021

A Systematic Survey for z < 0.04 Changing-Look AGNs

We have conducted a systematic survey for z $<$ 0.04 active Galactic nuclei (AGNs) that may have changed spectral class over the past decade. We use SkyMapper, Pan-STARRS and the Véron-Cetty & Véron (2010) catalogue to search the entire sky for these ``changing-look'' AGNs using a variety of selection methods, where Pan-STARRS has a coverage of 3$π$ steradians (sky north of Declination $-30^\circ$) and SkyMapper has coverage of $\sim$ 21,000$~\rm{deg^2}$ (sky south of Declination $0^\circ$). We use small aperture photometry to measure how colour and flux have changed over time, where a change may indicate a change in spectral type. Optical colour and flux are used as a proxy for changing H$α$ equivalent width, while WISE 3.4 $μ$m flux is used to look for changes in the hot dust component. We have identified four AGNs with varying spectra selected using our optical colour selection method. Three AGNs were confirmed from recent observations with WiFeS on the 2.3 m telescope at Siding Spring and the other was identified from archival spectra alone. From this, we identify two new changing look AGNs; NGC 1346 and 2MASX J20075129-1108346. We also recover Mrk 915 and Mrk 609, which are known to have varying spectra in the literature, but they do not meet our specific criteria for changing look AGNs.

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arXiv2020

HI study of isolated and pair galaxies: the MIR SFR-M$\star$ sequence

Using mid-infrared star formation rate and stellar mass indicators in $\textit{WISE}$, we construct and contrast the relation between star formation rate and stellar mass for isolated and paired galaxies. Our samples comprise a selection of AMIGA (isolated galaxies) and pairs of ALFALFA galaxies with HI detections such that we can examine the relationship between HI content (gas fraction, HI deficiency) and galaxy location on the main sequence (MS) in these two contrasting environments. We derive for the first time an HI scaling relation for isolated galaxies using $\textit{WISE}$ stellar masses, and thereby establish a baseline predictor of HI content that can be used to assess the impact of environment on HI content when compared with samples of galaxies in different environments. We use this updated relation to determine the HI deficiency of both our paired and isolated galaxies. Across all the quantities examined as a function of environment in this work (MS location, gas fraction, and HI deficiency), the AMIGA sample of isolated galaxies is found to have the lower dispersion: $σ_{\rm{AMIGA}} = 0.37$ versus $σ_{\rm{PAIRS}} = 0.55$ on the MS, $σ_{\rm{AMIGA}} = 0.44$ versus $σ_{\rm{PAIRS}} = 0.54$ in gas fraction, and $σ_{\rm{AMIGA}} = 0.28$ versus $σ_{\rm{PAIRS}} = 0.34$ in HI deficiency. We also note fewer isolated quiescent galaxies, 3 (0.6$\%$), compared to 12 (2.3$\%$) quiescent pair members. Our results suggest the differences in scatter measured between our samples are environment driven. Galaxies in isolation behave relatively predictably, and galaxies in more densely populated environments adopt a more stochastic behaviour, across a broad range of quantities.

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arXiv2020

GAMA+KiDS: Empirical correlations between halo mass and other galaxy properties near the knee of the stellar-to-halo mass relation

We use KiDS weak lensing data to measure variations in mean halo mass as a function of several key galaxy properties (namely: stellar colour, specific star formation rate, Sersic index, and effective radius) for a volume-limited sample of GAMA galaxies in a narrow stellar mass range ($M_* \sim 2$--$5 \times 10^{10}$ Msol). This mass range is particularly interesting, inasmuch as it is where bimodalities in galaxy properties are most pronounced, and near to the break in both the galaxy stellar mass function and the stellar-to-halo mass relation (SHMR). In this narrow mass range, we find that both size and Sersic index are better predictors of halo mass than either colour or SSFR, with the data showing a slight preference for Sersic index. In other words, we find that mean halo mass is more tightly correlated with galaxy structure than either past star formation history or current star formation rate. Our results lead to an approximate lower bound on the dispersion in halo masses among $\log M_* \approx {10.5}$ galaxies: we find that the dispersion is $\gtrsim 0.3$ dex. This would imply either that offsets from the mean SHMR are closely coupled to size/structure, or that the dispersion in the SHMR is larger than past results have suggested. Our results thus provide new empirical constraints on the relationship between stellar and halo mass assembly at this particularly interesting mass range.

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arXiv2020

The influence of angular momentum and environment on the HI gas of late-type galaxies

We use high-resolution HI data from the WHISP survey to study the HI and angular momentum properties of a sample of 114 late-type galaxies. We explore the specific baryonic angular momentum -- baryonic mass ($j_b - M_b$) relation, and find that an unbroken power law of the form $j_b \propto M_b^{0.55 \pm 0.02}$ fits the data well, with an intrinsic scatter of $\sim 0.13 \pm 0.01$ dex. We revisit the relation between the atomic gas fraction, $f_{atm}$, and the integrated atomic stability parameter $q$ (the $f_{atm} - q$ relation), originally introduced by Obreschkow et al., and probe this parameter space by populating it with galaxies from different environments, in order to study the influence of the environment on their $j_b$, $f_{atm}$ and $q$ values. We find evidence that galaxies with close neighbours show a larger intrinsic scatter about the $f_{atm} - q$ relation compared to galaxies without close-neighbours. We also find enhanced SFR among the deviating galaxies with close neighbours. In addition, we use the bulge-to-total (B/T) ratio as a morphology proxy, and find a general trend of decreasing B/T values with increasing disc stability and HI fraction in the $f_{atm} - q$ plane, indicating a fundamental link between mass, specific angular momentum, gas fraction and morphology of galaxies.

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arXiv2018

Calibrating the James Webb Space Telescope filters as star formation rate indicators

We have calibrated the 6.5 m James Webb Space Telescope (JWST) mid-infrared filters as star formation rate indicators, using JWST photometry synthesized from $Spitzer$ spectra of 49 low redshift galaxies, which cover a wider luminosity range than most previous studies. We use Balmer decrement corrected $\rm{Hα}$ luminosity and synthesized mid-infrared photometry to empirically calibrate the $Spitzer$, WISE and JWST filters as star formation rate indicators. Our $Spitzer$ and WISE calibrations are in good agreement with recent calibrations from the literature. While mid-infrared luminosity may be directly proportional to star formation rate for high luminosity galaxies, we find a power-law relationship between mid-infrared luminosity and star formation rate for low luminosity galaxies ($L_{\rm Hα} \leq 10^{43}~{\rm erg~s^{-1}}$). We find that for galaxies with a $\rm{Hα}$ luminosity of $\rm{10^{40}}~erg~s^{-1}$ (corresponding to a star formation rate of $\sim 0.055~{\rm M_\odot~yr^{-1}}$), the corresponding JWST mid-infrared $νL_ν$ luminosity is between $\rm{10^{40.50}}$ and $\rm{10^{41.00}}~erg~s^{-1}$. Power-law fits of JWST luminosity as a function of $\rm{Hα}$ luminosity have indices between 1.17 and 1.32. We find that the scatter in the JWST filter calibrations decreases with increasing wavelength from 0.39 to 0.20 dex, although F1000W is an exception where the scatter is just 0.24 dex.

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arXiv2018

A Multi-Frequency Study of the Milky Way-like Spiral Galaxy NGC 6744

We present a multi-frequency study of the intermediate spiral SAB(r)bc type galaxy NGC 6744, using available data from the Chandra X-Ray telescope, radio continuum data from the Australia Telescope Compact Array and Murchison Widefield Array, and Wide-field Infrared Survey Explorer infrared observations. We identify 117 X-ray sources and 280 radio sources. Of these, we find nine sources in common between the X-ray and radio catalogues, one of which is a faint central black hole with a bolometric radio luminosity similar to the Milky Way's central black hole. We classify 5 objects as supernova remnant candidates, 2 objects as likely supernova remnants, 17 as HII regions, 1 source as an AGN; the remaining 255 radio sources are categorised as background objects and one X-ray source is classified as a foreground star. We find the star-formation rate (SFR) of NGC 6744 to be in the range 2.8 - 4.7 $\rm{M_{\odot}~yr^{-1}}$ signifying the galaxy is still actively forming stars. The specific SFR of NGC 6744 is greater than that of late-type spirals such as the Milky Way, but considerably less that that of a typical starburst galaxy.

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arXiv2018

Galaxy And Mass Assembly (GAMA): The effect of galaxy group environment on active galactic nuclei

In galaxy clusters, efficiently accreting active galactic nuclei (AGN) are preferentially located in the infall regions of the cluster projected phase-space, and are rarely found in the cluster core. This has been attributed to both an increase in triggering opportunities for infalling galaxies, and a reduction of those mechanisms in the hot, virialised, cluster core. Exploiting the depth and completeness ($98\,$per cent at $r<19.8\,$mag) of the Galaxy And Mass Assembly survey (GAMA), we probe down the group halo mass function to assess whether AGN are found in the same regions in groups as they are in clusters. We select 451 optical AGN from 7498 galaxies with $\log_{10}(M_*/\text{M}_\odot) > 9.9$ in 695 groups with $11.53\leq \log_{10}(M_{200}/\text{M}_\odot) \leq 14.56$ at $z<0.15$. By analysing the projected phase-space positions of these galaxies we demonstrate that when split both radially, and into physically derived infalling and core populations, AGN position within group projected phase-space is dependent on halo mass. For groups with $\log_{10}(M_{200}/\text{M}_\odot)>13.5$, AGN are preferentially found in the infalling galaxy population with $3.6σ$ confidence. At lower halo masses we observe no difference in AGN fraction between core and infalling galaxies. These observations support a model where a reduced number of low-speed interactions, ram pressure stripping and intra-group/cluster medium temperature, the dominance of which increase with halo mass, work to inhibit AGN in the cores of groups and clusters with $\log_{10}(M_{200}/\text{M}_\odot)>13.5$, but do not significantly affect nuclear activity in cores of less massive structures.

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arXiv2017

Calibration of ultraviolet, mid-infrared and radio star formation rate indicators

We present calibrations for star formation rate indicators in the ultraviolet, mid-infrared and radio continuum bands, including one of the first direct calibrations of 150 MHz as a star formation rate indicator. Our calibrations utilize 66 nearby star forming galaxies with Balmer decrement corrected H-alpha luminosities, which span 5 orders of magnitude in star formation rate and have absolute magnitudes of -24<M_r<-12. Most of our photometry and spectrophotometry is measured from the same region of each galaxy, and our spectrophotometry has been validated with SDSS photometry, so our random and systematic errors are small relative to the intrinsic scatter seen in star formation rate indicator calibrations. We find WISE W4 (22.8 micron), Spitzer 24 micron and 1.4 GHz have tight correlations with Balmer decrement corrected H-alpha luminosity, with scatter of only 0.2 dex. Our calibrations are comparable to those from the prior literature for L* galaxies, but for dwarf galaxies our calibrations can give star formation rates that are far greater than those derived from much of the prior literature.

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