arXiv subjects

Aaron Robotham

Publications and source records attributed to Aaron Robotham.

At least 55 records · Page 3Linked to original sources

arXiv2023

Paper 1: The JWST PEARLS View of the El Gordo Galaxy Cluster and of the Structure It Magnifies

The massive galaxy cluster El Gordo (z=0.87) imprints multitudes of gravitationally lensed arcs onto James Webb Space Telescope (JWST) Near-Infrared Camera (NIRCam) images. Eight bands of NIRCam imaging were obtained in the ``Prime Extragalactic Areas for Reionization and Lensing Science'' (``PEARLS'') program. PSF-matched photometry across Hubble Space Telescope (HST) and NIRCam filters supplies new photometric redshifts. A new light-traces-mass lens model based on 56 image multiplicities identifies the two mass peaks and yields a mass estimate within 500 kpc of ~(7.0 +/- 0.30) x 10^14 Msun. A search for substructure in the 140 cluster members with spectroscopic redshifts confirms the two main mass components. The southeastern mass peak that contains the BCG is more tightly bound than the northwestern one. The virial mass within 1.7 Mpc is (5.1 +/- 0.60) x 10^14 Msun, lower than the lensing mass. A significant transverse velocity component could mean the virial mass is underestimated. We contribute one new member to the previously known z=4.32 galaxy group. Intrinsic (delensed) positions of the five secure group members span a physical extent of ~60 kpc. Thirteen additional candidates selected by spectroscopic/photometric constraints are small and faint with a mean intrinsic luminosity ~2.2 mag fainter than L*. NIRCam imaging admits a fairly wide range of brightnesses and morphologies for the group members, suggesting a more diverse galaxy population in this galaxy overdensity.

astro-ph.GA↗

arXiv2023

JWST's PEARLS: TN J1338-1942 -- I. Extreme jet triggered star-formation in a $z=4.11$ luminous radio galaxy

We present the first JWST observations of the $z=4.11$ luminous radio galaxy TN J1338-1942, obtained as part of the ``Prime Extragalactic Areas for Reionization and Lensing Science'' (``PEARLS'') project. Our NIRCam observations, designed to probe the key rest-frame optical continuum and emission line features at this redshift, enable resolved spectral energy distribution modelling that incorporates both a range of stellar population assumptions and radiative shock models. With an estimated stellar mass of $\log_{10}(M/\text{M}_{\odot}) \sim 10.9$, TN J1338--1942 is confirmed to be one of the most massive galaxies known at this epoch. Our observations also reveal extremely high equivalent-width nebular emission coincident with the luminous AGN jets that is best fit by radiative shocks surrounded by extensive recent star-formation. We estimate the total star-formation rate (SFR) could be as high as $\sim1600\,\text{M}_{\odot}\,\text{yr}^{-1}$, with the SFR that we attribute to the jet induced burst conservatively $\gtrsim500\,\text{M}_{\odot}\,\text{yr}^{-1}$. The mass-weighted age of the star-formation, $t_{\text{mass}} <4$ Myr, is consistent with the likely age of the jets responsible for the triggered activity and significantly younger than that measured in the core of the host galaxy. The extreme scale of the potential jet-triggered star-formation activity indicates the potential importance of positive AGN feedback in the earliest stages of massive galaxy formation, with our observations also illustrating the extraordinary prospects for detailed studies of high-redshift galaxies with JWST.

astro-ph.GA↗

arXiv2023

SKYSURF-4: Panchromatic HST All-Sky Surface-Brightness Measurement Methods and Results

The diffuse, unresolved sky provides most of the photons that the Hubble Space Telescope (HST) receives, yet remains poorly understood. HST Archival Legacy program SKYSURF aims to measure the 0.2-1.6 $μ$m sky surface brightness (sky-SB) from over 140,000 HST images. We describe a sky-SB measurement algorithm designed for SKYSURF that is able to recover the input sky-SB from simulated images to within 1% uncertainty. We present our sky-SB measurements estimated using this algorithm on the entire SKYSURF database. Comparing our sky-SB spectral energy distribution (SED) to measurements from the literature shows general agreements. Our SKYSURF SED also reveals a possible dependence on Sun angle, indicating either non-isotropic scattering of solar photons off interplanetary dust or an additional component to Zodiacal Light. Finally, we update Diffuse Light limits in the near-IR based on the methods from Carleton et al. (2022), with values of 0.009 MJy sr$^{-1}$ (22 nW m$^{-2}$ sr$^{-1}$) at 1.25 $μ$m, 0.015 MJy sr$^{-1}$ (32 nW m$^{-2}$ sr$^{-1}$) at 1.4 $μ$m, and 0.013 MJy sr$^{-1}$ (25 nW m$^{-2}$ sr$^{-1}$) at 1.6 $μ$m. These estimates provide the most stringent all-sky constraints to date in this wavelength range. SKYSURF sky-SB measurements are made public on the official SKYSURF website and will be used to constrain Diffuse Light in future papers.

astro-ph.IM↗

arXiv2023

JWST's PEARLS: dust attenuation and gravitational lensing in the backlit-galaxy system VV 191

We derive the spatial and wavelength behavior of dust attenuation in the multiple-armed spiral galaxy VV191b using backlighting by the superimposed elliptical system VV191a in a pair with an exceptionally favorable geometry for this measurement. Imaging using JWST and HST spans the wavelength range 0.3-4.5 microns with high angular resolution, tracing the dust in detail from 0.6 to 1.5 microns. Distinct dust lanes continue well beyond the bright spiral arms, and trace a complex web, with a very sharp radial cutoff near 1.7 Petrosian radii. We present attenuation profiles and coverage statistics in each band at radii 14-21 kpc. We derive the attenuation law with wavelength; the data both within and between the dust lanes clearly favor a stronger reddening behavior (R ~ 2.0 between 0.6 and 0.9 microns, approaching unity by 1.5 microns) than found for starbursts and star-forming regions of galaxies. Power-law extinction behavior lambda^(-beta) gives beta=2.1 from 0.6-0.9 microns. R decreases at increasing wavelengths (R~1.1 between 0.9 and 1.5 microns), while beta steepens to 2.5. Mixing regions of different column density flattens the wavelength behavior, so these results suggest a different grain population than in our vicinity. The NIRCam images reveal a lens arc and counterimage from a background galaxy at z~1, spanning 90 degrees azimuthally at 2.8" from the foreground elliptical galaxy nucleus, and an additional weakly-lensed galaxy. The lens model and imaging data give a mass/light ratio 7.6 in solar units within the Einstein radius 2.0 kpc.

astro-ph.GA↗

arXiv2022

JWST's PEARLS: A JWST/NIRCam view of ALMA sources

We report the results of James Webb Space Telescope/NIRCam observations of 19 (sub)millimeter (submm/mm) sources detected by the Atacama Large Millimeter Array (ALMA). The accurate ALMA positions allowed unambiguous identifications of their NIRCam counterparts. Taking gravitational lensing into account, these represent 16 distinct galaxies in three fields and constitute the largest sample of its kind to date. The counterparts' spectral energy distributions from rest-frame ultraviolet to near infrared provide photometric redshifts ($1 10^{10.5}$ Msol), which are similar to sub-millimeter galaxy (SMG) hosts studied previously. However, our sample is fainter in submm/mm than the classic SMG samples are, and our sources exhibit a wider range of properties. They have dust-embedded star-formation rates as low as 10 Msol yr$^{-1}$, and the sources populate both the star-forming main sequence and the quiescent categories. The deep NIRCam data allow us to study the rest-frame near-IR morphologies. Excluding two multiply imaged systems and one quasar, the majority of the remaining sources are disk-like and show either little or no disturbance. This suggests that secular growth is a potential route for the assembly of high-mass disk galaxies. While a few hosts have large disks, the majority have small disks (median half-mass radius of 1.6 kpc). At this time, it is unclear whether this is due to the prevalence of small disks at these redshifts or some unknown selection effects of deep ALMA observations. A larger sample of ALMA sources with NIRCam observations will be able to address this question.

astro-ph.GA↗

arXiv2022

Webb's PEARLS: Bright 1.5--2.0 micron Dropouts in the Spitzer/IRAC Dark Field

Using the first epoch of four-band NIRCam observations obtained by the James Webb Space Telescope (JWST) Prime Extragalactic Areas for Reionization and Lensing Science Program in the Spitzer IRAC Dark Field, we search for F150W and F200W dropouts. In 14.2 arcmin^2, we have found eight F150W dropouts and eight F200W dropouts, all brighter than 27.5 mag (the brightest being ~24 mag) in the band to the red side of the break. As they are detected in multiple bands, these must be real objects. Their nature, however, is unclear, and characterizing their properties is important for realizing the full potential of JWST. If the observed color decrements are due to the Lyman break, these objects should be at z >~ 11.7 and z >~ 15.4, respectively. The color diagnostics show that at least four F150W dropouts are far away from the usual contaminators encountered in dropout searches (red galaxies at much lower redshifts or brown dwarf stars). While the diagnostics of the F200W dropouts are less certain due to the limited number of passbands, at least one of them is likely not a known type of contaminant, and the rest are consistent with either high-redshift galaxies with evolved stellar populations or old galaxies at z ~ 3 to 8. If a significant fraction of our dropouts are indeed at z ~ 12, we have to face the severe problem of explaining their high luminosities and number densities. Spectroscopic identifications of such objects are urgently needed.

astro-ph.GA↗

arXiv2022

JWST's PEARLS: Prime Extragalactic Areas for Reionization and Lensing Science: Project Overview and First Results

We give an overview and describe the rationale, methods, and first results from NIRCam images of the JWST "Prime Extragalactic Areas for Reionization and Lensing Science" ("PEARLS") project. PEARLS uses up to eight NIRCam filters to survey several prime extragalactic survey areas: two fields at the North Ecliptic Pole (NEP); seven gravitationally lensing clusters; two high redshift proto-clusters; and the iconic backlit VV 191 galaxy system to map its dust attenuation. PEARLS also includes NIRISS spectra for one of the NEP fields and NIRSpec spectra of two high-redshift quasars. The main goal of PEARLS is to study the epoch of galaxy assembly, AGN growth, and First Light. Five fields, the JWST NEP Time-Domain Field (TDF), IRAC Dark Field (IDF), and three lensing clusters, will be observed in up to four epochs over a year. The cadence and sensitivity of the imaging data are ideally suited to find faint variable objects such as weak AGN, high-redshift supernovae, and cluster caustic transits. Both NEP fields have sightlines through our Galaxy, providing significant numbers of very faint brown dwarfs whose proper motions can be studied. Observations from the first spoke in the NEP TDF are public. This paper presents our first PEARLS observations, their NIRCam data reduction and analysis, our first object catalogs, the 0.9-4.5 $μ$m galaxy counts and Integrated Galaxy Light. We assess the JWST sky brightness in 13 NIRCam filters, yielding our first constraints to diffuse light at 0.9-4.5 μm. PEARLS is designed to be of lasting benefit to the community.

astro-ph.CO↗

arXiv2022

JWST's PEARLS: a new lens model for ACT-CL J0102$-$4915, "EL Gordo'', and the first red supergiant star at cosmological distances discovered by JWST

The first JWST data on the massive colliding cluster El Gordo confirm 23 known families of multiply lensed images and identify 8 new members of these families. Based on these families, which have been confirmed spectroscopically by MUSE, we derived an initial lens model. This model guided the identification of 37 additional families of multiply lensed galaxies, among which 28 are entirely new systems, and 9 were previously known. The initial lens model determined geometric redshifts for the 37 new systems. The geometric redshifts agree reasonably well with spectroscopic or photometric redshifts when those are available. The geometric redshifts enable two additional models that include all 60 families of multiply lensed galaxies spanning a redshift range $2 0.8$ and has an estimated virial mass close the maximum mass allowed by standard cosmological models. The JWST images also reveal the presence of small-mass perturbers that produce small lensing distortions. The smallest of these is consistent with being a dwarf galaxy at $z=0.87$ and has an estimated mass of $3.8\times10^9$~\Msol, making it the smallest substructure found at $z>0.5$. The JWST images also show several candidate caustic-crossing events. One of them is detected at high significance at the expected position of the critical curve and is likely a red supergiant star at $z=2.1878$. This would be the first red supergiant found at cosmological distances. The cluster lensing should magnify background objects at $z>6$, making more of them visible than in blank fields of similar size, but there appears to be a deficiency of such objects.

astro-ph.GA↗

arXiv2022

SKYSURF: Constraints on Zodiacal Light and Extragalactic Background Light through Panchromatic HST All-Sky Surface-Brightness Measurements: I. Survey Overview and Methods

We give an overview and describe the rationale, methods, and testing of the Hubble Space Telescope (HST) Archival Legacy project "SKYSURF." SKYSURF uses HST's unique capability as an absolute photometer to measure the ~0.2-1.7 $μ$m sky surface brightness (SB) from 249,861 WFPC2, ACS, and WFC3 exposures in ~1400 independent HST fields. SKYSURF's panchromatic dataset is designed to constrain the discrete and diffuse UV to near-IR sky components: Zodiacal Light (ZL; inner Solar System), Kuiper Belt Objects (KBOs; outer Solar System), Diffuse Galactic Light (DGL), and the discrete plus diffuse Extragalactic Background Light (EBL). We outline SKYSURF's methods to: (1) measure sky-SB levels between its detected objects; (2) measure the integrated discrete EBL, most of which comes from AB$\simeq$17-22 mag galaxies; and (3) estimate how much diffuse light may exist in addition to the extrapolated discrete galaxy counts. Simulations of HST WFC3/IR images with known sky-values and gradients, realistic cosmic ray (CR) distributions, and star plus galaxy counts were processed with nine different algorithms to measure the "Lowest Estimated Sky-SB" (LES) in each image between the discrete objects. The best algorithms recover the inserted LES values within 0.2% when there are no image gradients, and within 0.2-0.4% when there are 5-10% gradients. SKYSURF requires non-standard re-processing of these HST images that includes restoring the lowest sky-level from each visit into each drizzled image. We provide a proof of concept of our methods from the WFC3/IR F125W images, where any residual diffuse light that HST sees in excess of the Kelsall et al. (1998) Zodiacal model prediction does not depend on the total object flux that each image contains. This enables us to present our first SKYSURF results on diffuse light in Carleton et al. (2022).

astro-ph.CO↗

arXiv2022

SKYSURF: Constraints on Zodiacal Light and Extragalactic Background Light through Panchromatic HST All-Sky Surface-Brightness Measurements: II. First Limits on Diffuse Light at 1.25, 1.4, and 1.6 microns

We present the first results from the HST Archival Legacy project "SKYSURF." As described in Windhorst et al. 2022, SKYSURF utilizes the large HST archive to study the diffuse UV, optical, and near-IR backgrounds and foregrounds in detail. Here we utilize SKYSURF's first sky-surface brightness measurements to constrain the level of near-IR diffuse Extragalactic Background Light (EBL). Our sky-surface brightness measurements have been verified to an accuracy of better than 1%, which when combined with systematic errors associated with HST, results in sky brightness uncertainties of $\sim$2-4% $\simeq$ 0.005 MJy/sr in each image. We put limits on the amount of diffuse EBL in three near-IR filters (F125W, F140W, and F160W) by comparing our preliminary sky measurements of $> 30,000$ images to Zodiacal light models, carefully selecting the darkest images to avoid contamination from stray light. In addition, we investigate the impact that instrumental thermal emission has on our measurements, finding that it has a limited impact on F125W and F140W measurements, whereas uncertainties in the exact thermal state of HST results in significant uncertainties in the level of astrophysical diffuse light in F160W images. When compared to the Kelsall et al. (1998) Zodiacal model, an isotropic diffuse background of $30$ nW m$^{-2}$ sr$^{-1}$ remains, whereas using the Wright (1998) Zodiacal model results in no discernible diffuse background. Based primarily on uncertainties in the foreground model subtraction, we present limits on the amount of diffuse EBL of 29 nW m$^{-2}$ sr$^{-1}$, 40 nW m$^{-2}$ sr$^{-1}$, and 29 nW m$^{-2}$ sr$^{-1}$ for F125W, F140W, and F160W respectively. While this light is generally isotropic, our modeling at this point does not distinguish between a cosmological origin or a Solar System origin (such as a dim, diffuse, spherical cloud of cometary dust).

astro-ph.CO↗

arXiv2022

The eROSITA Final Equatorial Depth Survey (eFEDS): X-ray emission around star-forming and quiescent galaxies at $0.05<z<0.3$

We aim at characterizing the hot phase of the Circum-Galactic Medium in a large sample of galaxies. We stack X-ray events from the SRG/eROSITA eFEDS survey around central galaxies in the GAMA 9hr field to construct radially projected soft X-ray luminosity profiles as a function of their stellar mass and specific star formation rate. We consider samples of quiescent (star-forming) galaxies in the stellar mass range $2\times 10^{10}$ -- $10^{12}$ M$_\odot$ ($3\times 10^9$ -- $6\times10^{11}$ M$_\odot$). For quiescent galaxies, the X-ray profiles are clearly extended throughout the available mass range; however, the measured profile is likely biased high due to projection effects, as these galaxies tend to live in dense and hot environments. For the most massive star forming samples ($\geq10^{11}$ M$_\odot$), there is a hint of detection of extended emission. For star-forming galaxies with $< 10^{11}$ M$_\odot$ the X-ray stacked profiles are compatible with unresolved sources and consistent with the expected emission from faint Active Galactic Nuclei and X-ray binaries. We measure for the first time the mean relation between average X-ray luminosity and stellar mass separately for quiescent and star-forming galaxies. High-mass ($\geq 10^{11}$ M$_\odot$) star-forming or quiescent galaxies follow the expected scaling of virialized hot haloes, while lower mass star-forming galaxies show a less prominent luminosity and a weaker dependence on stellar mass, consistent with empirical models of the weak AGN population. When comparing our results with state-of-the art numerical simulations, we find an overall consistency on large ($>80$ kpc) scales at masses $\geq 10^{11}$ M$_\odot$, but disagreement on the small scales, where brighter than observed compact cores are predicted. Simulations also do not predict the clear differentiation that we observe between quiescent and star-forming galaxies.

astro-ph.GA↗

arXiv2022

MeerKAT uncovers the physics of an Odd Radio Circle

Odd Radio Circles (ORCs) are recently-discovered faint diffuse circles of radio emission, of unknown cause, surrounding galaxies at moderate redshift ($z ~ 0.2-0.6). Here we present detailed new MeerKAT radio images at 1284 MHz of the first ORC, originally discovered with the Australian Square Kilometre Array Pathfinder, with higher resolution (6 arcsec) and sensitivity (~ 2.4 uJy/bm). In addition to the new images, which reveal a complex internal structure consisting of multiple arcs, we also present polarisation and spectral index maps. Based on these new data, we consider potential mechanisms that may generate the ORCs.

astro-ph.GA↗

arXiv2021

Galaxy And Mass Assembly (GAMA) Survey: The Merging Potential of Brightest Group Galaxies

Using a volume-limited sample of 550 groups from the Galaxy And Mass Assembly (GAMA) Galaxy Group Catalogue spanning the halo mass range $12.8 < \log [M_{h}/M] < 14.2$, we investigate the merging potential of central Brightest Group Galaxies (BGGs). We use spectroscopically-confirmed close-companion galaxies as an indication of the potential stellar mass build-up of low-redshift BGGs, $z \leq 0.2$. We identify 17 close-companion galaxies with projected separations $r_{p} < 30$ kpc, relative velocities $Δv \leq 300$ km s$^{-1}$, and stellar-mass ratios $M_{BGG}/M_{CC} \leq 4$ relative to the BGG. These close-companion galaxies yield a total pair fraction of $0.03 \pm 0.01$. Overall, we find that BGGs in our sample have the potential to grow in stellar mass due to mergers by $2.2 \pm 1.5\%$ Gyr$^{-1}$. This is lower than the stellar mass growth predicted by current galaxy evolution models.

astro-ph.GA↗

arXiv2021

Multi-wavelength scaling relations in galaxy groups: a detailed comparison of GAMA and KiDS observations to BAHAMAS simulations

We study the scaling relations between the baryonic content and total mass of groups of galaxies, as these systems provide a unique way to examine the role of non-gravitational processes in structure formation. Using Planck and ROSAT data, we conduct detailed comparisons of the stacked thermal Sunyaev-Zel'dovich (tSZ) and X-ray scaling relations of galaxy groups found in the Galaxy And Mass Assembly (GAMA) survey and the BAHAMAS hydrodynamical simulation. We use weak gravitational lensing data from the Kilo Degree Survey (KiDS) to determine the average halo mass of the studied systems. We analyse the simulation in the same way, using realistic weak lensing, X-ray, and tSZ synthetic observations. Furthermore, to keep selection biases under control, we employ exactly the same galaxy selection and group identification procedures to the observations and simulation. Applying this comparison, we find that the simulations reproduce the richness, size, and stellar mass functions of GAMA groups, as well as the stacked weak lensing and tSZ signals in bins of group stellar mass. However, the simulations predict X-ray luminosities that are higher than observed for this optically-selected group sample. As the same simulations were previously shown to match the luminosities of X-ray-selected groups, this suggests that X-ray-selected systems may form a biased subset. Finally, we demonstrate that our observational processing of the X-ray and tSZ signals is free of significant biases. We find that our optical group selection procedure has, however, some room for improvement.

astro-ph.CO↗

arXiv2020

K-CLASH: spatially-resolving star-forming galaxies in field and cluster environments at $z \approx 0.2$-$0.6$

We present the KMOS-CLASH (K-CLASH) survey, a K-band Multi-Object Spectrograph (KMOS) survey of the spatially-resolved gas properties and kinematics of 191 (predominantly blue) H$α$-detected galaxies at $0.2 \lesssim z \lesssim 0.6$ in field and cluster environments. K-CLASH targets galaxies in four Cluster Lensing And Supernova survey with Hubble (CLASH) fields in the KMOS $IZ$-band, over $7'$ radius ($\approx2$-$3$ Mpc) fields-of-view. K-CLASH aims to study the transition of star-forming galaxies from turbulent, highly star-forming disc-like and peculiar systems at $z\approx1$-$3$, to the comparatively quiescent, ordered late-type galaxies at $z\approx0$, and to examine the role of clusters in the build-up of the red sequence since $z\approx1$. In this paper, we describe the K-CLASH survey, present the sample, and provide an overview of the K-CLASH galaxy properties. We demonstrate that our sample comprises star-forming galaxies typical of their stellar masses and epochs, residing both in field and cluster environments. We conclude K-CLASH provides an ideal sample to bridge the gap between existing large integral-field spectroscopy surveys at higher and lower redshifts. We find that star-forming K-CLASH cluster galaxies at intermediate redshifts have systematically lower stellar masses than their star-forming counterparts in the field, hinting at possible "downsizing" scenarios of galaxy growth in clusters at these epochs. We measure no difference between the star-formation rates of H$α$-detected, star-forming galaxies in either environment after accounting for stellar mass, suggesting that cluster quenching occurs very rapidly during the epochs probed by K-CLASH, or that star-forming K-CLASH galaxies in clusters have only recently arrived there, with insufficient time elapsed for quenching to have occured.

astro-ph.GA↗

arXiv2019

The Detailed Science Case for the Maunakea Spectroscopic Explorer, 2019 edition

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more.

astro-ph.IM↗

arXiv2018

WALLABY Early Science - I. The NGC 7162 Galaxy Group

We present Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) early science results from the Australian Square Kilometre Array Pathfinder (ASKAP) observations of the NGC 7162 galaxy group. We use archival HIPASS and Australia Telescope Compact Array (ATCA) observations of this group to validate the new ASKAP data and the data reduction pipeline ASKAPsoft. We detect six galaxies in the neutral hydrogen (HI) 21-cm line, expanding the NGC 7162 group membership from four to seven galaxies. Two of the new detections are also the first HI detections of the dwarf galaxies, AM 2159-434 and GALEXASC J220338.65-431128.7, for which we have measured velocities of $cz=2558$ and $cz=2727$ km s$^{-1}$, respectively. We confirm that there is extended HI emission around NGC 7162 possibly due to past interactions in the group as indicated by the $40^{\circ}$ offset between the kinematic and morphological major axes for NGC 7162A, and its HI richness. Taking advantage of the increased resolution (factor of $\sim1.5$) of the ASKAP data over archival ATCA observations, we fit a tilted ring model and use envelope tracing to determine the galaxies' rotation curves. Using these we estimate the dynamical masses and find, as expected, high dark matter fractions of $f_{\mathrm{DM}}\sim0.81-0.95$ for all group members. The ASKAP data are publicly available.

astro-ph.GA↗

arXiv2018

The Causes of the Red Sequence, the Blue Cloud, the Green Valley and the Green Mountain

The galaxies found in optical surveys fall in two distinct regions of a diagram of optical colour versus absolute magnitude: the red sequence and the blue cloud with the green valley in between. We show that the galaxies found in a submillimetre survey have almost the opposite distribution in this diagram, forming a `green mountain'. We show that these distinctive distributions follow naturally from a single, continuous, curved Galaxy Sequence in a diagram of specific star-formation rate versus stellar mass without there being the need for a separate star-forming galaxy Main Sequence and region of passive galaxies. The cause of the red sequence and the blue cloud is the geometric mapping between stellar mass/specific star-formation rate and absolute magnitude/colour, which distorts a continuous Galaxy Sequence in the diagram of intrinsic properties into a bimodal distribution in the diagram of observed properties. The cause of the green mountain is Malmquist bias in the submillimetre waveband, with submillimetre surveys tending to select galaxies on the curve of the Galaxy Sequence, which have the highest ratios of submillimetre-to-optical luminosity. This effect, working in reverse, causes galaxies on the curve of the Galaxy Sequence to be underrepresented in optical samples, deepening the green valley. The green valley is therefore not evidence (1) for there being two distinct populations of galaxies, (2) for galaxies in this region evolving more quickly than galaxies in the blue cloud and the red sequence, (c) for rapid quenching processes in the galaxy population.

astro-ph.GA↗