arXiv subjects

Ryan C. W. Houghton

Publications and source records attributed to Ryan C. W. Houghton.

13 recordsLinked to original sources

arXiv2020

Simulating gas kinematic studies of high-redshift galaxies with the HARMONI Integral Field Spectrograph

We present simulated observations of gas kinematics in galaxies formed in 10 pc resolution cosmological simulations with the hydrodynamical + N-body code RAMSES, using the new RAMSES2HSIM pipeline with the simulated observing pipeline (HSIM) for the ELT HARMONI IFU spectrograph. We post-process the galaxy's gas kinematics and Hα line emission for each simulation cell, and integrate the emission to produce an extinction-corrected input cube. We then simulate observations of the input cube with HARMONI, for a range of exposure times, spatial sampling, and spectral resolution. We analyze the mock observations to recover galaxy properties such as its kinematics and compare with the known simulation values. We investigate the cause of biases between the 'real' and 'observed' kinematic values, demonstrating the sensitivity of the inferred rotation curve to knowledge of the instrument's point spread function.

astro-ph.IM↗

arXiv2018

The stellar population and initial mass function of NGC 1399 with MUSE

We present spatially resolved measurements of the stellar initial mass function (IMF) in NGC 1399, the largest elliptical galaxy in the Fornax Cluster. Using data from the Multi Unit Spectroscopic Explorer (MUSE) and updated state-of-the-art stellar population synthesis models from Conroy et. al 2018, we use full spectral fitting to measure the low-mass IMF, as well as a number of individual elemental abundances, as a function of radius in this object. We find that the IMF in NGC 1399 is heavier than the Milky Way in its centre and remains radially constant at a super-salpeter slope out to 0.7 R$_{e}$. At radii larger than this, the IMF slope decreases to become marginally consistent with a Milky Way IMF just beyond R$_{e}$. The inferred central V-band M/L ratio is in excellent agreement with the previously reported dynamical M/L measurement from Houghton et. al 2006. The measured radial form of the M/L ratio may be evidence for a two-phase formation in this object, with the central regions forming differently to the outskirts. We also report measurements of a spatially resolved filament of ionised gas extending 4 arcseconds (404 pc at $D_{\mathrm{L}}=21.1$ Mpc) from the centre of NGC 1399, with very narrow equivalent width and low velocity dispersion ($65\pm14$ \kms). The location of the emission, combined with an analysis of the emission line ratios, leads us to conclude that NGC 1399's AGN is the source of ionising radiation.

astro-ph.GA↗

arXiv2018

The KMOS Cluster Survey (KCS) II - The Effect of Environment on the Structural Properties of Massive Cluster Galaxies at Redshift $1.39 < z <1.61$

We present results on the structural properties of massive passive galaxies in three clusters at $1.39<z<1.61$ from the KMOS Cluster Survey. We measure light-weighted and mass-weighted sizes from optical and near-infrared Hubble Space Telescope imaging and spatially resolved stellar mass maps. The rest-frame $R$-band sizes of these galaxies are a factor of $\sim2-3$ smaller than their local counterparts. The slopes of the relation between the stellar mass and the light-weighted size are consistent with recent studies in clusters and the field. Their mass-weighted sizes are smaller than the rest frame $R$-band sizes, with an average mass-weighted to light-weighted size ratio that varies between $\sim0.45$ and $0.8$ among the clusters. We find that the median light-weighted size of the passive galaxies in the two more evolved clusters is $\sim24\%$ larger than for field galaxies, independent of the use of circularized effective radii or semi-major axes. These two clusters also show a smaller size ratio than the less evolved cluster, which we investigate using color gradients to probe the underlying $M_{*}/L_{H_{160}}$ gradients. The median color gradients are $\nabla{z-H} \sim-0.4$ mag dex$^{-1}$, twice the local value. Using stellar populations models, these gradients are best reproduced by a combination of age and metallicity gradients. Our results favor the minor merger scenario as the dominant process responsible for the observed galaxy properties and the environmental differences at this redshift. The environmental differences support that clusters experience accelerated structural evolution compared to the field, likely via an epoch of enhanced minor merger activity during cluster assembly.

astro-ph.GA↗

arXiv2018

Radial measurements of IMF-sensitive absorption features in two massive ETGs

We make radial measurements of stellar initial mass function (IMF) sensitive absorption features in the two massive early-type galaxies NGC 1277 and IC 843. Using the Oxford Short Wavelength Integral Field SpecTrogaph (SWIFT), we obtain resolved measurements of the NaI0.82 and FeH0.99 indices, among others, finding both galaxies show strong gradients in NaI absorption combined with flat FeH profiles at $\sim0.4$Å. We find these measurements may be explained by radial gradients in the IMF, appropriate abundance gradients in [Na/Fe] and [Fe/H], or a combination of the two, and our data is unable to break this degeneracy. We also use full spectral fitting to infer global properties from an integrated spectrum of each object, deriving a unimodal IMF slope consistent with Salpeter in IC 843 ($x=2.27\pm0.17$) but steeper than Salpeter in NGC 1277 ($x=2.69\pm0.11$), despite their similar FeH equivalent widths. Independently, we fit the strength of the FeH feature and compare to the E-MILES and CvD12 stellar population libraries, finding agreement between the models. The IMF values derived in this way are in close agreement with those from spectral fitting in NGC 1277 ($x_{\mathrm{CvD}}=2.59^{+0.25}_{-0.48}$ , $x_{\mathrm{E-MILES}}=2.77\pm0.31$), but are less consistent in IC 843, with the IMF derived from FeH alone leading to steeper slopes than when fitting the full spectrum ($x_{\mathrm{CvD}}=2.57^{+0.30}_{-0.41}$, $x_{\mathrm{E-MILES}}=2.72\pm0.25$). This work highlights the importance of a large wavelength coverage for breaking the degeneracy between abundance and IMF variations, and may bring into doubt the use of the Wing-Ford band as an IMF index if used without other spectral information.

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arXiv2017

The KMOS Cluster Survey (KCS) III: fundamental plane of cluster galaxies at $z \simeq 1.80$ in JKCS 041

We present data for 16 galaxies in the overdensity JKCS 041 at $z \simeq 1.80$ as part of the K-band Multi-Object Spectrograph (KMOS) Cluster Survey (KCS). With 20-hour integrations, we have obtained deep absorption-line spectra from which we derived velocity dispersions for seven quiescent galaxies. We combined photometric parameters derived from Hubble Space Telescope images with the dispersions to construct a fundamental plane (FP) for quiescent galaxies in JKCS 041. From the zero-point evolution of the FP, we derived a formation redshift for the galaxies of $z_{form} = 3.0\pm0.3$, corresponding to a mean age of $1.4\pm0.2$ Gyrs. We tested the effect of structural and velocity dispersion evolution on our FP zero point and found a negligible contribution when using dynamical mass-normalized parameters ($\sim 3\%$), but a significant contribution from stellar-mass-normalized parameters ($\sim 42 \%$). From the relative velocities of the galaxies, we probed the three-dimensional structure of these 16 confirmed members of JKCS 041, and found that a group of galaxies in the south west of the overdensity had systematically higher velocities. We derived ages for the galaxies in the different groups from the FP. We found the east-extending group had typically older galaxies ($2.1\substack{+0.3\\-0.2}$ Gyrs), than those in the south-west group ($0.3\pm0.2$ Gyrs). Although based on small numbers, the overdensity dynamics, morphology, and age results could indicate that JKCS 041 is in formation and may comprise two merging groups of galaxies. The result could link large-scale structure to ages of galaxies for the first time at this redshift.

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arXiv2017

Local analogues of high-redshift star-forming galaxies: integral field spectroscopy of green peas

We use integral field spectroscopy, from the SWIFT and Palm3K instruments, to perform a spatially-resolved spectroscopic analysis of four nearby highly star-forming `green pea' (GP) galaxies, that are likely analogues of star-forming systems at z~2.5-3. By studying emission-line maps in H$α$, [NII]$λλ$6548,6584 and [SII]$λ$$λ$6716,6731, we explore the kinematic morphology of these systems and constrain properties such as gas-phase metallicities, electron densities and gas-ionization mechanisms. Two of our GPs are rotationally-supported while the others are dispersion-dominated systems. The rotationally-supported galaxies both show evidence for recent or ongoing mergers. However, given that these systems have intact disks, these interactions are likely to have low mass ratios (i.e. minor mergers), suggesting that the minor-merger process may be partly responsible for the high SFRs seen in these GPs. Nevertheless, the fact that the other two GPs appear morphologically undisturbed suggests that mergers (including minor mergers) are not necessary for driving the high star formation rates in such galaxies. We show that the GPs are metal-poor systems (25-40 per cent of solar) and that the gas ionization is not driven by AGN in any of our systems, indicating that AGN activity is not co-eval with star formation in these starbursting galaxies.

astro-ph.GA↗

arXiv2017

The KMOS Cluster Survey (KCS) I: The fundamental plane and the formation ages of cluster galaxies at redshift $1.4<z<1.6$

We present the analysis of the fundamental plane (FP) for a sample of 19 massive red-sequence galaxies ($M_{\star} >4\times10^{10} M_{\odot}$) in 3 known overdensities at $1.39 11$) in our sample, we translate the FP zero-point evolution into a mass-to-light-ratio $M/L$ evolution finding $Δ\log M/L_{B}=(-0.46\pm0.10)z$, $Δ\log M/L_{B}=(-0.52\pm0.07)z$, to $Δ\log M/L_{B}=(-0.55\pm0.10)z$, respectively. We assess the potential contribution of the galaxies structural and stellar velocity dispersion evolution to the evolution of the FP zero-point and find it to be $\sim$6-35 % of the FP zero-point evolution. The rate of $M/L$ evolution is consistent with galaxies evolving passively. By using single stellar population models, we find an average age of $2.33^{+0.86}_{-0.51}$ Gyr for the $\log M_{\star}/M_{\odot}>11$ galaxies in our massive and virialized cluster at $z=1.39$, $1.59^{+1.40}_{-0.62}$ Gyr in a massive but not virialized cluster at $z=1.46$, and $1.20^{+1.03}_{-0.47}$ Gyr in a protocluster at $z=1.61$. After accounting for the difference in the age of the Universe between redshifts, the ages of the galaxies in the three overdensities are consistent within the errors, with possibly a weak suggestion that galaxies in the most evolved structure are older.

astro-ph.GA↗

arXiv2016

Radial gradients in initial mass function sensitive absorption features in the Coma brightest cluster galaxies

Using the Oxford Short Wavelength Integral Field specTrograph (SWIFT), we trace radial variations of initial mass function (IMF) sensitive absorption features of three galaxies in the Coma cluster. We obtain resolved spectroscopy of the central 5kpc for the two central brightest-cluster galaxies (BCGs) NGC4889, NGC4874, and the BCG in the south-west group NGC4839, as well as unresolved data for NGC4873 as a low-$σ_*$ control. We present radial measurements of the IMF-sensitive features sodium NaI$_{\rm{SDSS}}$, calcium triplet CaT and iron-hydride FeH0.99, along with the magnesium MgI0.88 and titanium oxide TiO0.89 features. We employ two separate methods for both telluric correction and sky-subtraction around the faint FeH feature to verify our analysis. Within NGC4889 we find strong gradients of NaI$_{\rm{SDSS}}$ and CaT but a flat FeH profile, which from comparing to stellar population synthesis models, suggests an old, $α$-enhanced population with a Chabrier, or even bottom-light IMF. The age and abundance is in line with previous studies but the normal IMF is in contrast to recent results suggesting an increased IMF slope with increased velocity dispersion. We measure flat NaI$_{\rm{SDSS}}$ and FeH profiles within NGC4874 and determine an old, possibly slightly $α$-enhanced and Chabrier IMF population. We find an $α$-enhanced, Chabrier IMF population in NGC4873. Within NGC4839 we measure both strong NaI$_{\rm{SDSS}}$ and strong FeH, although with a large systematic uncertainty, suggesting a possible heavier IMF. The IMFs we infer for these galaxies are supported by published dynamical modelling. We stress that IMF constraints should be corroborated by further spectral coverage and independent methods on a galaxy-by-galaxy basis.

astro-ph.GA↗

arXiv2016

Sizes, Colour gradients and Resolved Stellar Mass Distributions for the Massive Cluster Galaxies in XMMUJ2235-2557 at z = 1.39

We analyse the sizes, colour gradients, and resolved stellar mass distributions for 36 massive and passive galaxies in the cluster XMMUJ2235-2557 at z=1.39 using optical and near-infrared Hubble Space Telescope imaging. We derive light-weighted Sérsic fits in five HST bands ($i_{775},z_{850},Y_{105},J_{125},H_{160}$), and find that the size decreases by ~20% going from $i_{775}$ to $H_{160}$ band, consistent with recent studies. We then generate spatially resolved stellar mass maps using an empirical relationship between $M_{*}/L_{H_{160}}$ and $(z_{850}-H_{160})$ and use these to derive mass-weighted Sérsic fits: the mass-weighted sizes are ~41% smaller than their rest-frame $r$-band counterparts compared with an average of ~12% at z~0. We attribute this evolution to the evolution in the $M_{*}/L_{H_{160}}$ and colour gradient. Indeed, as expected, the ratio of mass-weighted to light-weighted size is correlated with the $M_{*}/L$ gradient, but is also mildly correlated with the mass surface density and mass-weighted size. The colour gradients $(\nabla_{z-H})$ are mostly negative, with a median value of $\sim0.45$ mag dex$^{-1}$, twice the local value. The evolution is caused by an evolution in age gradients along the semi-major axis ($a$), with $\nabla_{age} = d \log(age) / d \log(a)$ $\sim-0.33$, while the survival of weaker colour gradients in old, local galaxies implies that metallicity gradients are also required, with $\nabla_{Z} = d \log(Z) / d \log(a)$ $\sim-0.2$. This is consistent with recent observational evidence for the inside-out growth of passive galaxies at high redshift, and favours a gradual mass growth mechanism, such as minor mergers.

astro-ph.GA↗

arXiv2015

The initial mass functions of M31 and M32 through far red stellar absorption features

Using the Oxford Short Wavelength Integral Field specTrograph (SWIFT), we investigate radial variations of several initial mass function (IMF) dependent absorption features in M31 and M32. We obtain high signal-to-noise spectra at six pointings along the major axis of M31 out to ~ 700" (2.7 kpc) and a single pointing of the central 10 pc for M32. In M31 the sodium NaI λ8190 index shows a flat equivalent width profile at ~ 0.4 Å through the majority of the bulge, with a strong gradient up to 0.8 Å in the central 10" (38 pc); the Wing-Ford FeH λ9916 index is measured to be constant at 0.4 Å for all radii; and calcium triplet CaT λ8498, 8542, 8662 shows a gradual increase through the bulge towards the centre. M32 displays flat profiles for all three indices, with FeH at ~ 0.5 Å, very high CaT at ~ 0.8 Å and low NaI at ~ 0.1 Å. We analyse these data using stellar population models. We find that M31 is well described on all scales by a Chabrier IMF, with a gradient in sodium enhancement of [Na/Fe] ~ +0.3 dex in the outer bulge, rising within the central 10" to perhaps [Na/Fe] ~ +1.0 dex in the nuclear region. We find M32 is described by a Chabrier IMF and young stellar age in line with other studies. Models show that CaT is much more sensitive to metallicity and [α/Fe] than to IMF. We note that the centres of M31 and M32 have very high stellar densities and yet we measure Chabrier IMFs in these regions.

astro-ph.GA↗

arXiv2015

On the distribution of galaxy ellipticity in clusters

We study the distribution of projected ellipticity n(epsilon) for galaxies in a sample of 20 rich (Richness >= 2) nearby (z < 0.1) clusters of galaxies. We find no evidence of differences in n(epsilon), although the nearest cluster in the sample (the Coma Cluster) is the largest outlier (P(same) < 0.05). We then study n(epsilon) within the clusters, and find that epsilon increases with projected cluster-centric radius R (hereafter the epsilon-R relation). This trend is preserved at fixed magnitude, showing that this relation exists over and above the trend of more luminous galaxies to be both rounder and more common in the centres of clusters. The epsilon-R relation is particularly strong in the subsample of intrinsically flattened galaxies (epsilon > 0.4), therefore it is not a consequence of the increasing fraction of round slow rotator galaxies near cluster centers. Furthermore, the epsilon-R relation persists for just smooth flattened galaxies and for galaxies with de Vaucouleurs-like light profiles, suggesting that the variation of the spiral fraction with radius is not the underlying cause of the trend. We interpret our findings in light of the classification of early type galaxies (ETGs) as fast and slow rotators. We conclude that the observed trend of decreasing epsilon towards the centres of clusters is evidence for physical effects in clusters causing fast rotator ETGs to have a lower average intrinsic ellipticity near the centres of rich clusters.

astro-ph.GA↗

arXiv2012

An Oxford SWIFT Integral Field Spectroscopy study of 14 early-type galaxies in the Coma cluster

As a demonstration of the capabilities of the new Oxford SWIFT integral field spectrograph, we present first observations for a set of 14 early-type galaxies in the core of the Coma cluster. Our data consist of I- and z-band spatially resolved spectroscopy obtained with the Oxford SWIFT spectrograph, combined with r-band photometry from the SDSS archive for 14 early- type galaxies. We derive spatially resolved kinematics for all objects from observations of the calcium triplet absorption features at \sim 8500 {AA} . Using this kinematic information we classify galaxies as either Fast Rotators or Slow Rotators. We compare the fraction of fast and slow rotators in our sample, representing the densest environment in the nearby Universe, to results from the ATLAS3D survey, finding the slow rotator fraction is \sim 50 per cent larger in the core of the Coma cluster than in the Virgo cluster or field, a 1.2 σ increase given our selection criteria. Comparing our sample to the Virgo cluster core only (which is 24 times less dense than the Coma core) we find no evidence of an increase in the slow rotator fraction. Combining measurements of the effective velocity dispersion {σ_e} with the photometric data we determine the Fundamental Plane for our sample of galaxies. We find the use of the average velocity dispersion within 1 effective radius, {σ_e}, reduces the residuals by 13 per cent with respect to comparable studies using central velocity dispersions, consistent with other recent integral field Fundamental Plane determinations.

astro-ph.CO↗

arXiv2011

The SAURON project - XVIII. The integrated UV-linestrength relations of early-type galaxies

Using far (FUV) and near (NUV) ultraviolet photometry from guest investigator programmes on the Galaxy Evolution Explorer (GALEX) satellite, optical photometry from the MDM Observatory and optical integral-field spectroscopy from SAURON, we explore the UV-linestrength relations of the 48 nearby early-type galaxies in the SAURON sample. Identical apertures are used for all quantities, avoiding aperture mismatch. We show that galaxies with purely old stellar populations show well-defined correlations of the integrated FUV-V and FUV-NUV colours with the integrated Mgb and Hbeta absorption linestrength indices, strongest for FUV-NUV. Correlations with the NUV-V colour, Fe5015 index and stellar velocity dispersion are much weaker. These correlations put stringent constraints on the origin of the UV-upturn phenomenon in early-type galaxies, and highlight its dependence on age and metallicity. In particular, despite recent debate, we recover the negative correlation between FUV-V colour and Mg linestrength originally publicised by Burstein et al. (1988), which we refer to as the "Burstein relation", suggesting a positive dependence of the UV-upturn on metallicity. We argue that the scatter in the correlations is real, and present mild evidence that a strong UV excess is preferentially present in slow-rotating galaxies. We also demonstrate that most outliers in the correlations are galaxies with current or recent star formation, some at very low levels. We believe that this sensitivity to weak star formation, afforded by the deep and varied data available for the SAURON sample, explains why our results are occasionally at odds with other recent but shallower surveys. This is supported by the analysis of a large, carefully-crafted sample of more distant early-type galaxies from the Sloan Digital Sky Survey (SDSS), more easily comparable with current and future large surveys.

astro-ph.CO↗