arXiv subjects

Lisa J. Kewley

Publications and source records attributed to Lisa J. Kewley.

At least 127 records · Page 7Linked to original sources

arXiv2015

A universal, turbulence-regulated star formation law: from Milky Way clouds to high-redshift disk and starburst galaxies

Whilst the star formation rate (SFR) of molecular clouds and galaxies is key in understanding galaxy evolution, the physical processes which determine the SFR remain unclear. This uncertainty about the underlying physics has resulted in various different star formation laws, all having substantial intrinsic scatter. Extending upon previous works that define the column density of star formation (Sigma_SFR) by the gas column density (Sigma_gas), we develop a new universal star formation (SF) law based on the multi-freefall prescription of gas. This new SF law relies predominantly on the probability density function (PDF) and on the sonic Mach number of the turbulence in the star-forming clouds. By doing so we derive a relation where the star formation rate (SFR) correlates with the molecular gas mass per multi-freefall time, whereas previous models had used the average, single-freefall time. We define a new quantity called maximum (multi-freefall) gas consumption rate (MGCR) and show that the actual SFR is only about 0.4% of this maximum possible SFR, confirming the observed low efficiency of star formation. We show that placing observations in this new framework (Sigma_SFR vs. MGCR) yields a significantly improved correlation with 3-4 times reduced scatter compared to previous SF laws and a goodness-of-fit parameter R^2=0.97. By inverting our new relationship, we provide sonic Mach number predictions for kpc-scale observations of Local Group galaxies as well as unresolved observations of local and high-redshift disk and starburst galaxies that do not have independent, reliable estimates for the turbulent cloud Mach number.

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arXiv2015

The Absence of an Environmental Dependence in the Mass-Metallicity Relation at z=2

We investigate the environmental dependence of the mass-metallicity relation at z=2 with MOSFIRE/Keck as part of the ZFIRE survey. Here, we present the chemical abundance of a Virgo-like progenitor at z=2.095 that has an established red sequence. We identified 43 cluster ($ =2.095\pm0.004$) and 74 field galaxies ($ =2.195\pm0.083$) for which we can measure metallicities. For the first time, we show that there is no discernible difference between the mass-metallicity relation of field and cluster galaxies to within 0.02dex. Both our field and cluster galaxy mass-metallicity relations are consistent with recent field galaxy studies at z~2. We present hydrodynamical simulations for which we derive mass-metallicity relations for field and cluster galaxies. We find at most a 0.1dex offset towards more metal-rich simulated cluster galaxies. Our results from both simulations and observations are suggestive that environmental effects, if present, are small and are secondary to the ongoing inflow and outflow processes that are governed by galaxy halo mass.

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arXiv2015

Kinematic classifications of local interacting galaxies: implications for the merger/disk classifications at high-z

The classification of galaxy mergers and isolated disks is key for understanding the relative importance of galaxy interactions and secular evolution during the assembly of galaxies. The kinematic properties of galaxies as traced by emission lines have been used to suggest the existence of a significant population of high-z star-forming galaxies consistent with isolated rotating disks. However, recent studies have cautioned that post-coalescence mergers may also display disk-like kinematics. To further investigate the robustness of merger/disk classifications based on kinematic properties, we carry out a systematic classification of 24 local (U)LIRGs spanning a range of galaxy morphologies: from isolated spiral galaxies, ongoing interacting systems, to fully merged remnants. We artificially redshift the WiFeS observations of these local (U)LIRGs to z=1.5 to make a realistic comparison with observations at high-z, and also to ensure that all galaxies have the same spatial sampling of ~900 pc. Using both kinemetry-based and visual classifications, we find that the reliability of kinematic classification shows a strong trend with the interaction stage of galaxies. Mergers with two nuclei and tidal tails have the most distinct kinematic properties compared to isolated disks, whereas a significant population of the interacting disks and merger remnants are indistinguishable from isolated disks. The high fraction of late-stage mergers showing disk-like kinematics reflects the complexity of the dynamics during galaxy interactions. However, the exact fractions of misidentified disks and mergers depend on the definition of kinematic asymmetries and the classification threshold when using kinemetry-based classifications. Our results suggest that additional indicators such as morphologies traced by stars or molecular gas are required to further constrain the merger/disk classifications at high-z.

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arXiv2015

Shocked Gas in IRAS F17207-0014: ISM Collisions and Outflows

We combine optical and near-infrared AO-assisted integral field observations of the merging ULIRG IRAS F17207-0014 from the Wide-Field Spectrograph (WiFeS) and Keck/OSIRIS. The optical emission line ratios [N II]/H$α$, [S II]/H$α$, and [O I]/H$α$ reveal a mixing sequence of shocks present throughout the galaxy, with the strongest contributions coming from large radii (up to 100% at $\sim$5 kpc in some directions), suggesting galactic-scale winds. The near-infrared observations, which have approximately 30 times higher spatial resolution, show that two sorts of shocks are present in the vicinity of the merging nuclei: low-level shocks distributed throughout our field-of-view evidenced by an H$_{2}$/Br$γ$ line ratio of $\sim$0.6-4, and strong collimated shocks with a high H$_{2}$/Br$γ$ line ratio of $\sim$4-8, extending south from the two nuclear disks approximately 400 pc ($\sim$0.5 arcsec). Our data suggest that the diffuse shocks are caused by the collision of the interstellar media associated with the two progenitor galaxies and the strong shocks trace the base of a collimated outflow coming from the nucleus of one of the two disks.

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arXiv2015

The SAMI Galaxy Survey: Shocks and Outflows in a normal star-forming galaxy

We demonstrate the feasibility and potential of using large integral field spectroscopic surveys to investigate the prevalence of galactic-scale outflows in the local Universe. Using integral field data from SAMI and the Wide Field Spectrograph, we study the nature of an isolated disk galaxy, SDSS J090005.05+000446.7 (z = 0.05386). In the integral field datasets, the galaxy presents skewed line profiles changing with position in the galaxy. The skewed line profiles are caused by different kinematic components overlapping in the line-of-sight direction. We perform spectral decomposition to separate the line profiles in each spatial pixel as combinations of (1) a narrow kinematic component consistent with HII regions, (2) a broad kinematic component consistent with shock excitation, and (3) an intermediate component consistent with shock excitation and photoionisation mixing. The three kinematic components have distinctly different velocity fields, velocity dispersions, line ratios, and electron densities. We model the line ratios, velocity dispersions, and electron densities with our MAPPINGS IV shock and photoionisation models, and we reach remarkable agreement between the data and the models. The models demonstrate that the different emission line properties are caused by major galactic outflows that introduce shock excitation in addition to photoionisation by star-forming activities. Interstellar shocks embedded in the outflows shock-excite and compress the gas, causing the elevated line ratios, velocity dispersions, and electron densities observed in the broad kinematic component. We argue from energy considerations that, with the lack of a powerful active galactic nucleus, the outflows are likely to be driven by starburst activities. Our results set a benchmark of the type of analysis that can be achieved by the SAMI Galaxy Survey on large numbers of galaxies.

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arXiv2015

Metallicity gradients in local field star-forming galaxies: Insights on inflows, outflows, and the coevolution of gas, stars and metals

We present metallicity gradients in 49 local field star-forming galaxies. We derive gas-phase oxygen abundances using two widely adopted metallicity calibrations based on the [OIII]/Hbeta, [NII]/Halpha and [NII]/[OII] line ratios. The two derived metallicity gradients are usually in good agreement within +/-0.14 dex/R25 (R25 is the B-band iso-photoal radius), but the metallicity gradients can differ significantly when the ionisation parameters change systematically with radius. We investigate the metallicity gradients as a function of stellar mass (8 MB > -22). When the metallicity gradients are expressed in dex/kpc, we show that galaxies with lower mass and luminosity, on average, have steeper metallicity gradients. When the metallicity gradients are expressed in dex/R25, we find no correlation between the metallicity gradients, and stellar mass and luminosity. We provide a local benchmark metallicity gradient of field star-forming galaxies useful for comparison with studies at high redshifts. We investigate the origin of the local benchmark gradient using simple chemical evolution models and observed gas and stellar surface density profiles in nearby field spiral galaxies. Our models suggest that the local benchmark gradient is a direct result of the coevolution of gas and stellar disk under virtually closed-box chemical evolution when the stellar-to-gas mass ratio becomes high (>>0.3). These models imply low current mass accretion rates (<0.3xSFR), and low mass outflow rates (<3xSFR) in local field star-forming galaxies.

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arXiv2015

A Turnover in the Galaxy Main Sequence of Star Formation at $M_{*} \sim 10^{10} M_{\odot}$ for Redshifts $z < 1.3$

The relationship between galaxy star formation rates (SFR) and stellar masses ($M_\ast$) is re-examined using a mass-selected sample of $\sim$62,000 star-forming galaxies at $z \le 1.3$ in the COSMOS 2-deg$^2$ field. Using new far-infrared photometry from $Herschel$-PACS and SPIRE and $Spitzer$-MIPS 24 $μ$m, along with derived infrared luminosities from the NRK method based on galaxies' locations in the restframe color-color diagram $(NUV - r)$ vs. $(r - K)$, we are able to more accurately determine total SFRs for our complete sample. At all redshifts, the relationship between median $SFR$ and $M_\ast$ follows a power-law at low stellar masses, and flattens to nearly constant SFR at high stellar masses. We describe a new parameterization that provides the best fit to the main sequence and characterizes the low mass power-law slope, turnover mass, and overall scaling. The turnover in the main sequence occurs at a characteristic mass of about $M_{0} \sim 10^{10} M_{\odot}$ at all redshifts. The low mass power-law slope ranges from 0.9-1.3 and the overall scaling rises in SFR as a function of $(1+z)^{4.12 \pm 0.10}$. A broken power-law fit below and above the turnover mass gives relationships of $SFR \propto M_{*}^{0.88 \pm 0.06}$ below the turnover mass and $SFR \propto M_{*}^{0.27 \pm 0.04}$ above the turnover mass. Galaxies more massive than $M_\ast \gtrsim 10^{10}\ M_{\rm \odot}$ have on average, a much lower specific star formation rate (sSFR) than would be expected by simply extrapolating the traditional linear fit to the main sequence found for less massive galaxies.

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arXiv2014

KECK/MOSFIRE spectroscopic confirmation of a Virgo-like cluster ancestor at z=2.095

We present the spectroscopic confirmation of a galaxy cluster at $z=2.095$ in the COSMOS field. This galaxy cluster was first reported in the ZFOURGE survey as harboring evolved massive galaxies using photometric redshifts derived with deep near-infrared (NIR) medium-band filters. We obtain medium resolution ($R \sim$ 3600) NIR spectroscopy with MOSFIRE on the Keck 1 telescope and secure 180 redshifts in a $12'\times12'$ region. We find a prominent spike of 57 galaxies at $z=2.095$ corresponding to the galaxy cluster. The cluster velocity dispersion is measured to be $σ_{\rm v1D}$ = 552 $\pm$ 52 km/s. This is the first study of a galaxy cluster in this redshift range ($z \gt 2.0$) with the combination of spectral resolution ($\sim$26 km/s) and the number of confirmed members (${>}50$) needed to impose a meaningful constraint on the cluster velocity dispersion and map its members over a large field of view. Our $Λ$CDM cosmological simulation suggests that this cluster will most likely evolve into a Virgo-like cluster with ${\rm M_{vir}}{=}10^{14.4\pm0.3} {\rm M_\odot}$ ($68\%$ confidence) at $z\sim$ 0. The theoretical expectation of finding such a cluster is $\sim$ $4\%$. Our results demonstrate the feasibility of studying galaxy clusters at $z > 2$ in the same detailed manner using multi-object NIR spectrographs as has been done in the optical in lower redshift clusters.

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arXiv2014

Catching Quenching Galaxies: The Nature of the WISE Infrared Transition Zone

We present the discovery of a prominent bifurcation between early-type galaxies and late-type galaxies, in [4.6]-[12] micron colors from the Wide Field Infrared Survey Explorer (WISE). We then use an emission-line diagnostic comparison sample to explore the nature of objects found both within, and near the edges of, this WISE infrared transition zone (IRTZ). We hypothesize that this birfurcation might be due to the presence of hot dust and PAH emission features in late-type galaxies. Using a sample of galaxies selected through the Shocked Poststarburst Galaxy Survey (SPOGS), we are able to identify galaxies with strong Balmer absorption (EW(Hdelta)>5 Angstroms) as well as emission lines inconsistent with star formation (deemed SPOG candidates, or SPOGs*) that lie within the optical green valley. Seyferts and low ionization nuclear emission line regions, whose u-r colors tend to be red, are strongly represented within the IRTZ, whereas SPOGs* tend to sit near the star-forming edge. Although AGN are well-represented in the IRTZ, we argue that the dominant IRTZ population are galaxies that are in late stages of transitioning across the optical green valley, shedding the last of their remnant interstellar media.

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arXiv2014

Starburst-AGN mixing: II. Optically-selected active galaxies

We use 4 galaxies from the Calar Alto Legacy Integral Field Area (CALIFA) survey with clear signs of accretion onto supermassive black holes to investigate the relative contribution of star-formation and active galactic nucleus (AGN) activity to the line-emission of each galaxy as a function of radius. The combination of star-formation and AGN activity produces curved "mixing sequences" on standard optical diagnostic diagrams, and the fraction of emission due to AGN activity decreases smoothly with distance from the centre of the galaxy. We use the AGN activity profiles to calculate the size of the AGN narrow line regions, which have radii of ~ 6.3 kpc. We calculate the fractional contribution of the star-formation and the AGN activity to the global Halpha, [O II] $λλ$ 3727,3729 and [O III] $λ$ 5007 luminosities of each galaxy, and show that both ionization sources contribute significantly to the emission in all three lines. We use weighted combinations of stellar and AGN photoionization models to produce mixing models, which are consistent with observations for 85 percent of spaxels across the galaxies in our sample. We also present a new diagnostic for starburst-AGN mixing which requires only the [O II], [O III] and Hbeta emission lines, and can therefore be used to calculate AGN fractions up to z ~ 0.85 in the optical and z ~ 3.5 in the near-infrared. We anticipate that this diagnostic will facilitate studies of the properties of AGN ionizing radiation fields and the relative optical contribution of star-formation and AGN activity over cosmic time.

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arXiv2014

S7 : Probing the physics of Seyfert Galaxies through their ENLR & HII Regions

Here we present the first results from the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) which aims to investigate the physics of ~140 radio-detected southern active Galaxies with z<0.02 through Integral Field Spectroscopy using the Wide Field Spectrograph (WiFeS). This instrument provides data cubes of the central 38 x 25 arc sec. of the target galaxies in the waveband 340-710nm with the unusually high resolution of R=7000 in the red (530-710nm), and R=3000 in the blue (340-560nm). These data provide the morphology, kinematics and the excitation structure of the extended narrow-line region, probe relationships with the black hole characteristics and the host galaxy, measures host galaxy abundance gradients and the determination of nuclear abundances from the HII regions. From photoionisation modelling, we may determine the shape of the ionising spectrum of the AGN, discover whether AGN metallicities differ from nuclear abundances determined from HII regions, and probe grain destruction in the vicinity of the AGN. Here we present some preliminary results and modelling of both Seyfert galaxies observed as part of the survey.

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arXiv2014

Galaxy emission line classification using 3D line ratio diagrams

Two-dimensional (2D) line ratio diagnostic diagrams have become a key tool in understanding the excitation mechanisms of galaxies. The curves used to separate the different regions - HII-like or else excited by an active galactic nucleus (AGN) - have been refined over time but the core technique has not evolved significantly. However, the classification of galaxies based on their emission line ratios really is a multi-dimensional problem. Here we exploit recent software developments to explore the potential of three-dimensional (3D) line ratio diagnostic diagrams. We introduce a specific set of 3D diagrams, the ZQE diagrams, which separate the oxygen abundance and the ionisation parameter of HII region-like spectra, and which also enable us to probe the excitation mechanism of the gas. By examining these new 3D spaces interactively, we define a new set of 2D diagnostics, the ZE diagnostics, which can provide the metallicity of objects excited by hot young stars, and which cleanly separate HII region-like objects from the different classes of AGNs. We show that these ZE diagnostics are consistent with the key log[NII]/H$α$ vs. log[OIII]/H$β$ diagnostic currently used by the community. They also have the advantage of attaching a probability that a given object belongs to one class or to the other. Finally, we discuss briefly why ZQE diagrams can provide a new way to differentiate and study the different classes of AGNs in anticipation of a dedicated follow-up study.

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arXiv2014

Metal-poor dwarf galaxies in the SIGRID galaxy sample. II. The electron temperature-abundance calibration and the parameters that affect it

In this paper, we use the Mappings photoionization code to explore the physical parameters that impact on the measurement of electron temperature and abundance in HII regions. In the previous paper we presented observations and measurements of physical properties from the spectra of seventeen HII regions in fourteen isolated dwarf irregular galaxies from the SIGRID sample. Here, we analyze these observations further, together with three additional published data sets. We explore the effects of optical thickness, electron density, ionization parameter, ionization source, and non-equilibrium effects on the relation between electron temperature and metallicity. We present a standard model that fits the observed data remarkably well at metallicities between 1/10 and 1 solar. We investigate the effects of optically thin HII regions, and show that they can have a considerable effect on the measured electron temperature, and that there is evidence that some of the observed objects are optically thin. We look at the role of the ionization parameter and find that lower ionization parameter values give better fits at higher oxygen abundance. We show that higher pressures combined with low optical depth, and also kappa electron energy distributions at low kappa values, can generate the apparent high electron temperatures in low metallicity HII regions, and that the former provides the better fit to observations. We examine the effects of these parameters on the strong line diagnostic methods. We extend this to three-dimensional diagnostic grids to confirm how well the observations are described by the grids.

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arXiv2014

Probing the physics of narrow-line regions of Seyfert galaxies I: The case of NGC 5427

We have used the Wide Field Spectrograph (WiFeS) on the ANU 2.3m telescope at Siding Spring to observe the nearby, nearly face-on, Seyfert 2 galaxy, NGC 5427. We have obtained integral field spectroscopy of both the nuclear regions and the HII regions in the spiral arms. We have constrained the chemical abundance in the interstellar medium of the extended narrow line region (ENLR) by measuring the abundance gradient in the circum-nuclear \ion{H}{ii} regions to determine the nuclear chemical abundances, and to use these to in turn determine the EUV spectral energy distribution for comparison with theoretical models. We find a very high nuclear abundance, $\sim 3.0$ times solar, with clear evidence of a nuclear enhancement of N and He, possibly caused by massive star formation in the extended ($\sim 100$pc) central disk structure. The circum-nuclear narrow-line region spectrum is fit by a radiation pressure dominated photoionisation model model with an input EUV spectrum from a Black Hole with mass $5\times10^7 M_{\odot}$ radiating at $\sim 0.1$ of its Eddington luminosity. The bolometric luminosity is closely constrained to be $\log L_{\mathrm bol.} = 44.3\pm 0.1$ erg s$^{-1}$. The EUV spectrum characterised by a soft accretion disk and a harder component extending to above 15keV. The ENLR region is extended in the NW-SE direction. The line ratio variation in circum-nuclear spaxels can be understood as the result of mixing \ion{H}{ii} regions with an ENLR having a radius-invariant spectrum.

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arXiv2014

Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. HII region observations and chemical abundances

In this paper we present the results of observations of seventeen HII regions in thirteen galaxies from the SIGRID sample of isolated gas rich irregular dwarf galaxies. The spectra of all but one of the galaxies exhibit the auroral [OIII] 4363A line, from which we calculate the electron temperature, Te, and gas-phase oxygen abundance. Five of the objects are blue compact dwarf (BCD) galaxies, of which four have not previously been analysed spectroscopically. We include one unusual galaxy which exhibits no evidence of the [NII] λλ 6548,6584A lines, suggesting a particularly low metallicity (< Zsolar/30). We compare the electron temperature based abundances with those derived using eight of the new strong line diagnostics presented by Dopita et al. (2013). Using a method derived from first principles for calculating total oxygen abundance, we show that the discrepancy between the Te-based and strong line gas-phase abundances have now been reduced to within ~0.07 dex. The chemical abundances are consistent with what is expected from the luminosity-metallicity relation. We derive estimates of the electron densities and find them to be between ~5 and ~100 cm-3. We find no evidence for a nitrogen plateau for objects in this sample with metallicities 0.5 > Zsolar > 0.15.

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arXiv2014

Starburst-AGN mixing: I. NGC 7130

We present an integral field spectroscopic study of the Luminous Infrared Galaxy (LIRG) NGC 7130, a known starburst-AGN composite galaxy. We employ standard emission line ratio diagnostics and maps of velocity dispersion and velocity field to investigate how the dominant ionising sources change as a function of radius. From the signatures of both star formation and AGN activity we show that NGC 7130 is a remarkably clean case of starburst-AGN mixing. We find a smooth transition from AGN dominated emission in the centre to pure star forming activity further out, from which we can estimate the radius of the extended narrow line region to be 1.8 +/- 0.8 kpc. We calculate that the fraction of [OIII] luminosity due to star formation and AGN activity is 30 +/- 2% and 70 +/- 3% respectively, and that the fraction of H-alpha luminosity due to star formation and AGN activity is 65 +/- 3% and 35 +/- 2% respectively. We conclude with a discussion of the importance and potential of starburst-AGN mixing for future studies of the starburst-AGN connection.

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arXiv2013

Spaxel Analysis: Probing the Physics of Star Formation in Ultraluminous Infrared Galaxies

This paper presents a detailed spectral pixel (spaxel) analysis of the ten Luminous Infrared Galaxies (LIRGs) previously observed with the Wide Field Spectrograph (WiFeS), an integral field spectrograph mounted on the ANU 2.3m telescope, and for which an abundance gradient analysis has already been presented by Rich et al. (2012). Here we use the strong emission line analysis techniques developed by Dopita et al. (2013) to measure the ionisation parameter and the oxygen abundance in each spaxel. In addition, we use the observed H$α$ flux to determine the surface rate of star formation ($M_{\odot}$ yr$^{-1}$ kpc$^{-2}$) and use the [\ion{S}{2}] $λ\lambda6717/6731$ ratio to estimate the local pressure in the ionised plasma. We discuss the correlations discovered between these physical quantities, and use them to infer aspects of the physics of star formation in these extreme star forming environments. In particular, we find a correlation between the star formation rate and the inferred ionisation parameter. We examine the possible reasons for this correlation, and determine that the most likely explanation is that the more active star forming regions have a different distribution of molecular gas which favour higher ionisation parameters in the ionised plasma.

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arXiv2013

Composite Spectra in Merging U/LIRGs Caused by Shocks

We present a key result from our optical integral field spectroscopic survey of 27 nearby ultra luminous and luminous infrared galaxies (U/LIRGs) from the Great Observatory All-sky LIRG Survey. Using spatially resolved multi-component emission line fitting to trace the emission line ratios and velocity dispersion of the ionized gas, we quantify for the first time the widespread shock ionization in gas-rich merging U/LIRGs. Our results show a fractional contribution to the total observed \Ha flux from radiative shocks increasing from a few percent during early merger stages to upwards of 60% of the observed optical emission line flux in late stage mergers. We compare our resolved spectroscopy to nuclear spectra and find that 3/4 of the galaxies in our sample that would be classified as "composite" based on optical spectroscopy are primarily characterized by a combination of star formation and merger-driven shocks. Our results have important implications for the interpretation of "composite" rest-frame optical spectra of U/LIRGs as starburst+AGN, as the shock emission combined with star formation can mimic "composite" optical spectra in the absence of any contribution from an AGN.

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