arXiv subjects

Lisa Kewley

Publications and source records attributed to Lisa Kewley.

53 records · Page 3Linked to original sources

arXiv2015

Probing the Physics of Narrow Line Regions in Active Galaxies II: The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7)

Here we describe the \emph{Siding Spring Southern Seyfert Spectroscopic Snapshot Survey} (S7) and present results on 64 galaxies drawn from the first data release. The S7 uses the Wide Field Spectrograph (WiFeS) mounted on the ANU 2.3m telescope located at the Siding Spring Observatory to deliver an integral field of $38\times25$~ arcsec at a spectral resolution of $R=7000$ in the red ($530-710$nm), and $R=3000$ in the blue ($340-560$nm). {From these data cubes we have extracted the Narrow Line Region (NLR) spectra from a 4 arc sec aperture centred on the nucleus. We also determine the H$β$ and [OIII]~$λ$5007 fluxes in the narrow lines, the nuclear reddening, the reddening-corrected relative intensities of the observed emission lines, and the H$β$ and \lOIII\ luminosities {determined from spectra for which the stellar continuum has been removed.} We present a set of images of the galaxies in [OIII]~$λ$5007, [NII]~$λ$6584 and H$α$ which serve to delineate the spatial extent of the extended narrow line region (ENLR) and {\bf also to} reveal the structure and morphology of the surrounding \HII\ regions. Finally, we provide a preliminary discussion of those Seyfert~1 and Seyfert~2 galaxies which display coronal emission lines in order to explore the origin of these lines.

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arXiv2014

IZI: Inferring the Gas Phase Metallicity (Z) and Ionization Parameter (q) of Ionized Nebulae using Bayesian Statistics

We present a new method for inferring the metallicity (Z) and ionization parameter (q) of HII regions and star-forming galaxies using strong nebular emission lines (SEL). We use Bayesian inference to derive the joint and marginalized posterior probability density functions for Z and q given a set of observed line fluxes and an input photo-ionization model. Our approach allows the use of arbitrary sets of SELs and the inclusion of flux upper limits. The method provides a self-consistent way of determining the physical conditions of ionized nebulae that is not tied to the arbitrary choice of a particular SEL diagnostic and uses all the available information. Unlike theoretically calibrated SEL diagnostics the method is flexible and not tied to a particular photo-ionization model. We describe our algorithm, validate it against other methods, and present a tool that implements it called IZI. Using a sample of nearby extra-galactic HII regions we assess the performance of commonly used SEL abundance diagnostics. We also use a sample of 22 local HII regions having both direct and recombination line (RL) oxygen abundance measurements in the literature to study discrepancies in the abundance scale between different methods. We find that oxygen abundances derived through Bayesian inference using currently available photo-ionization models in the literature can be in good (~30%) agreement with RL abundances, although some models perform significantly better than others. We also confirm that abundances measured using the direct method are typically 0.2 dex lower than both RL and photo-ionization model based abundances.

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arXiv2014

The energy source and dynamics of infrared luminous galaxy ESO 148-IG002

ESO 148-IG002 represents a transformative stage of galaxy evolution, containing two galaxies at close separation which are currently coalescing into a single galaxy. We present integral field data of this galaxy from the ANU Wide Field Spectrograph (WiFeS). We analyse our integral field data using optical line ratio maps and velocity maps. We apply active galactic nucleus (AGN), star-burst and shock models to investigate the relative contribution from star-formation, shock excitation and AGN activity to the optical emission in this key merger stage. We find that ESO 148-IG002 has a flat metallicity gradient, consistent with a recent gas inflow. We separate the line emission maps into a star forming region with low velocity dispersion that spatially covers the whole system as well as a southern high velocity dispersion region with a coherent velocity pattern which could either be rotation or an AGN-driven outflow, showing little evidence for pure star formation. We show that the two overlapping galaxies can be separated using kinematic information, demonstrating the power of moderate spectral resolution integral field spectroscopy.

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arXiv2014

The Universal Relation of Galactic Chemical Evolution: The Origin of the Mass-Metallicity Relation

We examine the mass-metallicity relation for $z\lesssim 1.6$. The mass-metallicity relation follows a steep slope with a turnover or `knee' at stellar masses around $10^{10} M_\odot$. At stellar masses higher than the characteristic turnover mass, the mass-metallicity relation flattens as metallicities begin to saturate. We show that the redshift evolution of the mass-metallicity relation depends only on evolution of the characteristic turnover mass. The relationship between metallicity and the stellar mass normalized to the characteristic turnover mass is independent of redshift. We find that the redshift independent slope of the mass-metallicity relation is set by the slope of the relationship between gas mass and stellar mass. The turnover in the mass-metallicity relation occurs when the gas-phase oxygen abundance is high enough that the amount of oxygen locked up in low mass stars is an appreciable fraction of the amount of oxygen produced by massive stars. The characteristic turnover mass is the stellar mass where the stellar-to-gas mass ratio is unity. Numerical modeling suggests that the relationship between metallicity and stellar-to-gas mass ratio is a redshift independent, universal relationship followed by all galaxies as they evolve. The mass-metallicity relation originates from this more fundamental universal relationship between metallicity and stellar-to-gas mass ratio. We test the validity of this universal metallicity relation in local galaxies where stellar mass, metallicity and gas mass measurements are available. The data are consistent with a universal metallicity relation. We derive an equation for estimating the hydrogen gas mass from measurements of stellar mass and metallicity valid for $z\lesssim1.6$ and predict the cosmological evolution of galactic gas masses.

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arXiv2013

Ultra-faint Ultraviolet Galaxies at z~2 Behind the Lensing Cluster Abell 1689: the Luminosity Function, Dust Extinction and Star Formation Rate Density

We identified the z~2 Lyman break galaxies using deep HST ultraviolet (F275W/F336W) imaging of Abell 1689. Because of the imaging depth and the large magnification provided by the cluster, we detect galaxies 100x fainter (-19.5< M_1500 <-13) than previous surveys at this redshift. We are able to calculate the intrinsic sensitivity of the observations as a function of source plane position, allowing determinations of effective volume as a function of luminosity. We fit the faint-end slope of the luminosity function to be alpha = -1.74 +/-0.08, consistent with the values obtained for 2.5 < z < 6. There is no turnover in the luminosity function down to MUV = -13. The trend of increasingly redder UV spectral slopes with luminosity at higher redshifts is observed in our sample, but with redder slopes at all luminosities and average reddening of < E(B - V) >= 0.15. We assume the stars in these galaxies are metal poor (0.2Z_sun) compared to their brighter counterparts (Z_sun), resulting in bluer assumed intrinsic UV slopes and larger derived dust extinction. The total UV luminosity density at z ~ 2 is 4.31x10^26 erg/s/Hz/Mpc^3, more than 70% of which is emitted by galaxies in the luminosity range of our sample. We determine the star formation rate density at z ~ 2 (assuming constant dust extinction correction of 4.2 over all luminosities and a Kroupa IMF) of 0.148 M/yr/Mpc^3, significantly higher than previous determinations because of the additional population of fainter galaxies and the larger dust correction factors.[abridged]

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arXiv2013

Empirical Constraints for the Magnitude and Composition of Galactic Winds

Galactic winds are a key physical mechanism for understanding galaxy formation and evolution, yet empirical and theoretical constraints for the character of winds are limited and discrepant. Recent empirical models find that local star-forming galaxies have a deficit of oxygen that scales with galaxy stellar mass. The oxygen deficit provides unique empirical constraints on the magnitude of mass loss, composition of outflowing material and metal reaccretion onto galaxies. We formulate the oxygen deficit constraints so they may be easily implemented into theoretical models of galaxy evolution. We parameterize an effective metal loading factor which combines the uncertainties of metal outflows and metal reaccretion into a single function of galaxy virial velocity. We determine the effective metal loading factor by forward-fitting the oxygen deficit. The effective metal loading factor we derive has important implications for the implementation of mass loss in models of galaxy evolution.

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arXiv2013

The Slow Flow Model of Dust Efflux in Local Star-Forming Galaxies

We develop a dust efflux model of radiation pressure acting on dust grains which successfully reproduces the relation between stellar mass, dust opacity and star formation rate observed in local star-forming galaxies. The dust content of local star-forming galaxies is set by the competition between the physical processes of dust production and dust loss in our model. The dust loss rate is proportional to the dust opacity and star formation rate. Observations of the relation between stellar mass and star formation rate at several epochs imply that the majority of local star-forming galaxies are best characterized as having continuous star formation histories. Dust loss is a consequence of sustained interaction of dust with the radiation field generated by continuous star formation. Dust efflux driven by radiation pressure rather than dust destruction offers a more consistent physical interpretation of the dust loss mechanism. By comparing our model results with the observed relation between stellar mass, dust extinction and star formation rate in local star-forming galaxies we are able to constrain the timescale and magnitude of dust loss. The timescale of dust loss is long and therefore dust is effluxed in a "Slow Flow". Dust loss is modest in low mass galaxies but massive galaxies may lose up to 70~80% of their dust over their lifetime. Our Slow Flow model shows that mass loss driven by dust opacity and star formation may be an important physical process for understanding normal star-forming galaxy evolution.

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arXiv2013

The Inner Kiloparsec of Mrk 273 with Keck Adaptive Optics

There is X-ray, optical, and mid-infrared imaging and spectroscopic evidence that the late-stage ultraluminous infrared galaxy merger Mrk 273 hosts a powerful active galactic nucleus (AGN). However, the exact location of the AGN and the nature of the nuclei have been difficult to determine due to dust obscuration and the limited wavelength coverage of available high-resolution data. Here we present near-infrared integral-field spectra and images of the nuclear region of Mrk 273 taken with OSIRIS and NIRC2 on the Keck II Telescope with laser guide star adaptive optics. We observe three spatially resolved components, and analyze the local molecular and ionized gas emission lines and their kinematics. We confirm the presence of the hard X-ray AGN in the southwest nucleus. In the north nucleus, we find a strongly rotating gas disk whose kinematics indicate a central black hole of mass 1.04 +/- 0.1 x 10^9 Msun. The H2 emission line shows an increase in velocity dispersion along the minor axis in both directions, and an increased flux with negative velocities in the southeast direction; this provides direct evidence for a collimated molecular outflow along the axis of rotation of the disk. The third spatially distinct component appears to the southeast, 640 and 750 pc from the north and southwest nuclei, respectively. This component is faint in continuum emission but shows several strong emission line features, including [Si vi] 1.964 μm which traces an extended coronal-line region. The geometry of the [Si vi] emission combined with shock models and energy arguments suggest that [Si vi] in the southeast component must be at least partly ionized by the SW AGN or a putative AGN in the northern disk, either through photoionization or through shock-heating from strong AGN- and circumnuclear starburst-driven outflows. This lends support to a scenario in which Mrk 273 may be a dual AGN system.

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arXiv2013

Ionized Outflows from Compact Steep Spectrum Sources

Massive outflows are known to exist, in the form of extended emission-line regions (EELRs), around about one-third of powerful FR II radio sources. We investigate the origin of these EELRs by studying the emission-line regions around compact-steep-spectrum (CSS) radio galaxies that are younger (10$^3$ to 10$^5$ years old) versions of the FR II radio galaxies. We have searched for and analyzed the emission-line regions around 11 CSS sources by taking integral field spectra using GMOS on Gemini North. We fit the [\ion{O}{3}] $λ5007$ line and present the velocity maps for each detected emission-line region. We find, in most cases, that the emission-line regions have multi-component velocity structures with different velocity dispersions and/or flux distributions for each component. The velocity gradients of the emission-line gas are mostly well aligned with the radio axis, suggesting a direct causal link between the outflowing gas and the radio jets. The complex velocity structure may be a result of different driving mechanisms related to the onset of the radio jets. We also present the results from the line-ratio diagnostics we used to analyze the ionization mechanism of the extended gas, which supports the scenario where the emission-line regions are ionized by a combination of AGN radiation and shock excitation.

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arXiv2013

The Chemical Evolution of Star-Forming Galaxies Over the Last 11 Billion Years

We calculate the stellar mass-metallicity relation at five epochs ranging to z~2.3. We quantify evolution in the shape of the mass-metallicity relation as a function of redshift; the mass-metallicity relation flattens at late times. There is an empirical upper limit to the gas-phase oxygen abundance in star-forming galaxies that is independent of redshift. From examination of the mass-metallicity relation and its observed scatter we show that the flattening at late times is a consequence of evolution in the stellar mass where galaxies enrich to this empirical upper metallicity limit; there is also evolution in the fraction of galaxies at a fixed stellar mass that enrich to this limit. The stellar mass where metallicities begin to saturate is ~0.7 dex smaller in the local universe than it is at z~0.8.

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arXiv2012

Addressing Decadal Survey Science through Community Access to Highly Multiplexed Spectroscopy with BigBOSS on the KPNO Mayall Telescope

This document summarizes the results of a community-based discussion of the potential science impact of the Mayall+BigBOSS highly multiplexed multi-object spectroscopic capability. The KPNO Mayall 4m telescope equipped with the DOE- and internationally-funded BigBOSS spectrograph offers one of the most cost-efficient ways of accomplishing many of the pressing scientific goals identified for this decade by the "New Worlds, New Horizons" report. The BigBOSS Key Project will place unprecedented constraints on cosmological parameters related to the expansion history of the universe. With the addition of an open (publicly funded) community access component, the scientific impact of BigBOSS can be extended to many important astrophysical questions related to the origin and evolution of galaxies, stars, and the IGM. Massive spectroscopy is the critical missing ingredient in numerous ongoing and planned ground- and space-based surveys, and BigBOSS is unique in its ability to provide this to the US community. BigBOSS data from community-led projects will play a vital role in the education and training of students and in maintaining US leadership in these fields of astrophysics. We urge the NSF-AST division to support community science with the BigBOSS multi-object spectrograph through the period of the BigBOSS survey in order to ensure public access to the extraordinary spectroscopic capability.

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arXiv2011

The Metallicity Evolution of Interacting Galaxies

Nuclear inflows of metal-poor interstellar gas triggered by galaxy interactions can account for the systematically lower central oxygen abundances observed in local interacting galaxies. Here, we investigate the metallicity evolution of a large set of simulations of colliding galaxies. Our models include cooling, star formation, feedback, and a new stochastic method for tracking the mass recycled back to the interstellar medium from stellar winds and supernovae. We study the influence of merger-induced inflows, enrichment, gas consumption, and galactic winds in determining the nuclear metallicity. The central metallicity is primarily a competition between the inflow of low-metallicity gas and enrichment from star formation. An average depression in the nuclear metallicity of ~0.07 is found for gas-poor disk-disk interactions. Gas-rich disk-disk interactions, on the other hand, typically have an enhancement in the central metallicity that is positively correlated with the gas content. The simulations fare reasonably well when compared to the observed mass-metallicity and separation-metallicity relationships, but further study is warranted.

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arXiv2007

Distinguishing Active Galactic Nuclei and Star Formation

Using the large emission line galaxy sample from the Sloan Digital Sky Survey we show that Star forming galaxies, Seyferts, and low-ionization nuclear emission-line regions (LINERs) form clearly separated branches on the standard optical diagnostic diagrams. We derive a new empirical classification scheme which cleanly separates these emission-line galaxies, using strong optical emission lines. Using this classification we identify a few distinguishing host galaxy properties of each class, which, along with the emission line analysis, suggest continuous evolution from one class to another. As a final note, we introduce models of both Starforming galaxies and AGN narrow line regions which can explain the distribution of galaxies on standard emission line ratio diagrams, and possibly suggest new diagnostics across the emission spectrum.

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arXiv2007

Nitrogen Production in Starburst Galaxies Detected by GALEX

We investigate the production of nitrogen in star forming galaxies with ultraviolet (UV) radiation detected by the Galaxy Evolution Explorer Satellite (GALEX). We use a sample of 8,745 GALEX emission line galaxies matched to the Sloan Digital Sky Survey (SDSS) spectroscopic sample. We derive both gas-phase oxygen and nitrogen abundances for the sample, and apply stellar population synthesis models to derive stellar masses and star formation histories of the galaxies. We compare oxygen abundances derived using three different diagnostics. We derive the specific star formation rates of the galaxies by modeling the 7-band GALEX+SDSS photometry. We find that galaxies that have log SFR/M$_*$ > -10.0 typically have values of log N/O ~0.05 dex less than galaxies with log SFR/M$_*$ < -10.0 and similar oxygen abundances.

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arXiv2007

No supernovae associated with two long-duration gamma ray bursts

It is now accepted that long duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard "collapsar" model predicts that a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we present observations of two nearby long-duration GRBs that challenge this simple view. In the GRBs 060505 and 060614 we demonstrate that no SN emission accompanied these long-duration bursts, down to limits hundreds of times fainter than the archetypal SN 1998bw that accompanied GRB 980425, and fainter than any Type Ic SN ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in star-forming regions. The absence of a SN to such deep limits is qualitatively different from all previous nearby long GRBs and suggests a new phenomenological type of massive stellar death. From the supplementary material: Now we have observed SN-less GRBs in star-forming regions, suggesting that a non-detection of a SN does not preclude a massive progenitor. The position of the GRB, i.e. in a star-forming region or in an older component, may be the only way to discriminate between merging compact objects and massive stars as progenitors. In fact, several host galaxies for short GRBs have been found to be as actively star-forming as some host galaxies of long-duration GRBs. The GRB labels "long" and "short" have become synonymous with "massive stars" and "other progenitors". These distinctions may need to be relaxed.

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arXiv2004

The X-Ray Derived Cosmological Star Formation History and the Galaxy X-Ray Luminosity Functions in the Chandra Deep Fields North and South

The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-Ray Luminosity Function for Galaxies in these Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~ (1 + z)^{2.7}. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0). This provides the most direct measure yet of the X-ray derived cosmic star-formation history of the Universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of AGN, finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the H-alpha LF indicating that additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, Radio, H-alpha) gives an interesting constraint on the IMF, and X-rays may be measuring directly the binary star formation history of the Universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future x-ray missions.

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arXiv2003

The Ionized Gas in Local Starburst Galaxies: Global and Small--Scale Feedback from Star Formation

Abridged: The small-- and intermediate--scale structure and the fraction of the ISM ionized by non--radiative processes is investigated in a small sample of four local starburst galaxies, imaged with the HST WFPC2. The sample comprises three dwarf galaxies, NGC3077, NGC4214, and NGC5253, and one giant spiral, NGC5236 (M83). The galaxies span a range in metallicity, luminosity, and environment, enabling the investigation of non--radiative ionization processes in a variety of galactic conditions. For this purpose, the four galaxies were imaged in the lines of H-beta(4861 A), [OIII](5007 A), H-alpha(6563 A), and [SII](6717,6731 A). The non--photoionized gas in these galaxies has been traced on scales ranging from a few tens of pc to a few hundred pc, providing a full budget for this ionized gas component. Using the `maximum starburst line' of Kewley et al. (2001) to discriminate between photoionized and non--photoionized gas, we find that in all four galaxies non--photoionization processes are responsible for a small fraction of the total H-alpha emission, at the level of 3%--4%. The starbursts in the three dwarf galaxies deposit a significant fraction, 70%-100%, of their mechanical energy into the surrounding interstellar medium and require time--extended (a few x 10^7 yr to ~10^8 yr) star formation, in order to account for the observed luminosity of the non--photoionized gas. In the massive spiral, the non--photoionized gas is concentrated in localized areas surrounded by active star--formation, with no evidence for extended structures (as those in the dwarfs). This confirms the picture that starbursts remain confined events in massive galaxies, likely due to the deep potential well.

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