arXiv subjects

K. C. Freeman

Publications and source records attributed to K. C. Freeman.

At least 55 records · Page 3Linked to original sources

arXiv2014

APASS Landolt-Sloan BVgri photometry of RAVE stars. I. Data, effective temperatures and reddenings

We provide APASS photometry in the Landolt BV and Sloan g'r'i' bands for all the 425,743 stars included in the latest 4th RAVE Data Release. The internal accuracy of the APASS photometry of RAVE stars, expressed as error of the mean of data obtained and separately calibrated over a median of 4 distinct observing epochs and distributed between 2009 and 2013, is 0.013, 0.012, 0.012, 0.014 and 0.021 mag for B, V, g', r' and i' band, respectively. The equally high external accuracy of APASS photometry has been verified on secondary Landolt and Sloan photometric standard stars not involved in the APASS calibration process, and on a large body of literature data on field and cluster stars, confirming the absence of offsets and trends. Compared with the Carlsberg Meridian Catalog (CMC-15), APASS astrometry of RAVE stars is accurate to a median value of 0.098 arcsec. Brightness distribution functions for the RAVE stars have been derived in all bands. APASS photometry of RAVE stars, augmented by 2MASS JHK infrared data, has been chi2 fitted to a densely populated synthetic photometric library designed to widely explore in temperature, surface gravity, metallicity and reddening. Resulting Teff and E(B-V), computed over a range of options, are provided and discussed, and will be kept updated in response to future APASS and RAVE data releases. In the process it is found that the reddening caused by an homogeneous slab of dust, extending for 140 pc on either side of the Galactic plane and responsible for E(B-V,poles)=0.036 +/- 0.002 at the galactic poles, is a suitable approximation of the actual reddening encountered at Galactic latitudes |b|>=25 deg.

astro-ph.SR↗

arXiv2014

Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal Galaxies

Dark matter (DM) halos of Sc-Im galaxies satisfy scaling laws analogous to the fundamental plane relations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii, higher central densities, and smaller central velocity dispersions. If dwarf spheroidal (dSph) and dwarf Magellanic irregular (dIm) galaxies lie on the extrapolations of these correlations, then we can estimate their baryon loss relative to that of brighter Sc-Im galaxies. We find that, if there had been no such enhanced baryon loss, then typical dSph and dIm galaxies would be brighter in absolute magnitude by 4 and 3.5 mag, respectively. Instead, these galaxies lost or retained as gas (in dIm galaxies) baryons that could have formed stars. Also, typical dSph and dIm galaxies have DM halos that are more massive than we thought, with velocity dispersions of about 30 km/s or circular-orbit rotation velocities of V_circ ~ 42 km/s. Comparison of DM and visible matter correlations confirms that, at V-band absolute magnitudes fainter than -18, dSph and dIm galaxies form a sequence of decreasing baryon-to-DM mass ratios in smaller dwarfs. We show explicitly that galaxy baryon content goes to (almost) zero at halo V_circ = 42 +- 4 km/s, in agreement with what we found from our estimate of baryon depletion. Our results suggest that there may be a large population of DM halos that are essentially dark and undiscovered. This helps to solve the problem that the fluctuation spectrum of cold DM predicts more dwarfs than we observe.

astro-ph.GA↗

arXiv2014

More pieces of the puzzle: Chemistry and substructures in the Galactic thick disk

We present a study of the chemical abundances of Solar neighbourhood stars associated to dynamical structures in the Milky Way's (thick) disk. These stars were identified as overdensity in the eccentricity range 0.3< ecc < 0.5 in the Copenhagen-Geneva Survey by Helmi et al. (2006). We find that the stars with these dynamical characteristics do not constitute a homogeneous population. A relatively sharp transition in dynamical and chemical properties appears to occur at a metallicity of [Fe/H] ~ -0.4. Stars with [Fe/H] > -0.4 have mostly lower eccentricities, smaller vertical velocity dispersions, are alpha-enhanced and define a rather narrow sequence in [alpha/Fe] vs [Fe/H], clearly distinct from that of the thin disk. Stars with [Fe/H] < -0.4 have a range of eccentricities, are hotter vertically, and depict a larger spread in [alpha/Fe]. We have also found tentative evidence of substructure possibly associated to the disruption of a metal-rich star cluster. The differences between these populations of stars is also present in e.g. [Zn/Fe], [Ni/Fe] and [SmII/Fe], suggesting a real physical distinction.

astro-ph.GA↗

arXiv2014

Kinematic modelling of the Milky Way using the RAVE and GCS stellar surveys

We investigate the kinematic parameters of the Milky Way disc using the RAVE and GCS stellar surveys. We do this by fitting a kinematic model to the data taking the selection function of the data into account. For stars in the GCS we use all phase-space coordinates, but for RAVE stars we use only $(l,b,v_{\rm los})$. Using MCMC technique, we investigate the full posterior distributions of the parameters given the data. We investigate the `age-velocity dispersion' relation for the three kinematic components ($σ_R,σ_ϕ,σ_z$), the radial dependence of the velocity dispersions, the Solar peculiar motion ($U_{\odot},V_{\odot}, W_{\odot} $), the circular speed $Θ_0$ at the Sun and the fall of mean azimuthal motion with height above the mid-plane. We confirm that the Besançon-style Gaussian model accurately fits the GCS data, but fails to match the details of the more spatially extended RAVE survey. In particular, the Shu distribution function (DF) handles non-circular orbits more accurately and provides a better fit to the kinematic data. The Gaussian distribution function not only fits the data poorly but systematically underestimates the fall of velocity dispersion with radius. We find that correlations exist between a number of parameters, which highlights the importance of doing joint fits. The large size of the RAVE survey, allows us to get precise values for most parameters. However, large systematic uncertainties remain, especially in $V_{\odot}$ and $Θ_0$. We find that, for an extended sample of stars, $Θ_0$ is underestimated by as much as $10\%$ if the vertical dependence of the mean azimuthal motion is neglected. Using a simple model for vertical dependence of kinematics, we find that it is possible to match the Sgr A* proper motion without any need for $V_{\odot}$ being larger than that estimated locally by surveys like GCS.

astro-ph.GA↗

arXiv2013

A new stellar chemo-kinematic relation reveals the merger history of the Milky Way disc

The velocity dispersions of stars near the Sun are known to increase with stellar age, but age can be difficult to determine so a proxy like the abundance of alpha elements (e.g., Mg) with respect to iron, [alpha/Fe], is used. Here we report an unexpected behavior found in the velocity dispersion of a sample of giant stars from the RAdial Velocity Experiment (RAVE) survey with high quality chemical and kinematical information, in that it decreases strongly for stars with [Mg/Fe] > 0.4 dex (i.e., those that formed in the first Gyr of the Galaxy's life). These findings can be explained by perturbations from massive mergers in the early Universe, which have affected more strongly the outer parts of the disc, and the subsequent radial migration of stars with cooler kinematics from the inner disc. Similar reversed trends in velocity dispersion are also found for different metallicity subpopulations. Our results suggest that the Milky Way disc merger history can be recovered by relating the observed chemo-kinematic relations to the properties of past merger events.

astro-ph.GA↗

arXiv2013

In the thick of it: metal-poor disc stars in RAVE

By selecting in the RAVE-DR4 survey the stars located between 1 and 2 kpc above the Galactic plane, we question the consistency of the simplest three-component model (thin disc, thick disc, halo) for the Milky Way. We confirm that the metallicity and azimuthal velocity distribution functions of the thick disc are not Gaussian. In particular, we find that the thick disc has an extended metallicity tail going at least down to [M/H]=-2 dex, contributing roughly at 3% of the entire thick disc population and having a shorter scale-length compared to the canonical thick disc. The mean azimuthal velocity of these metal-poor stars allows us to estimate the correlation between the metallicity and the orbital velocity, which is an important constraint on the formation mechanisms of the Galactic thick disc. Given our simple approach, we find dVphi/d[M/H] ~ 50 km/s/dex, which is in a very good agreement with previous literature values. We complete the study with a brief discussion on the implications of the formation scenarios for the thick disc, and suggest that given the above mentioned characteristics, a thick disc mainly formed by radial migration mechanisms seems unlikely.

astro-ph.GA↗

arXiv2013

Chromospherically Active Stars in the RAVE Survey. I. The Catalogue

RAVE, the unbiased magnitude limited survey of the southern sky stars, contained 456,676 medium-resolution spectra at the time of our analysis. Spectra cover the CaII IRT range which is a known indicator of chromospheric activity. Our previous work (Matijevič et al. 2012) classified all spectra using locally linear embedding. It identified 53,347 cases with a suggested emission component in calcium lines. Here we use a spectral subtraction technique to measure the properties of this emission. Synthetic templates are replaced by the observed spectra of non-active stars to bypass the difficult computations of non-LTE profiles of the line cores and stellar parameter dependence. We derive both the equivalent width of the excess emission for each calcium line on a 5Å wide interval and their sum EW_IRT for ~44,000 candidate active dwarf stars with S/N>20 and with no respect to the source of their emission flux. From these ~14,000 show a detectable chromospheric flux with at least 2σ confidence level. Our set of active stars vastly enlarges previously known samples. Atmospheric parameters and in some cases radial velocities of active stars derived from automatic pipeline suffer from systematic shifts due to their shallower calcium lines. We re-estimate the effective temperature, metallicity and radial velocities for candidate active stars. The overall distribution of activity levels shows a bimodal shape, with the first peak coinciding with non-active stars and the second with the pre main-sequence cases. The catalogue will be publicly available with the next RAVE public data releases.

astro-ph.SR↗

arXiv2013

Diffuse interstellar band at 8620 Å in RAVE: A new method for detecting the diffuse interstellar band in spectra of cool stars

Diffuse interstellar bands are usually observed in spectra of hot stars, where interstellar lines are rarely blended with stellar ones. The need for hot stars is a strong limitation in the number of sightlines we can observe and the distribution of sightlines in the Galaxy, as hot stars are rare and concentrated in the Galactic plane. We are introducing a new method, where interstellar lines can be observed in spectra of cool stars in large spectroscopic surveys. The method is completely automated and does not require prior knowledge of the stellar parameters. If known, the stellar parameters only reduce the computational time and are not involved in the extraction of the interstellar spectrum. The main step in extracting interstellar lines is a construction of the stellar spectrum, which is in our method done by finding other observed spectra that lack interstellar features and are otherwise very similar to the spectrum in question. Such spectra are then combined into a single stellar spectrum template, which matches the stellar component in an observed spectrum. We demonstrate the performance of this new method on a sample of 482,430 spectra observed in RAVE survey. However, many spectra have to be combined (48 on average) in order to achieve a S/N ratio high enough to measure the DIB's profile, hence limiting the spatial information about the ISM. Only one strong interstellar line is included in the RAVE spectral range, a diffuse interstellar band at 8620 Å. We compare its equivalent width with extinction maps and with Bayesian reddening, calculated for individual stars, and provide a linear relation between the equivalent width and reddening. Separately from the introduced method, we calculate equivalent widths of the diffuse interstellar band in spectra of hot stars with known extinction and compare all three linear relations with each other and with relations from the literature.

astro-ph.GA↗

arXiv2013

The wobbly Galaxy: kinematics north and south with RAVE red clump giants

The RAVE survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using the red clump, we examine the mean velocity components in 3D between an R of 6 and 10 kpc and a Z of -2 to 2 kpc, concentrating on North-South differences. Simple parametric fits to the R, Z trends for VPHI and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, VR, finding that the gradient is more marked below the plane, with a Z gradient also present. The vertical velocity, VZ, also shows clear structure, with indications of a rarefaction-compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the North-South differences in VR and VZ along the line-of-sight, with the VR estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disk, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.

astro-ph.GA↗

arXiv2013

The planetary nebula population in the halo of M87

We investigate the diffuse light in the outer regions of the nearby elliptical galaxy M87 in the Virgo cluster, using planetary nebulas (PNs) as tracers. The surveyed areas (0.43 squared degrees) cover M87 up to a radial distance of 150 kpc, in the ransition region between galaxy halo and intracluster light (ICL). All PNs are identified through the on-off band technique using automatic selection criteria based on the distribution of the detected sources in the colour-magnitude diagram and the properties of their point-spread function. We extract a catalogue of 688 objects down to m_5007=28.4, with an estimated residual contamination from foreground stars and background Lyalpha galaxies, which amounts to ~35% of the sample. This is one of the largest extragalactic PN samples in number of candidates, magnitude depth, and radial extent, which allows us to carry out an unprecedented photometric study of the PN population in the outer regions of M87. We find that the logarithmic density profile of the PN distribution is shallower than the surface brightness profile at large radii. This behaviour is consistent with the superposition of two components associated with the halo of M87 and with the ICL, which have different luminosity specific PN numbers, the ICL contributing three times more PNs per unit light. Because of the depth of this survey we are also able to study the shape of the PN luminosity function (PNLF) in the outer regions of M87. We find a slope for the PNLF that is steeper at fainter magnitudes than the standard analytical PNLF formula and adopt a generalised model that treats the slope as a free parameter. Comparing the PNLF of M87 and the M31 bulge, both normalised by the sampled luminosity, the M87 PNLF contains fewer bright PNs and has a steeper slope towards fainter magnitudes.

astro-ph.CO↗

arXiv2013

Planetary Nebula Spectrograph survey of S0 galaxy kinematics. II. Clues to the origins of S0 galaxies

The stellar kinematics of the spheroids and discs of S0 galaxies contain clues to their formation histories. Unfortunately, it is difficult to disentangle the two components and to recover their stellar kinematics in the faint outer parts of the galaxies using conventional absorption line spectroscopy. This paper therefore presents the stellar kinematics of six S0 galaxies derived from observations of planetary nebulae (PNe), obtained using the Planetary Nebula Spectrograph. To separate the kinematics of the two components, we use a maximum-likelihood method that combines the discrete kinematic data with a photometric component decomposition. The results of this analysis reveal that: the discs of S0 galaxies are rotationally supported; however, the amount of random motion in these discs is systematically higher than in comparable spiral galaxies; and the S0s lie around one magnitude below the Tully--Fisher relation for spiral galaxies, while their spheroids lie nearly one magnitude above the Faber--Jackson relation for ellipticals. All of these findings are consistent with a scenario in which spirals are converted into S0s through a process of mild harassment or "pestering," with their discs somewhat heated and their spheroid somewhat enhanced by the conversion process. In such a scenario, one might expect the properties of S0s to depend on environment. We do not see such an effect in this fairly small sample, although any differences would be diluted by the fact that the current location does not necessarily reflect the environment in which the transformation occurred. Similar observations of larger samples probing a broader range of environments, coupled with more detailed modelling of the transformation process to match the wide range of parameters that we have shown can now be measured, should take us from these first steps to the definitive answer as to how S0 galaxies form.

astro-ph.CO↗

arXiv2013

The Planetary Nebula Spectrograph survey of S0 galaxy kinematics. Data and Overview

The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them. We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins. Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems*. We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy. The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases.

astro-ph.CO↗

arXiv2013

Slipher and the Nature of the Nebulae

Why do some discoveries, which appear in hindsight to be obviously major discoveries, have so little impact when they were made. These are discoveries that were ahead of their time, but for some reason the scientific community was not ready to absorb them. I got interested in Slipher after learning that by 1914 he had observed nebular redshifts up to 1000 km/s. Why did this not convince people at the time that the nebulae were extragalactic ? Was this another example, like Zwicky's (1933) discovery of dark matter in the Coma cluster, of a discovery which was too far ahead of its time ? I conclude that Slipher's situation was different from Zwicky's. The significance of Slipher's nebular redshifts was partly recognised at the time, but I believe that its full significance was masked by van Maanen's work which turned out to be erroneous and reduced the impact of Slipher's discoveries.

physics.hist-ph↗

arXiv2012

Exploring the Morphology of RAVE Stellar Spectra

The RAdial Velocity Experiment (RAVE) is a medium resolution R~7500 spectroscopic survey of the Milky Way which already obtained over half a million stellar spectra. They present a randomly selected magnitude-limited sample, so it is important to use a reliable and automated classification scheme which identifies normal single stars and discovers different types of peculiar stars. To this end we present a morphological classification of 350,000 RAVE survey stellar spectra using locally linear embedding, a dimensionality reduction method which enables representing the complex spectral morphology in a low dimensional projected space while still preserving the properties of the local neighborhoods of spectra. We find that the majority of all spectra in the database ~90-95% belong to normal single stars, but there is also a significant population of several types of peculiars. Among them the most populated groups are those of various types of spectroscopic binary and chromospherically active stars. Both of them include several thousands of spectra. Particularly the latter group offers significant further investigation opportunities since activity of stars is a known proxy of stellar ages. Applying the same classification procedure to the sample of normal single stars alone shows that the shape of the projected manifold in two dimensional space correlates with stellar temperature, surface gravity and metallicity.

astro-ph.SR↗

arXiv2011

Carbon-Enhanced Metal-Poor Stars in the Inner and Outer Halo Components of the Milky Way

(Abridged) Carbon-enhanced metal-poor (CEMP) stars in the halo components of the Milky Way are explored, based on accurate determinations of the carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over 30000 calibration stars from the Sloan Digital Sky Survey (SDSS). Using our present criterion that low-metallicity stars exhibiting [C/Fe] ratios ("carbonicity") in excess of [C/Fe]$ = +0.7$ are considered CEMP stars, the global frequency of CEMP stars in the halo system for \feh\ $< -1.5$ is 8%; for \feh\ $< -2.0$ it is 12%; for \feh\ $<-2.5$ it is 20%. We also confirm a significant increase in the level of carbon enrichment with declining metallicity, growing from $<$[C/Fe]$>$ $\sim +1.0$ at \feh\ $= -1.5$ to $<$[C/Fe]$>$ $\sim +1.7$ at \feh\ $= -2.7$. The nature of the carbonicity distribution function (CarDF) changes dramatically with increasing distance above the Galactic plane, $|$Z$|$. For $|$Z$|$ $< 5$ kpc, relatively few CEMP stars are identified. For distances $|$Z$|$ $> 5$ kpc, the CarDF exhibits a strong tail towards high values, up to [C/Fe] $>$ +3.0. We also find a clear increase in the CEMP frequency with $|$Z$|$. For stars with $-2.0 <$ [Fe/H] $< -$1.5, the frequency grows from 5% at $|$Z$|$ $\sim 2$ kpc to 10% at $|$Z$|$ $\sim 10$ kpc. For stars with [Fe/H] $< -$2.0, the frequency grows from 8% at $|$Z$|$ $\sim 2$ kpc to 25% at $|$Z$|$ $\sim 10$ kpc. For stars with $-2.0 <$ [Fe/H] $< -$1.5, the mean carbonicity is $<$[C/Fe]$>$ $\sim +1.0$ for 0 kpc $<$ $|$Z$|$ $<$ 10 kpc, with little dependence on $|$Z$|$; for [Fe/H] $< -$2.0, $<$[C/Fe]$>$ $\sim +1.5$, again roughly independent of $|$Z$|$.

astro-ph.GA↗

arXiv2011

An HI Survey of Six Local Group Analogs. II. HI properties of group galaxies

We have conducted an HI 21 cm emission-line survey of six loose groups of galaxies chosen to be analogs to the Local Group. The survey was conducted using the Parkes Multibeam instrument and the Australia Telescope Compact Array (ATCA) over a ~1 Mpc^2 area and covering the full depth of each group, with a M(HI) sensitivity of ~7x10^5 M(sun). Our survey detected 110 sources, 61 of which are associated with the six groups. All of these sources were confirmed with ATCA observations or were previously cataloged by HIPASS. The sources all have optical counterparts and properties consistent with dwarf irregular or late-type spiral galaxies. We present here the HI properties of the groups and their galaxies. We derive an HI mass function for the groups that is consistent with being flatter than the equivalent field HIMF. We also derive a circular velocity distribution function, tracing the luminous dark matter halos in the groups, that is consistent with those of the Local Group and HIPASS galaxies, both of which are shallower than that of clusters or predictions from CDM models of galaxy formation.

astro-ph.CO↗

arXiv2011

The RAVE catalogue of stellar elemental abundances: first data release

We present chemical elemental abundances for $36,561$ stars observed by the RAdial Velocity Experiment (RAVE), an ambitious spectroscopic survey of our Galaxy at Galactic latitudes $|$b$|>25^{\circ}$ and with magnitudes in the range 9$<I_{DENIS}<$13. RAVE spectra cover the Ca-triplet region at 8410--8795Å with resolving power R$\sim$7500. This first data release of the RAVE chemical catalogue is complementary to the third RAVE data release of radial velocities and stellar parameters, and it contains chemical abundances for the elements Mg, Al, Si, Ca, Ti, Fe and Ni, with a mean error of $\sim$0.2 dex, as judged from accuracy tests performed on synthetic and real spectra. Abundances are estimated through a dedicated processing pipeline in which the curve of growth of individual lines is obtained from a library of absorption-line equivalent widths to construct a model spectrum that is then matched to the observed spectrum via a $χ^2$-minimization technique. We plan to extend this pipeline to include estimates for other elements, such as oxygen and sulfur, in future data releases.

astro-ph.GA↗

arXiv2011

Distance determination for RAVE stars using stellar models III: The nature of the RAVE survey and Milky Way chemistry

We apply the method of Burnett & Binney (2010) for the determination of stellar distances and parameters to the internal catalogue of the Radial Velocity Experiment (Steinmetz et al. 2006). Subsamples of stars that either have Hipparcos parallaxes or belong to well-studied clusters, inspire confidence in the formal errors. Distances to dwarfs cooler than ~6000 K appear to be unbiased, but those to hotter dwarfs tend to be too small by ~10% of the formal errors. Distances to giants tend to be too large by about the same amount. The median distance error in the whole sample of 216,000 stars is 28% and the error distribution is similar for both giants and dwarfs. Roughly half the stars in the RAVE survey are giants. The giant fraction is largest at low latitudes and in directions towards the Galactic Centre. Near the plane the metallicity distribution is remarkably narrow and centred on [M/H]-0.04 dex; with increasing |z| it broadens out and its median moves to [M/H] ~ -0.5. Mean age as a function of distance from the Galactic centre and distance |z| from the Galactic plane shows the anticipated increase in mean age with |z|.

astro-ph.GA↗