arXiv subjects

Chris Miller

Publications and source records attributed to Chris Miller.

30 records · Page 2Linked to original sources

arXiv2015

Planetary Candidates Observed by Kepler VI: Planet Sample from Q1-Q16 (47 Months)

\We present the sixth catalog of Kepler candidate planets based on nearly 4 years of high precision photometry. This catalog builds on the legacy of previous catalogs released by the Kepler project and includes 1493 new Kepler Objects of Interest (KOIs) of which 554 are planet candidates, and 131 of these candidates have best fit radii <1.5 R_earth. This brings the total number of KOIs and planet candidates to 7305 and 4173 respectively. We suspect that many of these new candidates at the low signal-to-noise limit may be false alarms created by instrumental noise, and discuss our efforts to identify such objects. We re-evaluate all previously published KOIs with orbital periods of >50 days to provide a consistently vetted sample that can be used to improve planet occurrence rate calculations. We discuss the performance of our planet detection algorithms, and the consistency of our vetting products. The full catalog is publicly available at the NASA Exoplanet Archive.

astro-ph.EP↗

arXiv2014

Design and development of a freeform active mirror for an astronomy application

The advent of extremely large telescopes will bring unprecedented light-collecting power and spatial resolution, but it will also lead to a significant increase in the size and complexity of focal-plane instruments. The use of freeform mirrors could drastically reduce the number of components in optical systems. Currently, manufacturing issues limit the common use of freeform mirrors at short wavelengths. This article outlines the use of freeform mirrors in astronomical instruments with a description of two efficient freeform optical systems. A new manufacturing method is presented which seeks to overcome the manufacturing issues through hydroforming of thin polished substrates. A specific design of an active array is detailed, which will compensate for residual manufacturing errors, thermoelastic deformation, and gravity-induced errors during observations. The combined hydroformed mirror and the active array comprise the Freeform Active Mirror Experiment, which will produce an accurate, compact, and stable freeform optics dedicated to visible and near-infrared observations.

astro-ph.IM↗

arXiv2011

From Star-Forming Spirals to Passive Spheroids: Integral Field Spectroscopy of E+A Galaxies

We present three dimensional spectroscopy of eleven E+A galaxies, selected for their strong H-delta absorption but weak (or non-existent) [OII]3727 and H-alpha emission. This selection suggests that a recent burst of star-formation was triggered but subsequently abruptly ended. We probe the spatial and spectral properties of both the young (~1Gyr) and old (few Gyr) stellar populations. Using the H-delta equivalent widths we estimate that the burst masses must have been at least 10% by mass (Mburst~10^10Mo), which is also consistent with the star-formation history inferred from the broad-band SEDs. On average the A-stars cover ~33% of the galaxy image, extending over 2-15kpc^2, indicating that the characteristic E+A signature is a property of the galaxy as a whole and not due to a heterogeneous mixture of populations. In approximately half of the sample, we find that the A-stars, nebular emission, and continuum emission are not co-located, suggesting that the newest stars are forming in a different place than those that formed ~1Gyr ago, and that recent star-formation has occurred in regions distinct from the oldest stellar populations. At least ten of the galaxies (91%) have dynamics that class them as "fast rotators" with magnitudes and dynamics comparable to local ellipticals and S0's. We also find a correlation between the spatial extent of the A-stars and dynamics such that the fastest rotators tend to have the most compact A-star populations, providing new constraints on models that aim to explain the transformation of later type galaxies into early types. Finally, we show that there are no obvious differences between the line extents and kinematics of E+A galaxies detected in the radio (AGN) compared to non-radio sources, suggesting that AGN feedback does not play a dramatic role in defining their properties, or that its effects are short.

astro-ph.CO↗

arXiv2011

A dichotomy for expansions of the real field

A dichotomy for expansions of the real field is established: Either the set of integers is definable or every nonempty bounded nowhere dense definable subset of the real numbers has Minkowski dimension zero.

math.LO↗

arXiv2006

Photometric Calibration of the DES

The Dark Energy Survey (DES) is a 5000 sq deg griz imaging survey to be conducted using a proposed 3 sq deg (2.2deg-diameter) wide-field mosaic camera on the CTIO Blanco 4m telescope. The primary scientific goal of the DES is to constrain dark energy cosmological parameters via four complementary methods: galaxy cluster counting, weak lensing, galaxy angular correlations, and Type Ia supernovae, supported by precision photometric redshifts. Here we present the photometric calibration plans for the DES, including a discussion of standard stars and field-to-field calibrations.

astro-ph↗

arXiv2005

Near-infrared imaging of 222 nearby Hdelta-strong galaxies from the SDSS

We present UFTI K-band imaging observations of 222 galaxies that are selected from the Sloan Digital Sky Survey to have unusually strong Hdelta absorption equivalent widths, W(Hd)>4A. Using GIM2D, the images are fit with two-dimensional surface brightness models consisting of a simple disk and bulge component to derive B/T, the fraction of luminosity in the bulge. We find that the galaxies with weak or absent Halpha or [OII] emission (known as k+a galaxies) are predominantly bulge-dominated (with a mode of B/T~0.6), while galaxies with nebular emission (known as e[a] galaxies) are mostly disk-dominated (B/T~0.1). The morphologies and (r-k) colours of most k+a galaxies are inconsistent with the hypothesis that they result from the truncation of star formation in normal, spiral galaxies. However, their (u-g) and (r-k) colours, as well as their Hdelta line strengths, form a sequence that is well matched by a model in which >5 per cent of the stellar mass has been produced in a recent starburst. The lack of scatter in the dust-sensitive (r-k) colours suggests that the unusual spectra of k+a galaxies are not due to the effects of dust. The e(a) galaxies, on the other hand, have a colour distribution that is distinct from the k+a population, and typical of normal or dusty (tauV~2) spiral galaxies. We conclude that many e(a) galaxies are not progenitors of k+a galaxies, but are a separate phenomenon. Both k+a and e(a) galaxies reside in environments (characterized by the local density of galaxies brighter than Mr=-20) that are typical of normal galaxies and that are inconsistent with overdense regions like rich galaxy clusters.

astro-ph↗

arXiv2004

GMOS Integral Field Spectroscopy of a Merging System with Enhanced Balmer Absorption

In this paper we present the three dimensional dynamics of the galaxy SDSS J101345.39+011613.66, selected for its unusually strong Balmer absorption lines (Wo(H-delta)=7.5A). Using the GMOS-South IFU in Nod & Shuffle mode we have mapped the continuum and optical absorption lines of this z=0.1055 field galaxy. This galaxy has a disturbed morphology, with a halo of diffuse material distributed asymmetrically toward the north. Using the [OII] emission line (Wo([OII])=4.1A) we find that the gas and hot OB stars are offset from the older stars in the system. The gas also has a spatially extended and elongated morphology with a velocity gradient of 100+/-20km/s across 6kpc in projection. Using the strong H-gamma and H-delta absorption lines we find that the A- stars are widely distributed across the system and are not centrally concentrated arguing that the A-star population has formed in molecular clouds outside the nucleus. By cross correlating the spectra from the datacube with an A-star template we find evidence that the A-star population has a 40km/s shear in the same direction as the gas. The disturbed morphology, strong colour gradients and strong H-delta and H-gamma absorption lines in SDSS J101345.39 argue that this is a recent tidal interaction/merger between a passive elliptical and star-forming galaxy. Although based on a single object, these results show that we can spatially resolve and constrain the dynamics of this short lived (yet important) phase of galaxy formation in which the evolutionary process take galaxies from star-forming to their quiescent end products.

astro-ph↗

arXiv2004

The bimodal galaxy color distribution: dependence on luminosity and environment

We analyse the u-r color distribution of 24346 galaxies with Mr<=-18 and z<0.08, drawn from the Sloan Digital Sky Survey first data release, as a function of luminosity and environment. The color distribution is well fit with two Gaussian distributions, which we use to divide the sample into a blue and red population. At fixed luminosity, the mean color of the blue (red) distribution is nearly independent of environment, with a weakly significant (~3sigma) detection of a trend for colors to become redder by 0.1-0.14 (0.03-0.06) mag with a factor ~100 increase in local density, as characterised by the surface density of galaxies within a +/-1000 km/s redshift slice. In contrast, at fixed luminosity the fraction of galaxies in the red distribution is a strong function of local density, increasing from ~10-30 per cent of the population in the lowest density environments, to ~70 per cent at the highest densities. The strength of this trend is similar for both the brightest (-23<Mr<-22) and faintest (-19<Mr<-18) galaxies in our sample. The fraction of red galaxies within the virialised regions of clusters shows no significant dependence on velocity dispersion. Even at the lowest densities explored, a substantial population of red galaxies exists, which might be fossil groups. We propose that most star-forming galaxies today evolve at a rate that is determined primarily by their intrinsic properties, and independent of their environment. Any environmentally triggered transformations from blue to red colors must either occur on a short timescale, or preferentially at high redshift, to preserve the simple Gaussian nature of the color distribution. The mechanism must be effective for both bright and faint galaxies.

astro-ph↗

arXiv2004

Galaxy ecology: groups and low-density environments in the SDSS and 2dFGRS

We analyse the observed correlation between galaxy environment and H-alpha emission line strength, using volume-limited samples and group catalogues of 24968 galaxies drawn from the 2dF Galaxy Redshift Survey (Mb<-19.5) and the Sloan Digital Sky Survey (Mr<-20.6). We characterise the environment by 1) Sigma_5, the surface number density of galaxies determined by the projected distance to the 5th nearest neighbour; and 2) rho1.1 and rho5.5, three-dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 Mpc and 5.5 Mpc, respectively. We find that star-forming and quiescent galaxies form two distinct populations, as characterised by their H-alpha equivalent width, EW(Ha). The relative numbers of star-forming and quiescent galaxies varies strongly and continuously with local density. However, the distribution of EW(Ha) amongst the star-forming population is independent of environment. The fraction of star-forming galaxies shows strong sensitivity to the density on large scales, rho5.5, which is likely independent of the trend with local density, rho1.1. We use two differently-selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for sigma=200-1000 km/s. Even in the lowest density environments, no more than ~70 per cent of galaxies show significant H-alpha emission. Based on these results, we conclude that the present-day correlation between star formation rate and environment is a result of short-timescale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy-galaxy interactions.

astro-ph↗

arXiv2003

A Merged Catalog of Clusters of Galaxies from Early SDSS Data

We present a catalog of 799 clusters of galaxies in the redshift range z_est = 0.05 - 0.3 selected from ~400 deg^2 of early SDSS commissioning data along the celestial equator. The catalog is based on merging two independent selection methods -- a color-magnitude red-sequence maxBCG technique (B), and a Hybrid Matched-Filter method (H). The BH catalog includes clusters with richness Λ>= 40 (Matched-Filter) and N_gal >= 13 (maxBCG), corresponding to typical velocity dispersion of σ_v >~ 400 km s^{-1} and mass (within 0.6 h^{-1) Mpc radius) >~ 5*10^{13} h^{-1} M_sun. This threshold is below Abell richness class 0 clusters. The average space density of these clusters is 2*10^{-5} h^3 Mpc^{-3}. All NORAS X-ray clusters and 53 of the 58 Abell clusters in the survey region are detected in the catalog; the 5 additional Abell clusters are detected below the BH catalog cuts. The cluster richness function is determined and found to exhibit a steeply decreasing cluster abundance with increasing richness. We derive observational scaling relations between cluster richness and observed cluster luminosity and cluster velocity dispersion; these scaling relations provide important physical calibrations for the clusters. The catalog can be used for studies of individual clusters, for comparisons with other sources such as X-ray clusters and AGNs, and, with proper correction for the relevant selection functions, also for statistical analyses of clusters.

astro-ph↗

arXiv2003

Limits on thermal conduction in galaxy clusters

We have calculated lower limits for the global effective thermal conductivity in a sample of 165 Abell clusters. We assumed that cluster X-ray luminosity is compensated by a conductive heat flux which we then compare with an upper limit to the temperature gradient inferred from the cluster temperature and radius. This gives a lower limit to the thermal conductivity and therefore to the relative suppression from the Spitzer conductivity, kappa_{Sp}. Not a single cluster requires super-Spitzer values of thermal conduction to balance the observed X-ray luminosity. The suppression coefficient f=kappa/kappa_{Sp} is clustered in a range 10^{-2} < f < 0.4. A weak dependence of $f$ versus $z$ is observed over $ 0 < z <0.41$. Possible biases and/or selection effects are discussed.

astro-ph↗

arXiv2002

Galaxy Star-Formation as a Function of Environment in the Early Data Release of the Sloan Digital Sky Survey

(Abridged) We present in this paper a detailed analysis of the effect of environment on the star-formation activity of galaxies within the EDR of the SDSS. We have used the Halpha emission line to derive the star-formation rate (SFR) for each galaxy within a volume-limited sample of 8598 galaxies with 0.05 < z < 0.095 and M(r)<= -20.45. We find that the SFR of galaxies is strongly correlated with the local (projected) galaxy density and thus we present here the density-SFR relation that is analogous to the density-morphology relation. The effect of density on the SFR of galaxies is seen in three ways. First, the overall distribution of SFRs is shifted to lower values in dense environments compared with the field population. Second, the effect is most noticeable for the strongly star-forming galaxies in the 75th percentile of the SFR distribution. Third, there is a ``break'' (or characteristic density) in the density-SFR relation at a local galaxy density of 1h-2 Mpc-2. To understand this break further, we have studied the SFR of galaxies as a function of clustercentric radius from 17 clusters and groups objectively selected from the SDSS EDR data. The distribution of SFRs of cluster galaxies begins to change, compared with the field population, at a clustercentric radius of 3-4 virial radii, which is consistent with the characteristic break in density that we observe in the density-SFR relation. Our tests suggest that the density-morphology relation alone is unlikely to explain the density-SFR relation we observe. Taken all together, these works demonstrate that the decrease in SFR of galaxies in dense environments is a universal phenomenon over a wide range in density (from 0.08 to 10h-2 Mpc-2) and redshift (out to z = 0.5).

astro-ph↗