arXiv subjects

Fiona Hoyle

Publications and source records attributed to Fiona Hoyle.

At least 19 recordsLinked to original sources

arXiv2016

On the Star Formation Properties of Void Galaxies

We measure the star formation properties of two large samples of galaxies from the SDSS in large-scale cosmic voids on time scales of 10 Myr and 100 Myr, using H$α$ emission line strengths and GALEX FUV fluxes, respectively. The first sample consists of 109,818 optically selected galaxies. We find that void galaxies in this sample have higher specific star formation rates (SSFRs; star formation rates per unit stellar mass) than similar stellar mass galaxies in denser regions. The second sample is a subset of the optically selected sample containing 8070 galaxies with reliable HI detections from ALFALFA. For the full HI detected sample, SSFRs do not vary systematically with large-scale environment. However, investigating only the HI detected dwarf galaxies reveals a trend towards higher SSFRs in voids. Furthermore, we estimate the star formation rate per unit HI mass (known as the star formation efficiency; SFE) of a galaxy, as a function of environment. For the overall HI detected population, we notice no environmental dependence. Limiting the sample to dwarf galaxies again reveals a trend towards higher SFEs in voids. These results suggest that void environments provide a nurturing environment for dwarf galaxy evolution allowing for higher specific star formation rates and efficiencies.

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arXiv2015

An observational detection of the bridge effect of void filaments

The bridge effect of void filaments is a phrase coined by Park & Lee (2009b) to explain the correlations found in a numerical experiment between the luminosity of the void galaxies and the degree of the straightness of their host filaments. Their numerical finding implies that a straight void filament provides a narrow channel for the efficient transportation of gas and matter particles from the surroundings into the void galaxies. Analyzing the Sloan void catalog constructed by Pan et al (2012), we identify the filamentary structures in void regions and determine the specific size of each void filament as a measure of its straightness. To avoid possible spurious signals caused by the Malmquist bias, we consider only those void filaments whose redshifts are in the range of 0=< z <= 0.02 and find a clear tendency that the void galaxies located in the more straight filaments are on average more luminous, which is in qualitative agreement with the numerical prediction. It is also shown that the strength of correlation increases with the number of the member galaxies of the void filaments, which can be physically understood on the grounds that the more stretched filaments can connect the dense surroundings even to the galaxies located deep in the central parts of the voids. This observational evidence may provide a key clue to the puzzling issue of why the void galaxies have higher specific star formation rates and bluer colors than their wall counterparts.

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arXiv2015

Spectral Properties of Galaxies in Void Regions

We present a study of spectral properties of galaxies in underdense large-scale structures, voids. Our void galaxy sample (75,939 galaxies) is selected from the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) with $\rm z < 0.107$. We find that there are no significant differences in the luminosities, stellar masses, stellar populations, and specific star formation rates between void galaxies of specific spectral types and their wall counterparts. However, the fraction of star-forming galaxies in voids is significantly higher ($\ge 9\%$) than that in walls. Void galaxies, when considering all spectral types, are slightly fainter, less massive, have younger stellar populations and of higher specific star formation rates than wall galaxies. These minor differences are totally caused by the higher fraction of star-forming galaxies in voids. We confirm that AGNs exist in voids, already found by \cite{co08}, with similar abundance as in walls. Type I AGNs contribute $\sim$ 1\%-2\% of void galaxies, similar to their fraction in walls. The intrinsic [O III] luminosities , spanning over $10^6\ L_{\sun} \sim 10^9\ L_{\sun}$, and Eddington ratios are similar comparing our void AGNs versus wall AGNs. Small scale statistics show that all spectral types of void galaxies are less clustered than their counterparts in walls. Major merger may not be the dominant trigger of black hole accretion in the luminosity range we probe. Our study implies that the growth of black holes relies weakly on large scale structures.

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arXiv2015

The Optical Luminosity Function of Void Galaxies in the SDSS and ALFALFA Surveys

We measure the r-band galaxy luminosity function (LF) across environments over the redshift range 0<$z$<0.107 using the SDSS. We divide our sample into galaxies residing in large scale voids (void galaxies) and those residing in denser regions (wall galaxies). The best fitting Schechter parameters for void galaxies are: log$Φ^*$= -3.40$\pm$0.03 log(Mpc$^{-3}$), $M^*$= -19.88$\pm$0.05, and $α$=-1.20$\pm$0.02. For wall galaxies, the best fitting parameters are: log$Φ^*$=-2.86$\pm$0.02 log(Mpc$^{-3}$), $M^*$=-20.80$\pm$0.03, and $α$=-1.16$\pm$0.01. We find a shift in the characteristic magnitude, $M^*$, towards fainter magnitudes for void galaxies and find no significant difference between the faint-end slopes of the void and wall galaxy LFs. We investigate how low surface brightness selections effects can affect the galaxy LF. To attempt to examine a sample of galaxies that is relatively free of surface brightness selection effects, we compute the optical galaxy LF of galaxies detected by the blind HI survey, ALFALFA. We find that the global LF of the ALFALFA sample is not well fit by a Schechter function, because of the presence of a wide dip in the LF around $M_r$=-18 and an upturn at fainter magnitudes ($α$~-1.47). We compare the HI selected r-band LF to various LFs of optically selected populations to determine where the HI selected optical LF obtains its shape. We find that sample selection plays a large role in determining the shape of the LF.

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arXiv2015

An observational signal of the void shape correlation and its link to the cosmic web

The shapes of cosmic voids are prone to distortions by the external tidal forces since their low-densities imply a lower internal resistance. This susceptibility of the void shapes to the tidal distortions makes them useful as an indicator of the large-scale tidal and density fields, despite the practical difficulty in defining them. Using the void catalog constructed by Pan et al. (2012) from the Seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7), we detect a clear 4 sigma signal of spatial correlations of the void shapes on the scale of 20 Mpc/h and show that the signal is robust against the projection of the void shapes onto the plane of sky. By constructing a simple analytic model for the void shape correlation, within the framework of tidal torque theory, we demonstrate that the void shape correlation function scales linearly with the two-point correlation function of the linear density field. We also find a direct observational evidence for the cross-correlation of the void shapes with the large-scale velocity shear field that was linearly reconstructed by Lee et al. (2014) from the SDSS DR7. We discuss the possibility of using the void shape correlation function to break the degeneracy between the density parameter and the power spectrum amplitude and to independently constrain the neutrino mass as well.

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arXiv2014

The HI Mass Function and Velocity Width Function of Void Galaxies in the Arecibo Legacy Fast ALFA Survey

We measure the HI mass function (HIMF) and velocity width function (WF) across environments over a range of masses $7.2<\log(M_{HI}/M_{\odot})<10.8$, and profile widths $1.3\log(km/s)<\log(W)<2.9\log(km/s)$, using a catalog of ~7,300 HI-selected galaxies from the ALFALFA Survey, located in the region of sky where ALFALFA and SDSS (Data Release 7) North overlap. We divide our galaxy sample into those that reside in large-scale voids (void galaxies) and those that live in denser regions (wall galaxies). We find the void HIMF to be well fit by a Schechter function with normalization $Φ^*=(1.37\pm0.1)\times10^{-2} h^3Mpc^{-3}$, characteristic mass $\log(M^*/M_{\odot})+2\log h_{70}=9.86\pm0.02$, and low-mass-end slope $α=-1.29\pm0.02$. Similarly, for wall galaxies, we find best-fitting parameters $Φ^*=(1.82\pm0.03)\times10^{-2} h^3Mpc^{-3}$, $\log(M^*/M_{\odot})+2\log h_{70}=10.00\pm0.01$, and $α=-1.35\pm0.01$. We conclude that void galaxies typically have slightly lower HI masses than their non-void counterparts, which is in agreement with the dark matter halo mass function shift in voids assuming a simple relationship between DM mass and HI mass. We also find that the low-mass slope of the void HIMF is similar to that of the wall HIMF suggesting that there is either no excess of low-mass galaxies in voids or there is an abundance of intermediate HI mass galaxies. We fit a modified Schechter function to the ALFALFA void WF and determine its best-fitting parameters to be $Φ^*=0.21\pm0.1 h^3Mpc^{-3}$, $\log(W^*)=2.13\pm0.3$, $α=0.52\pm0.5$ and high-width slope $β=1.3\pm0.4$. For wall galaxies, the WF parameters are: $Φ^*=0.022\pm0.009 h^3Mpc^{-3}$, $\log(W^*)=2.62\pm0.5$, $α=-0.64\pm0.2$ and $β=3.58\pm1.5$. Because of large uncertainties on the void and wall width functions, we cannot conclude whether the WF is dependent on the environment.

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arXiv2012

Photometric Properties of Void Galaxies in the Sloan Digital Sky Survey DR7 Data Release

Using the sample presented in Pan:2011, we analyse the photometric properties of 88,794 void galaxies and compare them to galaxies in higher density environments with the same absolute magnitude distribution. In Pan et al. (2011), we found a total of 1054 dynamically distinct voids in the SDSS with radius larger than 10h^-1 Mpc. The voids are underdense, with delta rho/rho < -0.9 in their centers. Here we study the photometric properties of these void galaxies. We look at the u - r colours as an indication of star formation activity and the inverse concentration index as an indication of galaxy type. We find that void galaxies are statistically bluer than galaxies found in higher density environments with the same magnitude distribution. We examine the colours of the galaxies as a function of magnitude, and we fit each colour distribution with a double-Gaussian model for the red and blue subpopulations. As we move from bright to dwarf galaxies, the population of red galaxies steadily decreases and the fraction of blue galaxies increases in both voids and walls, however the fraction of blue galaxies in the voids is always higher and bluer than in the walls. We also split the void and wall galaxies into samples depending on galaxy type. We find that late type void galaxies are bluer than late type wall galaxies and the same holds for early galaxies. We also find that early type, dwarf void galaxies are blue in colour. We also study the properties of void galaxies as a function of their distance from the center of the void. We find very little variation in the properties, such as magnitude, colour and type, of void galaxies as a function of their location in the void. The only exception is that the dwarf void galaxies may live closer to the center. The centers of voids have very similar density contrast and hence all void galaxies live in very similar density environments (ABRIDGED)

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arXiv2011

Cosmic Voids in Sloan Digital Sky Survey Data Release 7

We study the distribution of cosmic voids and void galaxies using Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Using the VoidFinder algorithm as described by Hoyle 2002, we identify 1054 statistically significant voids in the northern galactic hemisphere with radii > 10 h^{-1} Mpc. The filling factor of voids in the sample volume is 62%. The largest void is just over 30 h^{-1} Mpc in effective radius. The median effective radius is 17 h^{-1} Mpc. The voids are found to be significantly underdense, with density contrast δ< -0.85 at the edges of the voids. The radial density profiles of these voids are similar to predictions of dynamically distinct underdensities in gravitational theory. We find 8,046 galaxies brighter than M_r = -20.09 within the voids, accounting for 7% of the galaxies. We compare the results of VoidFinder on SDSS DR7 to mock catalogs generated from a SPH halo model simulation as well as other Λ-CDM simulations and find similar void fractions and void sizes in the data and simulations. This catalog is made publicly available at http://www.physics.drexel.edu/~pan/voidcatalog.html for download.

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arXiv2008

The Aspen--Amsterdam Void Finder Comparison Project

Despite a history that dates back at least a quarter of a century studies of voids in the large--scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void--finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi--analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than $m_B = -20 $ is found to be smaller than about -0.8 by all our void finding algorithms.

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arXiv2007

Active Galactic Nuclei in Void Regions

We present a comprehensive study of accretion activity in the most underdense environments in the universe, the voids, based on the SDSS DR2 data. Based on investigations of multiple void regions, we show that AGN's occurrence rate and properties differ from those in walls. AGN are more common in voids than in walls, but only among moderately luminous and massive galaxies (M_r < -20, log M_*/M_sun < 10.5), and this enhancement is more pronounced for the weakly accreting systems (i.e., L_[O III] < 10^39 erg/s). Void AGN hosted by moderately massive and luminous galaxies are accreting at equal or lower rates than their wall counterparts, show less obscuration than in walls, and similarly aged stellar populations. The very few void AGN in massive bright hosts accrete more strongly, are more obscured, and are associated with younger stellar emission than wall AGN. Thus, accretion strength is probably connected to the availability of fuel supply, and accretion and star-formation co-evolve and rely on the same source of fuel. Nearest neighbor statistics indicate that the weak accretion activity (LINER-like) is not influenced by the local environment. However, H IIs, Seyferts, and Transition objects prefer more grouped small scale structures, indicating that the rate at which galaxies interact with each other affects their activity. These trends support a potential H II -> Seyfert/Transition Object -> LINER evolutionary sequence that we show is apparent in many properties of actively line-emitting galaxies, in both voids and walls. The subtle differences between void and wall AGN might be explained by a longer, less disturbed duty cycle of these systems in voids.

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arXiv2007

Source Matching in the SDSS and RASS: Which Galaxies are Really X-ray Sources?

The current view of galaxy formation holds that all massive galaxies harbor a massive black hole at their center, but that these black holes are not always in an actively accreting phase. X-ray emission is often used to identify accreting sources, but for galaxies that are not harboring quasars (low-luminosity active galaxies), the X-ray flux may be weak, or obscured by dust. To aid in the understanding of weakly accreting black holes in the local universe, a large sample of galaxies with X-ray detections is needed. We cross-match the ROSAT All Sky Survey (RASS) with galaxies from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4) to create such a sample. Because of the high SDSS source density and large RASS positional errors, the cross-matched catalog is highly contaminated by random associations. We investigate the overlap of these surveys and provide a statistical test of the validity of RASS-SDSS galaxy cross-matches. SDSS quasars provide a test of our cross-match validation scheme, as they have a very high fraction of true RASS matches. We find that the number of true matches between the SDSS main galaxy sample and the RASS is highly dependent on the optical spectral classification of the galaxy; essentially no star-forming galaxies are detected, while more than 0.6% of narrow-line Seyferts are detected in the RASS. Also, galaxies with ambiguous optical classification have a surprisingly high RASS detection fraction. This allows us to further constrain the SEDs of low-luminosity active galaxies. Our technique is quite general, and can be applied to any cross-matching between surveys with well-understood positional errors.

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arXiv2005

A Map of the Universe

We have produced a new conformal map of the universe illustrating recent discoveries, ranging from Kuiper belt objects in the Solar system, to the galaxies and quasars from the Sloan Digital Sky Survey. This map projection, based on the logarithm map of the complex plane, preserves shapes locally, and yet is able to display the entire range of astronomical scales from the Earth's neighborhood to the cosmic microwave background. The conformal nature of the projection, preserving shapes locally, may be of particular use for analyzing large scale structure. Prominent in the map is a Sloan Great Wall of galaxies 1.37 billion light years long, 80% longer than the Great Wall discovered by Geller and Huchra and therefore the largest observed structure in the universe.

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arXiv2005

Minkowski Functionals of SDSS galaxies I : Analysis of Excursion Sets

We present a first morphometric investigation of a preliminary sample from the SDSS of 154287 galaxies with apparent magnitude 14.5<m_r<17.5 and redshift 0.001<z<0.4. We measure the Minkowski Functionals, which are a complete set of morphological descriptors. To account for the complicated wedge--like geometry of the present survey data, we construct isodensity contour surfaces from the galaxy positions in redshift space and employ two complementary methods of computing the Minkowski Functionals. We find that the observed Minkowski Functionals for SDSS galaxies are consistent with the prediction of a Lambda--dominated spatially--flat Cold Dark Matter model with random--Gaussian initial conditions, within the cosmic variance estimated from the corresponding mock catalogue. We expect that future releases of the SDSS survey will allow us to distinguish morphological differences in the galaxy distribution with regard to different morphological type and luminosity ranges.

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arXiv2004

The Mass Function of Void Galaxies in the SDSS Data Release 2

We estimate the Mass Function of void galaxies in the second public data release of the Sloan Digital Sky Survey from a sample of 1000 galaxies with local density contrasts of delta_v < -0.6. The galaxy sample is split into ellipticals and spirals using a color-Sersic index criteria. We estimate the virial masses of ellipticals using the measured spectral line-widths along with the observed size. Projection effects and uncertainties in halo properties make mass estimates of spirals more difficult. We use an inversion of the Tully-Fisher relation to estimate the isothermal rotational velocity, and introduce a scaling factor to estimate the halo extent. We then fit the measured mass function against a theoretical Press-Schechter model, and find that the distribution of galaxies in voids appears to be nearly unbiased compared to the mass.

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arXiv2004

Photometric Properties of Void Galaxies in the Sloan Digital Sky Survey

Using a nearest neighbor analysis, we construct a sample of void galaxies from the Sloan Digital Sky Survey (SDSS) and compare the photometric properties of these galaxies to the population of non-void (wall) galaxies. We trace the density field of galaxies using a volume-limited sample with z_{max}=0.089. Galaxies from the flux-limited SDSS with z\leq z_{max} and fewer than three volume-limited neighbors within 7h^{-1}Mpc are classified as void galaxies. This criterion implies a density contrast δρ/ ρ< -0.6 around void galaxies. From 155,000 galaxies, we obtain a sub-sample of 13,742 galaxies with z\leq z_{max}, from which we identify 1,010 galaxies as void galaxies. To identify an additional 194 faint void galaxies from the SDSS in the nearby universe, r~ 72 h^{-1}Mpc, we employ volume-limited samples extracted from the Updated Zwicky Catalog and the Southern Sky Redshift Survey with z_{max}=0.025 to trace the galaxy distribution. Our void galaxies span a range of absolute magnitude from M_r=-13.5 to M_r=-22.5. Using SDSS photometry, we compare the colors, concentration indices, and Sersic indices of the void and wall samples. Void galaxies are significantly bluer than galaxies lying at higher density. The population of void galaxies with M_r ~ M* +1 and brighter is on average bluer and more concentrated (later type) than galaxies outside of voids. The latter behavior is only partly explained by the paucity of luminous red galaxies in voids. These results generally agree with the predictions of semi-analytic models for galaxy formation in cold dark matter models, which indicate that void galaxies should be relatively bluer, more disklike, and have higher specific star formation rates.

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arXiv2004

Spectroscopic Properties of Void Galaxies in the Sloan Digital Sky Survey

We study the spectroscopic properties of a sample of 10^3 void galaxies from the Sloan Digital Sky Survey (SDSS) and compare these with the properties of galaxies in higher density regions (wall galaxies). This sample of void galaxies covers the range of absolute magnitude from M_r=-13.5 to M_r=-22.5 in regions with density contrast delta < -0.6. In this paper we compare the equivalent widths of Halpha, [OII], [NII], Hbeta, and [OIII] of void and wall galaxies of similar luminosities and find that void galaxies have larger values, indicating that they are still forming stars at a high rate. A comparison of the Balmer break, as measured by the parameter Dn(4000), reveals that void galaxies have younger stellar populations than wall galaxies. Using standard techniques, we estimate Halpha and [OII] star formation rates of the void and wall galaxies and along with estimates of the stellar masses, we compute specific star formation rates. In most cases, we find that void galaxies have similar SFRs to wall galaxies but they are fainter and smaller mass. This means that, consistent with the EWs, void galaxies have higher specific star formation rates than wall galaxies.

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arXiv2004

Mapping the Cosmic Web with the Sloan Digital Sky Survey

Wide-angle, moderately deep redshift surveys such as that conducted as part of the Sloan Digital Sky Survey (SDSS) allow study of the relationship between the structural elements of the large-scale distribution of galaxies -- including groups, cluster, superclusters, and voids -- and the dependence of galaxy formation and evolution on these enviroments. We present a progress report on mapping efforts with the SDSS and discuss recently constructed catalogs of clusters, voids, and void galaxies, and evidence for a 420Mpc/h supercluster or ``Great Wall.'' Analysis of multi-band photometry and moderate-resolution spectroscopy from the SDSS reveals environmental dependence of the star formation history of galaxies that extends over more than a factor of 100 in density, from clusters all the way to the deep interiors of voids. On average, galaxies in the rarified environments of voids exhibit bluer colors, higher specific star formation rates, lower dust content, and more disk-like morphology than objects in denser regions. This trend persists in comparisons of samples in low vs. high-density regions with similar luminosity and morphology, thus this dependence is not simply an extension of the morphology-density relation. Large-scale modulation of the halo mass function and the temperature of the intergalactic medium might explain this dependence of galaxy evolution on the large-scale environment.

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arXiv2003

Voids in the 2dF Galaxy Redshift Survey

We present an analysis of voids in the 2dF Galaxy Redshift Survey (2dFGRS). This analysis includes identification of void regions and measurement of void statistics. The 2dFGRS is the largest completed redshift survey to date, including a total of 245,591 galaxies covering 1500 deg$^2$ to a median depth of z~0.11. We use the voidfinder algorithm to identify a total of 289 voids in the 2dFGRS with radius larger than 10h^{-1}Mpc. These voids have an average effective radius of 14.89+- 2.65 h^{-1}Mpc in the North Galactic Pole region (NGP) and 15.61+- 2.84h^{-1}Mpc in the South Galactic Pole region (SGP). These voids are extremely underdense, with average density contrast of delta rho/rho= -0.94+-0.02. The centers of voids are even emptier, because the few galaxies within the voids typically lie close to the edges. The total volume of the universe filled by these void regions is approximately 40%. We measure the Void Probability Function (VPF) of the 2dFGRS for volume-limited samples with limiting absolute magnitudes from -16 to -21 in b_J. We measure the Underdensity Probability Function for samples with limiting absolute magnitudes from -18 to -21. We find that the SGP is more underdense than the NGP for all but the brightest sample under consideration. There is good agreement between the VPF's of the Center for Astrophysics Survey and the 2dFGRS. Comparison of VPF's measured for the 2dFGRS with the distribution of simulated dark-matter halos of similar number density indicates that voids in the matter distribution in LCDM simulations are not empty enough. However, semi-analytic models of galaxy formation that include feedback effects yield VPF's that show excellent agreement with the data.

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