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RE: Solitary galaxies in voids
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Title: Extended Hot Halos Around Isolated Galaxies Observed in the ROSAT All-Sky Survey
Authors: Michael E. Anderson, Joel N. Bregman, Xinyu Dai

We place general constraints on the luminosity and mass of hot X-ray emitting gas residing in extended "hot halos" around nearby massive galaxies. We examine stacked images of 2165 galaxies from the 2MASS Very Isolated Galaxy Catalogue (2MVIG), as well as subsets of this sample based on galaxy morphology and K-band luminosity. We detect X-ray emission at high confidence (ranging up to nearly 10\sigma) for each subsample of galaxies. The average L_X within 50 kpc is 1.0±0.1 (statistical) \pm0.2 (systematic) x10^40 erg/s, although the early-type galaxies are more than twice as luminous as the late-type galaxies. Using a spatial analysis, we also find evidence for extended emission around five out of seven subsamples (the full sample, the luminous galaxies, early-type galaxies, luminous late-type galaxies, and luminous early-type galaxies) at 92.7%, 99.3%, 89.3%, 98.7%, and 92.1% confidence, respectively. Several additional lines of evidence also support this conclusion and suggest that about 1/2 of the total emission is extended, and about 1/3 of the extended emission comes from hot gas. For the sample of luminous galaxies, which has the strongest evidence for extended emission, the average hot gas mass is 4x10^9 Msun within 50 kpc and the implied accretion rate is 0.4 Msun/yr.

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Isolated AMIGA galaxies
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Title: Star formation in isolated AMIGA galaxies: dynamical influence of bars
Authors: S. Verley, F. Combes, L. Verdes-Montenegro, G. Bergond, S. Leon

Star formation depends strongly both on the local environment of galaxies, and on the internal dynamics of the interstellar medium. To disentangle the two effects, we obtained, in the framework of the AMIGA project, Ha and Gunn r photometric data for more than 200 spiral galaxies lying in very low-density regions of the local Universe. We characterise the Ha emission, tracing current star formation, of the 45 largest and less inclined galaxies observed for which we estimate the torques between the gas and the bulk of the optical matter. We could subsequently study the Ha morphological aspect of these isolated spiral galaxies. Using Fourier analysis, we focus on the modes of the spiral arms and also on the strength of the bars, computing the torques between the gas and newly formed stars (Ha) and the bulk of the optical matter (Gunn r). We interpret the various bar/spiral morphologies observed in terms of the secular evolution experienced by galaxies in isolation. We also classify the different spatial distributions of star forming regions in barred galaxies. The observed frequency of particular patterns brings constraints on the lifetime of the various evolution phases. We propose an evolutive sequence accounting for the transitions between the different phases we could observe. Isolated galaxies appear not to be preferentially barred or unbarred. Through numerical simulations, trying to fit the Ha distributions yields constraints on the star formation law, which is likely to differ from a genuine Schmidt law. In particular, it is probable that the relative velocity of the gas in the bar also needs to be taken into account.

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Topology of large-scale structure
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Title: Topology of large-scale structure in the 2dF Galaxy Redshift Survey
Authors: J. Berian James, Geraint F. Lewis, Matthew Colless

We investigate the topology of the completed 2dF Galaxy Redshift Survey, drawing two flux-limited samples of the local Universe from the 2dFGRS catalogue, which contains over 220,000 galaxies at a median redshift of z = 0.11. The samples are cut at z = 0.2 and corrected for selection effects. We then use the three-dimensional genus statistic to probe the connectedness of the galaxy distribution on scales ranging from 8 to 20 Mpc, and compare these measurements with the analytical result for a Gaussian random density field, a generic prediction of inflationary models. We demonstrate consistency with inflation on the range of scales considered. We then introduce a parameterisation of the analytic genus curve formula that is sensitive to asymmetries in genus number as a function of density and use it to demonstrate that such phenomena are ruled out with 95% confidence between 8 and 16 Mpc.

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AMIGA sample of isolated galaxies
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Title: The AMIGA sample of isolated galaxies III. IRAS data and infrared diagnostics
Authors: U. Lisenfeld, L. Verdes-Montenegro, J. Sulentic, S. Leon, D. Espada, G. Bergond, J. Sabater, J.D. Santander-Vela, S. Verley

We describe the mid- (MIR) and far- (FIR) infrared properties of a large (~1000) sample of the most isolated galaxies in the local Universe. This sample is intended as a ''nurture-free'' zero point against which more environmentally influenced samples can be compared. We reprocess IRAS MIR/FIR survey data using the ADDSCAN/SCANPI utility for 1030 out of 1050 galaxies from the Catalogue of Isolated Galaxies (CIG) as part of the AMIGA project. We focus on diagnostics (FIR luminosity L_{FIR}, R=\log(L_FIR/L_B) and IRAS colours) thought to be sensitive to effects of environment or interaction. The distribution of \log(L_FIR) is sharply peaked from 9.0--10.5 with very few ( < 2%) galaxies above 10.5. The optically normalised luminosity diagnostic R= \log(L_FIR/L_B) shows a distribution sharply peaked between 0.0 and -1.0. These results were compared to the magnitude limited CfA sample that was selected without environmental discrimination. This modestly (e.g. compared to cluster, binary galaxy and compact group samples) environmentally affected sample shows significantly higher mean \log(L_{FIR}) and R, whereas the mean \log(L_B) is the same. Our sample shows a strong L_{FIR} vs. L_{B} correlation, with a slope steeper than one (L_FIR \propto L_B^1.41). Interacting galaxies were found above this correlation, showing an enhancement in L_FIR. With respect to the IRAS colours, we found higher F_60/F_100 value for ellipticals and late-type galaxies than for spirals, indicating a higher dust temperature. The results indicate that the FIR emission is a variable enhanced by interaction, and that our sample probably shows the lowest possible mean value. This attests to the utility of our sample for defining a nurture-free zero point.

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Properties of Voids
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Title: Properties of Voids in the 2dFGRS Galaxy Survey
Authors: Anton V. Tikhonov

A method for detecting voids in the galaxy distribution is presented. Using this method, we have identified 732 voids with a radius of the seed sphere Rseed > 4.0h^-1Mpc in a volume-limited sample of galaxies from the southern part of the 2dFGRS survey. 110 voids with Rseed > 9.0h^-1Mpc have a positive significance. The mean volume of such voids is 19000 h^-3 Mpc^3. Voids with Rseed > 9.0h^-1Mpc occupy 55% of the sample volume. We construct a dependence of the volumes of all the identified voids on their ranks and determine parameters of the galaxy distribution. The dependence of the volume of voids on their rank is consistent with a fractal model (Zipf's power law) of the galaxy distribution with a fractal dimension D ~ 2.1 (given the uncertainty in determining the dimension using our method and the results of a correlation analysis) up to scales of 25h^-1Mpc with the subsequent transition to homogeneity. The directions of the greatest elongations of voids and their ellipticities (oblateness) are determined from the parameters of equivalent ellipsoids. The directions of the greatest void elongations have an enhanced concentration to the directions perpendicular to the line of sight.

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Solitary galaxies in voids
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Solitary galaxies that live in vast, empty regions of space are forming stars at a surprisingly high rate, reveals the largest survey of galaxies ever made. The lonely galaxies may thrive because few galaxies are close enough to strip them of their star-forming gas.

An overwhelming 95% of galaxies live in clusters or groups, some of which teem with thousands of members. Gravity pulls the objects towards each other, forming bubble-like voids.
But clumps of gas can remain in the voids, allowing galaxies to spring up "like oases in the desert". Previously, about a dozen such galaxies had been studied.
Now, Fiona Hoyle, an astronomer at Widener University in Chester, Pennsylvania, US, and colleagues have identified 1000 void galaxies using the Sloan Digital Sky Survey (SDSS). The survey, which is more than halfway to its goal of systematically mapping a quarter of the sky, shows hundreds of dark voids that each shine with a few beacon-like galaxies.



The SDSS colour images confirm previous findings that the galaxies differ strikingly from their counterparts in clusters.
"They're bluer than expected, which means they've got a higher rate of star formation," - Fiona Hoyle.
"The sheer number of void galaxies now makes it pretty clear that this (higher star formation rate) is endemic," - John Huchra, an astronomer at the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, US.
Two mechanisms could explain the effect. Either the galaxies were late bloomers and formed their first stars recently - taking billions of years longer than most other galaxies to condense from gas - or they ignited their first stars long ago but managed to draw out the star-formation process.

Hoyle favours the second scenario: "Galaxies in clusters interact often and during these interactions the galaxies are stripped of their gas." In contrast, the void galaxies "probably haven't undergone significant interaction, so they've been able to keep hold of their gas supply".
Hoyle’s colleague Michael Vogeley at Drexel University in Philadelphia, Pennsylvania, US, likens the process to "a few rugged individuals who, without the competition of others, can live surprisingly well from the meagre offerings of the land in most rural areas."
But Huchra says the active star formation probably results from a combination of both effects. "It's a chicken and egg problem".

Hoyle believes the ongoing SDSS could turn up as many as 10,000 void galaxies. And she hopes the data will contain the answer to a long-standing mystery about the nature of blob-shaped "elliptical" galaxies.
Some astronomers believe these objects form solely through the merger of spiral galaxies, while others say they could condense directly from gas. Finding elliptical galaxies in relatively empty voids could show that the galaxies can take shape directly out of "primordial" gas.

The research was presented at a meeting of the American Astronomical Society in Minneapolis, Minnesota, US, last week and published in the Astrophysical Journal.
Journal reference: Astrophysical Journal (vol 624, p 571)

Voids in the 2dFGRS
A Map of the Universe

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