The COS-Halos Survey: An Empirical Description of the Metal-Line Absorption in the Low-Redshift Circumgalactic Medium. (arXiv:1212.0558v1 [astro-ph.CO]):
We present the equivalent width and column density measurements for low and
intermediate ionization states of the circumgalactic medium (CGM) surrounding
44 low-z, L ~ L* galaxies drawn from the COS-Halos survey. These measurements
are derived from far-UV transitions observed in HST/COS and Keck/HIRES spectra
of background quasars within an impact parameter R < 160 kpc to the targeted
galaxies. The data show significant metal-line absorption for 33 of the 44
galaxies, including quiescent systems, revealing the common occurance of a cool
(T ~ 10^{4 - 5} K), metal-enriched CGM. The detection rates and column
densities derived for these metal lines decrease with increasing impact
parameter, a trend we interpret as a declining metal surface density profile
for the CGM. A comparison of the relative column densities of adjacent
ionization states indicates the gas is predominantly ionized. The large surface
density in metals demands a large reservoir of metals and gas in the cool CGM
(very conservatively, M_ CGMcool > 10^9 MSun), which likely traces a distinct
density and/or temperature regime from the highly-ionized CGM traced by OVI
absorption. The large dispersion in absorption strengths (including
non-detections) suggests the cool CGM traces a wide range of densities or a mix
of local ionizing conditions. Lastly, the kinematics inferred from the
metal-line profiles are consistent with the cool CGM being bound to the dark
matters halos hosting the galaxies; this gas may serve as fuel for future
star-formation. Future work will leverage this dataset to provide estimates on
the mass, metallicity, dynamics, and origin of the cool CGM in low-z, L*
galaxies.
Showing posts with label WHIM. Show all posts
Showing posts with label WHIM. Show all posts
Monday, December 17, 2012
Thursday, December 13, 2012
Constraints on Hydrodynamical Subgrid Models from Quasar Absorption Line Studies of the Simulated Circumgalactic Medium. (arXiv:1212.2965v1 [astro-ph.GA])
Constraints on Hydrodynamical Subgrid Models from Quasar Absorption Line Studies of the Simulated Circumgalactic Medium. (arXiv:1212.2965v1 [astro-ph.GA]):
Cosmological hydrodynamical simulations of galaxy evolution are increasingly
able to produce realistic galaxies, but the largest hurdle remaining is in
constructing subgrid models that accurately describe the behavior of stellar
feedback. As an alternate way to test and calibrate such models, we propose to
focus on the circumgalactic medium. To do so, we generate a suite of
adaptive-mesh refinement (AMR) simulations for a Milky-Way-massed galaxy run to
z=0, systematically varying the feedback implementation. We then post-process
the simulation data to compute the absorbing column density for a wide range of
common atomic absorbers throughout the galactic halo, including H I, Mg II, Si
II, Si III, Si IV, C IV, N V, O VI, and O VII. The radial profiles of these
atomic column densities are compared against several quasar absorption line
studies, to determine if one feedback prescription is favored. We find that
although our models match some of the observations (specifically those ions
with lower ionization strengths), it is particularly difficult to match O VI
observations. There is some indication that the models with increased feedback
intensity are better matches. We demonstrate that sufficient metals exist in
these halos to reproduce the observed column density distribution in principle,
but the simulated circumgalactic medium lacks significant multiphase
substructure and is generally too hot. Furthermore, we demonstrate the failings
of inflow-only models (without energetic feedback) at populating the CGM with
adequate metals to match observations even in the presence of multiphase
structure. Additionally, we briefly investigate the evolution of the CGM from
z=3 to present. Overall, we find that quasar absorption line observations of
the gas around galaxies provide a new and important constraint on feedback
models.
Cosmological hydrodynamical simulations of galaxy evolution are increasingly
able to produce realistic galaxies, but the largest hurdle remaining is in
constructing subgrid models that accurately describe the behavior of stellar
feedback. As an alternate way to test and calibrate such models, we propose to
focus on the circumgalactic medium. To do so, we generate a suite of
adaptive-mesh refinement (AMR) simulations for a Milky-Way-massed galaxy run to
z=0, systematically varying the feedback implementation. We then post-process
the simulation data to compute the absorbing column density for a wide range of
common atomic absorbers throughout the galactic halo, including H I, Mg II, Si
II, Si III, Si IV, C IV, N V, O VI, and O VII. The radial profiles of these
atomic column densities are compared against several quasar absorption line
studies, to determine if one feedback prescription is favored. We find that
although our models match some of the observations (specifically those ions
with lower ionization strengths), it is particularly difficult to match O VI
observations. There is some indication that the models with increased feedback
intensity are better matches. We demonstrate that sufficient metals exist in
these halos to reproduce the observed column density distribution in principle,
but the simulated circumgalactic medium lacks significant multiphase
substructure and is generally too hot. Furthermore, we demonstrate the failings
of inflow-only models (without energetic feedback) at populating the CGM with
adequate metals to match observations even in the presence of multiphase
structure. Additionally, we briefly investigate the evolution of the CGM from
z=3 to present. Overall, we find that quasar absorption line observations of
the gas around galaxies provide a new and important constraint on feedback
models.
Friday, November 9, 2012
Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line Detections and Identifications (Paper I). (arXiv:1210.7177v1 [astro-ph.CO])
Chandra View of the Warm-Hot IGM toward 1ES 1553+113: Absorption Line Detections and Identifications (Paper I). (arXiv:1210.7177v1 [astro-ph.CO]):
About 30-40 percent of the expected number of baryons is still missing in the
local Universe (z \lesssim 0.4). They are predicted to be hiding in a web of
intergalactic gas at temperatures of about 10^5-10^7 K (the WHIM). Detecting
this matter has had limited success so far, because of its low-density and high
temperature, which makes it difficult to detect with current far-ultraviolet
and X-ray instrumentation.
Here we present the first results from our pilot 500 ks Chandra-LETG
observation of the soft X-ray brightest source in the z > 0.4 sky, the blazar
1ES 1553+113. We identify a total of 11 possible absorption lines, with
single-line statistical significances between 2.2-4.1 sigma. Six of these lines
are detected at high significance (3.6 < \sigma < 4.1), while the remaining
five are regarded as marginal detections in association with either other X-ray
lines detected at higher significance and/or FUV signposts. Three of these
lines are consistent with metal absorption at z~0. The remaining 8 lines may be
imprinted by intervening absorbers and are all consistent with being
high-ionization counterparts of FUV HI and/or OVI IGM signposts. In particular,
four of these eight absorption lines (4.1\sigma, 4.1\sigma, 3.8\sigma and
2.7\sigma), are identified as CV and CVI absorbers belonging to two WHIM
systems at z_X = 0.312 and z_X = 0.133, which also produce broad HI and OVI
absorption in the FUV. The true statistical significances of these two X-ray
absorption systems, after properly accounting for the number of redshift
trials, are 5.8\sigma and 3.8\sigma.
About 30-40 percent of the expected number of baryons is still missing in the
local Universe (z \lesssim 0.4). They are predicted to be hiding in a web of
intergalactic gas at temperatures of about 10^5-10^7 K (the WHIM). Detecting
this matter has had limited success so far, because of its low-density and high
temperature, which makes it difficult to detect with current far-ultraviolet
and X-ray instrumentation.
Here we present the first results from our pilot 500 ks Chandra-LETG
observation of the soft X-ray brightest source in the z > 0.4 sky, the blazar
1ES 1553+113. We identify a total of 11 possible absorption lines, with
single-line statistical significances between 2.2-4.1 sigma. Six of these lines
are detected at high significance (3.6 < \sigma < 4.1), while the remaining
five are regarded as marginal detections in association with either other X-ray
lines detected at higher significance and/or FUV signposts. Three of these
lines are consistent with metal absorption at z~0. The remaining 8 lines may be
imprinted by intervening absorbers and are all consistent with being
high-ionization counterparts of FUV HI and/or OVI IGM signposts. In particular,
four of these eight absorption lines (4.1\sigma, 4.1\sigma, 3.8\sigma and
2.7\sigma), are identified as CV and CVI absorbers belonging to two WHIM
systems at z_X = 0.312 and z_X = 0.133, which also produce broad HI and OVI
absorption in the FUV. The true statistical significances of these two X-ray
absorption systems, after properly accounting for the number of redshift
trials, are 5.8\sigma and 3.8\sigma.
Monday, October 15, 2012
Galactic Winds in Cosmological Simulations of the Circumgalactic Medium. (arXiv:1210.3582v1 [astro-ph.CO])
Galactic Winds in Cosmological Simulations of the Circumgalactic Medium. (arXiv:1210.3582v1 [astro-ph.CO]):
(Abridged) We explore new observationally-constrained sub-resolution models
of galactic outflows and investigate their impact on the circumgalactic medium
(CGM) over redshifts z = 2 - 4. We perform cosmological hydrodynamic
simulations, including star formation, chemical enrichment, and four cases of
SNe-driven outflows: no wind (NW), an energy-driven constant velocity wind
(CW), a radially varying wind (RVWa) where the outflow velocity has a positive
correlation with galactocentric distance (r), and a RVW with additional
dependence on halo mass (RVWb). Overall, we find that the outflows expel
metal-enriched gas away from galaxies, significantly quench star formation, and
enrich the CGM. At z = 2, the radial profiles of gas properties around galaxy
centers are most sensitive to the choice of the wind model for halo masses
(10^9 - 10^11) M_sun. We infer that the RVWb model is similar to the NW case,
except that it substantially enriches the CGM: the carbon metallicity (Z_C) is
10 times higher in RVWb than in NW at r > R_200; and the warm gas of 10^4 -
10^5 K and delta < 100 is enriched to 50 times higher than in NW. We also find
that the impact of models CW and RVWa are similar, with the following
differences. RVWa causes a greater suppression of star formation rate at z < 5,
and has a higher fraction of low-density (delta < 10), warm-hot (10^4 - 10^6 K)
gas than in CW. Outflows in CW produce a higher and earlier enrichment of some
IGM phases than RVWa. By visual inspection, we note that the RVWa model shows
more pronounced bipolar outflows and galactic disks. We present fitting
formulae for [Z_C-delta] and [Z_C-r], also for the abundance of CIV as a
function of r. We predict observational diagnostics to distinguish between
different outflow scenarios: Z_C of the CGM gas at r = (30 - 300) kpc/h
comoving, and CIV fraction of the inner gas at r < (4 - 5) kpc/h comoving.
(Abridged) We explore new observationally-constrained sub-resolution models
of galactic outflows and investigate their impact on the circumgalactic medium
(CGM) over redshifts z = 2 - 4. We perform cosmological hydrodynamic
simulations, including star formation, chemical enrichment, and four cases of
SNe-driven outflows: no wind (NW), an energy-driven constant velocity wind
(CW), a radially varying wind (RVWa) where the outflow velocity has a positive
correlation with galactocentric distance (r), and a RVW with additional
dependence on halo mass (RVWb). Overall, we find that the outflows expel
metal-enriched gas away from galaxies, significantly quench star formation, and
enrich the CGM. At z = 2, the radial profiles of gas properties around galaxy
centers are most sensitive to the choice of the wind model for halo masses
(10^9 - 10^11) M_sun. We infer that the RVWb model is similar to the NW case,
except that it substantially enriches the CGM: the carbon metallicity (Z_C) is
10 times higher in RVWb than in NW at r > R_200; and the warm gas of 10^4 -
10^5 K and delta < 100 is enriched to 50 times higher than in NW. We also find
that the impact of models CW and RVWa are similar, with the following
differences. RVWa causes a greater suppression of star formation rate at z < 5,
and has a higher fraction of low-density (delta < 10), warm-hot (10^4 - 10^6 K)
gas than in CW. Outflows in CW produce a higher and earlier enrichment of some
IGM phases than RVWa. By visual inspection, we note that the RVWa model shows
more pronounced bipolar outflows and galactic disks. We present fitting
formulae for [Z_C-delta] and [Z_C-r], also for the abundance of CIV as a
function of r. We predict observational diagnostics to distinguish between
different outflow scenarios: Z_C of the CGM gas at r = (30 - 300) kpc/h
comoving, and CIV fraction of the inner gas at r < (4 - 5) kpc/h comoving.
Wednesday, September 19, 2012
Warm-hot gas in groups and galaxies toward H2356-309. (arXiv:1209.4080v1 [astro-ph.CO])
Warm-hot gas in groups and galaxies toward H2356-309. (arXiv:1209.4080v1 [astro-ph.CO]):
We present a detailed analysis of the galaxy and group distributions around
three reported X-ray absorption line systems in the spectrum of the quasar
H2356-309. Previous studies associated these absorbers with known large-scale
galaxy structures (i.e., walls and filaments) along the line of sight. Such
absorption lines typically trace 10^{5-7} K gas, and may be evidence of the
elusive warm-hot intergalactic medium (WHIM) thought to harbor the bulk of the
low-redshift "missing baryons;" alternatively, they may be linked to individual
galaxies or groups in the filaments. Here we combine existing galaxy survey
data with new, highly complete multi-object Magellan spectroscopy to
investigate the detailed galaxy distribution near each absorber. All of these
three absorption systems nominally lie within the virial radii of nearby
galaxies and/or groups, and could therefore arise in these virialized
structures rather than (or in addition to) the WHIM. However, we find no
additional galaxies near a fourth "void" absorber recently reported by
Zappacosta et al., suggesting that this system may indeed trace gas
unassociated with any individual halo. We therefore conclude that most X-ray
absorbers are coincident with galaxy and/or group environments, though some
could still trace the large-scale filamentary WHIM gas predicted by
simulations.
We present a detailed analysis of the galaxy and group distributions around
three reported X-ray absorption line systems in the spectrum of the quasar
H2356-309. Previous studies associated these absorbers with known large-scale
galaxy structures (i.e., walls and filaments) along the line of sight. Such
absorption lines typically trace 10^{5-7} K gas, and may be evidence of the
elusive warm-hot intergalactic medium (WHIM) thought to harbor the bulk of the
low-redshift "missing baryons;" alternatively, they may be linked to individual
galaxies or groups in the filaments. Here we combine existing galaxy survey
data with new, highly complete multi-object Magellan spectroscopy to
investigate the detailed galaxy distribution near each absorber. All of these
three absorption systems nominally lie within the virial radii of nearby
galaxies and/or groups, and could therefore arise in these virialized
structures rather than (or in addition to) the WHIM. However, we find no
additional galaxies near a fourth "void" absorber recently reported by
Zappacosta et al., suggesting that this system may indeed trace gas
unassociated with any individual halo. We therefore conclude that most X-ray
absorbers are coincident with galaxy and/or group environments, though some
could still trace the large-scale filamentary WHIM gas predicted by
simulations.
Friday, September 14, 2012
The 21cm forest in the diffuse IGM as seen by LOFAR. (arXiv:1209.2615v1 [astro-ph.CO])
The 21cm forest in the diffuse IGM as seen by LOFAR. (arXiv:1209.2615v1 [astro-ph.CO]):
We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.
We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.
Tuesday, September 4, 2012
The Temperature-Density Relation in the Intergalactic Medium at Redshift =2.4. (arXiv:1209.0005v1 [astro-ph.CO])
The Temperature-Density Relation in the Intergalactic Medium at Redshift <z>=2.4. (arXiv:1209.0005v1 [astro-ph.CO]):
We present new measurements of the temperature-density (T-rho) relation for
neutral hydrogen in the 2.0 < z < 2.8 intergalactic medium (IGM) using a sample
of ~6000 individual HI absorbers fitted with Voigt profiles constrained in all
cases by multiple Lyman series transitions. We find model independent evidence
for a positive correlation between the column density of HI (NHI) and the
minimum observed velocity width of absorbers (bmin). With minimal
interpretation, this implies that the temperature-density relation in the IGM
is not "inverted", contrary to many recent studies. Fitting bmin as a function
of NHI results in line width - column density dependence of the form bdmin =
b_0 [NHI/N_(HI,0)]^(Gamma-1) with a minimum line width at mean density rhobar
[N_(HI, 0) = 10^13.6 cm^-2] of b_0 = 17.9 +- 0.2 km/s and a power-law index of
(Gamma-1) = 0.15 +- 0.02. Using analytic arguments, these measurements imply an
"equation of state" for the IGM at <z>= 2.4 of the form T = T_0
(rho/rhobar)^(gamma-1) with a temperature at mean density of T_0 = (1.94 +-
0.05) x 10^4 K and a power-law index (gamma -1) = 0.46 +- 0.05.
We present new measurements of the temperature-density (T-rho) relation for
neutral hydrogen in the 2.0 < z < 2.8 intergalactic medium (IGM) using a sample
of ~6000 individual HI absorbers fitted with Voigt profiles constrained in all
cases by multiple Lyman series transitions. We find model independent evidence
for a positive correlation between the column density of HI (NHI) and the
minimum observed velocity width of absorbers (bmin). With minimal
interpretation, this implies that the temperature-density relation in the IGM
is not "inverted", contrary to many recent studies. Fitting bmin as a function
of NHI results in line width - column density dependence of the form bdmin =
b_0 [NHI/N_(HI,0)]^(Gamma-1) with a minimum line width at mean density rhobar
[N_(HI, 0) = 10^13.6 cm^-2] of b_0 = 17.9 +- 0.2 km/s and a power-law index of
(Gamma-1) = 0.15 +- 0.02. Using analytic arguments, these measurements imply an
"equation of state" for the IGM at <z>= 2.4 of the form T = T_0
(rho/rhobar)^(gamma-1) with a temperature at mean density of T_0 = (1.94 +-
0.05) x 10^4 K and a power-law index (gamma -1) = 0.46 +- 0.05.
Tuesday, August 21, 2012
The nature of the unresolved extragalactic soft CXB. (arXiv:1208.4105v1 [astro-ph.CO])
The nature of the unresolved extragalactic soft CXB. (arXiv:1208.4105v1 [astro-ph.CO]):
In this paper we investigate the power spectrum of the unresolved 0.5-2 keV
CXB with deep Chandra 4 Ms observations in the CDFS. We measured a signal
which, on scales >30", is significantly higher than the Shot-Noise and is
increasing with the angular scale. We interpreted this signal as the joint
contribution of clustered undetected sources like AGN, Galaxies and
Inter-Galactic-Medium (IGM). The power of unresolved cosmic sources
fluctuations accounts for \sim 12% of the 0.5-2 keV extragalactic CXB. Overall,
our modeling predicts that \sim 20% of the unresolved CXB flux is made by low
luminosity AGN, \sim 25% by galaxies and \sim 55% by the IGM (Inter Galactic
Medium). We do not find any direct evidence of the so called Warm Hot
Intergalactic Medium (i.e. matter with 10^5K<T<10^7K and density contrast
{\delta} <1000), but we estimated that it could produce about 1/7 of the
unresolved CXB. We placed an upper limit to the space density of postulated
X-ray-emitting early black hole at z>7.5 and compared it with SMBH evolution
models.
In this paper we investigate the power spectrum of the unresolved 0.5-2 keV
CXB with deep Chandra 4 Ms observations in the CDFS. We measured a signal
which, on scales >30", is significantly higher than the Shot-Noise and is
increasing with the angular scale. We interpreted this signal as the joint
contribution of clustered undetected sources like AGN, Galaxies and
Inter-Galactic-Medium (IGM). The power of unresolved cosmic sources
fluctuations accounts for \sim 12% of the 0.5-2 keV extragalactic CXB. Overall,
our modeling predicts that \sim 20% of the unresolved CXB flux is made by low
luminosity AGN, \sim 25% by galaxies and \sim 55% by the IGM (Inter Galactic
Medium). We do not find any direct evidence of the so called Warm Hot
Intergalactic Medium (i.e. matter with 10^5K<T<10^7K and density contrast
{\delta} <1000), but we estimated that it could produce about 1/7 of the
unresolved CXB. We placed an upper limit to the space density of postulated
X-ray-emitting early black hole at z>7.5 and compared it with SMBH evolution
models.
Friday, August 17, 2012
Identifying the Baryons in a Multiphase Intergalactic Medium. (arXiv:1208.3249v1 [astro-ph.CO])
Identifying the Baryons in a Multiphase Intergalactic Medium. (arXiv:1208.3249v1 [astro-ph.CO]):
In this white paper, we summarize current observations of the baryon census
at low redshift (Shull, Smith, & Danforth 2012). Measurements of Lya, O-VI, and
broad Lya absorbers, together with more careful corrections for metallicity and
ionization fraction, can now account for approximately 60% of the baryons in
the intergalactic medium (IGM). An additional 5 +/- 3% may reside in the
circumgalactic medium (CGM), 7 +/- 2% in galaxies, and 4 +/- 1.5% in clusters.
This still leaves a substantial fraction, 29 +/- 13%, unaccounted for. We
suggest improvements in measuring the baryons in major components of the IGM
and CGM with future Ultraviolet and X-ray spectrographs. These missions could
find and map the missing baryons, the fuel for the formation and chemical
evolution of galaxies.
In this white paper, we summarize current observations of the baryon census
at low redshift (Shull, Smith, & Danforth 2012). Measurements of Lya, O-VI, and
broad Lya absorbers, together with more careful corrections for metallicity and
ionization fraction, can now account for approximately 60% of the baryons in
the intergalactic medium (IGM). An additional 5 +/- 3% may reside in the
circumgalactic medium (CGM), 7 +/- 2% in galaxies, and 4 +/- 1.5% in clusters.
This still leaves a substantial fraction, 29 +/- 13%, unaccounted for. We
suggest improvements in measuring the baryons in major components of the IGM
and CGM with future Ultraviolet and X-ray spectrographs. These missions could
find and map the missing baryons, the fuel for the formation and chemical
evolution of galaxies.
Tuesday, July 24, 2012
A filament of dark matter between two clusters of galaxies. (arXiv:1207.0809v1 [astro-ph.CO])
A filament of dark matter between two clusters of galaxies. (arXiv:1207.0809v1 [astro-ph.CO]):
It is a firm prediction of the concordance Cold Dark Matter (CDM)
cosmological model that galaxy clusters live at the intersection of large-scale
structure filaments. The thread-like structure of this "cosmic web" has been
traced by galaxy redshift surveys for decades. More recently the Warm-Hot
Intergalactic Medium (WHIM) residing in low redshift filaments has been
observed in emission and absorption. However, a reliable direct detection of
the underlying Dark Matter skeleton, which should contain more than half of all
matter, remained elusive, as earlier candidates for such detections were either
falsified or suffered from low signal-to-noise ratios and unphysical
misalignements of dark and luminous matter. Here we report the detection of a
dark matter filament connecting the two main components of the Abell 222/223
supercluster system from its weak gravitational lensing signal, both in a
non-parametric mass reconstruction and in parametric model fits. This filament
is coincident with an overdensity of galaxies and diffuse, soft X-ray emission
and contributes mass comparable to that of an additional galaxy cluster to the
total mass of the supercluster. Combined with X-ray observations, we place an
upper limit of 0.09 on the hot gas fraction, the mass of X-ray emitting gas
divided by the total mass, in the filament.
It is a firm prediction of the concordance Cold Dark Matter (CDM)
cosmological model that galaxy clusters live at the intersection of large-scale
structure filaments. The thread-like structure of this "cosmic web" has been
traced by galaxy redshift surveys for decades. More recently the Warm-Hot
Intergalactic Medium (WHIM) residing in low redshift filaments has been
observed in emission and absorption. However, a reliable direct detection of
the underlying Dark Matter skeleton, which should contain more than half of all
matter, remained elusive, as earlier candidates for such detections were either
falsified or suffered from low signal-to-noise ratios and unphysical
misalignements of dark and luminous matter. Here we report the detection of a
dark matter filament connecting the two main components of the Abell 222/223
supercluster system from its weak gravitational lensing signal, both in a
non-parametric mass reconstruction and in parametric model fits. This filament
is coincident with an overdensity of galaxies and diffuse, soft X-ray emission
and contributes mass comparable to that of an additional galaxy cluster to the
total mass of the supercluster. Combined with X-ray observations, we place an
upper limit of 0.09 on the hot gas fraction, the mass of X-ray emitting gas
divided by the total mass, in the filament.
The VMC Survey - VI. Quasars behind the Magellanic system. (arXiv:1207.2492v1 [astro-ph.CO])
The VMC Survey - VI. Quasars behind the Magellanic system. (arXiv:1207.2492v1 [astro-ph.CO]):
The number and spatial distribution of confirmed quasi-stellar objects (QSOs)
behind the Magellanic system is limited. This undermines their use as
astrometric reference objects for studies of proper motion and of the
interstellar medium along the line of sight. We search for criteria to identify
candidate QSOs using near-infrared observations from the VISTA survey of the
Magellanic Clouds system (VMC). The VMC survey provides photometry in the YJKs
bands and 12 epochs in the Ks band with unprecedented sensitivity and spatial
resolution. The (Y-J) vs. (J-Ks) diagram has been used to distinguish QSOs from
Milky Way and Magellanic Cloud stars. Then, the slope of variation in the Ks
band has been used to identify a sample of high confidence candidates. These
criteria were developed based on the properties of 117 known QSOs. YJKs
magnitudes and Ks light-curves of known QSOs behind the Magellanic system from
present VMC data are presented. About 75% of them show a slope of variation
>10^-4 mag/day and the shape of the light-curve is in general irregular and
without any clear periodicity. A method to identify QSOs based solely on the
VMC data is proposed using YJKs colours and Ks variability. The number of QSO
candidates found in the South Ecliptic Pole and the 30 Doradus tiles is 22 and
26, respectively, with negligible contamination by young stellar objects,
planetary nebulae, stars and normal galaxies. The high confidence in the nature
of the selected objects is supported by recent studies of possible
contaminants, but remains to be confirmed spectroscopically. In the entire VMC
survey area we expect to find about 1500 QSOs behind the LMC, 600 behind the
SMC, 300 behind the Bridge and 50 behind the Stream areas. The Ks light-curves
can help support investigations of the mechanism responsible for the
variations. (Abridged)
The number and spatial distribution of confirmed quasi-stellar objects (QSOs)
behind the Magellanic system is limited. This undermines their use as
astrometric reference objects for studies of proper motion and of the
interstellar medium along the line of sight. We search for criteria to identify
candidate QSOs using near-infrared observations from the VISTA survey of the
Magellanic Clouds system (VMC). The VMC survey provides photometry in the YJKs
bands and 12 epochs in the Ks band with unprecedented sensitivity and spatial
resolution. The (Y-J) vs. (J-Ks) diagram has been used to distinguish QSOs from
Milky Way and Magellanic Cloud stars. Then, the slope of variation in the Ks
band has been used to identify a sample of high confidence candidates. These
criteria were developed based on the properties of 117 known QSOs. YJKs
magnitudes and Ks light-curves of known QSOs behind the Magellanic system from
present VMC data are presented. About 75% of them show a slope of variation
>10^-4 mag/day and the shape of the light-curve is in general irregular and
without any clear periodicity. A method to identify QSOs based solely on the
VMC data is proposed using YJKs colours and Ks variability. The number of QSO
candidates found in the South Ecliptic Pole and the 30 Doradus tiles is 22 and
26, respectively, with negligible contamination by young stellar objects,
planetary nebulae, stars and normal galaxies. The high confidence in the nature
of the selected objects is supported by recent studies of possible
contaminants, but remains to be confirmed spectroscopically. In the entire VMC
survey area we expect to find about 1500 QSOs behind the LMC, 600 behind the
SMC, 300 behind the Bridge and 50 behind the Stream areas. The Ks light-curves
can help support investigations of the mechanism responsible for the
variations. (Abridged)
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