arXiv subjects

T. Fang

Publications and source records attributed to T. Fang.

At least 19 recordsLinked to original sources

arXiv2019

The Voyage of Metals in the Universe from Cosmological to Planetary Scales: the need for a Very High-Resolution, High Throughput Soft X-ray Spectrometer

Metals form an essential part of the Universe at all scales. Without metals we would not exist, and the Cosmos would look completely different. Metals are primarily born through nuclear processes in stars. They leave their cradles through winds or explosions, and then start their journey through space. This can lead them in and out of astronomical objects on all scales, ranging from comets, planets, stars, entire galaxies, groups and clusters of galaxies to the largest structures of the Universe. Their wanderings are fundamental in determining how these objects, and the entire universe, evolve. In addition, their bare presence can be used to trace what these structures look like. The scope of this paper is to highlight the most important open astrophysical problems that will be central in the next decades and for which a deep understanding of the Universe-wandering metals, their physical and kinematical states and their chemical composition represents the only viable solution. The majority of these studies can only be efficiently performed through High Resolution Spectroscopy in the soft X-ray band.

astro-ph.IM↗

arXiv2018

To be or not to be: hot WHIM absorption in the blazar PKS 2155-304 sight line?

The cosmological missing baryons at z<1 most likely hide in the hot (T$\gtrsim10^{5.5}$ K) phase of the Warm Hot Intergalactic Medium (WHIM). While the hot WHIM is hard to detect due to its high ionisation level, the warm (T$\lesssim10^{5.5}$ K) phase of the WHIM has been very robustly detected in the FUV band. We adopted the assumption that the hot and warm WHIM phases are co-located and thus used the FUV-detected warm WHIM as a tracer for the cosmologically interesting hot WHIM. We utilised the assumption by performing an X-ray follow-up in the sight line of a blazar PKS 2155-304 at the redshifts where previous FUV measurements of OVI, SiIV and BLA absorption have indicated the existence of the warm WHIM. We looked for the OVII He$\alpha$ and OVIII Ly$\alpha$ absorption lines, the most likely hot WHIM tracers. Despite of the very large exposure time ($\approx$ 1 Ms), the XMM-Newton/RGS1 data yielded no significant detection which corresponds to upper limits of $\log{N({\rm OVII})({\rm cm}^{-2}))} \le 14.5-15.2$ and $\log{N({\rm OVIII})({\rm cm}^{-2}))} \le 14.9-15.2$. An analysis of LETG/HRC data yielded consistent results. However, the LETG/ACIS data yielded a detection of an absorption line - like feature at $\lambda \approx$ 20 \AA\ at simple one parameter uncertainty - based confidence level of 3.7 $\sigma$, consistently with several earlier LETG/ACIS reports. Given the high statistical quality of the RGS1 data, the possibility of RGS1 accidentally missing the true line at $\lambda \sim$ 20 \AA\ is very low, 0.006%. Neglecting this, the LETG/ACIS detection can be interpreted as Ly$\alpha$ transition of OVIII at one of the redshifts (z$\approx$ 0.054) of FUV-detected warm WHIM. Given the very convincing X-ray spectral evidence for and against the existence of the $\lambda \sim$ 20 \AA\ feature, we cannot conclude whether or not it is a true astrophysical absorption line.

astro-ph.CO↗

arXiv2016

A possible Chandra and Hubble Space Telescope detection of extragalactic WHIM towards PG 1116+215

(Abridged) We have analyzed Chandra LETG and XMM-Newton RGS spectra towards the z=0.177 quasar PG 1116+215, a sightline that is rendered particularly interesting by the HST detection of several OVI and HI broad Lyman-alpha absorption lines that may be associated with the warm-hot intergalactic medium. We performed a search for resonance K-alpha absorption lines from OVII and OVIII at the redshifts of the detected far-ultraviolet lines. We detected an absorption line in the Chandra spectra at 5.2 sigma confidence level at wavelengths corresponding to OVIII K-alpha at z=0.0911+-0.0004+-0.0005 (statistical followed by systematic error). This redshift is within 3 sigma of that of a HI broad Lyman-alpha of b=130 km/s at z=0.09279+-0.00005. We have also analyzed the available XMM-Newton RGS data towards PG 1116+215. Unfortunately, the XMM-Newton data are not suitable to investigate this line because of instrumental features at the wavelengths of interest. At the same redshift, the Chandra and XMM-Newton spectra have OVII K-alpha absorption line features of significance 1.5 sigma and 1.8 sigma, respectively. We also analyzed the available SDSS spectroscopic galaxy survey data towards PG 1116+215 in the redshift range of interest. We found evidence for a galaxy filament that intersects the PG 1116+215 sightline and additional galaxy structures that may host WHIM. The combination of HST, Chandra, XMM-Newton and SDSS data indicates that we have likely detected a multi-temperature WHIM at z=0.091-0.093 towards PG 1116+215.

astro-ph.CO↗

arXiv2010

Studying the WHIM Content of the Galaxy Large-Scale Structures along the Line of Sight to H 2356-309

We make use of a 500ks Chandra HRC-S/LETG spectrum of the blazar H2356-309, combined with a lower S/N spectrum of the same target, to search for the presence of warm-hot absorbing gas associated with two Large-Scale Structures (LSSs) crossed by this sightline at z=0.062 (the Pisces-Cetus Supercluster, PCS) and at z=0.128 ("Farther Sculptor Wall", FSW). No statistically significant (>=3sigma) individual absorption is detected from any of the strong He- or H-like transitions of C, O and Ne at the redshifts of the structures. However we are still able to constrain the physical and geometrical parameters of the associated putative absorbing gas, by performing joint spectral fit of marginal detections and upper limits of the strongest expected lines with our self-consistent hybrid ionization WHIM spectral model. At the redshift of the PCS we identify a warm phase with logT=5.35_-0.13^+0.07 K and log N_H =19.1+/-0.2 cm^-2 possibly coexisting with a hotter and less significant phase with logT=6.9^+0.1_-0.8 K and log N_H=20.1^+0.3_-1.7 cm^-2 (1sigma errors). For the FSW we estimate logT=6.6_-0.2^+0.1 K and log N_H=19.8_-0.8^+0.4 cm^-2. Our constraints allow us to estimate the cumulative number density per unit redshifts of OVII WHIM absorbers. We also estimate the cosmological mass density obtaining Omega_b(WHIM)=(0.021^+0.031_-0.018) (Z/Z_sun)^-1, consistent with the mass density of the intergalactic 'missing baryons' for high metallicities.

astro-ph.CO↗

arXiv2006

AEGIS: Chandra Observation of DEEP2 Galaxy Groups and Clusters

We present a 200 ksec Chandra observation of seven spectroscopically selected, high redshift (0.75 < z < 1.03) galaxy groups and clusters discovered by the DEEP2 Galaxy Redshift Survey in the Extended Groth Strip (EGS). X-ray emission at the locations of these systems is consistent with background. The 3-sigma upper limits on the bolometric X-ray luminosities (L_X) of these systems put a strong constraint on the relation between L_X and the velocity dispersion of member galaxies sigma_gal at z~1; the DEEP2 systems have lower luminosity than would be predicted by the local relation. Our result is consistent with recent findings that at high redshift, optically selected clusters tend to be X-ray underluminous. A comparison with mock catalogs indicates that it is unlikely that this effect is entirely caused by a measurement bias between sigma_gal and the dark matter velocity dispersion. Physically, the DEEP2 systems may still be in the process of forming and hence not fully virialized, or they may be deficient in hot gas compared to local systems. We find only one possibly extended source in this Chandra field, which happens to lie outside the DEEP2 coverage.

astro-ph↗

arXiv2006

The All-wavelength Extended Groth Strip International Survey (AEGIS) Data Sets

In this the first of a series of Letters, we present a description of the panchromatic data sets that have been acquired in the Extended Groth Strip region of the sky. Our survey, the All-wavelength Extended Groth strip International Survey (AEGIS), is intended to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray (0.5 - 10 keV), GALEX ultraviolet (1200 - 2500 Angstrom), CFHT/MegaCam Legacy Survey optical (3600 - 9000 Angstroms), CFHT/CFH12K optical (4500 - 9000 Angstroms), Hubble Space Telescope/ACS optical (4400 - 8500 Angstroms), Palomar/WIRC near-infrared (1.2 - 2.2 microns), Spitzer/IRAC mid-infrared (3.6 - 8.0 microns), Spitzer/MIPS far-infrared (24 - 70 microns), and VLA radio continuum (6 - 20 cm). In addition, this region of the sky has been targeted for extensive spectroscopy using the DEIMOS spectrograph on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

astro-ph↗

arXiv2006

Simulations of thermally broadened HI Lya absorption arising in the warm-hot intergalactic medium

Recent far-ultraviolet (FUV) absorption line measurements of low-redshift quasars have unveiled a population of intervening broad HI Lya absorbers (BLAs) with large Doppler parameters (b> 40 km/s). If the large width of these lines is dominated by thermal line broadening, the BLAs may trace highly-ionized gas in the warm-hot intergalactic medium (WHIM) in the temperature range T ~ 10^5-10^6 K, a gas phase that is expected to contain a large fraction of the baryons at low redshift. In this paper we use a hydrodynamical simulation to study frequency, distribution, physical conditions, and baryon content of the BLAs at z=0. From our simulated spectra we derive a number of BLAs per unit redshift of (dN/dz)_BLA ~ 38 for HI absorbers with log (N(cm^-2)/b(km/s))>10.7, b>40 km/s, and log N(HII)<20.5. The baryon content of these systems is Omega_b(BLA)=0.0121/h_65, which represents ~25 percent of the total baryon budget in our simulation. Our results thus support the idea that BLAs represent a significant baryon reservoir at low redshift. BLAs predominantly trace shock-heated collisionally ionized WHIM gas at temperatures log T~4.4-6.2. About 27 percent of the BLAs in our simulation originate in the photoionized Lya forest (log T<4.3) and their large line widths are determined by non-thermal broadening effects such as unresolved velocity structure and macroscopic turbulence. Our simulation implies that for a large-enough sample of BLAs in FUV spectra it is possible to obtain a reasonable approximation of the baryon content of these systems solely from the measured HI column densities and b values.

astro-ph↗

arXiv2005

Simulation of Soft X-ray Emission Lines from the Missing Baryons

We study the soft X-ray emission (0.1 - 1 keV) from the Warm-Hot Intergalactic Medium (WHIM) in a hydrodynamic simulation of a Cold Dark Matter universe. Our main goal is to investigate how such emission can be explored with a combination of imaging and spectroscopy, and to motivate future X-ray missions. We first present high resolution images of the X-ray emission in several energy bands, in which emission from different ion species dominates. We pick three different areas to study the high resolution spectra of X-rays from the warm-hot IGM: (a) a galaxy group; (b) a filament and (c) an underluminous region. By taking into account the background X-ray emission from AGNs and foreground emission from the Galaxy, we compute composite X-ray spectra of the selected regions. We briefly investigate angular clustering of the soft-X-ray emission, finding a strong signal. Most interestingly, the combination of high spectral resolution and angular information allows us to map the emission from the WHIM in 3 dimensions. We cross-correlate the positions of galaxies in the simulation with this redshift map of emission and detect the presence of six different ion species (Ne IX, Fe XVII, O VII, O VIII, N VII, C VI) in the large-scale structure traced by the galaxies. Finally we show how such emission can be detected and studied with future X-ray satellites, with particular attention to a proposed mission, the Missing Baryon Explorer, or MBE. We present simulated observations of the WHIM gas with MBE.

astro-ph↗

arXiv2004

A Mass Measurement for the Missing Baryons in the Warm-Hot Intergalactic Medium via the X-ray Forest

Recent Cosmological measurements indicate that baryons comprise about four percent of the total mass-energy density of the Universe, which is in accord with the predictions arising from studies of the production of the lightest elements. It also is in agreement with the actual number of baryons detected at early times (redshifts>2). However, close to our own epoch (z<2), the number of baryons actually detected add up to just over half (~55 percent) of the number seen at z>2, meaning that about 45 percent are 'missing'. Here we report a determination of the mass-density of a previously undetected population of baryons, in the warm-hot phase of the intergalactic medium. We show that this mass-density is consistent, within the uncertainties, with the mass-density of the missing baryons.

astro-ph↗

arXiv2004

Probing the Statistics of the Temperature-Density Relation of the IGM

Gravitational instability induces a simple correlation between the large and small scale fluctuations of the Ly-alpha flux spectrum. However, non-gravitational processes involved in structure formation and evolution will alter such a correlation. In this paper we explore how scatter in the temperature-density relation of the IGM reduces the gravitationally induced scale-scale correlation. By examining whether or not observations of the correlation are close to that predicted by pure gravity, this puts constraints on the scatter in the temperature-density relation and in turn on any physical process which would lead to scatter, e.g. strong fluctuations in the UV background or radiative transfer effects. By applying this method to high resolution Keck spectra of Q 1422+231 and HS 1946+7658, we find the predicted correlation signal induced by gravity, and the diminishing of this correlation signal at small scales. This suggests extra physics affects the small-scale structure of the forest, and we can constrain the scatter in the temperature-density relation to a conservative 20% upper limit. A crude model suggests, if there is any spatial correlation of temperature, the coherence length scale must be smaller than ~ 0.3/h Mpc to be consistent with the Keck data.

astro-ph↗

arXiv2002

Chandra Detection of Local OVII He-alpha Absorption Along the Sight Line Toward 3C 273

With the Chandra X-ray Telescope we have detected a zero-redshift OVII He-alpha absorption line along the sight line toward 3C 273. This line detection is highly significant, with a signal-to-noise ratio (S/N) of 6.4. We explore two models, which associate this line with (1) the intragroup gas in the Local Group, and (2) the hot halo gas in the vicinity of our Milky Way. In the first model, we find that for a standard beta-model of the gas distribution in the Local Group, the temperature is constrained to 2.3e5 < T < 1.2e6 K and the baryon overdensity is delta_b ~ 100; both results are consistent with the properties of the so-called warm-hot intergalactic medium (WHIM) predicted by cosmological simulations. We also find the core radius of the gas distribution should be > 100 kpc. In the second model we discuss several possible Galactic origins for the absorption, and we comment on the possibility that the OVII is associated with the OVI absorption observed in this direction by the Far Ultraviolet Spectroscopic Explorer (FUSE). We find that there is a strong indication that collisional ionization is the dominant ionization source for the observed absorption.

astro-ph↗

arXiv2002

Chandra Detection of X-ray Absorption from Local Warm/Hot Gas

Recently, with the Chandra X-ray Telescope we have detected several local X-ray absorption lines along lines-of-sight towards distant quasars. These absorption lines are produced by warm/hot gas located in local intergalactic space and/or in our Galaxy. I will present our observations and discuss the origin of the X-ray absorption and its implications in probing the warm/hot component of local baryons.

astro-ph↗

arXiv2002

Chandra Detection of OVIII Ly-alpha Absorption from an Overdense Region in the Intergalactic Medium

We report the first detection of an OVIII Ly-alpha absorption line associated with an overdense region in the intergalactic medium (IGM) along the sightline towards PKS 2155-304 with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). The absorption line is detected at 4.5 sigma level with cz~16,600 km/s. At the same velocity Shull et al.(1998) detected a small group of spiral galaxies (with an overdensity of delta_gal ~ 100) and low metallicity HI Ly-alpha clouds. We constrain the intragroup gas that gives rise to the OVIII Ly-alpha line to a baryon density in the range 1e-5 < n_b < 7.5e-5 cm^-3 (50 < delta_b < 350) and a temperature of (4-5) x 10^6 K, assuming 0.1 solar abundance. These estimates are in accordance with those of the warm/hot intergalactic medium (WHIM) that are predicted from hydrodynamic simulations. Extrapolating from this single detection implies a large fraction of the ``missing baryons'' (~ 10%, or ~ 30-40% of the WHIM) are probed by the OVIII absorber.

astro-ph↗

arXiv2001

Highly Ionized Absorption in the X-ray Spectrum of Cyg X-1

Using the Chandra High Energy Transmission Grating Spectrometer (HETGS), we have found significant absorption features in the X-ray spectrum of Cyg X-1 taken in the continuous clocking mode. These features include resonance lines of He-like ions of S, Si, and Mg; the Ly alpha lines of H-like S, Si, Mg, and Ne; and several lower ionization lines of Fe XX, XXII, and XXIV. Preliminary analysis shows that the lines are resolved in many cases, giving line widths of order 300 km/s and are redshifted by 460 +/- 10 km/s. These features are interpreted in the context of an accreting stellar wind model that is ionized by the X-ray source. In addition, there are clear absorption features due to neutral Mg, Fe, and O in the interstellar medium.

astro-ph↗

arXiv2001

A Chandra HETG Observation of the Quasar H 1821+643 and Its Surrounding Cluster

We present the high-resolution X-ray spectrum of the low-redshift quasar H 1821+643 and its surrounding hot cluster observed with the Chandra High Energy Transmission Grating Spectrometer (HETGS). An iron emission line attributed to the quasar at ~6.43 keV (rest frame) is clearly resolved, with an equivalent width of ~100 eV. Although we cannot rule out contributions to the line from a putative torus, the diskline model provides an acceptable fit to this iron line. We also detect a weak emission feature at ~6.9 keV (rest frame). We suggest that both lines could originate in an accretion disk comprised of a highly ionized optically thin atmosphere sitting atop a mostly neutral disk. We search for absorption features from a warm/hot component of the intergalactic medium along the ~1.5Gpc/h line of sight to the quasar. No absorption features are detected at or above the 3 sigma level while a total of six OVI intervening absorption systems have been detected with HST and FUSE. Based on the lack of OVII and OVIII absorption lines and by assuming collisionally ionization, we constrain the gas temperature of a typical OVI absorber to 10^5 < T < 10^6 K, which is consistent with the results from hydrodynamic simulations of the intergalactic medium. The zeroth order image reveals the extended emission from the surrounding cluster. We have been able to separate the moderate CCD X-ray spectrum of the surrounding cluster from the central quasar and find that this is a hot cluster with a temperature of ~10 keV and a metal abundance of ~0.3 Zo. We also independently obtain the redshift of the cluster, which is consistent with the optical results. We estimate that the cluster makes negligible contributions to the 6.9 keV iron K line flux.

astro-ph↗

arXiv2001

Probing the Intergalactic Medium with the OVI Forest

Recent STIS and FUSE observations have detected OVI absorption lines at low redshift that are not clearly associated with any galactic system. In this paper, we argue that these lines are due to metal enriched hot gas in the intergalactic medium. Using numerical simulations of a cosmological-constant dominated cosmology, combined with reasonable assumptions about the metallicity distribution, we show that the number density and internal characteristics of these lines are correctly predicted. We find that the OVI is primarily produced by collisional ionization from gas at a few times 10^5 K for lines with equivalent widths larger than 40 mA, while weaker lines can also be produced by photo-ionization. The absorption occurs in diffuse gas in filaments at moderate overdensity (delta ~ 5-100).

astro-ph↗

arXiv2001

Simulating the X-ray Forest

Numerical simulations predict that a large number of baryons reside in intergalactic space at temperatures between 10^5-10^7 K. Highly-ionized metals, such as O VII and O VIII, are good tracers of this ``warm-hot intergalactic medium'', or WHIM. For collisionally-ionized gas, the ionization fraction of each ion peaks at some particular temperature (``peak temperatures''), so different ions can therefore trace the IGM at different temperatures. We performed a hydrodynamic simulation to study the metal distributions in the IGM. We then draw random lines-of-sight across the simulated region and synthesize resonance absorption line spectra in a similar way to simulating the Ly-alpha forest. By studying the distribution functions of H- and He-like O, Si and Fe in a collisionally-ionized IGM and comparing with semi-analytic results based on the Press-Schechter formalism, we find: (1) ions with higher peak temperatures (for instance, Fe XXVI) tend to concentrate around virialized halos, which can be well described by the Press-Schechter distribution, ions with lower peak temperatures are found both in small halos (such as groups of galaxies) and in filaments; (2) lower peak temperature ions are more abundant and should be easily observed; (3) peculiar velocities contribute a significant part to the broadening of the resonant absorption lines.

astro-ph↗

arXiv2001

Simulating the X-ray Forest in a Lambda CDM Universe

Numerical simulations predict that a large number of baryons reside in intergalactic space at temperatures between 10^5-10^7 K. Highly-ionized metals, such as O VII and O VIII, are good tracers of this ``warm-hot intergalactic medium'', or WHIM. For collisionally-ionized gas, the ionization fraction of each ion peaks at some particular temperature (``peak temperatures''), so different ions can therefore trace the IGM at different temperatures. We performed a hydrodynamic simulation to study the metal distributions in the IGM. By studying the distribution functions of H- and He-like O, Si and Fe in a collisionally-ionized IGM and comparing with semi-analytic results based on the Press-Schechter formalism, we find: (1) ions with higher peak temperatures (for instance, Fe XXVI) tend to concentrate around virialized halos, which can be well described by the Press-Schechter distribution, ions with lower peak temperatures are found both in small halos (such as groups of galaxies) and in filaments; (2) lower peak temperature ions are more abundant and should be easily observed; (3) peculiar velocities contribute a significant part to the broadening of the resonant absorption lines.

astro-ph↗