arXiv subjects

Vikram Khaire

Publications and source records attributed to Vikram Khaire.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of Weak O VI Absorption in Underdense Regions of the Low-Redshift Intergalactic Medium

We search for weak O VI absorption in the low-redshift intergalactic medium (IGM) using 82 high signal-to-noise quasar spectra from the Cosmic Origins Spectrograph on the Hubble Space Telescope. From this dataset we compile a clean sample of 396 intervening Lyman-alpha (Lya) lines with H I column densities log N (HI))< 14.5 and no individual O VI detections. Stacking at the location of the O VI doublet reveals absorption at $>5σ$ significance, with equivalent width of 1.7 $\pm$ 0.3 mA, corresponding to log N (O VI) = 12.14 $\pm$ 0.08. The stacked O VI signal associated with strong Lya (13.5 <= log N (HI) < 14.5) absorbers is significantly stronger than that associated with weaker Lya (12.5 <= log N (HI) < 13.5) absorbers. For the subset of 81 broad Lya absorbers (BLAs; b(HI) > 45 km/s), we obtain a marginal $\sim4σ$ O VI detection. Other than Si III, detected at 5$σ$, no metal lines are found. Cross-correlation with galaxies shows that 93% of Lya absorbers lack an associated bright galaxy ($>3L^\ast$) within 1~Mpc. A more complete low-redshift ($z<0.25$) search shows 66% Lya lack galaxies down to $L^\ast$. This suggests the O VI arises mainly from the diffuse IGM, not the circumgalactic medium. The stacked O VI signal suggests characteristic metallicities of $\approx 0.01Z_\odot$ under photoionisation and $\approx 0.001Z_\odot$ under collisional ionisation, though these estimates are model-dependent and assume O VI and H I trace the same phase. This study provides the first observational evidence for metal absorption in low-column-density Lya systems that individually show no detectable metals, placing important constraints on metal enrichment of the underdense IGM.

astro-ph.GA↗

arXiv2026

Non-Primordial Contribution to the Cosmic Microwave Background

The cosmic microwave background is interpreted as relic thermal radiation from the epoch of recombination at z~1090. Recent work by Gjergo & Kroupa (GK25) have challenged this assumption proposing that dust-enshrouded starbursts in the progenitors of massive early-type galaxies at z~15-20 contribute to the CMB at the percent level of the present-day CMB energy density. Existing test of this scenario rely on CMB monopole; here we present the first test against the CMB anisotropy power spectrum, which probes the dynamical imprint of the radiation content on the sound horizon and acoustic peaks. We introduce a single parameter $\varepsilon_{\rm CMB}$ that rescales the primordial photon energy density above a redshift (z=17). Constraining the model with the Planck 2018, lensing likelihoods and BAO measurements, we find a percent-level dust contribution to be fully consistent with the data. When we restrict $\varepsilon_{\rm CMB}\leq1$, the data place a 68 (95) per cent lower bound $\varepsilon_{\rm CMB}\gtrsim 0.971$(0.953), permitting a non-primordial contribution of up to ~3 (~5) per cent of the CMB energy density. When sampled with $\varepsilon_{\rm CMB}$ as a free parameter, the data yield a well-resolved posterior with $\varepsilon_{\rm CMB} = 1.0104^{+0.025}_{-0.024}$. The 1.4 per cent contribution conservatively estimated by GK25 lies within the 1$σ$ region of both cases, whereas a contribution approaching the full CMB energy density is excluded at 42$σ$. Treating z as a free parameter returns a flat posterior across 5<z<50 indicating the anisotropy constrains the dust contribution, but not when it occurs. The standard LCDM parameters are recovered without significant shifts, showing that current CMB precision cosmology has sufficient room to accommodate a dust contribution at the level predicted by GK25 without detectable consequence for parameter inference.

astro-ph.CO↗

arXiv2026

Yuti: A General-purpose Transit Simulator for Arbitrary Shaped Objects Orbiting Stars

We present a versatile transit simulator (Yuti) aimed at generating light curves for arbitrarily shaped objects transiting stars. Utilizing a Monte Carlo algorithm, it accurately models the stellar flux blocked by these objects, producing precise light curves. The simulator adeptly handles realistic background stars, integrating effects such as tidal distortions and limb darkening, alongside the rotational dynamics of transiting objects of arbitrary geometries. We showcase its wide-ranging utility through successful simulations of light curves for single- and multiplanet systems, tidally distorted planets, eclipsing binaries, and exocomets. Additionally, our simulator can simulate light curves for hypothetical alien megastructures of any conceivable shape, providing avenues to identify interesting candidates for follow-up studies. We demonstrate applications of Yuti in modeling a Dyson swarm in construction, Dyson rings, and Dyson disks, discussing how tidally locked Dyson disks can be distinguished from planetary light curves.

astro-ph.EP↗

arXiv2026

Uncertainty-Aware Deep Learning for the Ly$α$ Forest: CNN-Based Absorber Detection and Characterization

The Ly$α$ forest is a powerful probe of the intergalactic medium and small-scale matter distribution, but deriving absorber properties traditionally requires computationally expensive Voigt-profile fitting. We present a convolutional neural network (CNN) that identifies and characterizes H I Ly$α$ absorbers directly from quasar spectra. The model is trained on synthetic spectra generated from the IllustrisTNG simulation and fitted with the VIPER Voigt-profile fitting code to provide training labels. The network simultaneously predicts absorber presence, column density ($N_{\rm HI}$), Doppler parameter ($b_{\rm HI}$), and line centroid. On simulated spectra, the CNN achieves an F1 score of $\sim$0.8, with mean absolute errors of $\sim$0.18 in $\log N_{\rm HI}$ and $\sim$0.10 in $\log b_{\rm HI}$. It accurately reproduces the H I column density distribution function (CDDF) and the $b_{\rm HI}$--$N_{\rm HI}$ relation, recovering CDDF slopes consistent with VIPER and a lower-envelope relation with an RMS difference of only 0.36 km s$^{-1}$. Applied to high-resolution UVES spectra, performance decreases to an F1 score of $\sim$0.5, with mean absolute errors of $\sim$0.34 in $\log N_{\rm HI}$ and $\sim$0.21 in $\log b_{\rm HI}$. Latent-space analysis reveals a significant domain shift between the simulated and observational spectra, contributing to the reduced performance. Nevertheless, the CNN preserves the observed CDDF and $b_{\rm HI}$--$N_{\rm HI}$ distributions, yielding CDDF slopes consistent with VIPER and a lower-envelope RMS difference of 2.96 km s$^{-1}$. Monte Carlo dropout is implemented during inference to quantify predictive uncertainties. Together with its computational efficiency, the method provides a scalable and uncertainty-aware framework for Ly$α$ forest analysis in upcoming spectroscopic surveys.

astro-ph.GA↗

arXiv2026

A Measurement of the Thermal and Ionization State of the IGM at $z < 0.5$

We apply a machine-learning-based inference method that exploits the joint Doppler parameter-column density (b-NHI) distribution from Lya forest decomposition to measure the thermal and ionization state of the intergalactic medium (IGM) in four redshift bins spanning z = 0.06 to 0.48, using 82 archival quasar spectra from the Cosmic Origin Spectrograph (COS) on board Hubble Space Telescope (HST). Our results show that the low-z IGM (z < 0.5) is extremely hot and nearly isothermal, with log(T0/K) = 4.45 (+0.08 / -0.12) [T0 = 28183 (+5700 / -6804) K] and gamma = 1.06 (+0.13 / -0.09) at z = 0.1. This temperature lies approx 7sigma (and 7 times) above the canonical prediction (log T0 approx 3.60, i.e. T0 ~ 4000 K, with gamma ~ 1.6 at z = 0), where the IGM is expected to have cooled long after He II reionization. We also measure the hydrogen photoionization rate to be log (GammaHI/s^-1) = -13.70 (+0.10 / -0.08) at z = 0.1, which is about approx 4sigma below the range predicted by current UV-background synthesis models (approx -13.3). To investigate the discrepancy between these high temperatures and theoretical models, we assess the impact of small-scale turbulence. By exploring a parameter grid in turbulent velocity (vtur) and GammaHI, we find that a standard IGM thermal and ionization state combined with unresolved turbulence of vtur simeq 15 km s^-1 can successfully reproduce the observed line widths at z = 0.1. Comparisons with high-resolution Space Telescope Imaging Spectrograph (STIS) expanded data indicate that the observed line widths are unlikely to be caused by instrumental resolution effects. Our findings suggest that either new heating mechanisms or unresolved turbulence are required to explain the unexpectedly broad Lya lines observed in the low-z IGM.

astro-ph.CO↗

arXiv2026

High-S/N Quasar Observations with HST/COS: Deep Fields for Spectroscopy

Hubble is still in prime observing condition for making transformative discoveries in UV astronomy. In this white paper we describe the science case for a deep (S/N>30) UV spectroscopic survey with HST/COS targeting approximately 20 QSOs at 0.5<z<1.5 at good resolution (20 km/s). This survey would capitalize on our current UV capability, produce a legacy dataset enabling community science in many areas of galactic and extragalactic research, and pioneer a path for future UV science with the Habitable Worlds Observatory. Such high-S/N spectra are largely missing from the MAST archives, and would be analogous to the deep Hubble imaging fields (HDF, UDF, Frontier Fields) that have been enormously successful and far-reaching in their science impact. This legacy dataset would enable frontier science programs in several areas, including (1) studies of the CGM and IGM at unparalleled sensitivity, covering a wide range of UV metal lines and reaching very low H I column densities of log N=12.6 and low metallicities near [Z/H]=-2, enabling precision studies of the chemical abundances, ionization, temperature, and baryon and metal budgets of the CGM and IGM; (2) diffuse gas in the Milky Way and Local Group, including high-velocity clouds and gas streams from satellite mergers; (3) AGN outflows, which would be probed in the rest-frame extreme ultraviolet (EUV), covering continuum-generation mechanisms and diagnostics of gas in accretion-disk outflows.

astro-ph.GA↗

arXiv2025

Reconstructing galactic feedback history via the Lyman-$α$ forest with Habitable Worlds Observatory

Recent studies have focused on the low-$z$ Ly$α$ forest as a potential constraint on galactic feedback, as different AGN and stellar feedback models in hydrodynamic simulations produce varying intergalactic medium (IGM) statistics. However, existing low-$z$ Ly$α$ forest data provide insufficient observational constraints for simulations due to their low precision and the lack of observations from $z \sim 0.4$ to $1.8$. The Habitable Worlds Observatory, equipped with an ultraviolet (FUV / NUV) spectrograph, could provide transformative data for this science by increasing both absorbers in the redshift range where current data are lacking and increasing the precision of the Ly$α$ forest observational data at $z \leq 0.4$. This spectrograph should cover the wavelengths $1215-3402$ Å~and have a spectral resolution of $R =$~40,000. Distinguishing between certain active-galactic nuclei feedback models requires high precision ($\sim 5-10$\%) measurements of the Ly$α$ forest 1D transmitted flux power spectrum for $z\leq 1.8$. This requires $\sim 830$ QSO spectra with S/N~$\geq5$ - $25$ depending on redshift. This data would also enable a comprehensive study on the thermal state of the IGM across time, and address the tension between the observed and simulated Ly$α$ forest $b$-value distribution.

astro-ph.GA↗

arXiv2025

The SAP-1 Payload: A Technology Demonstration for Space-Based Microbiology Experiments

The SSPACE Astrobiology Payload (SAP) series, starting with the SAP-1 project is designed to conduct in-situ microbiology experiments in low earth orbit. This payload series aims to understand the behaviour of microbial organisms in space, particularly those critical for human health, and the corresponding effects due to microgravity and solar/galactic radiation. SAP-1 focuses on studying Bacillus clausii and Bacillus coagulans, bacteria beneficial to humans. It aims to provide a space laboratory for astrobiology experiments under microgravity conditions. The hardware developed for these experiments is indigenous and tailored to meet the unique requirements of autonomous microbiology experiments by controlling pressure, temperature, and nutrition flow to bacteria. A rotating platform, which forms the core design, is innovatively utilised to regulate the flow and mixing of nutrients with dormant bacteria. The technology demonstration models developed at SSPACE have yielded promising results, with ongoing efforts to refine, adapt for space conditions, and prepare for integration with nanosatellites or space modules. The anticipated payload will be compact, approximately 1U in size (10cm x 10cm x 10cm), consume less than 5W power, and offer flexibility for various microbiological studies.

astro-ph.IM↗

arXiv2024

Lyman Limit System with O VI in the Circumgalactic Environment of a Pair of Galaxies

We report on the analysis of a multiphase Lyman limit system (LLS) at $z=0.39047$ identified towards the background quasar FBQS J0209-0438. The O VI doublet lines associated with this absorber have a different profile from the low ionization metals and the H I. The Ly$α$ has a very broad H I ($b \approx 150$ km s$^{-1}$) component well-aligned with one of the O VI components. The Doppler $b$-parameters for the broad H I and O VI indicate gas with $T= (0.8-2.0)\times 10^6$ K, and a total hydrogen column density that is an order of magnitude larger than the cooler phase of gas responsible for the LLS. Observations by VLT/MUSE show two moderately star-forming galaxies within $ρ\lesssim 105$ kpc, and $|Δv|\lesssim 130$ km s$^{-1}$ of the absorber, one of them a dwarf galaxy ($M_*\approx 10^6$ M$_\odot$) overlapping with the quasar PSF, and the other a larger galaxy ($R_{1/2}\approx 4$ kpc) with $M_*\approx 3\times 10^{10}$ M$_\odot$ and $M_h\approx 7\times 10^{11}$ M$_\odot$, and the dwarf galaxy within its virial radius. Though the absorption is aligned with the extended major axis of the larger galaxy, the line-of-sight velocity of the absorbing gas is inconsistent with corotating accretion. The metallicity inferred for the LLS is lower than the gas phase [O/H] of the two galaxies. The mixture of cool and warm/hot gas phases for the absorbing gas and its proximity and orientation to the galaxy pair points to the LLS being a high-velocity gas in the combined halo environment of both galaxies.

astro-ph.GA↗

arXiv2024

FLAME: Fitting Ly$α$ Absorption lines using Machine learning

We introduce FLAME, a machine-learning algorithm designed to fit Voigt profiles to HI Lyman-alpha (Ly$α$) absorption lines using deep convolutional neural networks. FLAME integrates two algorithms: the first determines the number of components required to fit Ly$α$ absorption lines, and the second calculates the Doppler parameter $b$, the HI column density N$_{\rm HI}$, and the velocity separation of individual components. For the current version of FLAME, we trained it on low-redshift Ly$α$ forests observed with the far-ultraviolet gratings of the Cosmic Origin Spectrograph (COS) on board the Hubble Space Telescope (HST). Using these data, we trained FLAME on $\sim$ $10^6$ simulated Voigt profiles which we forward-modeled to mimic Ly$α$ absorption lines observed with HST-COS in order to classify lines as either single or double components and then determine Voigt profile-fitting parameters. FLAME shows impressive accuracy on the simulated data, identifying more than 98\% (90\%) of single (double) component lines. It determines $b$ values within $\approx \pm{8}~(15)$ km s$^{-1}$ and log $N_{\rm HI}/ {\rm cm}^2$ values within $\approx \pm 0.3~(0.8)$ for 90\% of the single (double) component lines. However, when applied to real data, FLAME's component classification accuracy drops by $\sim$ 10\%. Nevertheless, there is reasonable agreement between the $b$ and N$_{\rm HI}$ distributions obtained from traditional Voigt profile-fitting methods and FLAME's predictions. Our mock HST-COS data analysis, designed to emulate real data parameters, demonstrates that FLAME is able to achieve consistent accuracy comparable to its performance with simulated data. This finding suggests that the drop in FLAME's accuracy when used on real data primarily arises from the difficulty in replicating the full complexity of real data in the training sample.

astro-ph.CO↗

arXiv2024

Measurements of the Thermal and Ionization State of the Intergalactic Medium during the Cosmic Afternoon

We perform the first measurement of the thermal and ionization state of the intergalactic medium (IGM) across 0.9 < z < 1.5 using 301 \lya absorption lines fitted from 12 HST STIS quasar spectra, with a total pathlength of Δz=2.1. We employ the machine-learning-based inference method that uses joint b-N distributions obtained from \lyaf decomposition. Our results show that the HI photoionization rates, Γ, are in good agreement with the recent UV background synthesis models, with \log (Γ/s^{-1})={-11.79}^{0.18}_{-0.15}, -11.98}^{0.09}_{-0.09}, and {-12.32}^{0.10}_{-0.12} at z=1.4, 1.2, and 1 respectively. We obtain the IGM temperature at the mean density, T_0, and the adiabatic index, γ, as [\log (T_0/K), γ]= [{4.13}^{+0.12}_{-0.10}, {1.34}^{+0.10}_{-0.15}], [{3.79}^{+0.11}_{-0.11}, {1.70}^{+0.09}_{-0.09}] and [{4.12}^{+0.15}_{-0.25}, {1.34}^{+0.21}_{-0.26}] at z=1.4, 1.2 and 1 respectively. Our measurements of T_0 at z=1.4 and 1.2 are consistent with the expected trend from z<3 temperature measurements as well as theoretical expectations that, in the absence of any non-standard heating, the IGM should cool down after HeII reionization. Whereas, our T_0 measurements at z=1 show unexpectedly high IGM temperature. However, because of the relatively large uncertainty in these measurements of the order of ΔT_0~5000 K, mostly emanating from the limited redshift path length of available data in these bins, we can not definitively conclude whether the IGM cools down at z<1.5. Lastly, we generate a mock dataset to test the constraining power of future measurement with larger datasets. The results demonstrate that, with redshift pathlength Δz \sim 2 for each redshift bin, three times the current dataset, we can constrain the T_0 of IGM within 1500K. Such precision would be sufficient to conclusively constrain the history of IGM thermal evolution at z < 1.5.

astro-ph.CO↗

arXiv2023

Can the Low Redshift Lyman Alpha Forest Constrain AGN Feedback Models?

We investigate the potential of low-redshift Lyman alpha (Ly$α$) forest for constraining active galactic nuclei (AGN) feedback models by analyzing the Illustris and IllustrisTNG simulation at z=0.1. These simulations are ideal for studying the impact of AGN feedback on the intergalactic medium (IGM) as they share initial conditions with significant differences in the feedback prescriptions. Both simulations reveal that the IGM is significantly impacted by AGN feedback. Specifically, feedback is stronger in Illustris and results in reducing cool baryon fraction to 23% relative to 39% in IllustrisTNG. However, when comparing various statistics of Ly$α$ forest such as 2D and marginalized distributions of Doppler widths and H I column density, line density, and flux power spectrum with real data, we find that most of these statistics are largely insensitive to the differences in feedback models. This lack of sensitivity arises because of the fundamental degeneracy between the fraction of cool baryons and the H I photoionization rate ($Γ_{\rm HI}$) as their product determines the optical depth of the Ly$α$ forest. Since the $Γ_{\rm HI}$ cannot be precisely predicted from first principles, it needs to be treated as a nuisance parameter adjusted to match the observed Ly$α$ line density. After adjusting $Γ_{\rm HI}$, the distinctions in the considered statistics essentially fade away. Only the Ly$α$ flux power spectrum at small spatial scales exhibits potentially observable differences, although this may be specific to the relatively extreme feedback model employed in Illustris. Without independent constraints on either $Γ_{\rm HI}$ or cool baryon fraction, constraining AGN feedback with low-redshift Ly$α$ forest will be very challenging.

astro-ph.CO↗

arXiv2023

Searching for the Imprints of AGN Feedback on the Lyman Alpha Forest Around Luminous Red Galaxies

We explore the potential of using the low-redshift Lyman-$α$ (Ly$α$) forest surrounding luminous red galaxies (LRGs) as a tool to constrain active galactic nuclei (AGN) feedback models. Our analysis is based on snapshots from the Illustris and IllustrisTNG simulations at a redshift of $z=0.1$. These simulations offer an ideal platform for studying the influence of AGN feedback on the gas surrounding galaxies, as they share the same initial conditions and underlying code but incorporate different feedback prescriptions. Both simulations show significant impacts of feedback on the temperature and density of the gas around massive halos. Following our previous work, we adjusted the UV background in both simulations to align with the observed number density of Ly$α$ lines ($\rm dN/dz$) in the intergalactic medium and study the Ly$α$ forest around massive halos hosting LRGs, at impact parameters ($r_{\perp}$) ranging from 0.1 to 100 pMpc. Our findings reveal that $\rm dN/dz$, as a function of $r_{\perp}$, is approximately 1.5 to 2 times higher in IllustrisTNG compared to Illustris up to $r_{\perp}$ of $\sim 10$ pMpc. To further assess whether existing data can effectively discern these differences, we search for archival data containing spectra of background quasars probing foreground LRGs. Through a feasibility analysis based on this data, we demonstrate that ${\rm dN/dz} (r_{\perp})$ measurements can distinguish between feedback models of IllustrisTNG and Illustris with a precision exceeding 12$σ$. This underscores the potential of ${\rm dN/dz} (r_{\perp})$ measurements around LRGs as a valuable benchmark observation for discriminating between different feedback models.

astro-ph.GA↗

arXiv2023

The Impact of the WHIM on the IGM Thermal State Determined from the Low-$z$ Lyman-$α$ Forest

At $z \lesssim 1$, shock heating caused by large-scale velocity flows and possibly violent feedback from galaxy formation, converts a significant fraction of the cool gas ($T\sim 10^4$ K) in the intergalactic medium (IGM) into warm-hot phase (WHIM) with $T >10^5$K, resulting in a significant deviation from the previously tight power-law IGM temperature-density relationship, $T=T_0 (ρ/ {\barρ})^{γ-1}$. This study explores the impact of the WHIM on measurements of the low-$z$ IGM thermal state, $[T_0,γ]$, based on the $b$-$N_{H I}$ distribution of the Lyman-$α$ forest. Exploiting a machine learning-enabled simulation-based inference method trained on Nyx hydrodynamical simulations, we demonstrate that [$T_0$, $γ$] can still be reliably measured from the $b$-$N_{H I}$ distribution at $z=0.1$, notwithstanding the substantial WHIM in the IGM. To investigate the effects of different feedback, we apply this inference methodology to mock spectra derived from the IllustrisTNG and Illustris simulations at $z=0.1$. The results suggest that the underlying $[T_0,γ]$ of both simulations can be recovered with biases as low as $|Δ\log(T_0/\text{K})| \lesssim 0.05$ dex, $|Δγ| \lesssim 0.1$, smaller than the precision of a typical measurement. Given the large differences in the volume-weighted WHIM fractions between the three simulations (Illustris 38\%, IllustrisTNG 10\%, Nyx 4\%) we conclude that the $b$-$N_{H I}$ distribution is not sensitive to the WHIM under realistic conditions. Finally, we investigate the physical properties of the detectable Lyman-$α$ absorbers, and discover that although their $T$ and $Δ$ distributions remain mostly unaffected by feedback, they are correlated with the photoionization rate used in the simulation.

astro-ph.CO↗

arXiv2022

Measuring the thermal and ionization state of the low-$z$ IGM using likelihood free inference

We present a new approach to measure the power-law temperature density relationship $T=T_0 (ρ/ \barρ)^{γ-1}$ and the UV background photoionization rate $Γ_{\rm HI}$ of the IGM based on the Voigt profile decomposition of the Ly$α$ forest into a set of discrete absorption lines with Doppler parameter $b$ and the neutral hydrogen column density $N_{\rm HI}$. Previous work demonstrated that the shape of the $b$-$N_{\rm HI}$ distribution is sensitive to the IGM thermal parameters $T_0$ and $γ$, whereas our new inference algorithm also takes into account the normalization of the distribution, i.e. the line-density d$N$/d$z$, and we demonstrate that precise constraints can also be obtained on $Γ_{\rm HI}$. We use density-estimation likelihood-free inference (DELFI) to emulate the dependence of the $b$-$N_{\rm HI}$ distribution on IGM parameters trained on an ensemble of 624 Nyx hydrodynamical simulations at $z = 0.1$, which we combine with a Gaussian process emulator of the normalization. To demonstrate the efficacy of this approach, we generate hundreds of realizations of realistic mock HST/COS datasets, each comprising 34 quasar sightlines, and forward model the noise and resolution to match the real data. We use this large ensemble of mocks to extensively test our inference and empirically demonstrate that our posterior distributions are robust. Our analysis shows that by applying our new approach to existing Ly$α$ forest spectra at $z\simeq 0.1$, one can measure the thermal and ionization state of the IGM with very high precision ($σ_{\log T_0} \sim 0.08$ dex, $σ_γ\sim 0.06$, and $σ_{\log Γ_{\rm HI}} \sim 0.07$ dex).

astro-ph.CO↗

arXiv2021

How Robust are the Inferred Density and Metallicity of the Circumgalactic Medium?

Quantitative estimates of the basic properties of the circumgalactic medium(CGM), such as its density and metallicity, depend on the spectrum of incident UV background radiation. Models of UV background are known to have large variations, mainly because they are synthesized using poorly constrained parameters, which introduce uncertainty in the inferred properties of the CGM. Here, we quantify this uncertainty using a large set of new UV background models with physically motivated toy models of metal-enriched CGM. We find that, the inferred density and metallicity of low-density ($10^{-5}$ cm$^{-3}$) gas is uncertain by factors of 6.3 and 3.2, whereas high density ($10^{-3}$ cm$^{-3}$) gas by factors of 4 and 1.6, respectively. The variation in the shape of the UV background models is entirely responsible for such a variation in the metallicity while variation in the density arises from both normalization and shape of the UV background. Moreover, we find a harder (softer) UV background infers higher (lower) density and metallicity. We also study warm-hot gas at $T= 10^{5.5}$ K and find that metallicity is robustly estimated but the inferred density is uncertain by a factor of 3 to 5.4 for low to high-density gas. Such large uncertainties in density and metallicity may severely limit the studies of the CGM and demand better observational constraints on the input parameters used in synthesizing UV background.

astro-ph.GA↗

arXiv2020

Physical Conditions of Five O VI Absorption Systems Towards PG $1522+101$

We present the analysis of five O VI absorbers identified across a redshift path of z $\sim (0.6 - 1.3)$ towards the background quasar PG $1522+101$ with information on five consecutive ionization stages of oxygen from O II to O VI. The combined $HST$ and $Keck$ spectra cover UV, redshifted EUV, and optical transitions from a multitude of ions spanning ionization energies in the range of $\sim (13 - 300)$ eV. Low ionization (C II, O II, Si II, Mg II) and very high ionization species (Ne VIII, Mg X) are non-detections in all the absorbers. Three of the absorbers have coverage of He I, in one of which it is a $> 3 σ$ detection. The kinematic structures of these absorbers are extracted from C IV detected in $HIRES$ spectra. The farthest absorber in our sample also contains the detections of Ne V and Ne VI. Assuming co-spatial absorbing components, the ionization models show the medium to be multiphased with small-scale density-temperature inhomogeneities that are sometimes kinematically unresolved. In two of the absorbers, there is an explicit indication of the presence of a warm gas phase ($T \gtrsim 10^5$ K) traced by O VI. In the remaining absorbers, the column densities of the ions are consistent with a non-uniform photoionized medium. The sub-solar [C/O] relative abundances inferred for the absorbers point at enrichment from massive Type II supernovae. Despite metal enrichment, the inferred wide range for [O/H] $\sim$ [$-2.1, +0.2$] amongst the absorbers along with their anti-correlation with the observed H I suggest poor small-scale mixing of metals with hydrogen in the regions surrounding galaxies and the IGM.

astro-ph.GA↗

arXiv2020

Solar-Metallicity Gas in the Extended Halo of a Galaxy at $z \sim 0.12$

We present the detection and analysis of a weak low-ionization absorber at $z = 0.12122$ along the blazar sightline PG~$1424+240$, using spectroscopic data from both $HST$/COS and STIS. The absorber is a weak Mg II analogue, with incidence of weak C II and Si II, along with multi-component C IV and O VI. The low ions are tracing a dense ($n_{H} \sim 10^{-3}$ cm$^{-3}$) parsec scale cloud of solar or higher metallicity. The kinematically coincident higher ions are either from a more diffuse ($n_{H} \sim 10^{-5} - 10^{-4}$ cm$^{-3}$) photoionized phase of kiloparsec scale dimensions, or are tracing a warm (T $\sim 2 \times 10^{5}$ K) collisionally ionized transition temperature plasma layer. The absorber resides in a galaxy overdense region, with 18 luminous ($> L^*$) galaxies within a projected radius of $5$ Mpc and $750$ km s$^{-1}$ of the absorber. The multi-phase properties, high metallicity and proximity to a $1.4$ $L^*$ galaxy, at $ρ\sim 200$ kpc and $|Δv| = 11$ km s$^{-1}$ separation, favors the possibility of the absorption tracing circumgalactic gas. The absorber serves as an example of weak Mg II - O VI systems as a means to study multiphase high velocity clouds in external galaxies.

astro-ph.GA↗