arXiv subjects

Rongmon Bordoloi

Publications and source records attributed to Rongmon Bordoloi.

At least 73 records · Page 4Linked to original sources

arXiv2020

Revealing the Dark Threads of the Cosmic Web

Modern cosmology predicts that matter in our Universe has assembled today into a vast network of filamentary structures colloquially termed the Cosmic Web. Because this matter is either electromagnetically invisible (i.e., dark) or too diffuse to image in emission, tests of this cosmic web paradigm are limited. Wide-field surveys do reveal web-like structures in the galaxy distribution, but these luminous galaxies represent less than 10% of baryonic matter. Statistics of absorption by the intergalactic medium (IGM) via spectroscopy of distant quasars support the model yet have not conclusively tied the diffuse IGM to the web. Here, we report on a new method inspired by the Physarum polycephalum slime mold that is able to infer the density field of the Cosmic Web from galaxy surveys. Applying our technique to galaxy and absorption-line surveys of the local Universe, we demonstrate that the bulk of the IGM indeed resides in the Cosmic Web. From the outskirts of Cosmic Web filaments, at approximately the cosmic mean matter density (rho_m) and approx. 5 virial radii from nearby galaxies, we detect an increasing H I absorption signature towards higher densities and the circumgalactic medium, to approx. 200 rho_m. However, the absorption is suppressed within the densest environments, suggesting shock-heating and ionization deep within filaments and/or feedback processes within galaxies.

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arXiv2020

Project AMIGA: The Circumgalactic Medium of Andromeda

Project AMIGA (Absorption Maps In the Gas of Andromeda) is a large ultraviolet Hubble Space Telescope program, which has assembled a sample of 43 QSOs that pierce the circumgalactic medium (CGM) of Andromeda (M31) from R=25 to 569 kpc (25 of them probing gas from 25 kpc to about the virial radius-Rvir = 300 kpc-of M31). Our large sample provides an unparalleled look at the physical conditions and distribution of metals in the CGM of a single galaxy using ions that probe a wide range of gas phases (Si II, Si III, Si IV, C II, C IV, and O VI, the latter being from the Far Ultraviolet Spectroscopic Explorer). We find that Si III and O VI have near unity covering factor maintained all the way out to 1.2Rvir and 1.9Rvir, respectively. We show that Si III is the dominant ion over Si II and Si IV at any R. While we do not find that the properties of the CGM of M31 depend strongly on the azimuth, we show that they change remarkably around 0.3-0.5Rvir, conveying that the inner regions of the CGM of M31 are more dynamic and have more complicated multi-phase gas-structures than at R>0.5Rvir. We estimate the metal mass of the CGM within Rvir as probed by Si II, Si III, and Si IV is 2x10^7 Msun and by O VI is >8x10^7 Msun, while the baryon mass of the 10^4-10^5.5 K gas is ~4x10^10 (Z/0.3 Zsun)^(-1) Msun within Rvir. We show that different zoom-in cosmological simulations of L* galaxies better reproduce the column density profile of O VI with R than Si III or the other studied ions. We find that observations of the M31 CGM and zoom-in simulations of L* galaxies have both lower ions showing higher column density dispersion and dependence on R than higher ions, indicating that the higher ionization structures are larger and/or more broadly distributed.

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arXiv2019

The low density and magnetization of a massive galaxy halo exposed by a fast radio burst

Present-day galaxies are surrounded by cool and enriched halo gas extending to hundreds of kiloparsecs. This halo gas is thought to be the dominant reservoir of material available to fuel future star formation, but direct constraints on its mass and physical properties have been difficult to obtain. We report the detection of a fast radio burst (FRB 181112) with arcsecond precision, which passes through the halo of a foreground galaxy. Analysis of the burst shows the halo gas has low net magnetization and turbulence. Our results imply predominantly diffuse gas in massive galactic halos, even those hosting active supermassive black holes, contrary to some previous results.

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arXiv2019

The Mass Inflow and Outflow Rates of the Milky Way

We present new calculations of the mass inflow and outflow rates around the Milky Way, derived from a catalog of ultraviolet metal-line high velocity clouds (HVCs). These calculations are conducted by transforming the HVC velocities into the Galactic Standard of Rest (GSR) reference frame, identifying inflowing (v_GSR < 0 km/s) and outflowing (v_GSR > 0 km/s) populations, and using observational constraints on the distance, metallicity, dust content, covering fractions, and total hydrogen column density of each population. After removing HVCs associated with the Magellanic Stream and the Fermi Bubbles, we find inflow and outflow rates in cool (T~10^4 K) ionized gas of dM_in/dt >~ 0.53+/-0.17 (d/12 kpc) (Z/0.2 Z_sun)^-1 M_sun/yr and dM_out/dt >~ 0.16+/-0.06 (d/12 kpc) (Z/0.5 Z_sun)^-1 M_sun/yr. The excess of inflowing over outflowing gas suggests that the Milky Way is currently in an inflow-dominated phase, but the presence of substantial mass flux in both directions supports a Galactic fountain model, in which gas is constantly recycled between the disk and the halo. We also find that the metal flux in both directions (in and out) is indistinguishable. By comparing the outflow rate to the Galactic star formation rate, we present the first estimate of the mass loading factor (etc_HVC) of the disk-wide Milky Way wind, finding eta_HVC >~ 0.10+/-0.06 (d/12 kpc) (Z/0.5 Z_sun)^-1. Including the contributions from low- and intermediate-velocity clouds and from hot gas would increase these inflow and outflow estimates.

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arXiv2019

The COS Absorption Survey of Baryon Harbors (CASBaH): Warm-hot Circumgalactic Gas Reservoirs Traced by Ne VIII Absorption

We survey the highly ionized circumgalactic media (CGM) of 29 blindly selected galaxies at 0.49 < z_(gal) < 1.44 based on high-S/N ultraviolet spectra of z > 1 QSOs and the galaxy database from the COS Absorption Survey of Baryon Harbors (CASBaH). We detect the Ne VIII doublet in nine of the galaxies, and for gas with N(Ne VIII) > 10^13.3 cm^-2 (> 10^13.5 cm^-2), we derive a Ne VIII covering fraction f_c = 75 +15/-25% (44 +22/-20%) within impact parameter (rho) < 200 kpc of M_* = 10^(9.5-11.5) Msol galaxies and f_c = 70 +16/-22% (f_c = 42 +20/-17%) within rho < 1.5 virial radii. We estimate the mass in Ne VIII-traced gas to be M_gas(Ne VIII) > 10^9.5 Msol (Z/Zsol)^-1, or 6-20% of the expected baryonic mass if the Ne VIII absorbers have solar metallicity. Ionizing Ne VII to Ne VIII requires 207 eV, and photons with this energy are scarce in the CGM. However, for the median halo mass and redshift of our sample, the virial temperature is close to the peak temperature for the Ne VIII ion, and the Ne VIII-bearing gas is plausibly collisionally ionized near this temperature. Moreover, we find that photoionized Ne VIII requires cool and low-density clouds that would be highly underpressured (by approximately two orders of magnitude) relative to the putative, ambient virialized medium, complicating scenarios where such clouds could survive. Thus, more complex (e.g., non-equilibrium) models may be required; this first statistical sample of Ne VIII absorber/galaxy systems will provide stringent constraints for future CGM studies.

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arXiv2019

The production and escape of ionizing photons from galaxies over cosmic time (Astro2020 Science White Paper)

The ionizing photons produced by massive stars are key actors in galaxy evolution. Ionizing photon production and escape is poorly understood. Improved space-based, spatially-resolved, multiplexed spectroscopic capabilities covering observed wavelengths of 1000 to 3000 Angstrom, in concert with spectroscopy from the ELTs and JWST, would lead to definitive answers as to how ionizing photons are produced and leaked, what populations of galaxies are responsible for ionizing photon leakage, what determines whether escape is possible, and how ionizing galaxy populations evolve over cosmic time.

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arXiv2019

Astro2020 Science White Paper: Spatially Resolved UV Nebular Diagnostics in Star-Forming Galaxies

Diagnosing the physical and chemical conditions within star-forming galaxies (SFGs) is of paramount importance to understanding key components of galaxy formation and evolution: star-formation, gas enrichment, outflows, and accretion. Well established optical emission-line diagnostics used to discern such properties (i.e., metal content, density, strength/shape of ionizing radiation) will be observationally inaccessible for the earliest galaxies, emphasizing the need for robust, reliable interstellar medium (ISM) diagnostics at ultraviolet (UV) wavelengths. Calibrating these UV diagnostics requires a comprehensive comparison of the UV and optical emission lines in nearby SFGs. Optical integral field unit (IFU) surveys have revealed the inhomogeneous nature of the ISM in SFGs, which leads to non-systematic biases in the interpretation of unresolved sources. Spatial variations are especially important to consider at UV wavelengths, where the strongest emission features originate from only the highest excitation regions of the nebula and are challenging to distinguish from competing high-ionization sources (e.g., shocks, AGN, etc.). Since surveys collecting large-scale optical integral field unit (IFU) spectroscopy are already underway, this white paper calls for an IFU or multi-object far-UV (FUV) spectroscopic instrument with high sensitivity, high spatial resolution, and large field of view (FoV). Given the impact of large-scale optical IFU surveys over the past decade, this white paper emphasizes the scientific need for a comparable foundation of spatially-resolved far-UV spectroscopy survey of nearby galaxies that will lay the foundation of diagnostics critical to the interpretation of the distant universe.

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arXiv2019

Ultraviolet Perspectives on Diffuse Gas in the Largest Cosmic Structures

The past decade has seen an explosion of discoveries and new insights into the diffuse gas within galaxies, galaxy clusters, and the filaments composing the Cosmic Web. A new decade will bring fresh opportunities to further this progress towards developing a comprehensive view of the composition, thermal state, and physical processes of diffuse gas in the Universe. Ultraviolet (UV) spectroscopy, probing diffuse 10^4-10^6 K gas at high spectral resolution, is uniquely poised to (1) witness environmental galaxy quenching processes in action, such as strangulation and tidal- and ram-pressure stripping, (2) directly account for the baryon content of galaxy clusters in the cold-warm (T<10^6 K) gas, (3) determine the phase structure and kinematics of gas participating in the equilibrium-regulating exchange of energy at the cores of galaxy clusters, and (4) map cold streams and filaments of the Cosmic Web that feed galaxies and clusters. With a substantial UV undertaking beyond the Hubble Space Telescope, all of the above would be achievable over the entire epoch of galaxy cluster formation. Such capabilities, coupled with already-planned advancements at other wavelengths, will transform extragalactic astronomy by revealing the dominant formation and growth mechanisms of gaseous halos over the mass spectrum, settling the debate between early- and late-time metal enrichment scenarios, and revealing how the ecosystems in which galaxies reside ultimately facilitate their demise.

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arXiv2019

Understanding the circumgalactic medium is critical for understanding galaxy evolution

Galaxies evolve under the influence of gas flows between their interstellar medium and their surrounding gaseous halos known as the circumgalactic medium (CGM). The CGM is a major reservoir of galactic baryons and metals, and plays a key role in the long cycles of accretion, feedback, and recycling of gas that drive star formation. In order to fully understand the physical processes at work within galaxies, it is therefore essential to have a firm understanding of the composition, structure, kinematics, thermodynamics, and evolution of the CGM. In this white paper we outline connections between the CGM and galactic star formation histories, internal kinematics, chemical evolution, quenching, satellite evolution, dark matter halo occupation, and the reionization of the larger-scale intergalactic medium in light of the advances that will be made on these topics in the 2020s. We argue that, in the next decade, fundamental progress on all of these major issues depends critically on improved empirical characterization and theoretical understanding of the CGM. In particular, we discuss how future advances in spatially-resolved CGM observations at high spectral resolution, broader characterization of the CGM across galaxy mass and redshift, and expected breakthroughs in cosmological hydrodynamic simulations will help resolve these major problems in galaxy evolution.

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arXiv2019

Spectroscopic Observations of the Fermi Bubbles

Two giant plasma lobes, known as the Fermi Bubbles, extend 10 kpc above and below the Galactic Center. Since their discovery in X-rays in 2003 (and in gamma-rays in 2010), the Bubbles have been recognized as a new morphological feature of our Galaxy and a striking example of energetic feedback from the nuclear region. They remain the subject of intense research and their origin via AGN activity or nuclear star formation is still debated. While imaging at gamma-ray, X-ray, microwave, and radio wavelengths has revealed their morphology and energetics, spectroscopy at radio and UV wavelengths has recently been used to study the kinematics and chemical abundances of outflowing gas clouds embedded in the Bubbles (the nuclear wind). Here we identify the scientific themes that have emerged from the spectroscopic studies, determine key open questions, and describe further observations needed in the next ten years to characterize the basic physical conditions in the nuclear wind and its impact on the rest of the Galaxy. Nuclear winds are ubiquitous in galaxies, and the Galactic Center represents the best opportunity to study the constitution and structure of a nuclear wind in close detail.

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arXiv2018

On the CGM Fundamental Plane: The Halo Mass Dependency of Circumgalactic HI

We analyze the equivalent widths of HI Ly-$α$ ($W_{Lyα}$) absorption from the inner (R < 160 kpc) circumgalactic medium (CGM) of 85 galaxies at $z \sim 0$ with stellar masses $M*$ ranging $\rm{8 \leq log M* / M_{\odot} \leq 11.6}$. Across three orders of magnitude in stellar mass, the CGM of present-day galaxies exhibits a very high covering fraction of cool hydrogen gas ($f_C = 87\pm 4$\%) indicating that the CGM is ubiquitous in modern, isolated galaxies. When HI Ly-$α$ is detected, its equivalent width declines with increasing radius regardless of the galaxy mass, but the scatter in this trend correlates closely with $M*$. Using the radial and stellar mass correlations, we construct a planar surface describing the cool CGM of modern galaxies: $\log W^{\rm{s}}_{HI 1215} \; = \; (0.34 \pm 0.02) -( 0.0026 \pm 0.0005)\times (R) + (0.286 \pm 0.002) \times \log (M*/M_{\odot})$. The RMS scatter around this bivariate relation is $\sim$0.2 dex. We interpret the explicit correlation between $W_{Lyα}$ and $M*$ to arise from the underlying dark matter halo mass ($M_{halo}$), thereby suggesting a CGM fundamental plane between $W_{Lyα}$, $R$ and $M_{halo}$. This correlation can be used to estimate the underlying dark matter halo mass from observations of saturated HI Ly-$α$ in the CGM of a modern galaxy.

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arXiv2018

The COS-AGN survey: Revealing the nature of circum-galactic gas around hosts of active galactic nuclei

Active galactic nuclei (AGN) are thought to play a critical role in shaping galaxies, but their effect on the circumgalactic medium (CGM) is not well studied. We present results from the COS-AGN survey: 19 quasar sightlines that probe the CGM of 20 optically-selected AGN host galaxies with impact parameters $80 < ρ_{imp} < 300$ kpc. Absorption lines from a variety of species are measured and compared to a stellar mass and impact parameter matched sample of sightlines through non-AGN galaxies. Amongst the observed species in the COS-AGN sample (HI, CII, SiII, SiIII, CIV, SiIV, NV), only Ly$α$ shows a high covering fraction ($94^{+6}_{-23}$% for rest-frame equivalent widths EW $> 124$ mÅ) whilst many of the metal ions are not detected in individual sightlines. A sightline-by-sightline comparison between COS-AGN and the control sample yields no significant difference in EW distribution. However, stacked spectra of the COS-AGN and control samples show significant (> 3 sigma) enhancements in the EW of both Ly$α$ and SiIII at impact parameters $> 164$ kpc by a factor of $+0.45\pm0.05$ dex and $> +0.75$ dex respectively. The lack of detections of both high-ionization species near the AGN and strong kinematic offsets between the absorption systemic galaxy redshifts indicates that neither the AGN's ionization nor its outflows are the origin of these differences. Instead, we suggest the observed differences could result from either AGN hosts residing in haloes with intrinsically distinct gas properties, or that their CGM has been affected by a previous event, such as a starburst, which may also have fuelled the nuclear activity.

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arXiv2018

Probing the Southern Fermi Bubble in Ultraviolet Absorption using Distant AGNs

The Fermi Bubbles are two giant gamma-ray emitting lobes extending 55$^{\circ}$ above and below the Galactic Center. While the Northern Bubble has been extensively studied in ultraviolet (UV) absorption, little is known about the gas kinematics of the southern Bubble. We use UV absorption-line spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to probe the southern Fermi Bubble using a sample of 17 background AGN projected behind or near the Bubble. We measure the incidence of high-velocity clouds (HVC), finding that four out of six sightlines passing through the Bubble show HVC absorption, versus six out of eleven passing outside. We find strong evidence that the maximum absolute LSR velocity of the HVC components decreases as a function of galactic latitude within the Bubble, for both blueshifted and redshifted components, as expected for a decelerating outflow. We explore whether the column-density ratios SiIV/SiIII, SiIV/SiII and SiIII/SiII correlate with the absolute galactic latitude within the Bubble. These results demonstrate the use of UV absorption-line spectroscopy to characterize the kinematics and ionization conditions of embedded clouds in the Galactic Center outflow.

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arXiv2017

The Sunburst Arc: Direct Lyman α escape observed in the brightest known lensed galaxy

We present rest-frame ultraviolet and optical spectroscopy of the brightest lensed galaxy yet discovered, at redshift z = 2.4. This source reveals a characteristic, triple-peaked Lyman α profile which has been predicted by various theoretical works but to our knowledge has not been unambiguously observed previously. The feature is well fit by a superposition of two components: a double-peak profile emerging from substantial radiative transfer, and a narrow, central component resulting from directly escaping Lyman α photons; but is poorly fit by either component alone. We demonstrate that the feature is unlikely to contain contamination from nearby sources, and that the central peak is unaffected by radiative transfer effects apart from very slight absorption. The feature is detected at signal-to-noise ratios exceeding 80 per pixel at line center, and bears strong resemblance to synthetic profiles predicted by numerical models.

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arXiv2017

Probing the Outflowing Multiphase Gas ~1 kpc Below the Galactic Center

Comparison of ISM absorption in the UV spectrum of LS 4825, a B1 Ib-II star d=21+/-5 kpc from the Sun toward l = 1.67 deg and b = -6.63 deg, with ISM absorption toward an aligned foreground star at d < 7.0+/-1.7 kpc, allows us to isolate and study gas associated with the Milky Way nuclear wind. Spectra from the Space Telescope Imaging Spectrograph (STIS) show low ion absorption out to d < 7 kpc ( e.g., O I, C II, Mg II, Si II, Fe II, S II) only between 0 and 40 km/s, while absorption at d > 7 kpc, ~1 kpc below the galactic plane, is complex and spans -290 to + 94 km/ s. The intermediate and high ions Si III, C IV, Si IV and N V show extremely strong absorption with multiple components from -283 to 107 km/ s implying that the ISM ~1 kpc below the galactic center has a substantial reservoir of plasma, and more gas containing C IV and N V than in the Carina OB1 association at z = 0 kpc. Abundances and physical conditions are presented for many absorption components. The high ion absorption traces cooling transition temperature plasma probably driven by the outflowing hot gas, while the extraordinary large thermal pressure, p/k ~ 10^5 cm^-3 K^-1 in an absorption component at -114 km/s probably arises from the ram pressure of the outflowing hot gas. The observations are consistent with a flow whose ionization structure in the high ions can be understood through a combination of non-equilibrium radiative cooling and turbulent mixing.

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arXiv2017

The formation and physical origin of highly ionized cooling gas

We present a simple model that explains the origin of warm diffuse gas seen primarily as highly ionized absorption line systems in the spectra of background sources. We predict the observed column densities of several highly ionized transitions such as OVI, OVIII, NeVIII, NV, and MgX; and present a unified comparison of the model predictions with absorption lines seen in the Milky Way disk, Milky Way halo,starburst galaxies, the circumgalactic medium and the intergalactic medium at low and high redshifts. We show that diffuse gas seen in such diverse environments can be simultaneously explained by a simple model of radiatively cooling gas. We show that most of such absorption line systems are consistent with being collisionally ionized, and estimate the maximum likelihood temperature of the gas in each observation. This model satisfactorily explains why OVI is regularly observed around star-forming low-z L* galaxies, and why NV is rarely seen around the same galaxies. We further present some consequences of this model in quantifying the dynamics of the cooling gas around galaxies and predict the shock velocities associated with such flows. A unique strength of this model is that while it has only one free (but physically well-constrained) parameter, it nevertheless successfully reproduces the available data on O VI absorbers in the interstellar, circumgalactic, intra-group, and intergalactic media, as well as the available data on other absorption-line from highly ionized species.

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arXiv2017

Spatially Resolved Patchy Lyman-$α$ Emission Within the Central Kiloparsec of a Strongly Lensed Quasar Host Galaxy at z = 2.8

We report the detection of extended Lyman-$α$ emission from the host galaxy of SDSS~J2222+2745, a strongly lensed quasar at $z = 2.8$. Spectroscopic follow-up clearly reveals extended Lyman-$α$ in emission between two images of the central active galactic nucleus (AGN). We reconstruct the lensed quasar host galaxy in the source plane by applying a strong lens model to HST imaging, and resolve spatial scales as small as $\sim$200 parsecs. In the source plane we recover the host galaxy morphology to within a few hundred parsecs of the central AGN, and map the extended Lyman-$α$ emission to its physical origin on one side of the host galaxy at radii $\sim$0.5-2 kpc from the central AGN. There are clear morphological differences between the Lyman-$α$ and rest-frame ultraviolet stellar continuum emission from the quasar host galaxy. Furthermore, the relative velocity profiles of quasar Lyman-$α$, host galaxy Lyman-$α$, and metal lines in outflowing gas reveal differences in the absorbing material affecting the AGN and host galaxy. These data indicate the presence of patchy local intervening gas in front of the central quasar and its host galaxy. This interpretation is consistent with the central luminous quasar being obscured across a substantial fraction of its surrounding solid angle, resulting in strong anisotropy in the exposure of the host galaxy to ionizing radiation from the AGN. This work demonstrates the power of strong lensing-assisted studies to probe spatial scales that are currently inaccessible by other means.

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arXiv2017

COS-Burst: Observations of the Impact of Starburst-Driven Winds on the Properties of the Circum-Galactic Medium

We report on observations made with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) using background QSOs to probe the circum-galactic medium (CGM) around 17 low-redshift galaxies that are undergoing or have recently undergone a strong starburst (the COS-Burst program). The sightlines extend out to roughly the virial radius of the galaxy halo. We construct control samples of normal star-forming low-redshift galaxies from the COS/HST archive that match the starbursts in terms of galaxy stellar mass and impact parameter. We find clear evidence that the CGM around the starbursts differs systematically compared to the control galaxies. The Ly$α$, Si III, C IV, and possibly O VI absorption-lines are stronger as a function of impact parameter, and the ratios of the equivalent widths of C IV/Ly$α$ and Si III/Ly$α$ are both larger than in normal star-forming galaxies. We also find that the widths and the velocity offsets (relative to $v_{sys}$) of the Ly$α$ absorption-lines are significantly larger in the CGM of the starbursts, implying velocities of the absorbing material that are roughly twice the halo virial velocity. We show that these properties can be understood as a consequence of the interaction between a starburst-driven wind and the pre-existing CGM. These results underscore the importance of winds driven from intensely star-forming galaxies in helping drive the evolution of galaxies and the intergalactic medium. They also offer a new probe of the properties of starburst-driven winds and of the CGM itself.

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