arXiv subjects

Rongmon Bordoloi

Publications and source records attributed to Rongmon Bordoloi.

85 records · Page 5Linked to original sources

arXiv2016

Nuclear Outflow of the Milky Way: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble

We report new observations from a systematic, spectroscopic, ultraviolet absorption-line survey that maps the spatial and kinematic properties of the high-velocity gas in the Galactic Center region. We examine the hypothesis that this gas traces the biconical nuclear outflow. We use ultraviolet spectra of 47 background QSOs and halo stars projected inside and outside the northern Fermi Bubble from the Hubble Space Telescope to study the incidence of high velocity absorption around it. We use five lines of sight inside the northern Fermi Bubble to constrain the velocity and column densities of outflowing gas traced by O I, Al II, C II, C IV, Si II, Si III, Si IV and other species. All five lines of sight inside the northern Fermi Bubble exhibit blueshifted high velocity absorption components, whereas only 9 out of the 42 lines of sight outside the northern Fermi Bubble exhibit blueshifted high velocity absorption components. The observed outflow velocity profile decreases with Galactic latitude and radial distance (R) from the Galactic Center. The observed blueshifted velocities change from $v_{GSR}$=-265 km/s at R~2.3 kpc to $v_{GSR}$=-91 km/s at R~6.5 kpc. We derive the metallicity of the entrained gas along the 1H1613-097 sightline, which passes through the center of the northern Fermi Bubble, finding [O/H] $\gtrsim -0.54 \pm 0.15$. A simple kinematic model tuned to match the observed absorption component velocities along the five lines of sight inside the Bubble, constrains the outflow velocities to ~1000$-$1300 km/s, and the age of the outflow to be ~ 6$-$9 Myr. We estimate a minimum mass outflow rate for the nuclear outflow to be $\gtrsim$ 0.2 $\rm{ M_{\odot}\; yr^{-1}}$. Combining the age and mass outflow rates, we determine a minimum mass of total UV absorbing cool gas entrained in the Fermi Bubbles to be $\gtrsim \rm{ 2 \times 10^{6} M_{\odot}}$.

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arXiv2016

The Properties of the Circumgalactic Medium in Red and Blue Galaxies: Results from the COS-GASS+COS-Halos Surveys

We use the combined data from the COS-GASS and COS-Halos surveys to characterize the Circum-Galactic Medium (CGM) surrounding typical low-redshift galaxies in the mass range $\rm~M_*\sim~10^{9.5-11.5}~M_{\odot} $, and over a range of impact parameters extending to just beyond the halo virial radius ($\rm~R_{vir}$). We find the radial scale length of the distributions of the equivalent widths of the Lyman~$α$ and Si III absorbers to be 0.9 and 0.4 $\rm~R_{vir}$, respectively. The radial distribution of equivalent widths is relatively uniform for the blue galaxies, but highly patchy (low covering fraction) for the red galaxies. We also find that the Lyman~$α$ and Si III equivalent widths show significant positive correlations with the specific star-formation rate (sSFR) of the galaxy. We find a surprising lack of correlations between the halo mass (virial velocity) and either the velocity dispersions or velocity offsets of the Lyman~$α$ lines. The ratio of the velocity offset to the velocity dispersion for the Lyman~$α$ absorbers has a mean value of $\sim$ 4, suggesting that a given the line-of-sight is intersecting a dynamically coherent structure in the CGM rather than a sea of orbiting clouds. The kinematic properties of the CGM are similar in the blue and red galaxies, although we find that a significantly larger fraction of the blue galaxies have large Lyman~$α$ velocity offsets (>200 km s$^{-1}$). We show that - if the CGM clouds represent future fuel for star-formation - our new results could imply a large drop in the specific star-formation rate across the galaxy mass-range we probe.

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arXiv2016

A Deep Search For Faint Galaxies Associated With Very Low-redshift C IV Absorbers: III. The Mass- and Environment-dependent Circumgalactic Medium

Using Hubble Space Telescope Cosmic Origins Spectrograph observations of 89 QSO sightlines through the Sloan Digital Sky Survey footprint, we study the relationships between C IV absorption systems and the properties of nearby galaxies as well as large-scale environment. To maintain sensitivity to very faint galaxies, we restrict our sample to 0.0015 < z < 0.015, which defines a complete galaxy survey to L > 0.01 L* or stellar mass log M_* > 8 Msun. We report two principal findings. First, for galaxies with impact parameter rho < 1 rvir, C IV detection strongly depends on the luminosity/stellar mass of the nearby galaxy. C IV is preferentially associated with galaxies with log M_* > 9.5 Msun; lower mass galaxies rarely exhibit significant C IV absorption (covering fraction f = 9 +12-6% for 11 galaxies with log M_* < 9.5 Msun). Second, C IV detection within the log M_* > 9.5 Msun population depends on environment. Using a fixed-aperture environmental density metric for galaxies with rho < 160 kpc at z < 0.055, we find that 57+/-12% (8/14) of galaxies in low-density regions (regions with fewer than seven L > 0.15 L* galaxies within 1.5 Mpc) have affiliated C IV absorption; however, none (0/7) of the galaxies in denser regions show C IV. Similarly, the C IV detection rate is lower for galaxies residing in groups with dark-matter halo masses of log Mhalo > 12.5 Msun. In contrast to C IV, H I is pervasive in the CGM without regard to mass or environment. These results indicate that C IV absorbers with log N(C IV) > 13.5 cm^-2 trace the halos of log M_* > 9.5 Msun galaxies but also reflect larger scale environmental conditions.

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arXiv2016

The First Distance Constraint on the Renegade High Velocity Cloud Complex WD

We present medium-resolution, near-ultraviolet VLT/FLAMES observations of the star USNO-A0600-15865535. We adapt a standard method of stellar typing to our measurement of the shape of the Balmer epsilon absorption line to demonstrates that USNO-A0600-15865535 is a blue horizontal branch star, residing in the lower stellar halo at a distance of 4.4 kpc from the Sun. We measure the H & K lines of singly-ionized calcium and find two isolated velocity components, one originating in the disk, and one associated with high-velocity cloud complex WD. This detection demonstrated that complex WD is closer than ~4.4 kpc and is the first distance constraint on the +100 km/s Galactic complex of clouds. We find that Complex WD is not in corotation with the Galactic disk as has been assumed for decades. We examine a number of scenarios, and find that the most likely is that Complex WD was ejected from the solar neighborhood and is only a few kpc from the Sun.

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arXiv2016

Spatially Resolved Galactic Wind in Lensed Galaxy RCSGA 032727-132609

We probe the spatial distribution of outflowing gas along four lines of sight separated by up to 6 kpc in a gravitationally-lensed star-forming galaxy at z=1.70. Using MgII and FeII emission and absorption as tracers, we find that the clumps of star formation are driving galactic outflows with velocities of -170 to -250 km/sec. The velocities of MgII emission are redshifted with respect to the systemic velocities of the galaxy, consistent with being back-scattered. By contrast, the FeII fluorescent emission lines are either slightly blueshifted or at the systemic velocity of the galaxy. Taken together, the velocity structure of the MgII and FeII emission is consistent with arising through scattering in galactic winds. Assuming a thin shell geometry for the out owing gas, the estimated masses carried out by these outfows are large (> 30 - 50 $\rm{M_{\odot} yr^{-1}}$), with mass loading factors several times the star-formation rate. Almost 20% to 50% of the blueshifted absorption probably escapes the gravitational potential of the galaxy. In this galaxy, the outflow is "locally sourced", that is, the properties of the outflow in each line of sight are dominated by the properties of the nearest clump of star formation; the wind is not global to the galaxy. The mass outflow rates and the momentum flux carried out by outflows in individual star forming knots of this object are comparable to that of starburst galaxies in the local Universe.

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arXiv2015

A Deep Search For Faint Galaxies Associated With Very Low-redshift C IV Absorbers: II. Program Design, Absorption-line Measurements, and Absorber Statistics

To investigate the evolution of metal-enriched gas over recent cosmic epochs as well as to characterize the diffuse, ionized, metal-enriched circumgalactic medium (CGM), we have conducted a blind survey for C IV absorption systems in 89 QSO sightlines observed with the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS). We have identified 42 absorbers at z < 0.16, comprising the largest uniform blind sample size to date in this redshift range. Our measurements indicate an increasing C IV absorber number density per comoving path length (dN/dX = 7.5 +/- 1.1) and modestly increasing mass density relative to the critical density of the Universe (Omega(C IV) = 10.0 +/- 1.5 x 10^-8 ) from z ~ 1.5 to the present epoch, consistent with predictions from cosmological hydrodynamical simulations. Furthermore, the data support a functional form for the column density distribution function that deviates from a single power-law, also consistent with independent theoretical predictions. As the data also probe heavy element ions in addition to C IV at the same redshifts, we identify, measure, and search for correlations between column densities of these species where components appear aligned in velocity. Among these ion-ion correlations, we find evidence for tight correlations between C II and Si II, C II and Si III, and C IV and Si IV, suggesting that these pairs of species arise in similar ionization conditions. However, the evidence for correlations decreases as the difference in ionization potential increases. Finally, when controlling for observational bias, we find only marginal evidence for a correlation (86.8% likelihood) between the Doppler line width b(C IV) and column density N(C IV).

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arXiv2015

Connection Between the Circumgalactic Medium and the Interstellar Medium of Galaxies: Results from the COS-GASS Survey

We present a study exploring the nature and properties of the Circum-Galactic Medium (CGM) and its connection to the atomic gas content in the interstellar medium (ISM) of galaxies as traced by the HI 21cm line. Our sample includes 45 low-z (0.026-0.049) galaxies from the GALEX Arecibo SDSS Survey. Their CGM was probed via absorption in the spectra of background Quasi-Stellar Objects at impact parameters of 63 to 231kpc. The spectra were obtained with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. We detected neutral hydrogen (Ly$α$ absorption-lines) in the CGM of 92% of the galaxies. We find the radial profile of the CGM as traced by the Ly$α$ equivalent width can be fit as an exponential with a scale length of roughly the virial radius of the dark matter halo. We found no correlation between the orientation of sightline relative to the galaxy major axis and the Ly$α$ equivalent width. The velocity spread of the circumgalactic gas is consistent with that seen in the atomic gas in the interstellar medium. We find a strong correlation (99.8% confidence) between the gas fraction (M(HI)/M*) and the impact-parameter-corrected Ly$α$ equivalent width. This is stronger than the analogous correlation between corrected Ly$α$ equivalent width and SFR/M* (97.5% confidence). These results imply a physical connection between the HI disk and the CGM, which is on scales an order-of-magnitude larger. This is consistent with the picture in which the HI disk is nourished by accretion of gas from the CGM.

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arXiv2015

Baryon Cycling in the Low-Redshift Circumgalactic Medium: A Comparison of Simulations to the COS-Halos Survey

We analyze the low-redshift (z~0.2) circumgalactic medium by comparing absorption-line data from the COS-Halos Survey to absorption around a matched galaxy sample from two cosmological hydrodynamic simulations. The models include different prescriptions for galactic outflows, namely hybrid energy/momentum driven wind (ezw), and constant winds (cw). We extract for comparison direct observables including equivalent widths, covering factors, ion ratios, and kinematics. Both wind models are generally in good agreement with these observations for HI and certain low ionization metal lines, but show poorer agreement with higher ionization metal lines including SiIII and OVI that are well-observed by COS-Halos. These discrepancies suggest that both wind models predict too much cool, metal-enriched gas and not enough hot gas, and/or that the metals are not sufficiently well-mixed. This may reflect our model assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation includes a heuristic prescription to quench massive galaxies by super-heating its ISM gas, which we show yields sufficient low ionisation absorption to be broadly consistent with observations, but also substantial OVI absorption that is inconsistent with data, suggesting that gas around quenched galaxies in the real Universe does not cool. At impact parameters of <50 kpc, recycling winds dominate the absorption of low ions and even HI, while OVI almost always arises from metals ejected longer than 1 Gyr ago. The similarity between the wind models is surprising, since we show that they differ substantially in their predicted amount and phase distribution of halo gas. We show that this similarity owes mainly to our comparison here at fixed stellar mass rather than at fixed halo mass in our previous works, which suggests that CGM properties are more closely tied to the stellar mass of galaxies rather than halo mass.

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arXiv2014

Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic Signature of the Milky Way's Biconical Nuclear Outflow

Giant lobes of plasma extend 55 degrees above and below the Galactic Center, glowing in emission from gamma rays (the Fermi Bubbles) to microwaves (the WMAP haze) and polarized radio waves. We use ultraviolet absorption-line spectra from the Hubble Space Telescope to constrain the velocity of the outflowing gas within these regions, targeting the quasar PDS 456 (Galactic coordinates l,b=10.4, +11.2 degrees). This sightline passes through a clear biconical structure seen in hard X-ray and gamma-ray emission near the base of the northern Fermi Bubble. We report two high-velocity metal absorption components, at v_LSR=-235 and +250 km/s, which cannot be explained by co-rotating gas in the Galactic disk or halo. Their velocities are suggestive of an origin on the front and back side of an expanding biconical outflow emanating from the Galactic Center. We develop simple kinematic biconical outflow models that can explain these observed profiles with an outflow velocity of ~900 km/s and a full opening angle of ~110 degrees (matching the X-ray bicone). This indicates Galactic Center activity over the last ~2.5-4.0 Myr, in line with age estimates of the Fermi Bubbles. The observations illustrate the use of UV absorption-line spectroscopy to probe the properties of swept-up gas venting into the Fermi Bubbles.

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arXiv2014

The COS-Dwarfs Survey: The Carbon Reservoir Around sub-L* Galaxies

We report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z $\leq$ 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected 1D and 2D distribution of C IV absorption, we find that C IV absorption is detected out to ~ 0.5 R$_{vir}$ of the host galaxies. The C IV absorption strength falls off radially as a power law and beyond 0.5 R$_{vir}$, no C IV absorption is detected above our sensitivity limit of ~ 50-100 m$Å$. We find a tentative correlation between detected C IV absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L~L* galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass of $\gtrsim$ 1.2$\times 10^6$ $M_\odot$ out to ~ 110 kpc. This mass is comparable to the carbon mass in the ISM and more than the carbon mass currently in stars of these galaxies. The C IV absorption seen around these sub-L* galaxies can account for almost two-thirds of all $W_r$> 100 m$Å$ C IV absorption detected at low z. Comparing the C IV covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties.

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arXiv2014

The COS-Halos Survey: Physical Conditions and Baryonic Mass in the Low-Redshift Circumgalactic Medium

We analyze the physical conditions of the cool, photoionized (T $\sim 10^4$ K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of $L \sim L^*$ galaxies at $z \sim 0.2$. These data are well described by simple photoionization models, with the gas highly ionized (n$_{\rm HII}$/n$_{\rm H} \gtrsim 99\%$) by the extragalactic ultraviolet background (EUVB). Scaling by estimates for the virial radius, R$_{\rm vir}$, we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n$_{\rm H}$ = (10$^{-4.2 \pm 0.25}$)(R/R$_{\rm vir})^{-0.8\pm0.3}$. Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the HI column densities, we estimate a lower limit to the cool gas mass M$_{\rm CGM}^{\rm cool} > 6.5 \times 10^{10}$ M$_{\odot}$ for the volume within R $<$ R$_{\rm vir}$. Allowing for an additional warm-hot, OVI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10$^{12}$ M$_{\odot}$ scale.

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arXiv2013

The High-Ion Content and Kinematics of Low-Redshift Lyman Limit Systems

We study the high-ionization phase and kinematics of the circumgalactic medium around low-redshift galaxies using a sample of 23 Lyman Limit Systems (LLSs) at 0.08<z<0.93 observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. In Lehner et al. (2013), we recently showed that low-z LLSs have a bimodal metallicity distribution. Here we extend that analysis to search for differences between the high-ion and kinematic properties of the metal-poor and metal-rich branches. We find that metal-rich LLSs tend to show higher O VI columns and broader O VI profiles than metal-poor LLSs. The total H I line width (dv90 statistic) in LLSs is not correlated with metallicity, indicating that the H I kinematics alone cannot be used to distinguish inflow from outflow and gas recycling. Among the 17 LLSs with O VI detections, all but two show evidence of kinematic sub-structure, in the form of O VI-H I centroid offsets, multiple components, or both. Using various scenarios for how the metallicity in the high-ion and low-ion phases of each LLS compare, we constrain the ionized hydrogen column in the O VI phase to lie in the range log N(H II)~17.6-20. The O VI phase of LLSs is a substantial baryon reservoir, with M(high-ion)~10^{8.5-10.9}(r/150 kpc)^2 solar masses, similar to the mass in the low-ion phase. Accounting for the O VI phase approximately doubles the contribution of low-z LLSs to the cosmic baryon budget.

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arXiv2009

Photo-z Performance for Precision Cosmology

Current and future weak lensing surveys will rely on photometrically estimated redshifts of very large numbers of galaxies. In this paper, we address several different aspects of the demanding photo-z performance that will be required for future experiments, such as the proposed ESA Euclid mission. It is first shown that the proposed all-sky near-infrared photometry from Euclid, in combination with anticipated ground-based photometry (e.g. PanStarrs-2 or DES) should yield the required precision in individual photo-z of sigma(z) < 0.05(1+z) at I_AB < 24.5. Simple a priori rejection schemes based on the photometry alone can be tuned to recognise objects with wildly discrepant photo-z and to reduce the outlier fraction to < 0.25% with only modest loss of otherwise usable objects. Turning to the more challenging problem of determining the mean redshift of a set of galaxies to a precision of 0.002(1+z) we argue that, for many different reasons, this is best accomplished by relying on the photo-z themselves rather than on the direct measurement of from spectroscopic redshifts of a representative subset of the galaxies. A simple adaptive scheme based on the statistical properties of the photo-z likelihood functions is shown to meet this stringent systematic requirement. We also examine the effect of an imprecise correction for Galactic extinction and the effects of contamination by fainter over-lapping objects in photo-z determination. The overall conclusion of this work is that the acquisition of photometrically estimated redshifts with the precision required for Euclid, or other similar experiments, will be challenging but possible. (abridged)

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