arXiv subjects

Ranga-Ram Chary

Publications and source records attributed to Ranga-Ram Chary.

At least 73 records · Page 4Linked to original sources

arXiv2008

Molecular Gas in the z=1.2 Ultraluminous Merger GOODS J123634.53+621241.3

We report the detection of CO(2-1) emission from the z=1.2 ultraluminous infrared galaxy (ULIRG) GOODS J123634.53+621241.3 (also known as the sub-millimeter galaxy GN26). These observations represent the first discovery of high-redshift CO emission using the new Combined Array for Research in Millimeter-Wave Astronomy (CARMA). Of all high-redshift (z>1) galaxies within the GOODS-North field, this source has the largest far-infrared (FIR) flux observed in the Spitzer 70um and 160um bands. The CO redshift confirms the optical identification of the source, and the bright CO(2-1) line suggests the presence of a large molecular gas reservoir of about 7x10^10 M(sun). The infrared-to-CO luminosity ratio of L(IR)/L'(CO) = 80+/-30 L(sun) (K Km/s pc^2)^-1 is slightly smaller than the average ratio found in local ULIRGs and high-redshift sub-millimeter galaxies. The short star-formation time scale of about 70 Myr is consistent with a starburst associated with the merger event and is much shorter than the time scales for spiral galaxies and estimates made for high-redshift galaxies selected on the basis of their B-z and z-K colors.

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arXiv2008

Contribution to Unresolved Infrared Fluctuations from Dwarf Galaxies at Redshifts of 2-3

In order to understand the origin of clustered anisotropies detected in Spitzer images between 3.6 and 8 microns, we stack the Spitzer IRAC/Great Observatories Origins Deep Survey (GOODS) images at pixel locations corresponding to faint, z_{AB}~27 mag, optical sources with no obvious IR counterparts. We obtain a strong detection of the sources with a stacked median flux at 3.6 microns of 130+/-5 nJy above the background. The wealth of multi-wavelength imaging data in GOODS enables a similar stacking analysis to be undertaken at various wavelengths between the ultraviolet and near-infrared bands. We obtain strong stacked detections of these optically faint sources over the entire wavelength range which places constraints on the average properties of these sources. We find that the flux spectrum of the median, stacked source is consistent with a L<0.03 L_{*,UV} galaxy with a 90% confidence interval for the redshift of 1.9-2.7. These sources produce a 3.6 micron absolute background intensity between 0.1 and 0.35 nW/m^2/sr and the clustered IR light could account for ~30-50% of fluctuation power in the IR background at 4 arcminute angular scales. Although the exact redshift distribution of these sources is unknown, these galaxies appear to contain 5-20% of the co-moving stellar mass density at z~2.5.

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arXiv2008

The Stellar Initial Mass Function at the Epoch of Reionization

I provide estimates of the ultraviolet and visible light luminosity density at z~6 after accounting for the contribution from faint galaxies below the detection limit of deep Hubble and Spitzer surveys. I find the rest-frame V-band luminosity density is a factor of ~2-3 below the ultraviolet luminosity density at z~6. This implies that the maximal age of the stellar population at z~6, for a Salpeter initial mass function, and a single, passively evolving burst, must be <100 Myr. If the stars in z~6 galaxies are remnants of the star-formation that was responsible for ionizing the intergalactic medium, reionization must have been a brief process that was completed at z<7. This assumes the most current estimates of the clumping factor and escape fraction and a Salpeter slope extending up to 200 M_{\sun} for the stellar initial mass function (IMF; dN/dM \propto M^α, α=-2.3). Unless the ratio of the clumping factor to escape fraction is less than 60, a Salpeter slope for the stellar IMF and reionization redshift higher than 7 is ruled out. In order to maintain an ionized intergalactic medium from redshift 9 onwards, the stellar IMF must have a slope of α=-1.65 even if stars as massive as ~200 M_{\sun} are formed. Correspondingly, if the intergalactic medium was ionized from redshift 11 onwards, the IMF must have α~-1.5. The range of stellar mass densities at z~6 straddled by IMFs which result in reionization at z>7 is 1.3+/-0.4\times10^{7} Msun/Mpc^3.

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arXiv2007

Mid-Infrared Spectral Diagnosis of Submillimeter Galaxies

We present deep mid-infrared spectroscopy with the Spitzer Space Telescope of 13 submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey North (GOODS-N) field. We find strong polycyclic aromatic hydrocarbon (PAH) emission in all of our targets which allows us to measure mid-IR spectroscopic redshifts and place constraints on the contribution from star formation and active galactic nuclei (AGN) activity to the mid-IR emission. In the high signal-to-noise ratio composite spectrum of SMGs, we find that the hot dust continuum from an AGN contributes at most 30% of the mid-IR luminosity. We find that the mid-IR properties of SMGs are distinct from those of 24 micron -selected ULIRGs at z~2; the former are predominantly dominated by star formation while the latter are a more heterogeneous sample with many showing significant AGN activity. We fit the IRS spectrum and the mid-IR to radio photometry of SMGs with template spectral energy distributions to determine the best estimate of the total infrared luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multi-wavelength analysis shows that the total infrared luminosity, LIR, in SMGs, is dominated by star formation and not AGN activity. We find that high redshift SMGs lie on the relation between LIR and LPAH6.2 (or LPAH7.7 or LPAH11.3) that has been established for local starburst galaxies. This suggests that PAH luminosity can be used as a proxy for the star formation rate in SMGs. SMGs are consistent with being a short lived cool phase in a massive merger. Although there appears to be an AGN present in many SMGs, it does not appear to have become strong enough to heat the dust and dominate the mid- or far-infrared emission.

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arXiv2007

Spitzer Observations of Gamma-Ray Burst Host Galaxies: A Unique Window into High Redshift Chemical Evolution and Star-formation

We present deep Spitzer 3.6 micron observations of three z~5 GRB host galaxies. Our observations reveal that z~5 GRB hosts are a factor of 3 less luminous than the median rest-frame V-band luminosity of spectroscopically confirmed z~5 galaxies in the GOODS fields and the UDF. The strong connection between GRBs and massive star formation implies that not all star-forming galaxies at these redshifts are currently being accounted for in deep surveys and GRBs provide a unique way to measure the contribution to the star-formation rate density from galaxies at the faint end of the galaxy luminosity function. By correlating the co-moving star-formation rate density with co-moving GRB rates at lower redshifts, we estimate a lower limit to the star-formation rate density of 0.12+/-0.09 and 0.09+/-0.05 M_sun/yr/Mpc^3 at z~4.5 and z~6, respectively. Finally, we provide evidence that the average metallicity of star-forming galaxies evolves as (stellar mass density)^(0.69+/-0.17) between $z\sim5$ and $z\sim0$, probably indicative of the loss of a significant fraction of metals to the intergalactic medium, particularly in low-mass galaxies.

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arXiv2007

The Far-Infrared Luminosity Function from GOODS-N: Constraining the Evolution of Infrared Galaxies for z \leq 1

We present the IR luminosity function derived from ultra-deep 70 micron imaging of the GOODS-North field. The 70 micron observations are longward of the PAH and silicate features which complicate work in the MIR. We derive far-infrared luminosities for the 143 sources with S_{70} > 2 mJy (S/N > 3 σ). The majority (81%) of the sources have spectroscopic redshifts, and photometric redshifts are calculated for the remainder. The IR luminosity function at four redshifts (z ~ 0.28, 0.48, 0.78, and 0.97) is derived and compared to the local one. There is considerable degeneracy between luminosity and density evolution. If the evolving luminosity function is described as ρ(L, z) = (1 + z)^q ρ(L/(1 + z)^p, 0), we find q = -2.19p + 6.09. In the case of pure luminosity evolution, we find a best fit of p = 2.78^{+0.34}_{-0.32}. This is consistent with the results from 24 micron and 1.4 GHz studies. Our results confirm the emerging picture of strong evolution in LIRGs and ULIRGs at 0.4 < z < 1.1, but we find no evidence of significant evolution in the sub-LIRG (L < 10^{11} L_{\odot}) population for z < 0.4.

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arXiv2007

Search with EGRET for a Gamma Ray Line from the Galactic Center

We search data from the Energetic Gamma Ray Experiment Telescope (EGRET) for a gamma-ray line in the energy range 0.1-10 GeV from the 10 X 10 degree region around the Galactic center. Our null results lead to upper limits to the line flux from the Galactic center. Such lines may have appeared if the dark matter in the Galactic halo is composed of weakly-interacting massive particles (WIMPs) in the mass range 0.1-10 GeV. For a given dark-matter-halo model, our null search translates to upper limits to the WIMP two-photon annihilation cross section as a function of WIMP mass. We show that for a toy model in which Majorana WIMPs in this mass range annihilate only to electron-positron pairs, these upper limits supersede those derived from measurements of the 511-keV line and continuum photons from internal bremsstrahlung at the Galactic center.

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arXiv2007

HUDF-JD2: Mid-infrared Evidence for a z~2 Luminous Infrared Galaxy

The Hubble Ultra Deep Field source JD2 presented in Mobasher et al. (2005) is an unusual galaxy that is very faint at all wavelengths shortward of 1.1 micron. Photometric redshift fits to data at 0.4 to 8 microns yield a significant probability that it is an extremely massive galaxy at z~6.5. In this paper we present new photometry at 16 microns and 22 micron from Spitzer Infrared Spectrograph (IRS) peak-up imaging of the Great Observatories Origins Deep Survey (GOODS) fields. We find that the spectral energy distribution shows a factor of ~4 rise in flux density between the 16 micron and 22 micron bandpass which is most likely due to the entrance of polycyclic aromatic hydrocarbon emission features into the 22 micron and 24 micron passbands. The flux ratio between these bandpasses can be best fit by a z~1.7 luminous infrared galaxy with a bolometric luminosity of (2-6)\times10^{11} L_sun corresponding to a star-formation rate of 80 M_sun/yr. The predicted flux density values at other longer wavelengths are below the detection limits of current instrumentation but such sources could potentially be detected in lensed submillimeter surveys. Re-evaluation of the optical/near-infrared photometry continues to favor z>6 photometric redshift solutions, but we argue that the consistency of the multiwavelength parameters of this galaxy with other dusty starbursts favor the z~2 mid-infrared photometric redshift. The data presented here provide evidence that optically undetected near-infrared sources which are detected at 24 microns are most likely dusty, starburst galaxies at a redshift of z~2 with stellar masses >10^{10} M_sun.

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arXiv2006

Arp102B: An ADAF and a Torus ?

Arp102B is a nearby radio galaxy which displays the presence of double peaked Balmer emission lines. Sub-arcsec Keck mid-infrared imaging and Spitzer spectroscopy reveal a spatially compact mid-infrared source which displays tentative evidence for variability. The F$_ν\proptoν^{-1.2}$ spectral energy distribution is suggestive of an advection dominated accretion flow. The absence of dust features over the 5-40 micron range make it unlikely that thermal dust emission dominates the mid-infrared luminosity. We also detect the presence of molecular hydrogen in emission which is asymmetrically redshifted by ~500-1000 km/s from the systemic velocity of the galaxy. Since the forbidden, low ionization lines in this galaxy are at the systemic velocity, we suggest that the molecular hydrogen emission arises from a rotating molecular gas structure surrounding the nuclear black hole at a distance of ~1 pc.

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arXiv2006

Insights into Galaxy Evolution from Mid-infrared Wavelengths

In this paper, I have attempted to highlight key results from deep extragalactic surveys at mid-infrared wavelengths. I discuss advances in our understanding of dust enshrouded star-formation and AGN activity at 0 3 will become possible only with future facilities like ALMA. Currently, the presence of dust can only be assessed in a small fraction of the youngest starbursts at z>5 by looking for redshifted large equivalent width H_alpha emission in broadband filters like the IRAC 4.5 micron passband. H_alpha to UV ratios in these objects are a tracer of dust extinction and measuring this ratio in GOODS galaxies indicate dust in ~20% of star-forming galaxies at z>5. Finally, implications for reionization based on the measured stellar mass density and star-formation rates of galaxies at these redshifts are discussed.

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arXiv2006

IR Background Anisotropies in Spitzer GOODS images and constraints on first galaxies

We describe the angular power spectrum of unresolved 3.6 micron IR light in Spitzer GOODS fields. The amplitude of the anisotropy spectrum decreases with decreasing flux threshold to which resolved sources are removed from images. When all pixels brighter than a Vega magnitude of 24.6 are removed, the amplitude of the power spectrum at arcminute angular scales can be described with an extra component of z>8 sources with a IRB contribution around 0.4 nW m^-2 sr-1. The shape of the power spectrum, however, is more consistent with that expected for unresolved, faint galaxies at lower redshifts with Vega magnitudes fainter than 23 with a total 3.6 micron intensity between 0.1 to 0.8 nW m^-2 sr^-1. We confirm this assumption by showing that large-scale power decreases rapidly when the unresolved clustering spectrum is measured from a processed HDF-N IRAC image where locations of faint ACS sources with no IR counterparts were also masked. Based on resolved counts and unresolved fluctuations, we find that, at most, about 7.0 nW m^-2 sr^-1 can be ascribed to galaxies.

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arXiv2006

Infrared spectral energy distributions of submillimetre galaxies

Submillimetre (sub-mm) galaxies have very high infrared (IR) luminosities and are thousands of times more numerous at z~2 than local ultra-luminous IR galaxies. They therefore represent a key phase in galaxy evolution which can be missed in optical surveys. Determining their contribution to the global star formation rate requires dissecting their IR emission into contributions from starbursts (SB) and active galactic nuclei (AGN). There are several examples of AGN systems which masquerade as SBs in either the IR or X-ray, and SBs can often look like AGN in some wavebands. A combination of SB and AGN emission is not unreasonable, given models of merger-driven evolution. To assess in detail what powers the intense IR luminosity of sub-mm galaxies it is important to obtain a complete multi-wavelength picture. Mid-IR spectroscopy is a particularly good probe of where the intense IR luminosity is coming from. We present the first results from a program to obtain Spitzer IRS spectroscopy of a sample of high redshift galaxies in the GOODS-N field, a large fraction of which are sub-mm galaxies. This field is already home to the deepest X-ray, optical, IR and radio data. We piece together the sub-mm data with the Spitzer photometry and IRS spectra to provide a well sampled IR spectral energy distribution (SED) of sub-mm galaxies and determine the contribution to the bolometric luminosity from the AGN and SB components.

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arXiv2006

Clustering of the IR Background Light with Spitzer: Contribution from Resolved Sources

We describe the angular power spectrum of resolved sources at 3.6 microns (L-band) in Spitzer imaging data of the GOODS HDF-N, the GOODS CDF-S, and the NDWFS Bootes field in several source magnitude bins. We also measure angular power spectra of resolved sources in the Bootes field at K_S and J-bands using ground-based IR imaging data. In the three bands, J, K_S, and L, we detect the clustering of galaxies on top of the shot-noise power spectrum at multipoles between ell ~ 10^2 and 10^5. The angular power spectra range from the large, linear scales to small, non-linear scales of galaxy clustering, and in some magnitude ranges, show departure from a power-law clustering spectrum. We consider a halo model to describe clustering measurements and to establish the halo occup ation number parameters of IR bright galaxies at redshifts around one. We also extend our clustering results and completeness-corrected faint source number counts in GOODS fields to understand the underlying nature of unresolved sources responsible for IR background (IRB) anisotropies that were detected in deep Spitzer images. While these unresolved fluctuations were measured at sub-arcminute angular scales, if a high-redshift diffuse component associated with first galaxies exists in the IRB, then it's clustering properties are best studied with shallow, wide-field images that allow a measurement of the clustering spectrum from a few degrees to arcminute angular scales.

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arXiv2006

Spitzer Observations of the Prototypical Extremely Red Objects HR10 and LBDS53W091: Separating Dusty Starbursts from Old Elliptical Galaxies

We present Spitzer Space Telescope observations of the well-studied extremely red objects (EROs) HR10 and LBDS53W091 from 3.6 to 160 microns. These galaxies are the prototypes of the two primary classes of EROs: dusty starbursts and old, evolved galaxies, respectively. Both galaxies, as well as LBDS53W069, another example of an old, quiescent galaxy, are well-detected out to 8 microns. However, only the dusty starburst HR10 is detected in the far-infrared. All three EROs have stellar masses of a few times 10^11 M(sun). Using evolutionary model fits to their multiband photometry, we predict the infrared colors of similar EROs at 1<z<2. We find that blueward of observed 10 microns, the two ERO classes are virtually indistinguishable photometrically. Deep spectroscopy and 24 micron data allow the classes to be separated.

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arXiv2006

The Hubble Deep Field North SCUBA Super-map IV - Characterizing submillimetre galaxies using deep Spitzer imaging

We present SEDs, Spitzer colours, and IR luminosities for 850 micron selected galaxies in the GOODS-N field. Using the deep Spitzer Legacy images and new data and reductions of the VLA-HDF radio data, we find statistically secure counterparts for 60 per cent (21/35) of our submm sample, and identify tentative counterparts for another 12 objects. This is the largest sample of submm galaxies with statistically secure counterparts detected in the radio and with Spitzer. We find that in most cases the 850 micron emission is dominated by a single 24 micron source. A composite rest-frame SED shows that the submm sources peak at longer wavelengths than those of local ULIRGs of the same luminosity and therefore appear to be cooler. The SEDs of submm galaxies are also different from those of their high redshift neighbours, the near-IR selected BzK galaxies, whose mid-IR to radio SEDs are more like those of local ULIRGs. Using 24 micron. 850 micron and 1.4 GHz observations, we fit templates that span the mid-IR through radio to derived the integrated IR luminosity of the submm galaxies. By themselves, 24 micron and radio fluxes are able to predict LIR reasonably well because they are relatively insensitive to temperature. However, the submm flux by itself consistently overpredicts LIR when using spectral templates which obey the local ULIRG temperature-luminosity relation. The shorter Spitzer wavelengths sample the stellar bump at the redshifts of the submm sources, and we find that the Spitzer photometry alone provides a model independent estimate of the redshift. Using X-ray and mid-IR data, only 5 per cent of our secure counterparts show strong evidence for an active galactic nucleus dominating the IR luminosity.

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arXiv2006

Using Spitzer to probe the nature of submillimetre galaxies in GOODS-N

How does the submm galaxy population detected by SCUBA fit into galaxy evolution? How do these rare starbursting systems, which contribute significantly to high redshift star-formation, relate to other galaxy populations? Deep radio observations have been most useful for studying these systems, but still leave a significant fraction of the population unidentified. Now with the deep IRAC and MIPS images from the GOODS Spitzer Legacy program and a re-analysis of the deep radio data, we are able to identify counterparts for a large fraction of SCUBA galaxies in GOODS-N. All of these counterparts are detected by Spitzer. Given the vast multi-wavelength data in this field, we can study the spectral energy distributions (SEDs) of these systems and determine what is fueling their intense infrared luminosities. A rest-frame composite optical-to-radio SED for all spectroscopically identified submm sources shows that the average SCUBA galaxy is consistent with models of ultraluminous starburst galaxies, although cooler than those observed locally. Because of this, the submm flux alone consistently overestimates IR when using spectral templates which obey the local ULIRG temperature-luminosity relation. The wide range of 24/850 micron flux ratios as a function of redshift indicates the presence of strong mid-IR features, to be confirmed with deep IRS spectroscopy. The IRAC colours of the submm systems provide useful redshift constraints, since, at these redshifts, IRAC samples the stellar bump. The Spitzer photometry of this large sample of submm galaxies has allowed us to put constraints on many of the outstanding issues in submm astronomy.

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arXiv2005

Spitzer Number Counts of AGN in the GOODS fields

We present mid-infrared observations of AGN in the GOODS fields, performed with the Spitzer Space Telescope. These are the deepest infrared and X-ray fields to date and cover a total area of ~0.1 square degrees. AGN are selected on the basis of their hard (2-8 keV) X-ray emission. The median AGN infrared luminosity is at least 10 times larger than the median for normal galaxies with the same redshift distribution, suggesting that the infrared emission is dominated by the central nucleus. The X-ray to infrared luminosity ratios of GOODS AGN, most of which are at 0.5<z<1.5, are similar to the values obtained for AGN in the local Universe. The observed infrared flux distribution has an integral slope of ~1.5 and there are 1000 sources per square degree brighter than ~50 uJy at 3-6 microns. The counts approximately match the predictions of models based on AGN unification, in which the majority of AGN are obscured. This agreement confirms that the faintest X-ray sources, which are dominated by the host galaxy light in the optical, are obscured AGN. Using these Spitzer data, the AGN contribution to the extragalactic infrared background light is calculated by correlating the X-ray and infrared catalogues. This is likely to be a lower limit given that the most obscured AGN are missed in X-rays. We estimate the contribution of AGN missed in X-rays, using a population synthesis model, to be ~45% of the observed AGN contribution, making the AGN contribution to the infrared background at most ~2-10% in the 3-24 micron range, depending on wavelength, lower than most previous estimates. The AGN contribution to the infrared background remains roughly constant with source flux in the IRAC bands but decreases with decreasing flux in the MIPS 24 um band, where the galaxy population becomes more important.

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arXiv2005

New Views into High Redshift Star-formation from GOODS

The GOODS-North field centered around the historical Hubble Deep Field-North provides one of the richest multiwavelength datasets compiled, spanning radio to X-ray frequencies, for studying the formation and evolution of galaxies at high redshift. In particular, the unprecedented sensitivity of the Spitzer/GOODS 24 micron observations allows an unbiased measure of dust-enshrouded star-formation and AGN activity in typical L* galaxies rather than the extreme, hyperluminous galaxies which far-infrared surveys detect. We consider a spectroscopically selected sample of 226 galaxies at z$>$1.5 of which 135 are in the redshift range 1.5$<z<3$. Less than 1/4 of galaxies considered here are detected at 24 microns. They primarily have red UV slopes but consitute about only 20 percent of the red galaxies in this sample indicating that the red UV colors of the majority of objects is due to an evolved stellar population. Although 24 micron sources are sparse in number, their combined energy output in the 1.5$<z<3$ range exceeds the combined UV luminosity of the sample by a factor of about 30. We also find that AGN, identified by their X-ray to infrared luminosity ratios, account for less than 10 percent of the sources considered and contribute less than 30 percent of the total budget in this redshift range. The infrared luminous galaxies which are increasingly being found at high-z appear to dominate the global energetics of the Universe out to z~3 as has previously been predicted.

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