arXiv subjects

Sangeeta Malhotra

Publications and source records attributed to Sangeeta Malhotra.

At least 145 records · Page 8Linked to original sources

arXiv2004

A Galaxy at z = 6.545 and Constraints on the Epoch of Reionization

We report the discovery of a Lyman-alpha-emitting galaxy at redshift z=6.545 serendipitously identified in the course of spectroscopic follow-up of hard X-ray sources on behalf of the Serendipitous Extragalactic X-Ray Source Identification (SEXSI) survey. The line flux of the galaxy, 2.1e-17 erg/cm2/s, is similar to line fluxes probed by narrow-band imaging surveys; the 5.2 square-arcminutes surveyed implies a surface density of z~6.5 Lyman-alpha emitters somewhat higher than that inferred from narrow-band surveys. This source marks the sixth Lyman-alpha-emitting galaxy identified at z~6.5, a redshift putatively beyond the epoch of reionization when the damping wings of the neutral hydrogen of the intergalactic medium is capable of severely attenuating Lyman-alpha emission. By comparing the Lyman-alpha emitter luminosity functions at z~5.7 and z~6.5, we infer that the intergalactic medium may remain largely reionized from the local universe out to z~6.5.

astro-ph↗

arXiv2004

A Luminous Lyman-alpha Emitting Galaxy at Redshift z=6.535: Discovery and Spectroscopic Confirmation

We present a redshift z=6.535 galaxy discovered by its Lyman alpha emission in a 9180A narrowband image from the Large Area Lyman Alpha (LALA) survey. The Ly-a line luminosity (1.1e43 erg/s) is among the largest known for star forming galaxies at z=6.5. The line shows the distinct asymmetry that is characteristic of high-redshift Ly-a. The 2 sigma lower bound on the observer-frame equivalent width is > 530A. This is hard to reconcile with a neutral intergalactic medium unless the Ly-a line is intrinsically strong and is emitted from its host galaxy with an intrinsic Doppler shift of several hundred km/s. If the IGM is ionized, it corresponds to a rest frame equivalent width > 40A after correcting for Ly-a forest absorption. We also present complete spectroscopic followup of the remaining candidates with line flux > 2e-17 erg/cm2/s in our 1200 square arcminute narrowband image. These include another galaxy with a strong emission line at 9136A and no detected continuum flux, which however is most likely an [OIII] source at z=0.824 based on a weak detection of the [OIII] 4959A line.

astro-ph↗

arXiv2004

Three Lyman-alpha Emitters at z approx 6: Early GMOS/Gemini Data from the GLARE Project

We report spectroscopic detection of three z~6 Lyman-alpha emitting galaxies, in the vicinity of the Hubble Ultra Deep Field, from the early data of the Gemini Lyman-$α$ at Reionisation Era (GLARE) project. Two objects, GLARE#3001 (z =5.79) and GLARE#3011 (z =5.94), are new detections and are fainter in $z'$ (z'_{AB} =26.37 and 27.15) than any Lyman break galaxy previously detected in Lyman-alpha. A third object, GLARE#1042 (z =5.83) has previously been detected in line emission from the ground; we report here a new spectroscopic continuum detection. Gemini/GMOS-S spectra of these objects, obtained using nod & shuffle, are presented together with a discussion of their photometric properties. All three objects were selected for spectroscopy via the i-drop Lyman Break technique, the two new detections from the GOODS v1.0 imaging data. The red i'-z' colors and high equivalent widths of these objects suggest a high-confidence z>5 Lyman-alpha identification of the emission lines. This brings the total number of known z>5 galaxies within 9 arcmin of the Hubble Ultra Deep Field to four, of which three are at the same redshift (z=5.8 within 2000 km/s suggesting the existence of a large-scale structure at this redshift.

astro-ph↗

arXiv2003

[O I] 63 micron Emission from High and Low Luminosity AGN Galaxies

The Infrared Space Observatory was used to search for a tracer of the warm and dense neutral interstellar medium, the [O I] 63.18 micron line, in four ultraluminous IRAS sources lying at redshifts between 0.6 and 1.4. While these sources are quasars, their infrared continuum emission suggests a substantial interstellar medium. No [O I] flux was securely detected after probing down to a 3 sigma sensitivity level sufficient for detecting line emission in starbursts with similar continuum emission. However, if the detection threshold is slightly relaxed, one target is detected with 2.7 sigma significance. For this radio-quiet quasar there is likely a substantial dense and warm interstellar medium; the upper limits for the three radio-loud sources do not preclude the same conclusion. Using a new, uniformly-processed database of the ISO extragalactic far-infrared spectroscopy observations, it is shown that nearby Seyfert galaxies typically have higher [O I]-to-far-infrared ratios than do normal star-forming galaxies, so the lack of strong [O I] 63 micron emission from these high-redshift ultraluminous sources cannot be attributed to their active cores.

astro-ph↗

arXiv2003

Infrared Emission of Normal Galaxies from 2.5 to 12 Microns: ISO Spectra, Near-Infrared Continuum and Mid-Infrared Emission Features

We present ISO-PHOT spectra of the regions 2.5-4.9um and 5.8-11.6um for a sample of 45 disk galaxies from the U.S. ISO Key Project on Normal Galaxies. The spectra can be decomposed into three spectral components: (1) continuum emission from stellar photospheres, which dominates the near-infrared (2.5- 4.9um; NIR) spectral region; (2) a weak NIR excess continuum, which has a color temperature of ~ 1000K, carries a luminosity of a few percent of the total far-infrared luminosity L(FIR), and most likely arises from the ISM; and (3) the well-known broad emission features at 6.2, 7.7, 8.6 and 11.3 um, which are generally attributed to aromatic carbon particles. These aromatic features in emission (AFEs) dominate the mid-infrared (5.8-11.6 um; MIR) part of the spectrum, and resemble the so-called Type-A spectra observed in many non-stellar sources and the diffuse ISM in our own Galaxy. The relative strengths of the AFEs vary by 15-25% among the galaxies. However, little correlation is seen between these variations and either IRAS 60um-to-100um flux density ratio R(60/100) or the FIR-to-blue luminosity ratio L(FIR)/L(B), suggesting that the observed variations are not a direct consequence of the radiation field differences among the galaxies. We demonstrate that the NIR excess continuum and AFE emission are correlated, suggesting that they are produced by similar mechanisms and similar (or the same) material. On the other hand, as the current star-formation activity increases, the overall strengths of the AFEs and the NIR excess continuum drop significantly with respect to that of the far-infrared emission from large dust grains. This is likely a consequence of the preferential destruction in intense radiation fields of the small carriers responsible for the NIR/AFE emission.

astro-ph↗

arXiv2002

Spectroscopic Confirmation of Three Redshift 5.7 Lyman-alpha Emitters from the Large Area Lyman Alpha Survey

Narrow-band searches for Lyman alpha emission are an efficient way of identifying star-forming galaxies at high redshifts. We present Keck telescope spectra confirming redshifts z = 5.7 for three objects discovered in the Large Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory. All three spectra show strong, narrow emission lines with the asymmetric profile that is characteristically produced in high redshift Lyman alpha emitters by preferential HI absorption in the blue wing of the line. These objects are undetected in deep Bw, V, R, and 6600A narrow-band images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyman alpha forest absorption at redshift z = 5.7. All three objects show large equivalent widths (>= 150A in the rest-frame), suggesting at least one of the following: a top-heavy initial mass function, very low stellar metallicity, or the presence of an active nucleus. We consider the case for an active nucleus to be weak in all three objects due to the limited width of the Lyman alpha emission line (< 500 km/s) and the absence of any other indicator of quasar activity. The three confirmed high redshift objects were among four spectroscopically observed targets drawn from the sample of 18 candidates presented by Rhoads and Malhotra (2001). Thus, these spectra support the Lyman alpha emitter population statistics from our earlier photometric study, which imply little evolution in number density from z=5.7 to z=4.5 and provide strong evidence that the reionization redshift is greater than 5.7.

astro-ph↗

arXiv2001

Lyman Alpha Emitters at Redshift z=5.7

Lyman alpha galaxies at high redshifts offer a powerful probe of both the formation of galaxies and the reionization of the intergalactic medium. Lyman alpha line emission is an efficient tool for identifying young galaxies at high redshift, because it is strong in systems with young stars and little or no dust-- properties expected in galaxies undergoing their first burst of star-formation. Lyman alpha galaxies also provide a robust test of the reionization epoch that is independent of Gunn-Peterson trough observations in quasar spectra and is better able to distinguish line center optical depths tau=5 from tau=10^5. This is because neutral gas scatters Lyman alpha photons, dramatically ``blurring'' images of Lyman alpha galaxies embedded in a neutral intergalactic medium and rendering them undetectable. We present a photometrically selected sample of z=5.7 Lyman alpha emitters derived from the Large Area Lyman Alpha survey. The presence of these low-luminosity Lyman alpha sources at z=5.7 immediately implies that the reionization redshift was > 5.7. Comparing these objects to our earlier z=4.5 sample, we find that the number of z=5.7 emitters at fixed line luminosity marginally exceeds the no-evolution expectation, but falls well short of published model predictions. The equivalent width distribution is similar at the two redshifts. The large equivalent widths of the Lyman alpha line indicate young galaxies undergoing their first star formation.

astro-ph↗

arXiv2001

Large Equivalent width Lyman-alpha line emission at z=4.5: young galaxies in a young universe

The Large Area Lyman Alpha survey has found ~ 150 Lyman-alpha emitters at z=4.5. While stellar models predict a maximum Lyman-alpha equivalent width (EW) of 240 angstrom, 60% of the Lyman-alpha emitters have EWs exceeding this value. We attempt to model the observed EW distribution by combining stellar population models with an extrapolation of Lyman break galaxy luminosity function at z=4, incorporating observational selection effects and Malmquist bias. To reproduce the high EWs seen in the sample we need to postulate a stellar initial mass function (IMF) with extreme slope alpha = 0.5 (instead of 2.35); zero metallicity stars; or narrow-lined active galactic nuclei. The models also reveal that only 7.5-15% of galaxies need show Lyman-alpha emission to explain the observed number counts. This raises the possibility that either star-formation in high redshift galaxies is episodic or the Lyman-alpha galaxies we are seeing are the youngest 7.5-15% and that Lyman-alpha is strongly quenched by dust at about 10 Mega-years of age.

astro-ph↗

arXiv2001

The Large Area Lyman Alpha Survey

The Lyman-$α$ line is expected to be strong in the presence of active star formation and the absence of dust, making it a good tool for finding chemically primitive galaxies in the early universe. We report on a new survey for high redshift Lyman-$α$ sources, the Large Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on the new $8192^2$ pixel CCD Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching known high redshift Lyman-$α$ galaxies have an observed surface density corresponding to $11000 \pm 700$ per square degree per unit redshift at $z=4.5$. Early spectroscopic followup from the Keck telescope suggests that $\sim 1/3$ of these are actually at $z\approx 4.5$, and has confirmed five $z > 4$ Lyman-$α$ emitters so far. Combining our photometric survey with spectroscopic results, we estimate a net density of $\sim 4000 $ Lyman-$α$ emitters per square degree per unit redshift at $z\approx 4.5$. The star formation rate density (estimated both from UV continuum and from line emission) is comparable to that of the Lyman break galaxy population within present uncertainties. The most extreme Lyman-$α$ emitters in our sample have rest frame equivalent widths $> 100Å$, consistent with the expectations for the first burst of star formation in a primitive, dust-free galaxy.

astro-ph↗

arXiv2001

Far Infrared Spectroscopy of star-forming galaxies: Expectations for the Herschel Space Observatory

ISO has enabled far-infrared spectroscopy of a variety of galaxies. Using the [CII] (158 micron) and [OI](63 micron) lines, we can characterize the physical conditions in the star-forming ISM. These observations also form the basis of our expectations for what the Herschel Space Observatory will observe for high redshift galaxies. While [CII] is suppressed in ULIRGs and normal galaxies with high dust temperatures, it is stronger than expected in metal poor galaxies by factors of a few. Young galaxies at high redshifts might be expected to be both metal poor and actively star-forming, leading to contrary expectations for the [CII] line strength. The best prediction for [CII] detection is derived by using the observed proportionality between [CII] and mid-IR emission from PAHs. Using the observed [CII]/7 micron ratio and number counts from ISO deep surveys we predict that HSO will be able to detect 100 sources/square-degree in the [CII] line.

astro-ph↗

arXiv2001

Large Area Lyman Alpha survey: finding young galaxies at z=4.5

Strong Lyman-alpha emission is a signpost of young stars and the absence of dust and thus indicates young galaxies. To find such a population of young galaxies at z=4.5 we started the Large Area Lyman Alpha survey (LALA). This survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on a large format (36x36 square-arcminutes) camera on the 4 meter telescope at KPNO. The volume density and star-formation contribution of the Lyman-alpha emitters at z=4.5 is comparable to that of Lyman break galaxies. With many candidates and a few spectroscopic confirmations in hand we discuss what the properties of Ly-alpha emitters imply for galaxy and star formation in the early universe.

astro-ph↗

arXiv2000

Evidence for the Heating of Atomic Interstellar Gas by PAHs

We report a strong correlation between the [CII] 158 micron cooling line and the mid-infrared flux in the 5-10 micron range in a wide variety of star-forming galaxies. The mid-infrared flux is dominated by Aromatic Feature Emission (AFE), which is thought to arise from large polycyclic aromatic hydrocarbon molecules or `PAHs' and generally associated with the smallest interstellar grains. The [CII] line is the dominant gas coolant in most regions of atomic interstellar gas, and therefore reflects the heating input to the gas. The ratio of these two quantities, [CII]/AFE, remains nearly constant with the ratio of the IRAS 60 micron band flux to the 100 micron band flux, R(60/100). This is in contrast to the drop in the [CII]/FIR ratio with increasing R(60/100), which signal higher dust temperatures and more intense radiation fields. We interpret the stable [CII]/AFE ratio as evidence that gas heating is dominated by the PAHs or small grains which are also AFE carriers over a wide range of conditions. The trend of decreasing [CII]/FIR with increasing radiation field suggests a decrease in the importance of PAHs or small grains relative to large grains both in gas heating and in dust cooling. We summarize the observed trends and suggest two plausible scenarios.

astro-ph↗

arXiv2000

First Results from the Large Area Lyman Alpha Survey

We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching known high redshift Lyman alpha galaxies (i.e., observed equivalent width above 80 Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in an 80 Angstrom filter) have an observed surface density corresponding to 11000 +- 700 per square degree per unit redshift at z=4.5. Spatial variation in this surface density is apparent on comparison between counts in 6561 and 6730 Angstrom filters. Early spectroscopic followup results from the Keck telescope included three sources meeting our criteria for good Lyman alpha candidates. Of these, one is confirmed as a z=4.52 source, while another remains consistent with either z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies per square degree per unit redshift.

astro-ph↗

arXiv1999

Towards an Understanding of the Mid-Infrared Surface Brightness of Normal Galaxies

We report a mid-infrared color and surface brightness analysis of IC 10, NGC 1313, and NGC 6946, three of the nearby galaxies studied under the Infrared Space Observatory Key Project on Normal Galaxies. Images with < 9 arcsecond (170 pc) resolution of these nearly face-on, late-type galaxies were obtained using the LW2 (6.75 mu) and LW3 (15 mu) ISOCAM filters. Though their global I_nu(6.75 mu)/I_nu(15 mu) flux ratios are similar and typical of normal galaxies, they show distinct trends of this color ratio with mid-infrared surface brightness. We find that I_nu(6.75 mu)/I_nu(15 mu) ~< 1 only occurs for regions of intense heating activity where the continuum rises at 15 micron and where PAH destruction can play an important role. The shape of the color-surface brightness trend also appears to depend, to the second-order, on the hardness of the ionizing radiation. We discuss these findings in the context of a two-component model for the phases of the interstellar medium and suggest that star formation intensity is largely responsible for the mid-infrared surface brightness and colors within normal galaxies, whereas differences in dust column density are the primary drivers of variations in the mid-infrared surface brightness between the disks of normal galaxies.

astro-ph↗

arXiv1997

Microlensing of Globular Clusters as a Probe of Galactic Structure

The spatial distribution of compact dark matter in our Galaxy can be determined in a few years of monitoring Galactic globular clusters for microlensing. Globular clusters are the only dense fields of stars distributed throughout the three-dimensional halo and hence are uniquely suited to probe its structure. The microlensing optical depths towards different clusters have varying contributions from the thin disk, thick disk, bulge, and halo of the Galaxy. Although measuring individual optical depths to all the clusters is a daunting task, we show that interesting Galactic structure information can be extracted with as few as $40$--$120$ events in total for the entire globular cluster system (observable with 2--5 years of monitoring). The globular cluster microlensing is particularly sensitive to the core radius of the halo mass distribution and to the scale length, surface mass density, and radial scale height variations of the thin disk.

astro-ph↗

arXiv1997

Detection of the 2175 angstrom dust feature in Mg II absorption systems

The broad absorption bump at 2175 angstrom due to dust, which is ubiquitous in the Galaxy and is seen in the Magellanic clouds, is also seen in a composite spectrum of Mg II absorbers. The composite absorber spectrum is obtained by taking the geometric mean of 92 quasar spectra after aligning them in the restframe of 96 absorbers. By aligning the spectra according to absorber redshifts we reinforce the spectral features of the absorbers, and smooth over possible bumps and wiggles in the emission spectra as well as small features in the flat fielding of the spectra. The width of the observed absorption feature is 200-300 angstrom (FWHM), or 0.4-0.6 (micron)^{-1} and the central wavelength is 2240 angstrom. These are somewhat different from the central wavelength of 2176 angstrqom and FWHM=0.8-1.25 (micron)^{-1} found in the Galaxy. Simulations show that this discrepancy between the properties of the 2175 angstrom feature in Mg II absorbers and Galactic ISM can be mostly explained by the different methods used to measure them.

astro-ph↗

arXiv1997

A Robust Determination of the Time Delay in 0957+561A,B and a Measurement of the Global Value of Hubble's Constant

Photometric monitoring of the gravitational lens system 0957+561A,B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g band event in the trailing (B) image light curve at the precise time predicted from the observation of an event during 1995 in the leading (A) image with a delay of 415 days. This success confirms the "short delay," and the lack of any feature at a delay near 540 days rejects the "long delay" for this system, resolving a long-standing controversy. A series of statistical analyses of our light curve data yield a best fit delay of 417 +/- 3 days (95% confidence interval). Recent improvements in the modeling of the lens system (consisting of a galaxy and cluster) allow us to derive a value of the global (at z = 0.36) value of Hubble's constant H_0 using Refsdal's method, a simple and direct distance determination based on securely understood physics and geometry. The result is H_0 = 63 +/- 12 km/s/Mpc (for Omega = 1) where this 95% confidence interval is dominated by remaining lens model uncertainties.

astro-ph↗

arXiv1996

Through a Lens Darkly: Evidence for Dusty Gravitational Lenses

Foreground galaxies that amplify the light from background quasars may also dim that light if the galaxies contain enough dust. Extinction by dust in lenses could hide the large number of lensed systems predicted for a flat universe with a large value of the cosmological constant $Λ$. We look for one signature of dust, namely reddening, by examining optical-infrared colors of gravitationally lensed images of quasars. We find that the lensed systems identified in radio and infrared searches have redder optical-IR colors than optically selected ones. This could be due to a bias against selecting reddened (hence extincted) quasars in the optical surveys, or due to the differences in the intrinsic colors of optical and radio quasars. Comparison of the radio-selected lensed and unlensed quasars shows that the lensed ones have redder colors. We therefore conclude that at least part of the color difference between the two lens samples is due to dust. From the color difference between lensed and unlensed radio quasars (and assuming Galactic extinction law) we can reconcile a large cosmological constant ($Λ=0.9$) with the number of lensed systems observed in flux limited optical surveys. These results substantially weaken the strongest constraint on cosmological scenarios that invoke a non-zero cosmological constant to explain age discrepancy problems, satisfy predictions of inflationary models of the early universe and play a role in large scale structure formation models. They also raise the prospect of using gravitational lenses to study the interstellar medium in high redshift galaxies.

astro-ph↗