arXiv subjects

Sangeeta Malhotra

Publications and source records attributed to Sangeeta Malhotra.

At least 109 records · Page 6Linked to original sources

arXiv2011

Searching for z~7.7 Lyman Alpha Emitters in the COSMOS Field with NEWFIRM

The study of Ly-alpha emission in the high-redshift universe is a useful probe of the epoch of reionization, as the Ly-alpha line should be attenuated by the intergalactic medium (IGM) at low to moderate neutral hydrogen fractions. Here we present the results of a deep and wide imaging search for Ly-alpha emitters in the COSMOS field. We have used two ultra-narrowband filters (filter width of ~8-9 °A) on the NEWFIRM camera, installed on the Mayall 4m telescope at Kitt Peak National Observatory, in order to isolate Ly-alpha emitters at z = 7.7; such ultra-narrowband imaging searches have proved to be excellent at detecting Ly-alpha emitters. We found 5-sigma detections of four candidate Ly-alpha emitters in a survey volume of 2.8 x 10^4 Mpc^3 (total survey area ~760 arcmin^2). Each candidate has a line flux greater than 8 x 10^-18 erg s^-1 cm^-2. Using these results to construct a luminosity function and comparing to previously established Ly-alpha luminosity functions at z = 5.7 and z = 6.5, we find no conclusive evidence for evolution of the luminosity function between z = 5.7 and z = 7.7. Statistical Monte Carlo simulations suggest that half of these candidates are real z = 7.7 targets, and spectroscopic follow-up will be required to verify the redshift of these candidates. However, our results are consistent with no strong evolution in the neutral hydrogen fraction of the IGM between z = 5.7 and z = 7.7, even if only one or two of the z = 7.7 candidates are spectroscopically confirmed.

astro-ph.CO↗

arXiv2011

X-ray Constraints on the Lyman-Alpha Escape Fraction

We have coadded X-ray flux of all known Lyman Alpha Emitters (LAEs) in the 4 Msec Chandra Deep Field South (CDF-S) region, to place sensitive upper limits on the average unobscured star-formation rate (SFR_X) in these galaxies. A very small fraction of Lyman-Alpha galaxies in the field are individually detected in the X-rays, implying a low fraction of AGN activity. After excluding the few X-ray detected LAEs, we stack the undetected LAEs located in the 4 Ms CDF-S data and 250 ks Extended CDF-S (ECDFS) data, and compute a 1-σupper limit on SFR_X < 14, 28, 28, 140, 440, 880 M$_{\sun}$ yr$^{-1}$ for LAEs located at z = 2.1, 3.1, 3.2, 4.5, 5.7 and 6.5, respectively. The upper limit of SFR_X in LAEs can be then be compared to SFR$_{Lyα}$ derived from Lyman-Alpha line and thus can constrain on the Lyman-Alpha escape fraction ($f^{Esc}_{Lyα}$). We derive a lower limit on f(Lyman-Alpha Escape) > 14% (84 % confidence level, 1-σlower limit) for LAEs at redshift z ~ 2.1 and z ~ 3.1-3.2. At z > 4, the current LAE samples are not of sufficient size to constrain SFR_X well. By averaging all the LAEs at z> 2, the X-ray non-detection constrains f(Lyman-Alpha Escape) > 17% (84 % confidence level, 1-σlower limit), and rejects f(Lyman-Alpha Escape) < 5.7% at the 99.87% confidence level from 2.1 < z < 6.5.

astro-ph.CO↗

arXiv2011

A Link to the Past: Using Markov Chain Monte Carlo Fitting to Constrain Fundamental Parameters of High-Redshift Galaxies

We have a developed a new method for fitting spectral energy distributions (SEDs) to identify and constrain the physical properties of high-redshift (4 < z < 8) galaxies. Our approach uses an implementation of Bayesian based Markov Chain Monte Carlo (PiMC^2) that allows us to compare observations to arbitrarily complex models and to compute 95% credible intervals that provide robust constraints for the model parameters. The work is presented in 2 sections. In the first, we test PiMC^2 using simulated SEDs to not only confirm the recovery of the known inputs but to assess the limitations of the method and identify potential hazards of SED fitting when applied specifically to high redshift (z>4) galaxies. Our tests reveal five critical results: 1) the ability to confidently constrain metallicity, population ages, and Av all require photometric accuracy better than what is currently achievable (i.e. less than a few percent); 2) the ability to confidently constrain stellar masses (within a factor of two) can be achieved without the need for high-precision photometry; 3) the addition of IRAC photometry does not guarantee that tighter constraints of the stellar masses and ages can be defined; 4) different assumptions about the star formation history can lead to significant biases in mass and age estimates; and 5) we are able to constrain stellar age and Av of objects that are both young and relatively dust free. In the second part of the paper we apply PiMC^2 to 17 4 6 galaxies. Using PiMC^2, we are able to constrain the stellar mass of these objects and in some cases their stellar age and find no evidence that any of these sources formed at a redshift much larger than z_f=8, a time when the Universe was ~ 0.6 Gyr old.

astro-ph.GA↗

arXiv2011

Predicting the Merger Fraction of Lyman alpha Emitters from Redshift z~3 to z~7

Rapid mass assembly, likely from mergers or smooth accretion, has been predicted to play a vital role in star-formation in high-redshift Lyman-alpha (Lya) emitters. Here we predict the major merger, minor merger, and smooth accreting Lya emitter fraction from z~3 to z~7 using a large dark matter simulation, and a simple physical model that is successful in reproducing many observations over this large redshift range. The central tenet of this model, different from many of the earlier models, is that the star-formation in Lya emitters is proportional to the mass accretion rate rather than the total halo mass. We find that at z~3, nearly 35% of the Lya emitters accrete their mass through major (3:1) mergers, and this fraction increases to about 50% at z~7. This imply that the star-formation in a large fraction of high-redshift Lya emitters is driven by mergers. While there is discrepancy between the model predictions and observed merger fractions, some of this difference (~15%) can be attributed to the mass-ratio used to define a merger in the simulation. We predict that future, deeper observations which use a 3:1 definition of major mergers will find >30% major merger fraction of Lya emitters at redshifts >3.

astro-ph.CO↗

arXiv2011

Search for z~6.96 Ly-alpha emitters with Magellan/IMACS in the COSMOS field

We report a search for z~6.96 Ly-alpha emitters (LAEs) using a Narrow-Band filter, centered at 9680 Angstroms, with the IMACS instrument on the Magellan telescope at Las Campanas Observatory. We obtain a sample of 6 Ly-alpha emitter candidates of luminosity ~10^42 erg/s in a total area of 465 square arcmin corresponding to a comoving volume of ~ 72000 Mpc^3. From this result, we derive a Ly-alpha luminosity function (LF) at z~6.96 and compare our sample with the only z~6.96 Ly-alpha emitter spectroscopically confirmed to date (Iye et al. 2006). We find no evolution between the z=5.7 and z~7 Ly-alpha luminosity functions, if a majority of our candidates are confirmed. Spectroscopic confirmation for this sample will enable more robust conclusions.

astro-ph.CO↗

arXiv2011

Sizing up Lyman-alpha and Lyman Break Galaxies

We show that populations of high redshift galaxies grow more luminous as they grow in linear size. This is because the brightness per unit area has a distinct upper limit due to the self-regulating nature of star-formation. As a corollary, we show that the observed increase in characteristic luminosity of Lyman Break Galaxies (L*) with time can be explained by their increase in size, which scales as 1/H(z). In contrast, Lyman-alpha selected galaxies have a characteristic, constant, small size between redshift z=2.25 and 6.5. Coupled with a characteristic surface brightness, this can explain their non-evolving ultraviolet continuum luminosity function. This compact physical size seems to be a critical determining factor in whether a galaxy will show Lyman-alpha emission. We base these conclusions on new size measurements for a sample of about 170 Lyman-alpha selected galaxies with Hubble Space Telescope broad-band imaging, over the redshift range 2.25 < z < 6. We combine these with a similar number of Lyman-alpha selected galaxies with half-light radii from the literature, and derive surface brightnesses for the entire combined sample.

astro-ph.CO↗

arXiv2011

First spectroscopic measurements of [OIII] emission from Lyman-alpha selected field galaxies at z ~ 3.1

We present the first spectroscopic measurements of the [OIII] 5007 A line in two z ~ 3.1 Lyman-alpha emitting galaxies (LAEs) using the new near-infrared instrument LUCIFER1 on the 8.4m Large Binocular Telescope (LBT). We also describe the optical imaging and spectroscopic observations used to identify these Lya emitting galaxies. Using the [OIII] line we have measured accurate systemic redshifts for these two galaxies, and discovered a velocity offset between the [OIII] and Ly-alpha lines in both, with the Lya line peaking 342 and 125 km/s redward of the systemic velocity. These velocity offsets imply that there are powerful outflows in high-redshift LAEs. They also ease the transmission of Lya photons through the interstellar medium and intergalactic medium around the galaxies. By measuring these offsets directly, we can refine both Lya-based tests for reionization, and Lya luminosity function measurements where the Lya forest affects the blue wing of the line. Our work also provides the first direct constraints on the strength of the [OIII] line in high-redshift LAEs. We find [OIII] fluxes of 7 and 36 x 10^-17 erg s^-1 cm^-2 in two z ~ 3.1 LAEs. These lines are strong enough to dominate broad-band flux measurements that include the line (in thiscase, K_s band photometry). Spectral energy distribution fits that do not account for the lines would therefore overestimate the 4000 A (and/or Balmer) break strength in such galaxies, and hence also the ages and stellar masses of such high-z galaxies.

astro-ph.CO↗

arXiv2010

Dust Extinction and Metallicities of Star-Forming Lyman Alpha Emitting Galaxies at Low Redshift

We present the results of an optical spectroscopic study of 12 GALEX-discovered star-forming Lyman alpha emitting galaxies (LAEs) at z ~ 0.3. We measure the emission line fluxes from these galaxies by fitting their observed spectra to stellar population models in order to correct for underlying stellar absorption. We revisit earlier stellar population model fitting results, finding that excluding now-known AGNs lowers the typical stellar population age and stellar mass of this sample to ~ 300 Myr and ~ 4 x 10^9 Msol, respectively. We calculate their dust extinction using the Balmer decrement, and find a typical visual attenuation of Av ~ 0.3 mag, similar to that seen in many high-redshift LAEs. Comparing the ratio of Lyalpha/Halpha and the Lyman alpha equivalent widths to the measured dust extinction, we find that the ISMs in these objects appear to be neither enhancing nor seriously attenuating the Lyman alpha equivalent widths, as would be the case in a quasi-clumpy ISM. Lastly, we perform a detailed analysis of the gas-phase metallicities of these galaxies, and we find that most galaxies in our sample have Z < 0.4 Zsol. We find that at a fixed stellar mass, these low-redshift LAE analogs are offset by ~ 0.6 dex lower in metallicity from the general galaxy population at similar redshifts based on the local mass-metallicity relationship. This implies that galaxies with Lyman alpha in emission may be systematically more metal poor than star-forming galaxies at the same stellar mass and redshift, similar to preliminary results at z ~ 2.

astro-ph.CO↗

arXiv2010

Spectroscopic Study of the HST/ACS PEARS Emission-Line Galaxies

We present spectroscopy of 76 emission-line galaxies (ELGs) in CDF-S taken with the LDSS3 spectrograph on Magellan Telescope. These galaxies are selected to have emission lines with ACS grism data in the Hubble Space Telescope Probing Evolution and Reionization Spectroscopically (PEARS) grism Survey. The ACS grism spectra cover the wavelength range 6000-9700 Å and most PEARS grism redshifts are based on a single emission line + photometric redshifts from broad-band colors; the Magellan spectra cover a wavelength range from 4000 Å to 9000 Å, and provide a check on redshifts derived from PEARS data. We find an accuracy of $σ_z$ = 0.006 for the ACS grism redshifts with only one catastrophic outlier. We probe for AGN in our sample via several different methods. In total we find 7 AGNs and AGN candidates out of 76 galaxies. Two AGNs are identified from the X-ray full-band luminosity, $L_{X-ray,FB}>10^{43}$ erg$\;$s$^{-1}$, the line widths and the power-law continuum spectra. Two unobscured faint AGN candidates are identified from the X-ray full-band luminosity $L_{X-ray,FB}\sim10^{41}$ erg$\;$s$^{-1}$, the hardness ratio and the column density, and the emission-line and X-ray derived SFRs. Two candidates are classified based on the line ratio of [NII]\lambda6584/H$α$ versus [OIII]$λ$5007/H$β$ (BPT diagram), which are between the empirical and theoretical demarcation curves, i.e, the transition region from star-forming galaxies to AGNs. One AGN candidate is identified from the high-ionization emission line HeIIÅ4686.

astro-ph.CO↗

arXiv2010

The Luminosity Function of Lyman alpha Emitters at Redshift z=7.7

Lyman alpha (Lya) emission lines should be attenuated in a neutral intergalactic medium (IGM). Therefore the visibility of Lya emitters at high redshifts can serve as a valuable probe of reionization at about the 50% level. We present an imaging search for z=7.7 Lya emitting galaxies using an ultra-narrowband filter (filter width= 9A) on the NEWFIRM imager at the Kitt Peak National Observatory. We found four candidate Lya emitters in a survey volume of 1.4 x 10^4 Mpc^3, with a line flux brighter than 6x10^-18 erg/cm^2/s (5 sigma in 2" aperture). We also performed a detailed Monte-Carlo simulation incorporating the instrumental effects to estimate the expected number of Lya emitters in our survey, and found that we should expect to detect one Lya emitter, assuming a non-evolving Lya luminosity function (LF) between z=6.5 and z=7.7. Even if one of the present candidates is spectroscopically confirmed as a z~8 Lya emitter, it would indicate that there is no significant evolution of the Lya LF from z=3.1 to z~8. While firm conclusions would need both spectroscopic confirmations and larger surveys to boost the number counts of galaxies, we successfully demonstrate the feasibility of sensitive near-infrared (1.06 um) narrow-band searches using custom filters designed to avoid the OH emission lines that make up most of the sky background.

astro-ph.CO↗

arXiv2010

A Physical Model of Lyman Alpha Emitters

We present a simple physical model for populating dark matter halos with Lyman Alpha Emiiters(LAEs) and predict the physical properties of LAEs at z~3-7. The central tenet of this model is that the Ly-alpha luminosity is proportional to the star formation rate (SFR) which is directly related to the halo mass accretion rate. The only free parameter in our model is then the star-formation efficiency (SFE). An efficiency of 2.5% provides the best-fit to the Ly-alpha luminosity function (LF) at redshift z=3.1, and we use this SFE to construct Ly-alpha LFs at other redshifts. Our model reproduce the Ly-alpha LFs, stellar ages, SFR ~1-10; Msun/yr, stellar masses ~ 10^7-10^8 Msun and the clustering properties of LAEs at z~3-7. We find the spatial correlation lengths ro ~ 3-6 Mpc/h, in agreement with the observations. Finally, we estimate the field-to-field variation ~ 30% for current volume and flux limited surveys, again consistent with observations. Our results suggest that the star formation, and hence Ly-alpha emission in LAEs is powered by the accretion of new material, and that the physical properties of LAEs do not evolve significantly over a wide range of redshifts. Relating the accreted mass, rather than the total mass of halos, to the Ly-alpha luminosity of LAEs naturally gives rise to the duty cycle of LAEs.

astro-ph.CO↗

arXiv2010

Hubble Space Telescope Imaging of Lyman Alpha Emission at z=4.4

We present the highest redshift detections of resolved Lyman alpha emission, using Hubble Space Telescope/ACS F658N narrowband-imaging data taken in parallel with the Wide Field Camera 3 Early Release Science program in the GOODS CDF-S. We detect Lyman alpha emission from three spectroscopically confirmed z = 4.4 Lyman alpha emitting galaxies (LAEs), more than doubling the sample of LAEs with resolved Lyman alpha emission. Comparing the light distribution between the rest-frame ultraviolet continuum and narrowband images, we investigate the escape of Lyman alpha photons at high redshift. While our data do not support a positional offset between the Lyman alpha and rest-frame ultraviolet (UV) continuum emission, the half-light radii in two out of the three galaxies are significantly larger in Lyman alpha than in the rest-frame UV continuum. This result is confirmed when comparing object sizes in a stack of all objects in both bands. Additionally, the narrowband flux detected with HST is significantly less than observed in similar filters from the ground. These results together imply that the Lyman alpha emission is not strictly confined to its indigenous star-forming regions. Rather, the Lyman alpha emission is more extended, with the missing HST flux likely existing in a diffuse outer halo. This suggests that the radiative transfer of Lyman alpha photons in high-redshift LAEs is complicated, with the interstellar-medium geometry and/or outflows playing a significant role in galaxies at these redshifts.

astro-ph.CO↗

arXiv2009

A Plethora of AGN Among Lyman Alpha Galaxies at Low Redshift

We investigate the fraction of z ~ 0.3 Lyman alpha emitting galaxies (LAEs) which host active galactic nucleus activity, which is typically from 1 - 5% at 2 < z < 6. Using optical spectroscopy of 23 LAEs at 0.2 < z < 0.45 selected with GALEX UV data, we probe for AGN with a variety of methods, including line widths, diagnostic line ratios, high-ionization emission, infrared activity and X-ray luminosity. We found that our sample of low-redshift LAEs has an AGN fraction of 43 ^{+18}{-26}%, significantly higher than at high redshift. While previous results have shown that low-redshift LAEs have a lower space density than their high-redshift analogs, these results show that star-forming LAEs at low-redshift are rarer still. Accounting for differences in available AGN classification methods, we conclude that rest-frame optical spectroscopy is necessary to identify low-luminosity AGNs in LAEs at high redshift, and that limits on the X-ray flux are necessary to determine the extent to which the AGN contaminates the Lyman alpha fluxes.

astro-ph.CO↗

arXiv2009

Early-type galaxies in the PEARS survey: Probing the stellar populations at moderate redshift

Using HST/ACS slitless grism spectra from the PEARS program, we study the stellar populations of morphologically selected early-type galaxies in the GOODS North and South fields. The sample - extracted from a visual classification of the (v2.0) HST/ACS images and restricted to redshifts z>0.4 - comprises 228 galaxies (F775W<24 ABmag) out to z~1.3 over 320 arcmin2, with a median redshift zM=0.75. This work significantly increases our previous sample from the GRAPES survey in the HUDF (18 galaxies over ~11 arcmin2; Pasquali et al. 2006b). The grism data allow us to separate the sample into `red' and `blue' spectra, with the latter comprising 15% of the total. Three different grids of models parameterising the star formation history are used to fit the low-resolution spectra. Over the redshift range of the sample - corresponding to a cosmic age between 5 and 10 Gyr - we find a strong correlation between stellar mass and average age, whereas the **spread** of ages (defined by the RMS of the distribution) is roughly ~1 Gyr and independent of stellar mass. The best-fit parameters suggest it is formation epoch and not formation timescale, that best correlates with mass in early-type galaxies. This result - along with the recently observed lack of evolution of the number density of massive galaxies - motivates the need for a channel of (massive) galaxy formation bypassing any phase in the blue cloud, as suggested by the simulations of Dekel et al. (2009).

astro-ph.CO↗

arXiv2009

Ly-alpha emitting galaxies at redshift z~4.5 in the LALA Cetus field

We present a large sample of Ly-alpha emitting galaxies spectroscopically confirmed at z~4.5, based on IMACS spectroscopic observations of candidate LAEs in the LALA Cetus field. We identify 110 z~4.5 LAEs based on single line detections with no continuum emission blueward of the line. The Ly-alpha confirmation rate varies from <50% to 76% for candidates selected in different narrowband filters at slightly different redshifts. We find a drop in the LAE density at redshift 4.50+-0.03 from redshift 4.39-4.47 by a factor of 66%, which could be a large scale void in the distribution of star-forming galaxies (~18Mpc along the line of sight and ~80Mpc across). The sample includes many objects with equivalent widths >200A. These large EW candidates are spectroscopically confirmed at the same rate as candidates with more modest EWs. A composite spectrum of all 110 confirmed LAEs shows the characteristic asymmetry of the Ly-alpha line. It also places new and stringent upper limits on the CIV 1549/Ly-alpha and HeII 1640/Ly-alpha line ratios, providing a new upper limit on the fraction of active galactic nuclei in Ly-alpha selected galaxy samples, and on the contribution of Pop III populations. Finally, we calculate the Ly-alpha luminosity function for our z~4.5 sample, which is consistent with those at other redshifts, showing that there is no evolution in Ly-alpha luminosity function from z~3.1-6.6.

astro-ph.CO↗

arXiv2009

Improved Photometric Redshifts with Surface Luminosity Priors

We apply Bayesian statistics with prior probabilities of galaxy surface luminosity (SL) to improve photometric redshifts. We apply the method to a sample of 1266 galaxies with spectroscopic redshifts in the GOODS North and South fields at 0.1 < z < 2.0. We start with spectrophotometric redshifts (SPZs) based on Probing Evolution and Reionization Spectroscopically grism spectra, which cover a wavelength range of 6000-9000A, combined with (U)BViz(JHK) broadband photometry in the GOODS fields. The accuracy of SPZ redshifts is estimated to be σ(Δ(z))=0.035 with an systematic offset of -0.026, where Δ(z)=Δz / (1+z), for galaxies in redshift range of 0.5 < z < 1.25. The addition of the SL prior probability helps break the degeneracy of SPZ redshifts between low redshift 4000 A break galaxies and high-redshift Lyman break galaxies which are mostly catastrophic outliers. For the 1138 galaxies at z < 1.6, the fraction of galaxies with redshift deviation Δ(z) > 0.2 is reduced from 15.0% to 10.4%, while the rms scatter of the fractional redshift error does not change much.

astro-ph.CO↗

arXiv2009

Evolution of Lyman Alpha Galaxies: Stellar Populations at z ~ 0.3

We present the results of a stellar population analysis of 30 Lyman alpha emitting galaxies (LAEs) at z ~ 0.3, previously discovered with the Galaxy Evolution Explorer (GALEX). With a few exceptions, we can accurately fit model spectral energy distributions to these objects, representing the first time this has been done for a large sample of LAEs at z < 3, a gap of ~ 8 Gyr in the history of the Universe. From the 26/30 LAEs which we can fit, we find an age and stellar mass range of 200 Myr - 10 Gyr and 10^9 - 10^11 Msol, respectively. These objects thus appear to be significantly older and more massive than LAEs at high-redshift. We also find that these LAEs show a mild trend towards higher metallicity than those at high redshift, as well as a tighter range of dust attenuation and interstellar medium geometry. These results suggest that low-redshift LAEs have evolved significantly from those at high redshift.

astro-ph↗

arXiv2009

The Star Formation Camera

The Star Formation Camera (SFC) is a wide-field (~15'x19, >280 arcmin^2), high-resolution (18x18 mas pixels) UV/optical dichroic camera designed for the Theia 4-m space-borne space telescope concept. SFC will deliver diffraction-limited images at lambda > 300 nm in both a blue (190-517nm) and a red (517-1075nm) channel simultaneously. Our aim is to conduct a comprehensive and systematic study of the astrophysical processes and environments relevant for the births and life cycles of stars and their planetary systems, and to investigate and understand the range of environments, feedback mechanisms, and other factors that most affect the outcome of the star and planet formation process. This program addresses the origins and evolution of stars, galaxies, and cosmic structure and has direct relevance for the formation and survival of planetary systems like our Solar System and planets like Earth. We present the design and performance specifications resulting from the implementation study of the camera, conducted under NASA's Astrophysics Strategic Mission Concept Studies program, which is intended to assemble realistic options for mission development over the next decade. The result is an extraordinarily capable instrument that will provide deep, high-resolution imaging across a very wide field enabling a great variety of community science as well as completing the core survey science that drives the design of the camera. The technology associated with the camera is next generation but still relatively high TRL, allowing a low-risk solution with moderate technology development investment over the next 10 years. We estimate the cost of the instrument to be $390M FY08.

astro-ph.IM↗