arXiv subjects

Emmanuel Momjian

Publications and source records attributed to Emmanuel Momjian.

104 records · Page 6Linked to original sources

arXiv2011

Imaging of the CCS 22.3 GHz emission in the Taurus Molecular Cloud complex

Thioxoethenylidene (CCS) is an abundant interstellar molecule, and a good tracer of high density and evolutionary stage of dense molecular clouds. It is also a suitable candidate for Zeeman splitting observations for its high splitting factor and narrow thermal linewidths. We report here EVLA 22.3 GHz observations of three dense molecular cores TMC-1, TMC-1C and L1521B in the Taurus Molecular Cloud complex to image the CCS 2_1-1_0 transition. For all three sources, the clumpy CCS emission is most likely tracing the starless cores. However, these compact structures account for only ~ 1-13% of the integrated emission detected in single-dish observations, indicating the presence of significant large scale diffuse emission in favorable conditions for producing CCS.

astro-ph.GA↗

arXiv2010

Deep 21-cm HI Observations at z~0.1: The Precursor to the Arecibo Ultra Deep Survey

The "ALFA Ultra Deep Survey" (AUDS) is an ongoing 21-cm spectral survey with the Arecibo 305m telescope. AUDS will be the most sensitive blind survey undertaken with Arecibo's 300 MHz Mock spectrometer. The survey searches for 21-cm HI line emission at redshifts between 0 and 0.16. The main goals of the survey are to investigate the HI content and probe the evolution of HI gas within that redshift region. In this paper, we report on a set of precursor observations with a total integration time of 53 hours. The survey detected a total of eighteen 21-cm emission lines at redshifts between 0.07 and 0.15 in a region centered around ra~0:00h, dec= 15:42deg. The rate of detection is consistent with the one expected from the local HI mass function. The derived relative HI density at the median redshift of the survey is rho_HI[z=0.125]=(1.0+/-0.3)*rho_0, where rho_0 is the HI density at zero redshift.

astro-ph.CO↗

arXiv2010

A GBT Survey for HI 21 cm Absorption in the Disks and Halos of Low-Redshift Galaxies

We present an HI 21 cm absorption survey with the Green Bank Telescope (GBT) of galaxy-quasar pairs selected by combining data from the Sloan Digital Sky Survey (SDSS) and the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. Our sample consists of 23 sightlines through 15 low-redshift foreground galaxy - background quasar pairs with impact parameters ranging from 1.7 kpc up to 86.7 kpc. We detected one absorber in the GBT survey from the foreground dwarf galaxy, GQ1042+0747, at an impact parameter of 1.7 kpc and another possible absorber in our follow-up Very Large Array (VLA) imaging of the nearby foreground galaxy, UGC 7408. Both of the absorbers are narrow (FWHM of 3.6 and 4.8 km/s), have sub Damped Lyman alpha column densities, and most likely originate in the disk gas of the foreground galaxies. We also detected H I emission from three foreground galaxies, including UGC 7408. Although our sample contains both blue and red galaxies, the two H I absorbers as well as the H I emissions are associated with blue galaxies. We discuss the physical conditions in the 21 cm absorbers and some drawbacks of the large GBT beam for this type of survey.

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arXiv2010

Total Molecular Gas Masses of z~3 Lyman-Break Galaxies: CO(1-0) Emission in MS1512-cB58 and the Cosmic Eye

We report the detection of CO(1-0) emission toward the lensed L*_UV Lyman-break galaxies (LBGs) MS1512-cB58 (z=2.73) and the Cosmic Eye (z=3.07), using the Expanded Very Large Array. The strength of the CO line emission reveals molecular gas reservoirs with masses of (4.6+/-1.1) x 10^8 (mu_L/32)^-1 (alpha_CO/0.8) Msun and (9.3+/-1.6) x 10^8 (mu_L/28)^-1 (alpha_CO/0.8) Msun, respectively. These observations suggest by ~30%-40% larger gas reservoirs than estimated previously based on CO(3-2) observations due to subthermal excitation of the J=3 line. These observations also suggest gas mass fractions of 0.46+/-0.17 and 0.16+/-0.06. The CO(1-0) emission in the Cosmic Eye is slightly resolved on scales of 4.5"+/-1.5", consistent with previous studies of nebular emission lines. This suggests that the molecular gas is associated with the most intensely star-forming regions seen in the ultraviolet (UV). We do not resolve the CO(1-0) emission in cB58 at ~2" resolution, but find that the CO(1-0) emission is also consistent with the position of the UV-brightest emission peak. The gas masses, gas fractions, moderate CO line excitation, and star formation efficiencies in these galaxies are consistent with what is found in nearby luminous infrared galaxies. These observations thus currently represent the best constraints on the molecular gas content of `ordinary' (i.e., ~L*_UV) z~3 star-forming galaxies. Despite comparable star formation rates, the gas properties of these young LBGs seem to be different from the recently identified optical/infrared-selected high-z massive, gas-rich star-forming galaxies, which are more gas-rich and massive, but have lower star formation efficiencies, and presumably trace a different galaxy population.

astro-ph.CO↗

arXiv2010

A Radio Spectral Line Study of the 2-Jy IRAS-NVSS Sample: Part I

We present results from an on-going survey for the HI 21 cm line and the OH 18 cm lines in IR galaxies with the Arecibo 305 m Radio Telescope. The observations of 85 galaxies extracted from the 2 Jy IRAS-NVSS sample in the R.A. (B1950) range 20 h-00 h are reported in this paper. We detected the HI 21 cm line in 82 of these galaxies, with 18 being new detections, and the OH 18 cm lines in 7 galaxies, with 4 being new detections. In some cases, the HI spectra show the classic double-horned or single-peaked emission profiles. However, the majority exhibit distorted HI spectral features indicating that the galaxies are in interacting and/or merging systems. From these HI and OH observations, various properties of the sample are derived and reported.

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arXiv2010

Using 21cm Absorption in Small Impact Parameter Galaxy-QSO Pairs to Probe Low-Redshift Damped and Sub-Damped Lyman-alpha System

To search for low-redshift damped Lyman-alpha (DLA) and sub-DLA quasar absorbers, we have conducted a 21cm absorption survey of radio-loud quasars at small impact parameters to foreground galaxies selected from the Sloan Digital Sky Survey (SDSS). Here we present the first results from this survey based on observations of SDSS J104257.58+074850.5 ($z_{QSO}$ = 2.66521), a quasar at an angular separation from a foreground galaxy ($z_{gal}$ = 0.03321) of 2.5" (1.7 kpc in projection). The foreground galaxy is a low-luminosity spiral with on-going star formation (0.004 M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$) and a metallicity of $-0.27 \pm 0.05$ dex. We detect 21cm absorption from the galaxy with the Green Bank Telescope (GBT), the Very Large Array (VLA), and the Very Long Baseline Array (VLBA). The absorption appears to be quiescent disk gas corotating with the galaxy and we do not find any evidence for outflowing cold neutral gas. The width of the main absorption line indicates that the gas is cold, $T_{k} < 283$ K, and the HI column is surprisingly low given the impact parameter of 1.7 kpc; we find that N(HI) $\leq 9.6 \times 10^{19}$ cm$^{-2}$ (GBT) and N(HI) $\leq 1.5 \times 10^{20}$ cm$^{-2}$ (VLBA). VLBA marginally resolves the continuum source and the absorber, and a lower limit of 27.1 $\times$ 13.9 pc is derived for the size of the absorbing cloud. In turn, this indicates a low density for a cold cloud, n(HI) $<$ 3.5 cm$^{-3}$. We hypothesize that this galaxy, which is relatively isolated, is becoming depleted in HI because it is converting its interstellar matter into stars without a replenishing source of gas, and we suggest future observations to probe this and similar galaxies.

astro-ph.CO↗

arXiv2009

Constraints on the presence of water megamaser emission in z~2.5 ultraluminous infrared starburst galaxies

We present Expanded Very Large Array and Arecibo observations of two lensed submm galaxies at z~2.5, in order to search for redshifted 22.235 GHz water megamaser emission. Both SMM J14011+0252 and SMM J16359+6612 have multi-wavelength characteristics consistent with ongoing starburst activity, as well as CO line emission indicating the presence of warm molecular gas. Our observations do not reveal any evidence for H2O megamaser emission in either target, while the lensing allows us to obtain deep limits to the H_2O line luminosities, L(H2O) < 7470 Lsun (3-sigma) in the case of SMM J14011+0252, and L(H2O) < 1893 Lsun for SMM J16359+6612, assuming linewidths of 80 km/s. Our search for, and subsequent non-detection of H2O megamaser emission in two strongly lensed starburst galaxies, rich in gas and dust, suggests that such megamaser emission is not likely to be common within the unlensed population of high-redshift starburst galaxies. We use the recent detection of strong H2O megamaser emission in the lensed quasar, MG J0414+0534 at z = 2.64 to make predictions for future EVLA C-band surveys of H2O megamaser emission in submm galaxies hosting AGN.

astro-ph.CO↗

arXiv2008

The Arecibo Legacy Fast ALFA Survey VII: A Neutral Hydrogen Cloud Complex in the Virgo Cluster

We present observations of an HI cloud complex most likely located in the Virgo galaxy cluster, first reported by Kent et al. (2007). The complex consists of five clouds, detected in the data set of the ALFALFA extragalactic HI survey at Arecibo. The clouds have radial velocities between cz~480 and 610 km/s. At the Virgo cluster distance, they are spread over a projected span of 170 kpc and have HI masses ranging from 0.48 to 1.7 x 10^8 Msun. The overall HI mass of the complex is 5.1 x 10^8 Msun. The clouds' velocity widths vary between 50 and 250 km/s. New results of follow-up aperture synthesis observations conducted with the Very Large Array are also presented, which yield a higher resolution view of two of the clouds in the complex. These two resolved clouds show no evidence of symmetry in the gas distribution or of any ordered motions. The possibility that the complex is a group of primordial objects, embedded in their own dark matter halo is thought to be unlikely. Scenarios in which the clouds have been removed from the disk of a galaxy traveling at high speed through the intracluster medium are considered. The most likely among those is thought to be one where the clouds were separated from NGC 4445 at a time >0.5 Gyr ago. The orbital velocity of the clouds and the putative parent galaxy would now be seen at a relatively large angle with respect to the line of sight.

astro-ph↗

arXiv2008

Formation of a Quasar Host Galaxy through a Wet Merger 1.4 Billion Years after the Big Bang

We present high-resolution Very Large Array imaging of the molecular gas in the host galaxy of the high redshift quasar BRI 1335-0417 (z=4.41). Our CO(2-1) observations have a linear resolution of 0.15" (1.0 kpc) and resolve the molecular gas emission both spatially and in velocity. The molecular gas in BRI 1335-0417 is extended on scales of 5 kpc, and shows a complex structure. At least three distinct components encompassing about two thirds of the total molecular mass of 9.2 x 10^10 M_sun are identified in velocity space, which are embedded in a structure that harbors about one third of the total molecular mass in the system. The brightest CO(2-1) line emission region has a peak brightness temperature of 61+/-9 K within 1 kpc diameter, which is comparable to the kinetic gas temperature as predicted from the CO line excitation. This is also comparable to the gas temperatures found in the central regions of nearby ultra-luminous infrared galaxies, which are however much more compact than 1 kpc. The spatial and velocity structure of the molecular reservoir in BRI 1335-0417 is inconsistent with a simple gravitationally bound disk, but resembles a merging system. Our observations are consistent with a major, gas-rich (`wet') merger that both feeds an accreting supermassive black hole (causing the bright quasar activity), and fuels a massive starburst that builds up the stellar bulge in this galaxy. Our study of this z>4 quasar host galaxy may thus be the most direct observational evidence that `wet' mergers at high redshift are related to AGN activity.

astro-ph↗

arXiv2007

Optically Unseen HI Detections towards the Virgo Cluster detected in the Arecibo Legacy Fast ALFA Survey

We report the discovery by the Arecibo Legacy Fast ALFA (ALFALFA) survey of eight HI features not coincident with stellar counterparts in the Virgo Cluster region. All of the HI clouds have cz < 3000 km/s and, if at the Virgo distance, HI masses between 1.9 x 10**7 and 1.1 x 10**9 solar masses. Four of the eight objects were reported or hinted at by previous studies and "rediscovered" by ALFALFA. While some clouds appear to be associated with optical galaxies in their vicinity, others show no clear association with a stellar counterpart. Two of them are embedded in relatively dense regions of the cluster and are associated with M49 and M86; they were previously known. The others are mostly located in peripheral regions of the cluster. Especially notable are a concentration of objects towards the so-called M cloud, 3 to 5 degrees to the NW of M87, and a complex of several clouds projected roughly halfway between M87 and M49. The object referred to as VIRGOHI21 and proposed to be a "dark galaxy" is also detected and shown to be a tidal feature associated with NGC 4254.

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arXiv2005

The Arecibo Legacy Fast ALFA Survey: II. Results of Precursor Observations

In preparation for the full Arecibo Legacy Fast ALFA extragalactic HI survey, precursor observations were carried out in Aug--Sep 2004 with the 7-beam Arecibo L-band feed array (ALFA) receiver system and the WAPP spectral processors. While these observations were geared mainly at testing and debugging survey strategy, hardware and software, approximately 36 hours of telescope time yielded science--quality data. From those observations, an initial list of 730 tentative detections of varying degree of reliability was extracted. Ninety--eight high signal-to-noise candidates were deemed to be bona fide HI line detections. To test our ability to discriminate cosmic signals from RFI and noise, 165 candidates ranging in reliability likelihood were re--observed with the single beam L--band wide system at Arecibo in Jan--Feb 2005. Of those, 41% were confirmed as real. We present the results of both the ALFA and single beam observations for the sample of 166 confirmed HI sources, as well as our assessment of their optical counterparts. Of the 166 sources, 62 coincide with previously known HI sources, while optical redshifts were available for an additional 18 galaxies; thus, 52% of the redshifts reported here were previously unknown. Of the 166 HI detections, 115 are identified with previously cataloged galaxies, of either known or unknown redshift, leaving 51 objects identified for the first time. Because of the higher sensitivity of the Arecibo system, fewer than 10% of the 166 HI sources would have been detected by a HIPASS--like survey of the same region. Three of the objects have HI masses less than 10^7 solar masses. The full ALFALFA survey which commenced in February 2005 should detect more than 100 times as many objects of similarly low HI mass over the next 5 years.

astro-ph↗

arXiv2004

Absorption Line Study of Halo Gas in NGC 3067 Toward the Background Quasar 3C 232

We present new H I 21 cm absorption data and ultraviolet spectroscopy from HST/STIS of the QSO/galaxy pair 3C 232/NGC 3067. The QSO sightline lies near the minor axis and 1.8 arcmin (11 kpc) above the plane of NGC 3067, a nearby luminous (cz = 1465 km/s, L = 0.5L*) starburst galaxy with a moderate star formation rate of 1.4 Solar masses per year. The UV spectra show that the Si IV and C IV doublets have the same three velocity components at cz = 1369, 1417, and 1530 km/s found in Ca II H & K, Na I D, Mg I, Mg II, and Fe II, implying that the low and high ionization gas are both found in three distinct absorbing clouds (only the strongest component at 1420 km/s is detected in H I 21 cm). The new Lyman alpha observation allows the first measurements of the spin and kinetic temperatures of halo gas: T_s = 435 +/- 140 K and T_k/T_s ~ 1. However, while a standard photoionization model can explain the low ions, the C IV and Si IV are explained more easily as collisionally-ionized boundary layers of the photoionized clouds. Due to their small inferred space velocity offsets (-260, -130, and +170 km/s) relative to the nucleus of NGC 3067 and the spatial coincidence of low and high ionization gas, we propose that these absorbers are analogous to Galactic high velocity clouds (HVCs). A comparison of the NGC 3067 clouds and Galactic HVCs finds similar H I column densities, kinematics, metallicities, spin temperatures, and inferred sizes. We find no compelling evidence that any halo gas along this sightline is escaping the gravitational potential of NGC 3067, despite its modest starburst.

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arXiv2003

VLBA observations of z > 4 radio-loud quasars

We present high resolution (< 20 mas) observations of the radio continuum emission at 1.4 GHz from three high-redshift quasars: J1053-0016 (z=4.29), 1235-0003 (z=4.69), and J0913+5919 (z=5.11), thereby doubling the number of z > 4 radio-loud quasars that have been imaged at mas resolution. The observations were carried out with the Very Long Baseline Array (VLBA) of the NRAO. All three sources are unresolved in these observations, with source size limits of a few mas. In all cases the flux densities measured by the VLBA are within 10% of those measured with the VLA, implying that the sources are not highly variable on yearly timescales. We find no indication for multiple images that might be produced by strong gravitational lensing on scales from 20 mas (VLBA) to a few arcseconds (VLA), to dynamic range limits of ~100.

astro-ph↗

arXiv1998

Luminosity indicators in dusty photoionized environments

The luminosity of the central source in ionizing radiation is an essential parameter in a photoionized environment, and one of the most fundamental physical quantities one can measure. We outline a method of determining luminosity for any emission-line region using only infrared data. In dusty environments, grains compete with hydrogen in absorbing continuum radiation. Grains produce infrared emission, and hydrogen produces recombination lines. We have computed a very large variety of photoionization models, using ranges of abundances, grain mixtures, ionizing continua, densities, and ionization parameters. The conditions were appropriate for such diverse objects as H II regions, planetary nebulae, starburst galaxies, and the narrow and broad line regions of active nuclei. The ratio of the total thermal grain emission relative to H$β$ (IR/H$β$) is the primary indicator of whether the cloud behaves as a classical Strömgren sphere (a hydrogen-bounded nebula) or whether grains absorb most of the incident continuum (a dust-bounded nebula). We find two global limits: when $IR/Hβ<100$ infrared recombination lines determine the source luminosity in ionizing photons; when $IR/Hβ\gg100$ the grains act as a bolometer to measure the luminosity.

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