Narayanan, Gopal

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Research Assistant Professor, Department of Astronomy, College of Natural Sciences
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Astrophysics and Astronomy
Development of advanced m\m and sub-mm receivers.
Molecular clouds and star formation
Radio astronomy

Search Results

Now showing 1 - 10 of 13
  • Publication
    Detection of Infall Signatures toward Serpens SMM4
    (2002-01-01) Narayanan, Gopal; Moriarty-Schieven, G; Walker, CK; Butner, HM
    We present the detection of kinematic infall signatures toward the Class 0 protostellar system SMM4 in the Serpens cloud core. We have observed the dense molecular gas toward the embedded source using millimeter and submillimeter line transitions of density sensitive molecular tracers. High signal-to-noise ratio maps obtained in HCO+ J = 1 → 0, J = 3 → 2, and J = 4 → 3, and CS J = 2 → 1 show the blue-bulge infall signature. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. The line profiles of HCO+ and CS exhibit the characteristic blue asymmetric line profile signature consistent with infall. In addition, HCO+ and CS optical depth profiles obtained using isotopic observations show a red asymmetry also consistent with an infall interpretation. Using three-dimensional radiative transfer models based on the rotating, collapse model of Terebey, Shu, & Cassen, we derive infall parameters of the source. To determine the direction and orientation of molecular outflows in the larger Serpens cluster, wide-field mapping of CO J = 1 → 0 emission was also performed.
  • Publication
    Star formation in bright-rimmed clouds. I. Millimeter and submillimeter molecular line surveys
    (2002-01-01) De Vries, CH; Narayanan, Gopal; Snell, Ronald L.
    We present the results of the first detailed millimeter and submillimeter molecular line survey of bright-rimmed clouds, observed at FCRAO in the CO (J = 1 → 0), C18O (J = 1 → 0), HCO+ (J = 1 → 0), H13CO+ (J = 1 → 0), and N2H+ (J = 1 → 0) transitions, and at the Heinrich Hertz Telescope in the CO (J = 2 → 1), HCO+ (J = 3 → 2), HCO+ (J = 4 → 3), H13CO+ (J = 3 → 2), and H13CO+ (J = 4 → 3) molecular line transitions. The source list is composed of a selection of bright-rimmed clouds from the catalog of such objects compiled by Sugitani et al. We also present observations of three Bok globules done for comparison with the bright-rimmed clouds. We find that the appearance of the millimeter CO and HCO+ emission is dominated by the morphology of the shock front in the bright-rimmed clouds. The HCO+ (J = 1 → 0) emission tends to trace the swept-up gas ridge and overdense regions, which may be triggered to collapse as a result of sequential star formation. Five of the seven bright-rimmed clouds we observe seem to have an outflow; however, only one shows the spectral line blue-asymmetric signature that is indicative of infall in the optically thick HCO+ emission. We also present evidence that in bright-rimmed clouds the nearby shock front may heat the core from outside-in, thereby washing out the normally observed line infall signatures seen in isolated star-forming regions. We find that the derived core masses of these bright-rimmed clouds are similar to other low- and intermediate-mass star-forming regions.
  • Publication
    Multiple parsec-scale outflows in the NGC 2071 cluster
    (2008-01-01) Stojimirovic, I; Snell, Ronald L.; Narayanan, Gopal
    We present high-sensitivity large-scale mapping results of the NGC 2071 cluster in Orion and its surrounding environment in 12CO and 13CO J = 1→ 0 molecular line tracers. The data were obtained at the FCRAO 14 m telescope with an angular resolution of ~45''. We show that the known outflow in NGC 2071 has a larger extent than previously reported, reaching 1.7 pc at its lowest velocities. We also report the discovery of two new outflows: the bipolar north outflow, located approximately 4' north of the main outflow, and the southeast outflow, which originates near the main outflow and has only a redshifted component. We compare our molecular-line data to the H2 data of Eislöffel and find a morphological match between the CO and H2 outflows. We also identify possible driving sources for the two new outflows from the SCUBA image of this region. We use multiple lines of evidence, including the mass-velocity relation, position-velocity maps, and the morphological relationship between the H2 and CO emission, to distinguish between entrainment models for each of these flows. Jet entrainment is the dominant entrainment mechanism in the three outflows observed in NGC 2071. The mass of the main outflow, over velocities where the outflow dominates, is 14.8 M. Likewise, the mass of the southeast redshifted-only outflow is 7.6 M, suggesting that it may also be driven by a luminous YSO. The mass of the north outflow is much smaller (1.4 M) and is more consistent with outflows driven by low-mass stars.
  • Publication
    Large-scale structure of the molecular gas in Taurus revealed by high linear dynamic range spectral line mapping
    (2008-01-01) Goldsmith, PF; Heyer, M; Narayanan, Gopal; Snell, Ronald L.; Li, D; Brunt, C
    We report the results of a 100 deg2 survey of the Taurus molecular cloud region in 12CO and 13CO J = 1→ 0. The image of the cloud in each velocity channel includes 3 × 106 Nyquist-sampled pixels on a 20'' grid. The high sensitivity and large spatial dynamic range of the maps reveal a very complex, highly structured cloud morphology, including filaments, cavities, and rings. The axes of the striations seen in the 12CO emission from relatively diffuse gas are aligned with the direction of the magnetic field. We have developed a statistical method for analyzing the pixels in which 12CO but not 13CO is detected, which allows us to determine the CO column in the diffuse portion of the cloud, as well as in the denser regions in which we detect both isotopologues. Using a column-density-dependent model for the CO fractional abundance, we derive the mass of the region mapped to be 2.4 × 104 M, more than twice as large as would be obtained using a canonical fixed fractional abundance of 13CO, and a factor of 3 greater than would be obtained considering only the high column density regions. We determine that half the mass of the cloud is in regions having column density below 2.1 × 1021 cm−2. The distribution of young stars in the region covered is highly nonuniform, with the probability of finding a star in a pixel with a specified column density rising sharply for N(H2) = 6 × 1021 cm−2. We determine a relatively low star formation efficiency (mass of young stars/mass of molecular gas), between 0.3% and 1.2%, and an average star formation rate during the past 3 Myr of 8 × 10−5 s
  • Publication
    (2011-01-01) Snell, Ronald L.; Narayanan, Gopal; Yun, Min; Heyer, M; Chung, A; Irvine, William M.; Erickson, NR; Liu, G
    The 3 mm wavelength spectra of 10 galaxies have been obtained at the Five College Radio Astronomy Observatory using a new, very broadband receiver and spectrometer, called the Redshift Search Receiver (RSR). The RSR has an instantaneous bandwidth of 37 GHz covering frequencies from 74 to 111 GHz and has a spectral resolution of 31 MHz (~100 km s–1). During tests of the RSR on the FCRAO 14 m telescope the complete 3 mm spectra of the central regions of NGC 253, Maffei 2, NGC1068, IC 342, M82, NGC 3079, NGC 3690, NGC 4258, Arp 220, and NGC 6240 were obtained. Within the wavelength band covered by the RSR, 20 spectral lines from 14 different atomic and molecular species were detected. Based on simultaneous fits to the spectrum of each galaxy, a number of key molecular line ratios are derived. A simple model which assumes the emission arises from an ensemble of Milky Way like Giant Molecular Cloud cores can adequately fit the observed line ratios using molecular abundances based on Galactic molecular cloud cores. Variations seen in some line ratios, such as 13CO/HCN and HCO+/HCN, can be explained if the mean density of the molecular gas varies from galaxy to galaxy. However, NGC 3690, NGC 4258, and NGC 6240 show very large HCO+/HCN ratios and require significant abundance enhancement of HCO+ over HCN, possible due to the proximity to active galactic nucleus activity. Finally, the mass of dense molecular gas is estimated and we infer that 25%-85% of the total molecular gas in the central regions of these galaxies must have densities greater than 104 cm–3.
  • Publication
    Spectra of Nearby Galaxies Measured with a New Very Broadband Receiver
    (2008-01-01) Narayanan, Gopal; Snell, Ronald L.; Erickson, NR; Chung, Aeree; Heyer, Mark H.; Yun, Min; Irvine, William M.
    Three-millimeter-wavelength spectra of a number of nearby galaxies have been obtained at the Five College Radio Astronomy Observatory (FCRAO) using a new, very broadband receiver. This instrument, which we call the Redshift Search Receiver, has an instantaneous bandwidth of 36 GHz and operates from 74 to 110.5 GHz. The receiver has been built at UMass/FCRAO to be part of the initial instrumentation for the Large Millimeter Telescope (LMT) and is intended primarily for determination of the redshift of distant, dust-obscured galaxies. It is being tested on the FCRAO 14 m by measuring the 3 mm spectra of a number of nearby galaxies. There are interesting differences in the chemistry of these galaxies.
  • Publication
    Kinematics of Protostellar Objects in the ρ Ophiuchus A Region
    (2006-01-01) Narayanan, Gopal; Logan, DW
    We present the detection of infall, rotation, and outflow kinematic signatures toward both a protostellar source, VLA 1623, and what was initially thought to be a pre-protostellar core, SM 1N, in the ρ Ophiuchus A region. The kinematic signatures of early star formation were detected in the dense molecular gas surrounding the embedded sources using high signal-to-noise ratio millimeter and submillimeter data. Centroid velocity maps made with HCO+ J = 4 → 3 and J = 1 → 0 line emission exhibit the blue bulge signature of infall, which is predicted to be seen when infall motion dominates over rotational motion. Further evidence for infalling gas is found in the HCO+ blue asymmetric line profiles and red asymmetric opacity profiles. We also performed CO J = 3 → 2 and J = 1 → 0 observations to determine the direction, orientation, and extent of molecular outflows, and we report the discovery of a new bipolar outflow possibly driven by SM 1N.
  • Publication
    (2011-01-01) Chung, A; Yun, Min; Narayanan, Gopal; Heyer, M; Erickson, NR
    The feedback from galactic outflows is thought to play an important role in shaping the gas content, star formation history, and ultimately the stellar mass function of galaxies. Here we present evidence for massive molecular outflows associated with ultra-luminous infrared galaxies (ULIRGs) in the co-added Redshift Search Receiver 12CO (1-0) spectrum. Our stacked spectrum of 27 ULIRGs at z = 0.043-0.11 (νrest = 110-120 GHz) shows broad wings around the CO line with ΔV(FWZI) 2000 km s–1. Its integrated line flux accounts for up to 25% ± 5% of the total CO line luminosity. When interpreted as a massive molecular outflow wind, the associated mechanical energy can be explained by a concentrated starburst with star formation rate (SFR) ≥100 M yr–1, which agrees well with their SFR derived from the FIR luminosity. Using the high signal-to-noise stacked composite spectrum, we also probe 13CO and 12CN emission in the sample and discuss how the chemical abundance of molecular gas may vary depending on the physical conditions of the nuclear region.
  • Publication
    SuperCam: a 64-pixel heterodyne imaging array for the 870-micron atmospheric window (Proceedings Paper)
    (2006-01-01) Groppi, C; Walker, C; Kulesa, C; Putz, P; Golish, D; Gensheimer, P; Hedden, A; Bussmann, S; Weinreb, S; Kuiper, T; Kooi, J; Jones, G; Bardin, J; Mani, H; Lichtenberger, A; Narayanan, Gopal
    We report on the development of SuperCam, a 64 pixel, superheterodyne camera designed for operation in the astrophysically important 870 μm atmospheric window. SuperCam will be used to answer fundamental questions about the physics and chemistry of molecular clouds in the Galaxy and their direct relation to star and planet formation. The advent of such a system will provide an order of magnitude increase in mapping speed over what is now available and revolutionize how observational astronomy is performed in this important wavelength regime. Unlike the situation with bolometric detectors, heterodyne receiver systems are coherent, retaining information about both the amplitude and phase of the incident photon stream. From this information a high resolution spectrum of the incident light can be obtained without multiplexing. SuperCam will be constructed by stacking eight, 1×8 rows of fixed tuned, SIS mixers. The IF output of each mixer will be connected to a low-noise, broadband MMIC amplifier integrated into the mixer block. The instantaneous IF bandwidth of each pixel will be ~2 GHz, with a center frequency of 5 GHz. A spectrum of the central 500 MHz of each IF band will be provided by the array spectrometer. Local oscillator power is provided by a frequency multiplier whose output is divided between the pixels by using a matrix of waveguide power dividers. The mixer array will be cooled to 4K by a closed-cycle refrigeration system. SuperCam will reside at the Cassegrain focus of the 10m Heinrich Hertz telescope (HHT). A prototype single row of the array will be tested on the HHT in 2006, with the first engineering run of the full array in late 2007. The array is designed and constructed so that it may be readily scaled to higher frequencies.
  • Publication
    (2009-01-01) Chung, A; Narayanan, Gopal; Yun, Min; Heyer, M; Erickson, NR
    We present 12 CO J = 1 → 0 observations of ultraluminous infrared galaxies (ULIRGs) obtained using the Redshift Search Receiver (RSR) on the 14 m telescope of the Five College Radio Astronomy Observatory. The RSR is a novel, dual-beam, dual-polarization receiver equipped with an ultra-wideband spectrometer backend that is being built as a facility receiver for the Large Millimeter Telescope. Our sample consists of 29 ULIRGs in the redshift range of 0.04-0.11, including 10 objects with no prior 12 CO measurements. We have detected 27 systems (a detection rate of 93%), including 9 ULIRGs that are detected in CO for the first time. Our study has increased the number of local ULIRGs with CO measurements by ~15%. The CO line luminosity L'CO, correlates well with far-infrared luminosity L FIR, following the general trend of other local ULIRGs. However, compared to previous surveys we probe deeper into the low CO luminosity end of the ULIRG population as a single study by including a number of CO faint objects in the sample. As a result, we find (1) a smoother transition between the ULIRG population and local quasi-stellar objects (QSOs) in L FIR-L'CO ("star formation efficiency") space, and (2) a broader range of L FIR/L'CO flux ratio (~ 60-103 L /(K km s–1 pc2)) than previously reported. In our new survey, we also have found a small number of ULIRGs with extreme L FIR/L'CO, which had been known to be rare. The mid-IR color and radio-excess of 56 local ULIRGs as a function of FIR-to-CO flux ratio is examined and compared with those of spirals/starburst galaxies and low-z QSOs. In this paper, using a large sample of local ULIRGs we explore the origin of their current power source and potential evolution to QSOs.