arXiv subjects

K. Y. Lo

Publications and source records attributed to K. Y. Lo.

45 records · Page 3Linked to original sources

arXiv1998

Mid-Infrared Images of Luminous Infrared Galaxies in a Merging Sequence

We report mid-infrared observations of several luminous infrared galaxies (LIGs) carried out with the Infrared Space Observatory. Our sample was chosen to represent different phases of a merger sequence of galaxy-galaxy interaction with special emphasis on early/intermediate stages of merging. The mid-infrared emission of these LIGs shows extended structures for the early and intermediate mergers, indicating that most of the mid-infrared luminosities are not from a central active galactic nucleus (AGN). Both the infrared hardness (indicated by the IRAS 12, 25, and 60 $\micron$ flux density ratios) and the peak-to-total flux density ratios of these LIGs increase as projected separation of these interacting galaxies become smaller, consistent with increasing star formation activities that are concentrated to a smaller area as the merging process advances. These observations provide among the first observational constraint of largely theoretically based scenarios.

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arXiv1998

Luminous IR Galaxies in a Merger Sequence: BIMA CO Imaging

In order to trace observationally the conditions in the interstellar medium (ISM) that lead to starbursts, we have used the newly expanded Berkeley-Illinois-Maryland Association (BIMA) millimeter-wave array to map the molecular ISM in a sample of LIGs chosen to represent different phases of the interacting/merging process. CO images of 6 galaxy mergers at the early/intermediate stage are shown here and preliminary results are summarized.

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arXiv1998

Intrinsic Size and Shape of Sgr A*: 3.6 AU by <1 AU

By means of near-simultaneous multi-wavelength VLBA measurements, we determine for the first time the intrinsic size of Sgr A* to be 3.6 AU by <1 AU with the major axis oriented essentially north-south. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at wavelength shorter than 1 mm, we can discern the intrinsic source size at 7~mm because (1) the scattering size along the minor axis is half that along the major axis, and (2) the near simultaneous multi-wavelength mapping of Sgr A* makes it possible to extrapolate precisely the minor axis scattering angle at 7~mm. The intrinsic size and shape place direct constraints on the various theoretical models for Sgr A*.

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arXiv1998

Intrinsic Size OF Sgr A*: 72 Schwarzschild Radii

Recent proper motion studies of stars at the very center of the Galaxy strongly suggest that Sagittarius (Sgr) A*, the compact nonthermal radio source at the Galactic Center, is a 2.5 million solar mass black hole. By means of near-simultaneous multi-wavelength Very Long Baseline Array measurements, we determine for the first time the intrinsic size and shape of Sgr A* to be 72 Rsc by < 20 Rsc, with the major axis oriented essentially north-south, where Rsc (= 7.5 x 10^{11} cm) is the Schwarzschild radius for a 2.5 million solar mass black hole. Contrary to previous expectation that the intrinsic structure of Sgr A* is observable only at wavelengths shorter than 1 mm, we can discern the intrinsic source size at 7 mm because (1) the scattering size along the minor axis is half that along the major axis, and (2) the near simultaneous multi-wavelength mapping of Sgr A* with the same interferometer makes it possible to extrapolate precisely the minor axis scattering angle at 7 mm. The intrinsic size and shape place direct constraints on the various emission models for Sgr A*. In particular, the advection dominated accretion flow model may have to incorporate a radio jet in order to account for the structure of Sgr A*.

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arXiv1997

ISO measurements of [CII] line variations in galaxies

We report measurements of the [CII] fine structure line at 157.714 micron in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared luminosity, [CII]/FIR, measures the ratio of the cooling of gas to that of dust; and thus the efficiency of the grain photoelectric heating process. This ratio varies by a factor of 40 in the current sample. About two-thirds of the galaxies have [CII]/FIR = 2-7 x 10^{-3}. The other one-third show trends of decreasing [CII]/FIR with increasing dust temperature, indicated by the ratio of infrared emission at 60 and 100 microns; and with increasing star-formation activity, measured by the ratio of far-infrared and blue band luminosity. We find three FIR bright galaxies with undetected [CII] line at 3-sigma upper limits of [CII]/FIR < 0.5-2 x 10^{-4}. The trend in the [CII]/FIR ratio with the temperature of dust and with star-formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed PDR lines have among the highest ratios of FIR to Blue luminosity and the ratio of 60 and 100 micron emission. Their lack of [CII] lines may be due to a continuing trend of decreasing [CII]/FIR with increasing star-formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case the upper limits on [CII]/FIR imply a ratio of UV flux to gas density G_0/n > 10 cm$^3. The low [CII]/FIR could also be due to either weak [CII] because of self-absorption or strong FIR continuum from regions weak in [CII], such as dense HII regions or plasma ionized by hard radiation of AGNs.

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arXiv1996

Pre-Starbursts in Luminous IR Galaxies ?

We present first results of our on-going BIMA Key Project: imaging the CO(1-0) emission from a sample of 10 luminous IR galaxies (LIRGs) that are at various merging stages, with special emphasis on systems apparently in the early/intermediate stages of merging. We present here CO images with $\sim 5''$ resolution. An important result is the recognition of a plausible pre-starburst phase in some early LIRG mergers (e.g., Arp 302 and NGC 6670). Our initial analysis suggests that a merger-induced starburst phase may not begin before the nuclear separation between the merging galaxies reaches roughly 10 kpc. The surface gas density seems to increase from a few times $10^2 \Msun pc^{-2} to >10^3 \Msun pc^{-2}$ while the prominent CO extent systematically decreases as merging progresses.

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arXiv1996

Arp 302: Non-starburst Luminous Infrared Galaxies

Arp 302, a luminous infrared source (L_{IR} = 4.2x10^{11} Lsun), consisting of two spiral galaxies (VV340A and VV340B) with nuclear separation of 40'', has the highest CO luminosity known. Observations with the BIMA array at 5'' X 7'' resolution reveal that the CO emission is extended over 23.0 kpc in the edge-on spiral galaxy, VV340A, corresponding to 6.7x10^{10} Msun of H_2. In the companion face-on galaxy, VV340B, the CO emission is extended over ~10.0 kpc, with 1.1x10^{10} Msun of H_2. The large CO extent is in strong contrast to starburst systems, such as Arp 220, in which the CO extent is typically $\le$ 1 kpc. Furthermore, $L_{IR}/M$(H_2) is found to be $\le$ 6.0 Lsun/Msun throughout both galaxies. Thus the high IR luminosity of Arp 302 is apparently not due to starbursts in the nuclear regions, but is due to its unusually large amount of molecular gas forming stars at a rate similar to giant molecular clouds in the Milky Way disk. Arp 302 consists of a pair of very gas-rich spiral galaxies that may be interacting and in a phase before a likely onset of starbursts.

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arXiv1996

ISOCAM Observations of NGC 6946: Mid-IR Structure

The nearby spiral galaxy NGC 6946 was observed with ISO-CAM in the mid-infrared, achieving 7 arcsec resolution and sub-MJy/steradian sensitivity. Images taken with CAM filters LW2 (7 microns) and LW3 (15 microns) are analysed to determine the morphology of this galaxy and understand better the emission mechanisms. The mid-infrared emission follows an exponential disk with a scale length 75 arcsec. This is 60% of the scale length in the optical R-band and radio continuum. The nuclear starburst region is too bright for reliable measurement in these images. Its surface brightness exceeds the inner disk by at least a factor of 12. The arms and interarm regions are clearly detected, with each of these components contributing about equally to the disk emission. The arm-interarm contrast is 2-4 in the mid-IR, close to that measured in the visible R band light and lower than the contrast in $Hα$, suggesting that non-ionizing radiation contributes significantly to dust heating.

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arXiv1996

The Mid-Infrared Color of NGC 6946

We analyze the new mid-infrared maps of NGC 6946 for variations in the color ratio of the 7-to-15 micron emission. Our preliminary findings are that this mid-infrared color is remarkably constant between arms and inter-arm regions, and as a function of radius in the disk, excluding the nuclear region. As surface brightness ranges by more than an order of magnitude and the radius runs from about 0.5 to 3kpc, the color ratio remains constant to about +/-20%. Our interpretation is that (1) hard UV radiation from OB stars does not dominate the heating of the grains radiating in the mid-infrared; and (2) that surface brightness variations are driven primarily by surface-filling fraction in the disk, and by radiation intensity increases in starburst environments, such as the nucleus of NGC 6946.

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