arXiv subjects

Yu Gao

Publications and source records attributed to Yu Gao.

378 records · Page 21Linked to original sources

arXiv2003

Non-Nuclear Hyper/Ultraluminous X-Ray Sources in the Starbursting Cartwheel Ring Galaxy

We report the Chandra/ACIS-S detection of more than 20 ultraluminous X-ray sources (ULXs, L_{0.5-10 keV} >~ 3 x 10^{39} ergs/sec) in the Cartwheel collisional ring galaxy system, of which over a dozen are located in the outer active star-forming ring. A remarkable hyperluminous X-ray source (HLX, L_{0.5-10 keV} >~ 10^{41} ergs/sec assuming isotropic radiation), which dominates the X-ray emission from the Cartwheel ring, is located in the same segment of the ring as most ULXs. These powerful H/ULXs appear to be coincident with giant HII region complexes, young star clusters, and radio and mid-infrared hot-spots: all strong indicators of recent massive star formation. The X-ray spectra show that H/ULXs have similar properties as those of the {\it most luminous} ULXs found in the nearest starbursts and galaxy mergers such as the Antennae galaxies and M82. The close association between the X-ray sources and the starbursting ring strongly suggests that the H/ULXs are intimately associated with the production and rapid evolution of short-lived massive stars. The observations represent the most extreme X-ray luminosities discovered to date associated with star-forming regions--rivaling the X-ray luminosities usually associated with active galactic nuclei.

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arXiv2002

Azimuthal and Kinematic Segregation of Neutral and Molecular Gas in Arp 118: The Yin-Yang Galaxy NGC 1144

We present new high-resolution HI observations of the disk of the collisional infrared luminous (L$_{\rm IR}=2.2\times10^{11}$ L$_{\sun}$) galaxy NGC 1144, which reveal an apparent large-scale azimuthal and kinematic segregation of neutral hydrogen relative to the molecular gas distribution. Even among violently collisional galaxies, the CO/HI asymmetry in NGC 1144 is unusual, both in the inner regions, and in the outer disk. We suggest that we are observing Arp 118 at a special moment, shortly after a high-speed collision between NGC 1144 and its elliptical companion NGC 1143. HI emission with an average molecular fraction f$_{mol}$ $<$ 0.5 is observed on one side (NW) of the rotating disk of NGC 1144, while the other side (SE) is dominated by dense molecular complexes in which f$_{mol}$ is almost unity. The interface region between the warm-- and cool--cloud dominated regions, lies on a deep spiral-like dust-lane which we identify as a shock-wave responsible for the relative shift in the dominance of HI and H$_2$ gas. A strong shock being fed by diffuse HI clouds with unusually large ($>$ 400 km s$^{-1}$) rotational velocities can explain: 1) the CO/HI asymmetries, 2) a large velocity jump (185 km s$^{-1}$) across the arm as measured by HI absorption against a radio bright continuum source which straddles the arm, and 3) the asymmetric distribution of star formation and off-nuclear molecular gas resulting from likely streaming motions associated with the strong-shock. The new results provide for the first time a coherent picture of Arp 118's many peculiarities, and underlines the potentially complex changes in the gas-phase that can accompany large gravitational perturbations of gas-rich galaxies.

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arXiv2001

Distorted HI Gas in the Widely Separated LIRG Arp 256

We present new interferometric HI and CO (1-0) observations of the luminous infrared source, Arp 256. Arp 256 consists of two spiral galaxies in an early stage of merging, with a projected nuclear separation of 29 kpc (54") and an infrared luminosity of 2.0E11 L_sun. Despite the large separation of the galaxies' nuclei and mildly disrupted stellar components, the HI disks are found to be strongly disrupted, and the southern galaxy in Arp 256 shows an elevated star formation efficiency, which is consistent with a nuclear starburst. Both of these results run contrary to expectations, posing interesting questions on the physical mechanisms involved in stimulating star formation during an interaction.

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arXiv2001

Nature of widely separated ultraluminous infrared galaxies

In the complete sample of ultraluminous infrared galaxies (ULIRGs) compiled by Kim (1995) about 5% consists of widely separated galaxies which are presumably in the early phase of interaction. This fact is contrary to the conventional view that ULIRGs are in the final stages of the merger of two gas-rich disk galaxies. We have undertaken high resolution CO(J=1-0) observations for the ultraluminous infrared galaxies that have nuclear separations larger than 20 kpc. We have detected the CO emission in 5 out of 6 systems, but only in one component of the ULIRG pairs. 4 of them have LINER spectral type and 1 galaxy has Seyfert II spectral type. In K'-band images these components are also brighter than the other components which have either HII-region spectra or no detectable emission lines. Using the standard conversion factor, the molecular gas content is estimated to be a few times 10 $^{10}$ M$_\odot$, similar to that of the other ultraluminous galaxies. The result indicates that the galaxy containing the molecular gas is also the source of most, if not all, of the huge far-infrared luminosity of the system. The optical and K'-band imaging observations and optical spectra suggest multiple merger scenario for 1 system. If the remaining systems are in an early stage of a binary tidal interaction, the commonly accepted interpretation of the ULIRG phenomenon as the final merger stage of two disk galaxies may need to be re-examined.

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arXiv2001

Gas Distribution and Starburst Activity in the Widely Separated Interacting Galaxies NGC 6670

We present high resolution H I 21 cm line, 20 cm radio continuum, and CO(1-0) line observations of the luminous infrared galaxy NGC 6670. NGC 6670 consists of two edge-on disk galaxies (NGC 6670E and NGC 6670W) with a projected nuclear separation of ~16 kpc. While there are no optically identified tidal features and the molecular disks are not distorted much, we have detected a 90 kpc long H I tail which suggests that the galaxies are interacting and have already experienced at least one close encounter. Our observations suggest that the previous encounter between the galaxies had a large impact parameter and that the system is still in an early stage of interaction. Even though NGC 6670 is in an early stage of interaction, we find evidence of starburst. The CO(1-0) observations show large molecular gas reservoirs in the central regions and the 20 cm radio continuum observations reveal enhanced star formation activity in the nuclear regions of both galaxies. We estimate the ratio L_IR/M_H2, which is often used as an indicator of star formation efficiency, in the nuclear regions of NGC 6670E and NGC 6670W to be 18 and 11 L_sun/M_sun, respectively. The nuclear star formation efficiency of these two galaxies has been elevated to the level observed in other nearby starburst galaxies (L_IR/M_H2 > 10 L_sun/M_sun). Other indicators of starburst activity such as CO brightness temperature and infrared surface brightness are also observed.

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arXiv2001

CO Detections of Edge-On Low Surface Brightness Galaxies

We have obtained deep CO(1-0) observations of 8 nearby, edge-on, low surface brightness (LSB) spiral galaxies using the NRAO 12-m telescope. We report detections of 3 of our targets at >4 sigma level as well as one marginal (~3 sigma) detection. These are among the first direct detections of molecular gas in late-type, LSB spirals. Using a Galactic CO-to-H2 conversion factor, we derive MH2~1.4-3.5 x 10^{7}Msun and MH2/MHI~0.010-0.055 for the detected sources, with 3-sigma upper limits of MH2<0.7-5.7 x 10^{7}Msun and MH2/MHI<0.006-0.029 for the undetected objects.

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arXiv2000

Molecular Gas and the Modest Star Formation Efficiency in the ``Antennae'' Galaxies: Arp~244=NGC 4038/39

(abridged) We report here a factor of 5.7 higher total CO flux in Arp~244 (the ``Antennae'' galaxies) than that previously accepted in the literature (thus a total molecular gas mass of 1.5x10$^{10}$ Msun), based on our fully sampled CO(1-0) observations at the NRAO 12m telescope. Our observations show that the molecular gas peaks predominately in the disk-disk overlap region between the nuclei, similar to the far-infrared (FIR) and mid-infrared (MIR) emission. The bulk of the molecular gas is forming into stars with a normal star formation efficiency (SFE) L_{IR}/M(H_2) \approx 4.2 Lsun/Msun, same as that of giant molecular clouds in the Galactic disk. Additional supportive evidence is the extremely low fraction of the dense molecular gas in Arp~244, revealed by our detections of the HCN(1-0) emission. We estimate the local SFE indicated by the ratio map of the radio continuum to CO(1-0) emission. Remarkably, the local SFE stays roughly same over the bulk of the molecular gas distribution. Only some localized regions show the highest radio-to-CO ratios that we have identified as the sites of the most intense starbursts with SFE >~ 20 Lsun/Msun. These starburst regions are confined exclusively in the dusty patches seen in the HST images near the CO and FIR peaks where presumably the violent starbursts are heavily obscured. Nevertheless, recent large-scale star formation is going on throughout the system, yet the measured level is more suggestive of a moderate starburst (SFE >~ 10 Lsun/Msun) or a weak to normal star formation (SFE ~ 4 Lsun/Msun). The overall starburst from the bulk of the molecular gas is yet to be initiated as most of the gas further condenses into kpc scale in the final coalescence.

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arXiv2000

The ISM of Low Surface Brightness Galaxies

Using Monte Carlo radiative transfer techniques, we model the dust content of the edge-on low surface brightness (LSB) galaxy UGC 7321 and explore the effects of dust on its disk color gradients. Dust alone cannot explain the large radial disk color gradients observed in this galaxy and we find that a significant fraction (~50%) of the dust in UGC 7321 appears to be contained in a clumped medium, indicating that at least some LSB galaxies can support a modest multi-phase ISM structure. In addition, we report some of the first direct detections of molecular gas (CO) in LSB galaxies.

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arXiv2000

Molecular Gas Concentrations Outside the Merging Disks

We present our preliminary BIMA CO(1-0) images of II~Zw~96 which show huge molecular gas concentrations outside the merging disks. The dominant extra-disk CO concentrations in II~Zw~96 correspond to the two star-forming ``knots'' hidden by dust, whereas the others are associated with the two spiral disks. The unique huge CO concentrations, $\approxgt 5$ kpc away from the merging nuclei, account for $\approxgt 50 %$ of the total CO emission, yet only at a moderate gas surface density as compared with that of ultraluminous infrared galaxies (ULIGs). We tentatively conclude that the huge extra-disk gas concentrations are most likely the overlap region of the gas disks, not necessarily coinciding with the stellar disks overlap region. The gas disks overlap region could be formed prior to the merging of the stellar disks and become the overlap regions found between the disks like that in Arp~244 (the ``Antennae'' galaxies). These overlap regions would eventually nurture the {\it extreme starbursts} and evolve into the extended gas disk surrounding the double nuclei in final merging in most ULIGs.

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arXiv2000

CO in Stephan's Quintet: First Evidence of Molecular Gas in the Intragroup Starburst

We present the first interferometric evidence of CO(1-0) emission in the intragroup starburst (IGS) region of HCG~92 (Stephan's Quintet). BIMA's large primary beam covers fully both the IGS region and the dominant group member galaxy Seyfert NGC 7319, as well as partially the intruder galaxy NGC 7318B. CO emission of $\approxgt 4σ$ is detected in all of them. The detection of the CO emission associated with the IGS is further supported by comparisons with observations in other wavebands (e.g., HI, mid-IR, \ha). Assuming the standard conversion factor, 3.6--8.0$\times 10^8~\Msun$ molecular gas is found to be associated with the IGS. Confining to the region of the CO emission peak, a relatively high star-formation efficiency of 8.6$\Lsun/\Msun$ is derived for the IGS, which is nearly comparable to that of local starburst galaxies (e.g., M82).

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arXiv2000

Mapping IR Enhancements in Closely Interacting Spiral-Spiral Pairs. I. ISO~CAM and ISO~SWS Observations

Mid-infrared (MIR) imaging and spectroscopic observations are presented for a well defined sample of eight closely interacting (CLO) pairs of spiral galaxies that have overlapping disks and show enhanced far-infrared (FIR) emission. The goal is to study the star formation distribution in CLO pairs, with special emphasis on the role of 'overlap starbursts'. Observations were made with the Infrared Space Observatory (ISO) using the CAM and SWS instruments. The ISO~CAM maps, tracing the MIR emission of warm dust heated by young massive stars, are compared to new ground based H$α$ and R-band images. We identify three possible subgroups in the sample, classified according to the star formation morphology: (1) advanced mergers (Arp~157, Arp~244 and Arp~299), (2) severely disturbed systems (Arp~81 and Arp~278), and (3) less disturbed systems (Arp~276, KPG 347 and KPG 426). Localized starbursts are detected in the overlap regions in all five pairs of subgroups (1) and (2), suggesting that they are a common property in colliding systems. Except for Arp~244, the 'overlap starburst' is usually fainter than the major nuclear starburst in CLO pairs. Star formation in 'less disturbed systems' is often distributed throughout the disks of both galaxies with no 'overlap starburst' detected in any of them. These systems also show less enhanced FIR emission, suggesting that they are in an earlier interaction stage than pairs of the other two subgroups where the direct disk collisions have probably not yet occurred.

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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

Molecular Gas Depletion and Starbursts in Luminous Infrared Galaxy Mergers

Most luminous infrared galaxies (LIGs) are closely interacting/merging systems rich in molecular gas. We study here the relationship between the stage of the galaxy-galaxy interactions, the molecular gas mass, and the star formation rate as deduced from the infrared luminosity L_IR in LIGs. We find a correlation between the CO(1-0) luminosity (a measure of molecular mass M(H_2)) and the projected separation of merger nuclei (indicator of merging stages) in a sample of 50 LIG mergers, which shows that the molecular gas content decreases as merging advances. The starburst is due to enhanced star formation in pre-existing molecular clouds and not to the formation of more molecular clouds from HI. The molecular content is being rapidly depleted due to the starbursts as merging progresses. This is further supported by an anti-correlation between L_IR/M(H_2), the global measure of the star formation rate per unit gas mass, and the projected separation implying an enhanced star formation ``efficiency'' in late stage mergers compared to that of early mergers. This is the first evidence connecting the depletion of molecular gas with starbursts in interacting galaxies.

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arXiv1998

Progressive Star Bursts and High Velocities in the Infrared Luminous, Colliding Galaxy Arp 118

In this paper we demonstrate for the first time the connection between the spatial and temporal progression of star formation and the changing locations of the very dense regions in the gas of a massive disk galaxy (NGC 1144) in the aftermath of its collision with a massive elliptical (NGC 1143). These two galaxies form the combined object Arp 118, a collisional ring galaxy system. The results of 3D, time-dependent, numerical simulations of the behavior of the gas, stars, and dark matter of a disk galaxy and the stars and dark matter in an elliptical during a collision are compared with multiwavelength observations of Arp 118. The collision that took place approximately 22 Myr ago generated a strong, non-linear density wave in the stars and gas in the disk of NGC 1144, causing the gas to became clumped on a large scale. This wave produced a series of superstarclusters along arcs and rings that emanate from the central point of impact in the disk. The locations of these star forming regions match those of the regions of increased gas density predicted the time sequence of models. The models also predict the large velocity gradients observed across the disk of NGC 1144. These are due to the rapid radial outflow of gas coupled to large azimuthal velocities in the expanding ring, caused by the impact of the massive intruder.

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arXiv1997

Molecular Gas in the Spectacular Ring Galaxy NGC 1144

We have detected extremely wide (1100 km/s) CO(1-0) emission from NGC 1144, an interacting, luminous infrared galaxy that is the dominant component of the Arp 118 system. The observations show that NGC 1144 is one of the most CO luminous galaxies in the local universe, with a CO luminosity twice that of Arp 220. Maps with the IRAM interferometer show that the CO is not in or very near the Seyfert 2 nucleus, but in the 20 kpc diameter ring that extends halfway between NGC 1144 and the elliptical galaxy NGC 1143. The greatest gas concentration, with 40% of the CO luminosity, is in the southern part of the ring, in NGC 1144. Another 15% of the CO luminosity comes from the dominant 10 $μ$m source, a giant extranuclear HII region. The ring of molecular gas, the off-center nucleus, the ring extending halfway to the intruder, and the velocity of the intruder nearly equal to the escape velocity all show that Arp 118 is a ring galaxy produced by a collision of a massive spiral with an elliptical. The most spectacular property is the velocity range, which in Arp 118 is 2 to 3 times higher than in a typical ring galaxy. Arp 118 is a rare example of a very luminous extended starburst with a scale of about 5-10 kpc, and a luminosity of 3 $\times 10^{11} L_{\odot}$.

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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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