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M. S. Yun

Publications and source records attributed to M. S. Yun.

At least 91 records · Page 5Linked to original sources
arXiv2002

HI Observations of the Stephan's Quintet

Using the VLA, we have made spectral-line and continuum observations of the neutral hydrogen in the direction of the compact group of galaxies Stephan's Quintet. The high-velocity clouds between 5600 and 6600 km/s, the disk of the foreground galaxy, NGC 7320, at 800 km/s, the extended continuum ridge near the center of the group, and 3 faint dwarf-like galaxies in the surrounding field were imaged with C, CS, and D arrays. Four of the HI clouds previously detected are confirmed. The two largest HI features are coincident with and concentrated mainly along separate large tidal tails that extend eastward. The most diffuse of the four clouds is resolved into two clumps, one coincide with tidal features south of NGC 7318a and the other devoid of any detectable stellar or Halfa sources. The two compact clouds, along the same line of sight, have peak emission at luminous infrared and bright Halfa sources probably indicative of star-forming activity. The total amount of HI detected at high redshifts is ~ 10**10Msol. As in previous HI studies of the group, no detectable emission was measured at the positions of any high-redshift galaxies so that any HI still bound to their disks must be less than 2.4 x 10**7Msol.

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arXiv2001

Where is the Neutral Atomic Gas in Hickson Groups?

We have analyzed the total HI contents of 72 Hickson compact groups of galaxies (HCGs) and the detailed spatial distributions and kinematics of HI within a subset of 16 groups using the high angular resolution observations obtained with the VLA in order to investigate a possible evolutionary scenario for these densest systems in the present day galaxy hierarchy. For the more homogeneous subsample of 48 groups, we found a mean HI deficiency of DefHI = 40 (40% of the expected HI for the LBs and morphological types of the member galaxies). The individual galaxies show larger degrees of deficiency than the groups globally, DefHI = 0.62 (24% of the expected HI), due in most cases to efficient gas stripping from individual galaxies into the group environment visible in the VLA maps. The degree of deficiency is found to be similar to the central galaxies of Virgo and Coma cluster, and Coma I group, in spite of the significantly different characteristics of these environments. Phase transformation of the HI should explain the deficiency. The detection rate of HCGs at X-ray wavelengths is larger for HI deficient groups. In the evolutionary scenario we propose, the amount of detected HI would decrease further with evolution, by continuous tidal stripping and/or heating. The H2 content also tends to be lower than expected for the galaxies in HI deficient groups, this may suggest that the HI stripping by frequent tidal interaction breaks the balance between the disruption of molecular clouds by star formation and the replenishment from the ambient HI.

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arXiv2001

A 250 GHz Survey of High Redshift QSOs from the Sloan Digital Sky Survey

We present observations at 250 GHz (1.2 mm), 43 GHz, and 1.4 GHz of a sample of 41 QSOs at z > 3.7 found in the Sloan Digital Sky Survey. We detect 16 sources with a 250 GHz flux density greater than 1.4 mJy. The combination of centimeter and millimeter wavelength observations indicates that the 250 GHz emission is most likely thermal dust emission. Assuming a dust temperature of 50 K, the implied dust masses for the 16 detected sources are in the range 1.5e8 to 5.9e8 Msun, and the dust emitting regions are likely to be larger than 1 kpc in extent. The radio-through-optical spectral energy distributions for these sources are within the broad range defined by lower redshift, lower optical luminosity QSOs. We consider possible dust heating mechanisms, including UV emission from the active nucleus (AGN) and a starburst concurrent with the AGN, with implied star formation rates between 500 and 2000 Msun/year.

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arXiv2000

The Central Velocity Field in NGC 253 : Possible Indication of a Bar

We have investigated whether motion of gas in a bar-like potential can account for the peculiar but systematic velocity field observed in the nuclear region of the starburst galaxy NGC 253. This unusual velocity field with gradients along both major and minor axes was revealed in a high resolution ($1.8^{\arcsec}\times 1.0^{\arcsec}$) H92$α$ recombination line observation by Anantharamiah and Goss (1996). A simple logarithmic potential is used to model the bar. Assuming that the bulk of the gas flows along closed and non-intersecting x1 (bar) and x2 (anti-bar) orbits of the bar potential, we have computed the expected velocity field and position-velocity diagrams and compared them with the observations. A comparison of the integrated CO intensity maps with the spatial distribution of the x1 and x2 orbits in the model indicates that the nuclear molecular gas in NGC 253 lies mainly on the x2 orbits. We also find that the velocity field observed in the central 100 pc region in the H92$α$ recombination line is well accounted for by the bar model if most of the ionized gas resides in the inner x2 orbits. However, the model is unable to explain the velocity field on a larger scale of $\sim 500$ pc observed using the OVRO interferometer with a resolution of $5^{\arcsec}\times 3^{\arcsec}$. The direction of the observed CO velocity field appears twisted compared to the model. We suggest that this perturbation in the velocity field may be due to an accretion event that could have occurred $10^7$ years ago.

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arXiv2000

The Scatter in the Relationship between Redshift and the Radio-to-Submm Spectral Index

We derive the scatter in the relationship between redshift and radio-to-submm spectral index, alpha^{350}_{1.4}, using the observed spectral energy distributions of 17 low redshift star forming galaxies. A mean galaxy model is derived, along with the rms scatter in alpha^{350}_{1.4}. The scatter is roughly constant with redshift. Constant rms scatter, combined with the flattening of the mean alpha^{350}_{1.4} -- z relationship with increasing redshift, leads to increasing uncertainty for redshift estimates at high redshifts. Normalizing by the dust temperature in the manner proposed by Blain decreases the scatter in alpha^{350}_{1.4} for most of the sample, but does not remove outliers, and free-free absorption at rest frequencies above 1.4 GHz is not likely to be a dominant cause for scatter in the alpha^{350}_{1.4} -- z relationship. We re-derive the cumulative redshift distribution of the 14 field galaxies in a recent submm and radio source sample of Smail et al.. The most likely median redshift for the distribution is 2.7, with a conservative lower limit of z = 2, as was also found by Smail et al. based on the original alpha^{350}_{1.4} -- z models. The normalization and shape of the redshift distribution for the faint submm sources are consistent with those expected for forming elliptical galaxies.

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arXiv2000

Dust Emission from High Redshift QSOs

We present detections of emission at 250 GHz (1.2 mm) from two high redshift QSOs from the Sloan Digital Sky Survey sample using the bolometer array at the IRAM 30m telescope. The sources are SDSSp 015048.83+004126.2 at z = 3.7, and SDSSp J033829.31+002156.3 at z = 5.0, which is the third highest redshift QSO known, and the highest redshift mm emitting source yet identified. We also present deep radio continuum imaging of these two sources at 1.4 GHz using the Very Large Array. The combination of cm and mm observations indicate that the 250 GHz emission is most likely thermal dust emission, with implied dust masses of 1e8 M_solar. We consider possible dust heating mechanisms, including UV emission from the active nucleus (AGN), and a massive starburst concurrent with the AGN, with implied star formation rates > 1e3 M_solar/year.

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arXiv1999

The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies

We have mapped the neutral atomic gas (HI) in the three disk-disk merger systems NGC 520, Arp 220, and Arp 299. These systems differ from the majority of the mergers mapped in HI, in that their stellar and gaseous tidal features do not coincide. In particular, they exhibit large stellar tidal features with little if any accompanying neutral gas and large gas-rich tidal features with little if any accompanying starlight. On smaller scales, there are striking anti-correlations where the gaseous and stellar tidal features appear to cross. We explore several possible causes for these differences, including dust obscuration, ram pressure stripping, and ionization effects. No single explanation can account for all of the observed differences. The fact that each of these systems shows evidence for a starburst driven superwind expanding in the direction of the most striking anti-correlations leads us to suggest that the superwind is primarily responsible for the observed differences, either by sweeping the features clear of gas via ram pressure, or by excavating a clear sightline towards the starburst and allowing UV photons to ionize regions of the tails.

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arXiv1999

Mass Distribution in Compact Groups

New redshift surveys of galaxies in the field of compact groups have discovered a population of faint galaxies which act as satellites orbiting in the potential well of the bright group. Here we analyze the mass distribution of the groups by comparing the mass derived from the bright members and the mass obtained from the satellite galaxies. Our analysis indicates the presence of a dark halo around the main group with a mass roughly four times that measured for the dominant galaxies of the compact group. We found that heavier halos are ruled out by the observations when comparing the distribution of positions and redshifts of the satellite galaxies with the distribution of satellites surrounding isolated spiral galaxies. The results agree with a picture where compact groups may form a stable system with galaxies moving in a common dark halo.

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arXiv1999

Sensitive Radio Observations of High Redshift Dusty QSOs

We present sensitive radio continuum imaging at 1.4 GHz and 4.9 GHz of seven high redshift QSOs selected for having a 240 GHz continuum detection, which is thought to be thermal dust emission. We detect radio continuum emission from four of the sources: BRI 0952-0115, BR 1202-0725, LBQS 1230+1627B, and BRI 1335-0417. The radio source in BR 1202-0725 is resolved into two components, coincident with the double mm and CO sources. We compare the results at 1.4 GHz and 240 GHz to empirical and semi-analytic spectral models based on star forming galaxies at low redshift. The radio-to-submm spectral energy distribution for BR 1202-0725, LBQS 1230+1627B, and BRI 1335-0417 are consistent with that expected for a massive starburst galaxy, with implied massive star formation rates of order 1000 solar masses per year (without correcting for possible amplification by gravitational lensing). The radio-to-submm spectral energy distribution for BRI 0952-0115 suggests a low-luminosity radio jet source driven by the AGN.

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arXiv1999

Tracing interactions in HCGs through the HI

We present a global study of HI spectral line mapping for 16 Hickson Compact Groups (HCGs) combining new and unpublished VLA data, plus the analysis of the HI content of individual galaxies. Sixty percent of the groups show morphological and kinematical signs of perturbations (from multiple tidal features to concentration of the HI in a single enveloping cloud) and sixty five of the resolved galaxies are found to be HI deficient with respect to a sample of isolated galaxies. In total, 77% of the groups suffer interactions among all its members which provides strong evidence of their reality. We find that dynamical evolution does not always produce HI deficiency, but when this deficiency is observed, it appears to correlate with a high group velocity dispersion and in some cases with the presence of a first-ranked elliptical. The X-ray data available for our sample are not sensitive enough for a comparison with the HI mass; however this study does suggest a correlation between HI deficiency and hot gas since velocity dispersions are known from the literature to correlate with X-ray luminosity.

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arXiv1999

Studying High Redshift Star Forming Galaxies at Centimeter and Millimeter Wavelengths

We discuss various aspects of centimeter and millimeter wavelength continuum and line observations of high redshift star forming galaxies. Perhaps the most important lesson is that sensitive observations at submm through cm wavelengths reveal a population of active star forming galaxies at high redshift which are unseen in deep optical surveys due to dust obscuration. Current models suggest that this population represents the formation of the spheroidal components of galaxies at z between 2 and 5, constituting about half of the total amount of cosmic star formation from the big bang to the present. High resolution imaging at cm wavelengths provides sub-arcsecond astrometry, and can be used to search for gravitational lensing and/or for the presence of an AGN. Radio continuum observations provide unique information on the magnetic fields in early galaxies, and give a gross indication of the star formation rate, while the radio-to-submm spectral index provides a rough indication of source redshift. Low J transitions of CO are redshifted into the cm bands for z > 2, allowing for sensitive searches for CO emission over large volumes at high redshift. We present recent results from the Very Large Array (VLA), and from the new 230 GHz MPIfR bolometer array at the IRAM 30m telescope. A wide field survey with the bolometer array indicates a cut-off in the source distribution function at FIR luminosities > 3e12 L_sun. Lastly, we summarize the scientific promise of the New VLA.

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arXiv1999

Submillimeter Imaging of the Luminous Infrared Galaxy Pair VV 114

We report on 450- and 850-micron observations of the interacting galaxy pair, VV114E+W (IC1623), taken with the SCUBA camera on the James Clerk Maxwell Telescope, and near-infrared observations taken with UFTI on the UK Infrared Telescope. The system VV114 is in an early stage of a gas-rich merger. We detect submillimeter (sub-mm) emission extended over 30 arcsec (12 kpc) and find a good correlation between the spatial distribution of the sub-mm and CO emission. Both the CO and sub-mm emission peak near the reddest region of VV114E and extend toward VV114W. The bulk of the sub-mm emission resides in the central region showing the largest CO velocity gradients, which is thought to mark the kinematic centroid of the merger remnant. We derived a total dust mass of 1.2x10^{8} M_sun, assuming a power-law distribution of dust temperatures. The sub-mm observations suggest that the majority of the dust is relatively cool (T_d ~20--25 K), and the total dust mass is about 4 times higher than that inferred from the IRAS data alone. The system will likely evolve into a compact starburst similar to Arp220.

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arXiv1999

A 180 Kpc Tidal Tail in the Luminous Infrared Merger Arp 299

We present VLA HI observations and UH88 deep optical B- and R-band observations of the IR luminous merger Arp 299 (= NGC 3690 + IC 694). These data reveal a gas-rich, optically faint tidal tail with a length of over 180 kpc. The size of this tidal feature necessitates an old interaction age for the merger (~750 Myr since first periapse), which is currently experiencing a very young star burst (~20 Myr). The observations reveal a most remarkable structure within the tidal tail: it appears to be composed of two parallel filaments separated by ~20 kpc. One of the filaments is gas rich with little if any starlight, while the other is gas poor. We believe that this bifurcation results from a warped disk in one of the progenitors. The quantities and kinematics of the tidal HI suggest that Arp 299 results from the collision of a retrograde Sab-Sb galaxy (IC 694) and a prograde Sbc-Sc galaxy (NGC 3690) that occurred 750 Myr ago and which will merge into a single object in ~60 Myr. We suggest that the present IR luminous phase in this system is due in part to the retrograde spin of IC 694. Finally, we discuss the apparent lack of tidal dwarf galaxies within the tail.

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arXiv1999

Molecular Gas in the z=2.565 Submillimeter Galaxy SMM J14011+0252

We report the detection of CO(3-2) emission from the submillimeter-selected luminous galaxy SMM J14011+0252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z=2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of 5x10^{10}h_{75}^{-2} M(sun), after correcting for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM J02399-0136, which was the first submillimeter selected CO source found at high redshift. The CO detections of these two high-redshift submillimeter galaxies suggest the presence of massive reservoirs of molecular gas which is consistent with the inferred high rates of star-formation (1000 M(sun)/yr). These two systems appear to be associated with merger events which may evolve into present day luminous elliptical galaxies.

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arXiv1998

Counterrotating Nuclear Disks in Arp 220

The ultraluminous infrared galaxy Arp 220 has been observed at 0.5" resolution in CO(2-1) and 1 mm continuum using the newly expanded Owens Valley Millimeter Array. The CO and continuum peaks at the double nuclei and the surrounding molecular gas disk are clearly resolved. We find steep velocity gradients across each nucleus (dV ~ 500 km/s within r= 0.3") whose directions are not aligned with each other and with that of the outer gas disk. We conclude that the double nuclei have their own gas disks (r ~ 100 pc). They are counterrotating with respect to each other and embedded in the outer gas disk (r ~ 1 kpc) rotating around the dynamical center of the system. The masses of each nucleus are M_dyn > 2* 10^9 M_sun based on the CO kinematics. Although there is no evidence of an old stellar population in the optical or near infrared spectroscopy of the nuclei (probably due to the much brighter young population), it seems likely that these nuclei were 'seeded' from the pre-merger nuclei in view of their counterrotating gas kinematics. The gas disks probably constitute a significant fraction (~ 50 %) of the mass in each nucleus. The CO and continuum brightness temperatures imply that the nuclear gas disks have high area filling factors (~ 0.5-1) and have extremely high visual extinctions (Av ~ 1000 mag). The molecular gas must be hot (>= 40 K) and dense (>= 10^4-5 cm^-3), given the large mass and small scale-height of the nuclear disks. The continuum data suggest that the large luminosity (be it starburst or AGN) must originate within 100 pc of the two nuclear gas disks which were presumably formed through concentration of gas from the progenitor outer galaxy disks.

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arXiv1998

CO Distribution and Kinematics Along the Bar in the Strongly Barred Spiral NGC 7479

We report on the 2.5 arcsec (400 pc) resolution CO (J = 1 -> 0) observations covering the whole length of the bar in the strongly barred late-type spiral galaxy NGC 7479. CO emission is detected only along a dust lane that traverses the whole length of the bar, including the nucleus. The emission is strongest in the nucleus. The distribution of emission is clumpy along the bar outside the nucleus, and consists of gas complexes that are unlikely to be gravitationally bound. The CO kinematics within the bar consist of two separate components. A kinematically distinct circumnuclear disk, < 500 pc in diameter, is undergoing predominantly circular motion with a maximum rotational velocity of 245 km/s at a radius of 1 arcsec (160 pc). The CO-emitting gas in the bar outside the circumnuclear disk has substantial noncircular motions which are consistent with a large radial velocity component, directed inwards. The CO emission has a large velocity gradient across the bar dust lane, ranging from 0.5 to 1.9 km/s/pc after correcting for inclination, and the projected velocity change across the dust lane is as high as 200 km/s. This sharp velocity gradient is consistent with a shock front at the location of the bar dust lane. A comparison of H-alpha and CO kinematics across the dust lane shows that although the H-alpha emission is often observed both upstream and downstream from the dust lane, the CO emission is observed only where the velocity gradient is large. We also compare the observations with hydrodynamic models and discuss star formation along the bar.

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arXiv1998

CO (J = 4 --> 3) and 650 Micron Continuum Observations of z=0.93 Hyperluminous Infrared Galaxy FSC 15307+3252

We report the results of our CO J = 4 --> 3 line and rest frame 650 micron continuum observations of the z=0.93 hyperluminous infrared galaxy FSC 15307+3252 using the Owens Valley Millimeter Array. No line or continuum emission was detected, but the derived limits provide a useful constraint on the temperature, emissivity, and mass of the cold dust associated with FSC 15307+3252 and its molecular gas content. The 3 sigma upper limit on the velocity integrated CO (4-3) line flux is 1.6 Jy km/s (for 300 km/s line width). This corresponds to a surprisingly small total molecular gas mass limit of 5 x 10^9 h^{-2} solar mass for this galaxy with infrared luminosity L(FIR) > 10^{13} solar luminosity. Combined with existing photometry data, our 3 sigma upper limit of 5.1 mJy for the 239 GHz (650 micron rest wavelength) continuum flux yields a total dust mass of 0.4-1.5 times 10^8 solar mass. The CO luminosity (thus molecular gas content) and the resulting gas-to-dust ratio are lower than the values typical for the more gas-rich infrared galaxies, but they are within the observed ranges. On the other hand, FSC 15307+3252 has a dust content and infrared luminosity 40 and 200 times larger than the infrared bright elliptical-like galaxies NGC 1275 and Cygnus A. The FIR luminosity to dust mass ratios, L(FIR)/M(dust), for all three galaxies hosting a powerful AGN (FSC 10214+4724, FSC 15307+3252, Cygnus A) are larger than reasonably expected for a galaxy dominated by a starburst and four times larger than Arp 220. Therefore the bulk of the observe FIR luminosity in these galaxies is likely powered by their luminous active nucleus.

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