Zhi-Yu Zhang

A KILOPARSEC-SCALE BINARY ACTIVE GALACTIC NUCLEUS CONFIRMED BY THE EXPANDED VERY LARGE ARRAY

We report the confirmation of a kiloparsec-scale binary active galactic nucleus (AGN) with high-resolution radio images from the Expanded Very Large Array (EVLA). SDSS J150243.1+111557 is a double-peaked [O III] AGN at z = 0.39 selected from the Sloan Digital Sky Survey. Our previous near-infrared adaptive optics imaging reveals two nuclei separated by 1. 4 (7.4 kpc), and our optical integral-field spectroscopy suggests that they are a type-1-type-2 AGN pair. However, these data alone cannot rule out the single AGN scenario where the narrow emission-line region associated with the secondary is photoionized by the broad-line AGN in the primary. Our new EVLA images at 1.4, 5.0, and 8.5 GHz show two steep-spectrum compact radio sources spatially coincident with the optical nuclei. The radio power of the type-2 AGN is an order-of-magnitude in excess of star-forming galaxies with similar extinction-corrected [O II] lambda 3727 luminosities, indicating that the radio emission is powered by accretion. Therefore, SDSS J150243.1+111557 is one of the few confirmed kiloparsec-scale binary AGN systems. Spectral energy distribution modeling shows that SDSS J150243.1+111557 is a merger of two similar to 10(11) M-circle dot galaxies. With both black hole masses around 10(8) M-circle dot, the AGNs are accreting at similar to 10 times below the Eddington limit.

DENSE GAS TRACERS AND STAR FORMATION LAWS IN ACTIVE GALAXIES: APEX SURVEY OF HCN J = 4 → 3, HCO+ J = 4 → 3, AND CS J = 7 → 6

We report HCN J = 4 --> 3, HCO+ J = 4 --> 3, and CS J = 7 --> 6 observations in 20 nearby star-forming galaxies with the Atacama Pathfinder EXperiment 12 m telescope. Combined with four HCN, three HCO+, and four CS detections from the literature, we probe the empirical link between the luminosity of molecular gas (L(gas)) and that of infrared emission (L-IR), up to the highest gas densities (similar to 10(6) cm(-3)) that have been probed so far. For nearby galaxies with large radii, we measure the IR luminosity within the submillimeter beam size (14 -18 ) to match the molecular emission. We find linear slopes for L(CS J=7-6)-L-IR and L(HCNJ=4-3)-L-IR, and a slightly super-linear slope for L(HCO+ J=4-3)-L-IR. The correlation of L(CS J=7-6)-L-IR even extends over eight orders of luminosity magnitude down to Galactic dense cores, with a fit of log(L-IR) = 1.00(+/-0.01)xlog(L(CS J=7-6)) + 4.03(+/-0.04). Such linear correlations appear to hold for all densities >10(4) cm(-3), and indicate that star formation rate is not related to the free-fall timescale for dense molecular gas.

Spectral lags caused by the curvature effect of fireballs

Recently, Shen et al. have studied the contributions of the curvature effect of fireballs to the spectral lag and have shown that the observed lags can be accounted for by the effect. Here, we check their results by performing a more precise calculation with both formulae presented by Shen et al. and Qin et al. Several other aspects which were not considered by Shen et al. are investigated. We find that in the case of ultrarelativistic motions, both formulae are identical as long as the whole fireball surface is concerned. In our analysis, the previous conclusion that the detected spectral lags can be accounted for by the curvature effect is confirmed, while the conclusion that the lag has no dependence on the radius of fireballs is not true. We find that introducing extreme physical parameters is not the only outlet to explain these observed large lags. Even for the larger lags (similar to 5 s), a wider local pulse (Delta t(theta,FWHM)= 10(7) s) can account for it. Some conclusions not presented in Shen et al. or those modified in our analysis are listed below: (i) lag proportional to Gamma(-epsilon) with epsilon > 2; (ii) lag is proportional to the local pulse width and the full width at half-maximum of the observed light curves; (iii) a large lag requires a large alpha(0) and a small beta(0) as well as a large E-0,E-p; (iv) when the rest-frame spectrum varies with time, the lag would become larger; (v) lag decreases with the increase of R-c; (vi) lag proportional to E within the certain energy range for a given Lorentz factor; (vii) lag is proportional to the opening angle of uniform jets when theta(j) < 0.6 Gamma(-1).

LARGE-SCALE KINEMATICS, ASTROCHEMISTRY, AND MAGNETIC FIELD STUDIES OF MASSIVE STAR-FORMING REGIONS THROUGH HC3N, HNC, AND C2H MAPPINGS

We have mapped 27 massive star-forming regions associated with water masers using three dense gas tracers: HC3N 10-9, HNC 1-0, and C2H 1-0. The FWHM sizes of HNC clumps and C2H clumps are about 1.5 and 1.6 times higher than those of HC3N, respectively, which can be explained by the fact that HC3N traces more dense gas than HNC and C2H. We found evidence for an increase in the optical depth of C2H with a radius from the center to the outer regions in some targets, supporting the chemical model of C2H. The C2H optical depth is found to decline as molecular clouds evolve to a later stage, suggesting that C2H might be used as a chemical clock for molecular clouds. The large-scale kinematic structure of clouds was investigated with three molecular lines. All of these sources show significant velocity gradients. The magnitudes of gradient are found to increase toward the inner region, indicating the differential rotation of clouds. Both the ratio of rotational to gravitational energy and the specific angular momentum seem to decrease toward the inner region, implying the obvious angular momentum transfer, which might be caused by magnetic braking. The average magnetic field strength and number density of molecular clouds is derived using the uniform magnetic sphere model. The derived magnetic field strengths range from 3 to 88 mu G, with a median value of 13 mu G. The mass-to-flux ratio of the molecular cloud is calculated to be much higher than the critical value with derived parameters, which agrees well with numerical simulations.

A test of the power-law relationship between gamma-ray burst pulse-width ratio and energy expected in fireballs and uniform jets

Recently, under the assumption that the Doppler effect of the relativistically expanding fireball surface is important, Qin et al. showed that in most cases the power law relationship between the pulse width and energy of gamma-ray bursts (GRBs)would exist in a certain energy range. We check this prediction with two GRB samples which contain well identified pulses. A power law anti-correlation between the full pulse width and energy and a power law correlation between the pulse width ratio and energy are seen in the light curves of the majority (around 65%) of bursts of the two samples within the energy range of BATSE, suggesting that these bursts are likely to arise from the emission associated with the shocks occurred on a relativistically expanding fireball surface. For the rest of the bursts, the relationships between these quantities were not predicted previously. We propose to consider other spectral evolutionary patterns or other radiation mechanisms such as a varying synchrotron or Comptonized spectrum to check if the observed relationships for these rest bursts can also be accounted for by the Doppler model. In addition, we find that the upper limits of the width ratio for the two samples do not exceed 0.9, in agrement with what predicted previously by the Doppler model. The plateau/power law/plateau and the peaked features predicted and detected previously by Qin et al. are generally observed, with the exceptions being noticed only in a few cases. According to the distinct values of two power law indices of FWHM and ratio and energy, we divide the bursts into three subsets which are located in different areas of the two indices plane. We suspect that different locations of the two indices might correspond to different mechanisms.

12CO,13CO and C18O observations along the major axes of nearby bright infrared galaxies

We present simultaneous observations of 12 CO, 13 CO and C 18 O J =1−0 emission in 11 nearby ( cz <1000 km s −1 ) bright infrared galaxies. Both 12 CO and 13 CO are detected in the centers of all the galaxies, except for 13 CO in NGC 3031. We have also detected C 18 O, CS J =2−1 and HCO + J =1−0 emission in the nuclear regions of M82 and M51. These are the first systematical extragalactic detections of 12 CO and its isotopes from the PMO 14 m telescope. We have conducted half-beam-spaced mapping of M82 over an area of 4 × 2.5 and major axis mapping of NGC 3627, NGC 3628, NGC 4631 and M51. The radial distributions of 12 CO and 13 CO in NGC 3627, NGC 3628 and M51 can be well fitted by an exponential profile. The 12 CO/ 13 CO intensity ratio, R , decreases monotonically with the galactocentric radius in all mapped sources. The average R in the center and disk of the galaxies are 9.9±3.0 and 5.6±1.9, respectively, much lower than the peculiar R (~24) found in the center of M82. The intensity ratios of 13 CO/C 18 O, 13 CO/HCO + and 13 CO/CS (either our or literature data) show little variation with galactocentric radius, in sharp contrast with the greatly varied R . This supports the notion that the observed gradient in R could be the result of the variations of the physical conditions across the disks. The H 2 column density derived from C 18 O shows that the Galactic standard conversion factor (X-factor) overestimates the amount of the molecular gas in M82 by a factor of ~2.5. These observations suggest that the X-factor in active star-forming regions (i.e., nuclear regions) should be lower than that in normal star-forming disks and the gradient in R can be used to trace the variations of the X-factor.

Physical implication of the Kocevski¿Ryde¿Liang pulse function of gamma-ray bursts

Kocevski, Ryde & Liang have proposed a semi-empirical function (the KRL function) of gamma-ray burst (GRB) pulses, which could well describe those pulses comprising a fast rise and an exponential decay (FRED) phase. Meanwhile, a theoretical model that could give rise to this kind of pulse, based on the Doppler effect of the expanding fireball surface, has been put forward in detail by Qin and co-workers. To provide a physical explanation to the parameters of the KRL function, we try to fit light curves of the Doppler model with the KRL function so that parameters in both models can be directly related. We pay attention only to single GRB pulses whose profiles are that of FRED and hence employ the sample presented by Kocevski, Ryde & Liang (the KRL sample) to study this issue. We find from our analysis that, for light curves, which arise from exponential rise and exponential decay local pulses, of the Doppler model, the ratio of the rise index r to the decay index d, derived when fitted by the KRL function, increases quickly first. It then remains nearly invariant with the relative width (relative to the time-scale of the initial fireball radius R(c)/C) of local pulses when the width exceeds 2 (the relative width is dimensionless). The rise and decay times of pulses are found to be related to the Lorentz factor by a power law, where the power-law index associated with the rise time is less than that of the decay time, and both are close to -2. In addition, the mean asymmetry shows a slight trend of decreasing with Lorentz factors. In plots of decay indices versus asymmetry, there is a descending phase and after this phase there is a rising portion. We find that these long GRBs of the KRL sample are mainly associated with those light curves arising from comoving pulses with the relative width being larger than 0.1. We show in our analysis that the effect of the comoving pulse shape on the KRL function parameters of the resulting pulses is considerable and can be distinguished by the decay index d when the relative comoving pulse width is less than 2 (when the relative width is larger than 2, it would be difficult to discern the difference in the resulting pulse shapes).

ALMA Constrains the Stellar Initial Mass Function of Dusty Starburst Galaxies

The Messenger 172 – June 2018 (for example, in excess of 1000 M⊙ yr –1, see Ivison et al., 1998) — have their ultraviolet and optical stellar light heavily obscured by dust (see Figure 1). However, according to theories and cosmological simulations, it is in exactly these systems where the most extreme IMF variations would arise. Are there any other sensible, indirect methods to probe the IMF in these important, dust-shrouded systems?

High resolution observations of the 6 cm H2CO maser in NGC 6240

We present high resolution (similar to 1 ) H2CO maser and 5 GHz radio continuum observations toward nearby merging galaxy NGC 6240 made with the Very Large Array in an A configuration. Two concentrations of H2CO emission at about a 6 sigma level have been detected, one of which is associated with the strongest CO peak in the overlap region while the other is about 2 southwest of the southern galaxy. Both H2CO concentrations are associated with near infrared H-2 emission, which is thought to be from shocked molecular gas. The total H2CO line luminosity in NGC 6240 is about 60% of that in Arp 220. Based on the distribution of H2CO emission in NGC 6240, which has both active galactic nuclei and an extreme starburst, the H2CO megamaser is likely to be related to the effect of the starburst instead of nuclear activity. Radio continuum cannot be the inversion mechanism of H2CO megamasers, because the two H2CO concentrations in NGC 6240 are not associated with radio continuum emission. Instead, with the association of near infrared H-2 emission, shock dynamics may produce the inverted population of H2CO needed to generate megamasers.

The molecular gas in Luminous Infrared Galaxies: a new emergent picture

Results from a large, multi-J CO, {13}CO, and HCN line survey of Luminous Infrared Galaxies (L_{IR}>=10^{10} L_{odot}) in the local Universe (z 10^{12} L_{odot}) the Photon Dominated Regions (PDRs) can encompass at most sim few% of their molecular gas mass while the large U_{CR} and the strong turbulence in these merger/starbursts, can volumetrically heat much of their molecular gas to T_{kin}sim(100-200)K, unhindered by the high dust extinctions. Moreover the strong supersonic turbulence in ULIRGs relocates much of their molecular gas at much higher average densities than in isolated spirals. This renders low-J CO lines incapable of constraining the properties of the bulk of the molecular gas in ULIRGs, with substantial and systematic underestimates of its mass possible when only such lines are used. A comparative study of multi-J HCN lines and CO SLEDs from J=1--0 up to J=13--12 of NGC 6240 and Arp 193 offers a clear example of two merger/starbursts whose similar low-J CO SLEDs, and L_{IR}/L_{CO,1-0}, L_{HCN, 1-0}/L_{CO,1-0} ratios, yield no indications about their strongly diverging CO SLEDs beyond J=4--3, and ultimately the different physical conditions in their molecular ISM. The much larger sensitivity of ALMA and its excellent site in the Atacama desert now allows the observations necessary to ....