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Register a new userHow to hide large scale outflows: size constraints on the jets of Sgr A*
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Despite significant strides made towards understanding accretion, outflow, and emission processes in the Galactic Center supermassive black hole Sagittarius A*, the presence of jets has neither been rejected nor proven. We investigate here whether the combined spectral and morphological properties of the source at radio through near infrared wavelengths are consistent with the predictions for inhomogeneous jets. In particular, we construct images of jets at a wavelength of 7mm based on models that are consistent with the spectrum of Sgr A*. We then compare these models through closure quantities with data obtained from the Very Long Baseline Array at 7mm. We find that the best-fit jet models give comparable or better fits than best-fit Gaussian models for the intrinsic source found in previous analyses. The best fitting jet models are bipolar, are highly inclined to the line of sight ($theta ge$ 75 degrees), may favor a position angle on the sky of 105 degrees, and have compact bases with sizes of a few gravitational radii.
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We present a procedure to share a secret spatial direction in the absence of a common reference frame using a multipartite quantum state. The procedure guarantees that the parties can determine the direction if they perform joint measurements on the state, but fail to do so if they restrict themselves to local operations and classical communication (LOCC). We calculate the fidelity for joint measurements, give bounds on the fidelity achievable by LOCC, and prove that there is a non-vanishing gap between the two of them, even in the limit of infinitely many copies. The robustness of the procedure under particle loss is also studied. As a by-product we find bounds on the probability of discriminating by LOCC between the invariant subspaces of total angular momentum N/2 and N/2-1 in a system of N elementary spins.
Covert networks are social networks that often consist of harmful users. Social Network Analysis (SNA) has played an important role in reducing criminal activities (e.g., counter terrorism) via detecting the influential users in such networks. There are various popular measures to quantify how influential or central any vertex is in a network. As expected, strategic and influential miscreants in covert networks would try to hide herself and her partners (called {em leaders}) from being detected via these measures by introducing new edges. Waniek et al. show that the corresponding computational problem, called Hiding Leader, is NP-Complete for the degree and closeness centrality measures. We study the popular core centrality measure and show that the problem is NP-Complete even when the core centrality of every leader is only $3$. On the contrary, we prove that the problem becomes polynomial time solvable for the degree centrality measure if the degree of every leader is bounded above by any constant. We then focus on the optimization version of the problem and show that the Hiding Leader problem admits a $2$ factor approximation algorithm for the degree centrality measure. We complement it by proving that one cannot hope to have any $(2-varepsilon)$ factor approximation algorithm for any constant $varepsilon>0$ unless there is a $varepsilon/2$ factor polynomial time algorithm for the Densest $k$-Subgraph problem which would be considered a significant breakthrough.
While it is generally thought that molecular outflows from young stellar objects (YSOs) are accelerated by underlying stellar winds or highly collimated jets, the actual mechanism of acceleration remains uncertain. The most favoured model, at least for low and intermediate mass stars, is that the molecules are accelerated at jet-driven bow shocks. Here we investigate, through high resolution numerical simulations, the efficiency of this mechanism in accelerating ambient molecular gas without causing dissociation. The efficiency of the mechanism is found to be surprisingly low suggesting that more momentum may be present in the underlying jet than previously thought. We also compare the momentum transferring efficiencies of pulsed versus steady jets. We find that pulsed jets, and the corresponding steady jet with the same average velocity, transfer virtually the same momentum to the ambient gas. The additional momentum ejected sideways from the jet beam in the case of the pulsed jet only serves to accelerate post-shock jet gas which forms a, largely atomic, sheath around the jet beam. For both the steady and pulsing jets, we find a power law relationship between mass and velocity ($m(v) propto v^{-gamma}$) which is similar to what is observed. We also find that increasing the molecular fraction in the jet decreases $gamma$ as one might expect. We reproduce the so-called Hubble law for molecular outflows and show that it is almost certainly a local effect in the presence of a bow shock.
Using hydrodynamic simulations, we study the mass loss due to supernova-driven outflows from Milky Way type disk galaxies, paying particular attention to the effect of the extended hot halo gas. We find that the total mass loss at inner radii scales roughly linearly with total mass of stars formed, and that the mass loading factor at the virial radius can be several times its value at inner radii because of the swept up hot halo gas. The temperature distribution of the outflowing material in the inner region ($sim $10 kpc) is bimodal in nature, peaking at $10^5$ K and $10^{6.5}$ K, responsible for optical and X-ray emission, respectively. The contribution of cold/warm gas with temperature $le 10^{5.5}$ K to the outflow rate within 10 kpc is $approx 0.3hbox{--}0.5$. The warm mass loading factor, $eta_{3e5}$ ($Tle 3 times 10^5$ K) is related to the mass loading factor at the virial radius ($eta_{v}$) as $eta_{v} approx 25, eta_{3e5}, left(mbox{SFR}/{rm M}_odot{rm yr}^{-1} right)^{-0.15}$ for a baryon fraction of 0.1 and a starburst period of 50 Myr. We also discuss the effect of multiple bursts that are separated by both short and long periods. The outflow speed at the virial radius is close to the sound speed in the hot halo, $lesssim 200$ km s$^{-1}$. We identify two `sequences of outflowing cold gas at small scales: a fast ($approx 500$ km~s$^{-1}$) sequence, driven by the unshocked free-wind; and a slow sequence ($approx pm 100$ km s$^{-1}$) at the conical interface of the superwind and the hot halo.
Mass outflow rates and loading factors are typically used to infer the quenching potential of galactic-scale outflows. However, these generally rely on observations of a single gas phase which can severely underestimate the total ejected gas mass. To address this, we use observations of high mass ($geqslant$10$^{10}$ M$_{odot}$), normal star-forming galaxies at $zsim$0 from the MaNGA, xCOLD GASS, xGASS and ALFALFA surveys and a stacking of NaD, H$alpha$, CO(1-0) and HI 21cm tracers with the aim of placing constraints on an average, total mass outflow rate and loading factor. We find detections of outflows in both neutral and ionised gas tracers, with no detections in stacks of molecular or atomic gas emission. Modelling of the outflow components reveals velocities of $|$v$_{text{NaD}}|$=131 km s$^{-1}$ and $|$v$_{text{H}alpha}|$=439 km s$^{-1}$ and outflow rates of $dot{M}_{text{NaD}}$=7.55 M$_{odot}$yr$^{-1}$ and $dot{M}_{text{H}alpha}$=0.10 M$_{odot}$yr$^{-1}$ for neutral and ionised gas, respectively. Assuming a molecular/atomic outflow velocity of 200 km s$^{-1}$, we derive upper limits of $dot{M}_{text{CO}}<$19.43 M$_{odot}$yr$^{-1}$ and $dot{M}_{text{HI}}<$26.72 M$_{odot}$yr$^{-1}$ for the molecular and atomic gas, respectively. Combining the detections and upper limits, we find average total outflow rates of $dot{M}_{text{tot}}lesssim$27 M$_{odot}$yr$^{-1}$ and a loading factor of $eta_{text{tot}}lesssim$6.39, with molecular gas likely contributing $lesssim$72% of the total mass outflow rate, and neutral and ionised gas contributing $sim$28% and $<$1%, respectively. Our results suggest that, to first order, a degree of quenching via ejective feedback could occur in normal galaxies when considering all gas phases, even in the absence of an AGN.
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