The thermally-unstable warm neutral medium: key for modeling the interstellar medium

We present 21-cm absorption measurements towards 12 radio continuum sources with previously identified thermally-unstable warm neutral medium (WNM). These observations were obtained with the Expanded Very Large Array (EVLA) and were complemented with the HI emission spectra obtained with the Arecibo Observatory. Out of 12 sources, HI absorption was detected along 5 lines of sight (seven new absorption features in total), resulting in a detection rate of ~42%. While our observations are sensitive to the WNM with a spin temperature T_s<3000 K, we detected only two wide absorption lines with T_s=400-900 K. These temperatures lie above the range allowed for the cold neutral medium (CNM) by the thermal equilbrium models and signify the thermally unstable WNM. Several absorption features have an optical depth of only a few x10^{-3}. While this is close or lower than what is theoretically expected for the CNM, we show that these weak lines are important for constraining the fraction of the thermally unstable WNM. Our observations demonstrate that, for the first time, high bandpass stability can be achieved with the VLA, allowing detection of absorption lines with a peak optical depth of ~10^{-3}.Comment: 10 pages. Accepted by Ap

Effective partitioning method for computing weighted Moore-Penrose inverse

We introduce a method and an algorithm for computing the weighted Moore-Penrose inverse of multiple-variable polynomial matrix and the related algorithm which is appropriated for sparse polynomial matrices. These methods and algorithms are generalizations of algorithms developed in [M.B. Tasic, P.S. Stanimirovic, M.D. Petkovic, Symbolic computation of weighted Moore-Penrose inverse using partitioning method, Appl. Math. Comput. 189 (2007) 615-640] to multiple-variable rational and polynomial matrices and improvements of these algorithms on sparse matrices. Also, these methods are generalizations of the partitioning method for computing the Moore-Penrose inverse of rational and polynomial matrices introduced in [P.S. Stanimirovic, M.B. Tasic, Partitioning method for rational and polynomial matrices, Appl. Math. Comput. 155 (2004) 137-163; M.D. Petkovic, P.S. Stanimirovic, Symbolic computation of the Moore-Penrose inverse using partitioning method, Internat. J. Comput. Math. 82 (2005) 355-367] to the case of weighted Moore-Penrose inverse. Algorithms are implemented in the symbolic computational package MATHEMATICA

Physical Properties of Complex C Halo Clouds

Observations from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey of the tail of Complex C are presented and the halo clouds associated with this complex cataloged. The properties of the Complex C clouds are compared to clouds cataloged at the tail of the Magellanic Stream to provide insight into the origin and destruction mechanism of Complex C. Magellanic Stream and Complex C clouds show similarities in their mass distributions (slope = -0.7 and -0.6, respectively) and have a common linewidth of 20 - 30 km/s (indicative of a warm component), which may indicate a common origin and/or physical process breaking down the clouds. The clouds cataloged at the tail of Complex C extend over a mass range of 10^1.1 to 10^4.8 solar masses, sizes of 10^1.2 to 10^2.6 pc, and have a median volume density of 0.065 cm^(-3) and median pressure of (P/k) = 580 K cm^{-3}. We do not see a prominent two-phase structure in Complex C, possibly due to its low metallicity and inefficient cooling compared to other halo clouds. From assuming the Complex C clouds are in pressure equilibrium with a hot halo medium, we find a median halo density of 5.8 x 10^(-4) cm^(-3), which given a constant distance of 10 kpc, is at a z-height of ~3 kpc. Using the same argument for the Stream results in a median halo density of 8.4 x 10^(-5) x (60kpc/d) cm^(-3). These densities are consistent with previous observational constraints and cosmological simulations. We also assess the derived cloud and halo properties with three dimensional grid simulations of halo HI clouds and find the temperature is generally consistent within a factor of 1.5 and the volume densities, pressures and halo densities are consistent within a factor of 3.Comment: Accepted for publication in AJ. 54 pages, including 6 tables and 16 figure

The Conditions for Star Formation at Low Metallicity: Results from the LMC

We present our recent work on the conditions under which star formation occurs in a metal-poor environment, the Large Magellanic Cloud ([Fe/H] ~ -0.4). Water masers are used as beacons of the current star formation in HII regions. Comparing their location with the dust morphology imaged with the Spitzer Space Telescope, and additional Halpha imaging and groundbased near-infrared observations, we conclude that the LMC environment seems favourable to sequential star formation triggered by massive star feedback (Oliveira et al. 2006). Good examples of this are 30 Doradus and N 113. There are also HII regions, such as N 105A, where feedback may not be responsible for the current star formation although the nature of one young stellar object (YSO) suggests that feedback may soon start making an impact. The chemistry in one YSO hints at a stronger influence from irradiation effects in a metal-poor environment where shielding by dust is suppressed (van Loon 2005)Comment: in "Triggered Star Formation in a Turbulent ISM", IAU symposium, poster contribution; a better quality version of this manuscript can be found at http://www.astro.keele.ac.uk/~jacco/papers/proc.ps a full version of the poster can be found at http://www.astro.keele.ac.uk/~jacco/papers/IAUS237_LMC_2006.pd

Evaluacija genotoksičnih efekata tiroksina primenom in vivo citogenetičkog testa na Swiss albino miševima

Thyroid hormones enhance aerobic metabolism favoring oxidative stress which may lead to covalent damage of various molecules including DNA. Previous investigations revealed that thyroid hormones induce DNA damage on human lymphocytes and sperm in the in vitro Comet assay. However, cytogenetic evaluation of genotoxic effects of thyroxine gave equivocal results: increase of sister chromatid exchanges, and no incerase of micronuclei in cultured human lymphocytes. Therefore, the aim of the present study was to further evaluate the possible genotoxic effects of thyroxine using in vivo cytogenetic test on Swiss albino mice. Three experimental concentrations of thyroxine were used (0.1 mg/kg, 0.5 mg/kg and 2.5 mg/kg). The mice were divided into several groups depending on the duration of the treatment with thyroxine. Thus, we treated mice for 1, 3, 7 and 10 days. Positive (Nmethyl- N'-nitro-N-nitrosoguanidine) and negative controls were also formed for the same time periods. Cytogenetic endpoinds (numerical and structural aberrations, chormosome gaps and breaks) were analysed in bone marrow cells from femures. The results obtained in this investigation showed that thyroxine has not induced chromosome damage or aberrations. This is in agreement with our previous analysis of micronuclei in human peripheral blood lymophocytes treated with thyroxine. On the other hand, we observed a decrease of mitotic index especially in animals treated for a longer period of time with the highest dose of thyroxine. Therefore, it can be concluded that thyroxine does not induce genotoxic effects which could be detected by cytogenetic analysis.Tireoidni hormoni podstiču aerobni metabolizam favorizujući oksidativni stres koji može da dovede do kovalentnih oštećenja različitih molekula uključujući i DNK. U prethodnim istraživanjima otkriveno je da tireoidni hormoni indukuju oštećenja molekula DNK u humanim limfocitima i spermi u in vitro Komet testu. Međutim, citogenetička evaluacija genotoksičnih efekata tiroksina dala je kontradiktorne rezultate: povećanje razmena sestrinskih hromatida bez porasta učesalosti mikronukleusa u kulturama humanih limfocita. Stoga je cilj istraživanja u ovom radu bio da dodatno ispitamo moguće genotoksične efekte tiroksina koristeć i in vitro citogenetički test na Swiss albino miševima. Upotrebljene su tri eksperimentalne koncentracije tiroksina (0,1 mg/kg, 0,5 mg/kg and 2.5 mg/kg). Miševi su podeljeni u nekoliko grupa zavisno od dužine tretmana tiroksinom: 1, 3, 7 i 10 dana. U istim vremenskim periodima miševi su tretirani pozitivnom (N-metil-N'-nitro-N-nitrozogvanidin) i negativnom kontrolom. Analizirani su citogenetički parametri (numeričke i strukturne aberacije hromozoma, gapovi i prekidi na hromozomima) u ćelijama kostne srži izolovanim iz femura. Rezultati dobijeni u ovom istraživanju ukazuju da tiroksin ne indukuje hromozomske prekide i aberacije, što je u saglasnosti sa našim prethodnim zapažanjima na humanim limfocitima u kulturi. Istovremeno, primetili smo smanjenje mitotskog indeksa, naročito kod životinja tretiranih u dužem vremenskom periodu sa visokim dozama tiroksina. Prema tome, može se zaključiti da tiroksin ne indukuje genotoksične efekte koji mogu da se detektuju citogenetičkim analizama

Compact HI clouds from the GALFA-HI survey

The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR), at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin. The unprecedented resolution and sensitivity of the GALFA-HI survey have resulted in the detection of numerous isolated, very compact HI clouds at low Galactic velocities, which are distinctly separated from the HI disk emission. In the limited area of ~4600 deg$^2$ surveyed so far, we have detected 96 of such compact clouds. The detected clouds are cold with a median T$_{k,max}$ (the kinetic temperature in the case in which there is no non-thermal broadening) of 300 K. Moreover, these clouds are quite compact and faint, with median values of 5 arcmin in angular size, 0.75 K in peak brightness temperature, and $5 \times 10^{18}$ cm$^{-2}$ in HI column density. Most of the clouds deviate from Galactic rotation at the 20-30 km/s level, and a significant fraction show evidence for a multiphase medium and velocity gradients. No counterparts for these clouds were found in other wavebands. From the modeling of spatial and velocity distributions of the whole compact cloud population, we find that the bulk of the compact clouds are related to the Galactic disk, and their distances are likely to be in the range of 0.1 to a few kpc. We discuss various possible scenarios for the formation and maintenance of this cloud population and its significance for Galactic ISM studies.Comment: Accepted for publication in the Astrophysical Journa