Pc Size Research Articles

On the universality of star formation efficiency in galaxies

We analyze high-resolution hydrodynamics simulations of an isolated disk dwarf galaxy with an explicit model for unresolved turbulence and turbulence-based star formation prescription. We examine the characteristic values of the star formation efficiency per free-fall time, ϵff, and its variations with local environment properties, such as metallicity, UV flux, and surface density. We show that the star formation efficiency per free-fall time in ≈10 pc star-forming regions of the simulated disks has values in the range ϵff≈0.01−0.1, similar to observational estimates, with no trend with metallicity and only a weak trend with the UV flux. Likewise, estimated using projected patches of 500 pc size does not vary with metallicity and shows only a weak trend with average UV flux and gas surface density. The characteristic values of ϵff≈0.01−0.1 arise naturally in the simulations via the combined effect of dynamical gas compression and ensuing stellar feedback that injects thermal and turbulent energy. The compression and feedback regulate the virial parameter, αvir, in star-forming regions, limiting it to αvir≈3−10. Turbulence plays an important role in the universality of ϵff because turbulent energy and its dissipation are not sensitive to metallicity and UV flux that affect thermal energy. Our results indicate that the universality of observational estimates of ϵff can be plausibly explained by the turbulence-driven and feedback-regulated properties of star-forming regions.

Millimeter-wave CO and SiO Observations toward the Broad-velocity-width Molecular Feature CO 16.134–0.553: A Smith Cloud Scenario?

We report the results of the CO J = 1–0 and SiO J = 2–1 mapping observations toward the broad-velocity-width molecular feature (BVF) CO 16.134–0.553 with the Nobeyama Radio Observatory 45 m telescope. The high-quality CO map shows that the 5 pc size BVF bridges two separate velocity components at V LSR ≃ 40 km s−1 and 65 km s−1 in the position–velocity space. The kinetic power of CO 16.134–0.553 amounts to 7.8 × 102 L ⊙, whereas no apparent driving sources were identified. Prominent SiO emission was detected from the BVF and its root in the V LSR ≃ 40 km s−1 component. In the CO Galactic plane survey data, CO 16.134–0.553 appears to correspond to the Galactic eastern rim of a 15 pc diameter expanding CO shell. A 1° diameter H i emission void and 4° long vertical H i filament were also found above and below the CO shell, respectively. We propose that the high-velocity plunge of a dark matter subhalo with a clump of baryonic matter was responsible for the formation of the H i void, CO 16.134–0.553/CO shell, and the H i filament.

Evidence for Low-power Radio Jet–ISM Interaction at 10 pc in the Dwarf AGN Host NGC 4395

Black-hole-driven outflows in galaxies hosting active galactic nuclei (AGN) may interact with their interstellar medium (ISM) affecting star formation (SF). Such feedback processes, reminiscent of those seen in massive galaxies, have been reported recently in some dwarf galaxies. However, such studies have usually been on kiloparsec and larger scales and our knowledge of the smallest spatial scales to which these feedback processes can operate is unclear. Here we demonstrate radio jet−ISM interaction on the scale of an asymmetric triple radio structure of ∼10 pc size in NGC 4395. This triple radio structure is seen in the 15 GHz continuum image and the two asymmetric jet-like structures are situated on either side of the radio core that coincides with the optical Gaia position. The high-resolution radio image and the extended [O iii]λ5007 emission, indicative of an outflow, are spatially coincident and are consistent with the interpretation of a low-power radio jet interacting with the ISM. Modelling of the spectral lines using MAPPINGS, and estimation of temperature using optical integral field spectroscopic data suggest shock ionization of the gas. The continuum emission at 237 GHz, though weak, was found to spatially coincide with the AGN. However, the CO(2−1) line emission was found to be displaced by around 20 pc northward of the AGN core. The spatial coincidence of molecular H2 λ2.4085 along the jet direction, the morphology of ionized [O iii]λ5007, and displacement of the CO(2−1) emission argues for conditions less favorable for SF in the central ∼10 pc region.

A review on hydrogen generation by phthalocyanines

Hydrogen, the most abundant element in the universe, holds a great promise as an alternative to conventional fuels. Among various approaches, the renewable energy-driven electro- and photocatalytic generations have lately gained prominence owing to the plethora of advantages that they offer. It is evident that when employing these methods for the H 2 production, catalyst is one of the significant components of the reaction, determining how much H 2 could be evolved by the proposed system. This has led to a rise in the number of scientific papers devoted to the development of novel and active catalysts as potential candidates for catalyzing the hydrogen evolution reaction. In this regard, phthalocyanine (Pc)-based catalysts offer powerful pathway due to the particular properties, including the ease of large-scale manufacturing, unique redox behavior, high efficiency, ease of separation, and recyclable nature. In this review, we have provided a comprehensive overview of publications on the generation of hydrogen by electrolysis and photolysis of various substrates, both of which are accomplished through the use of a variety of Pc-based catalysts. A number of critical variables were discovered to be consequential for the ultimate H 2 generation rate in the catalytic H 2 evolution systems such as the effects of metal center, substitution, support, size, and crystallinity of Pc. • The renewable energy-driven electro(photo) catalytic hydrogen production. • Phthalocyanine-based electro(photo) catalysts for green hydrogen generation. • Water splitting towards hydrogen evolution. • Phthalocyanines as photosensitizer, semiconductor, and catalysts in hydrogen evolution.

Uptake of Upconverting Nanoparticles by Breast Cancer Cells: Surface Coating versus the Protein Corona.

Fluorophores with multifunctional properties known as rare-earth-doped nanoparticles (RENPs) are promising candidates for bioimaging, therapy, and drug delivery. When applied in vivo, these nanoparticles (NPs) have to retain long blood-circulation time, bypass elimination by phagocytic cells, and successfully arrive at the target area. Usually, NPs in a biological medium are exposed to proteins, which form the so-called “protein corona” (PC) around the NPs and influence their targeted delivery and accumulation in cells and tissues. Different surface coatings change the PC size and composition, subsequently deciding the fate of the NPs. Thus, detailed studies on the PC are of utmost importance to determine the most suitable NP surface modification for biomedical use. When it comes to RENPs, these studies are particularly scarce. Here, we investigate the PC composition and its impact on the cellular uptake of citrate-, SiO2-, and phospholipid micelle-coated RENPs (LiYF4:Yb3+,Tm3+). We observed that the PC of citrate- and phospholipid-coated RENPs is relatively stable and similar in the adsorbed protein composition, while the PC of SiO2-coated RENPs is larger and highly dynamic. Moreover, biocompatibility, accumulation, and cytotoxicity of various RENPs in cancer cells have been evaluated. On the basis of the cellular imaging, supported by the inhibition studies, it was revealed that RENPs are internalized by endocytosis and that specific endocytic routes are PC composition dependent. Overall, these results are essential to fill the gaps in the fundamental understanding of the nano-biointeractions of RENPs, pertinent for their envisioned application in biomedicine.

Revisiting Attenuation Curves: The Case of NGC 3351*

Abstract Multiwavelength images from the far-UV (∼0.15 μm) to the submillimeter of the central region of the galaxy NGC 3351 are analyzed to constrain its stellar populations and dust attenuation. Despite hosting a ∼1 kpc circumnuclear starburst ring, NGC 3351 deviates from the IRX–β relation, the relation between the infrared-to-UV luminosity ratio and the UV continuum slope β that other starburst galaxies follow. To understand the reason for the deviation, we leverage the high angular resolution of archival near-UV-to-near-IR Hubble Space Telescope images to divide the ring into ∼60–180 pc size regions and model each individually. We find that the UV slope of the combined intrinsic (dust-free) stellar populations in the central region is redder than what is expected for a young model population. This is due to the region’s complex star formation history, which boosts the near-UV emission relative to the far-UV. The resulting net attenuation curve has a UV slope that lies between those of the starburst attenuation curve (Calzetti et al. 2000) and the Small Magellanic Cloud extinction curve; the total-to-selective attenuation value, R ′ (V) = 4.93, is larger than both. As found for other star-forming galaxies, the stellar continuum of NGC 3351 is less attenuated than the ionized gas, with E(B − V)star = 0.40 E(B − V)gas. The combination of the “red” intrinsic stellar population and the new attenuation curve fully accounts for the location of the central region of NGC 3351 on the IRX–β diagram. Thus, the observed characteristics result from the complex mixture of stellar populations and dust column densities in the circumnuclear region. Despite being a sample of one, these findings highlight the difficulty of defining attenuation curves of general applicability outside the regime of centrally concentrated starbursts.

Feasibility of using polycarbonate as a substrate of thin film composite membrane in forward osmosis

AbstractIn the present study, thin film composite (TFC) forward osmosis (FO) membranes with polycarbonate (PC (and polyether sulfone (PES) as substrates were fabricated to investigate the impact of the structural parameters of substrate on the performance of the membranes. Firstly, the substrates were prepared by Loeb‐Sourirajan method. Characterization techniques including FESEM, contact angle measurement, pure water flux, gas permeability test, and tensile test were applied to investigate the properties of the substrates. After preparing suitable substrates, active layers were fabricated via interfacial polymerization (IP) technique. The performance and characterization test showed that PC is a relatively hydrophilic polymer with a good property for using as a substrate of FO TFC membrane but as the result of gas permeability test show, this membrane has large surface pore size in comparison with PES membrane. Mean pore size of PC and PES membrane is 378 and 139 nm, respectively. Also, the results show that the effective surface porosity of PC (285, 1/m) is more than PES (213, 1/m) substrate.

Bounds on the size of PC and URC formulas

In this paper, we investigate CNF encodings, for which unit propagation is strong enough to derive a contradiction if the encoding is not consistent with a partial assignment of the variables (unit refutation complete or URC encoding) or additionally to derive all implied literals if the encoding is consistent with the partial assignment (propagation complete or PC encoding). We prove an exponential separation between the sizes of PC and URC encodings without auxiliary variables and strengthen the known results on their relationship to the PC and URC encodings that can use auxiliary variables. Besides of this, we prove that the sizes of any two irredundant PC formulas representing the same function differ at most by a factor polynomial in the number of the variables and present an example of a function demonstrating that a similar statement is not true for URC formulas. One of the separations above implies that a q-Horn formula may require an exponential number of additional clauses to become a URC formula. On the other hand, for every q-Horn formula, we present a polynomial size URC encoding of the same function using auxiliary variables. This encoding is not q-Horn in general.

FORENSIC MEDICINE CRITERIA FOR DIFFERENTIAL DIAGNOSIS OF ALCOHOL AND CARBON GAS POISONING BY STOX-POLYARIMETRY

Ethyl alcohol and carbon monoxide are among the most common toxic substances that can be fatal. However, morphological changes in this pathological condition are insufficiently studied. A new step was the development and application of laser polarimetric methods for microscopic study of optically inhomogeneous biological structures using statistical analysis of digital polarization maps. This work is aimed at studying and substantiating the information possibilities of the objective method of Stokes-polarimetric microscopy of histological sections of biological tissues (BT) in the differential diagnosis of alcohol and carbon monoxide poisoning.Aim of the work. To develop a set of forensic objective criteria to increase the possibility of differential diagnosis of ethanol and carbon monoxide poisoning according to Stokes-polarimetric mapping of polycrystalline structure of blood films and human myocardium.Materials and methods. The object of the study were histological sections of the myocardium and polycrystalline blood films obtained from 68 corpses of both sexes, aged 18 to 75 years. BT samples from deaths due to ischemic heart disease (ІHD) were used for control. The study was performed by Stokes polarimetric mapping of microscopic images of biological layers.Results. Comparative analysis of polarization maps of the crystallization parameter (CP) and histograms of the distributions of the 4th parameter of the Stokes vector showed greater sensitivity of the Stokes-polarimetric mapping method of digital microscopic images of polycrystalline blood films to degenerative-dystrophic changes for alcohol and carbon monoxide poisoning. histological sections of the myocardium. Moreover, the most informative were the statistical moments of the 3rd - 4th orders, which characterize the coordinate distributions of the PC size of microscopic images of samples of polycrystalline blood films.Conclusion. The possibility of statistically significant (p1; p2<0.05) differentiation of deaths due to alcohol and carbon monoxide poisoning by calculating statistical moments of higher orders (asymmetry As and excess Ek), which characterize the PC size distributions of microscopic images of polycrystalline blood films, has been established.

Real Time Measurement of Airplane Flutter via Distributed Acoustic Sensing

This research group has recently used the new technology Distributed Acoustic Sensing (DAS) for the monitoring and the measurement of airplane flutter. To the authors’ knowledge, this is the first such use for this new technology. Traditionally, the measurement of airplane flutter requires the mounting of a very large number of sensors on the wing being monitored, and extensive wiring must be connected to all these sensors. The new system and technology introduced in this paper dramatically reduces the hardware requirements in such an application: all the traditional sensors and wiring are replaced with one fiber optic cable with a diameter of 2 mm. An electro-optical system with the size of a desktop PC monitors simultaneously one or more of such fiber optic cables and detects/characterizes any mechanical disturbances on the cables. Theoretical and experimental results are given.

A panchromatic spatially resolved analysis of nearby galaxies – II. The main sequence – gas relation at sub-kpc scale in grand-design spirals

ABSTRACT In this work, we analyse the connection between gas availability and the position of a region with respect to the spatially resolved main-sequence (MS) relation. Following the procedure presented in Enia et al. (2020), for a sample of five face-on, grand design spiral galaxies located on the MS we obtain estimates of stellar mass and star formation rate surface densities (Σ⋆ and ΣSFR) within cells of 500 pc size. Thanks to H i 21cm and 12CO(2–1) maps of comparable resolution, within the same cells we estimate the surface densities of the atomic (ΣH i) and molecular ($\Sigma _{\rm {H_2}}$) gas and explore the correlations among all these quantities. Σ⋆, ΣSFR, and $\Sigma _{\rm {H_2}}$ define a 3D relation whose projections are the spatially resolved MS, the Kennicutt–Schmidt law and the molecular gas MS. We find that $\Sigma _{\rm {H_2}}$ steadily increases along the MS relation and is almost constant perpendicular to it. ΣH i is nearly constant along the MS and increases in its upper envelope. As a result, ΣSFR can be expressed as a function of Σ⋆ and ΣH i, following the relation log ΣSFR = 0.97log Σ⋆ + 1.99log ΣH i − 11.11. We show that the total gas fraction significantly increases towards the starburst regions, accompanied by a weak increase in star formation efficiency. Finally, we find that H2/H i varies strongly with the distance from the MS, dropping dramatically in regions of intense star formation, where the UV radiation from newly formed stars dissociates the H2 molecule, illustrating the self-regulating nature of the star formation process.

Wolf–Rayet Galaxies in SDSS-IV MaNGA. I. Catalog Construction and Sample Properties

Abstract Wolf–Rayet (WR) galaxies are a rare galaxy type that hosts living high-mass stars during the WR phase (i.e., WR stars) and can thus provide constraints on the stellar initial mass function, massive star formation, stellar evolution models, etc. Spatially resolved spectroscopy should in principle identify WR galaxies more efficiently than single-fiber surveys of galactic centers, as WR stars should be more preferentially found in disks. Using integral field unit data from the ongoing SDSS-IV MaNGA survey, we have performed a thorough search for WR galaxies. We first identify H ii regions in each datacube and carry out full spectral fitting to the stacked spectra. We then visually inspect the residual spectrum of each H ii region and identify significant WR “blue bumps” at 4600–4750 Å. The WR catalog includes 267 WR regions of ∼500 pc (radius) sizes, distributed in 90 galaxies from MaNGA Product Launch-7. We find that WR regions are exclusively found in galaxies with the bluest colors and highest star formation rates. Most WR galaxies have late-type morphologies and show relatively large asymmetry in their images, implying WR regions exist more in interacting galaxies. We estimate the stellar mass function of WR galaxies and the mass-dependent detection rate (typically ∼2%). This rate is about 40 times higher than that in catalogs from SDSS single-fiber surveys and half that of the CALIFA-based catalog. The difference in detection rates can be explained mainly by three factors: spatial coverage, spectral signal-to-noise ratio, and redshift ranges of parent samples. We tabulate WR galaxy properties for future studies.

Dust Destruction by Drift-induced Sputtering in Active Galactic Nuclei

Abstract Recent mid-infrared high spatial resolution observations have revealed that active galactic nuclei (AGNs) may host a polar dust region with the size of several pc, and such dust may be carried by radiation from the central engine. The polar dust emission often exhibits very weak or absence of the silicate 10 μm emission feature. A possible explanation is that the polar dust is dominated by micron-sized large grains because these grains do not show the silicate feature, while it remains unclear how large grains are preferentially supplied to the polar region. Here, we propose a new scenario describing the prevalence of large grains at the polar region. We show that grains are accelerated to the hypersonic drift velocity by the radiation pressure from AGN, and the hypersonic drift results in dust destruction via kinetic sputtering. Sputtering destroys small grains faster than the large ones, and thus larger grains will be preferentially blown over longer distance. Although the hypersonic drift, or kinetic sputtering, tends to be suppressed for very small grains due to the Coulomb drag, they might also be disrupted by Coulomb explosion. Removal of small grains and/or survival of large grains may explain the lack of a silicate 10 μm emission feature in polar dust emission.

Unveiling Vela – variability of interstellar lines in the direction of the Vela supernova remnant – III. Na D and Ca ii K

ABSTRACT High-resolution optical spectra were obtained in 2017–2019 with The Southern African Large Telescope of 15 stars in the direction of the Vela supernova remnant. Interstellar Ca ii H & K and Na i D lines are discussed in this paper. In particular, the line profiles are compared with profiles at a comparable spectral resolution obtained in 1993–1996 by Cha and Sembach. Ten of the lines of sight show changes to one or more of the components in that line of sight. Changes include small changes (1–2 km s−1) in radial velocity and/or increases/decreases in equivalent width over the two decades between the periods of observation. Changes are more obvious in the Ca K line than in the Na D lines. These changes are attributed to gas disturbed by interactions between the supernova ejecta and the surrounding interstellar medium. A representative time-scale may be 20–50 yr. Small-scale variations in line profiles across the face of the remnant suggest, as previously remarked, that a linear scale for interactions is a small fraction of the 40 pc size of the present remnant.

Modeling ionized gas in low-metallicity environments: the Local Group dwarf galaxy IC 10

Context. Star formation activity is an important driver of galaxy evolution and is influenced by the physical properties of the interstellar medium. Dwarf galaxies allow us to understand how the propagation of radiation and the physical conditions of the different ISM phases are affected by the low-metallicity environment. Aims. Our objective is to investigate the physical properties of the ionized gas of the low-metallicity dwarf galaxy, IC 10, at various spatial scales: from individual H II regions to the entire galaxy scale and examine whether diagnostics for integrated measurements introduce bias in the results. Methods. We modeled the ionized gas combining the mid- and far-infrared fine-structure cooling lines observed with Spitzer/IRS and Herschel/PACS, with the photoionization code CLOUDY. The free parameters of the models are the age of the stellar cluster, the density, and the ionization parameter of the ionized gas as well as the depth of the cloud. The latter is used to investigate the leakage of the ionizing photons from the analyzed regions of IC 10. We investigated H II regions in the main star-forming body, on scales of ~25 pc, three in the main star-forming region in the center of the galaxy and two on the first arc. We then considered larger sizes on the scale of ~200 pc. Results. Most clumps have almost-identical properties, density ~102–102.6 cm−3, ionization parameter between 10−2.2 and 10−1.6, and age of the stellar cluster ~5.5 Myr. All of them are matter-bounded regions, allowing ionizing photons to leak. The relatively uniform physical properties of the clumps suggest a common origin for their star formation activity, which could be related to the feedback from stellar winds or supernovae of a previous generation of stars. The properties derived for ~200 pc size “zones” have similar properties as the H II regions they encompass, but with the larger regions tending to be more radiation bounded. Finally, we investigated the fraction of [CII] 157.7 μm, [SiII] 34.8 μm and [FeII] 25.9 μm emission arising from the ionized gas phase and we find that most of the emission originates from the neutral gas, not from the ionized gas.

Natural course of pineal cysts\u2014a radiographic study

BackgroundPineal cysts (PCs) are a benign lesion of the pineal gland that have been known to the medical community for a long time. With a prevalence rate of approximately 1% in the general population, PC is often a reason for medical counseling. The natural course of PC morphology has not been well described. In this study, we present a longitudinal magnetic resonance imaging (MRI) study of patients with PCs, with special focus on those who showed an increase or decrease in PC size.MethodsWe enrolled all patients with a PC who were referred to our department between January 2000 and January 2018. Each patient underwent a clinical examination, and the patient’s age, sex, and presenting signs and symptoms were noted. MRI was performed during periodic examinations, and a clinical and radiological course was reassessed.ResultsIn total, 133 patients (99 women, 34 men) were enrolled. The mean maximum diameter was 12.7 ± 5.2 mm (range 7–35 mm). PCs increased in size during the follow-up in seven patients (5.3%) and decreased in size in 10 (7.5%). The remaining cysts (n = 116, 87.2%) were stable over the follow-up period. Analyzing patients according to cyst size change, we found a significant difference in the mean age between the PC progression group and PC regression group (p = 0.01). The mean size of the PCs at the time of diagnosis did not differ significantly between the two groups (p = 0.81). We diagnosed two cases of pineal apoplexy.ConclusionWe found that PCs are a dynamic structure that may change in size during the patient’s lifetime. Patients with an increase in PC size were significantly younger than patients with a decrease in size. Therefore, PC growth in the first, second, and third decennium is normal and does not justify medical intervention. Surgery is indicated in cases of hydrocephalus and Parinaud’s syndrome or in atypical cysts when neoplasia is suspected. The size of a PC does not predict PC behavior in terms of a future increase or decrease in size.

Telaah morfometrik Pinctada margaritifera untuk pengembangan usaha budidaya

This study aimed to analyze the size of shell length, shell width and total weight of Pinctada margaritifera shells, and water quality parameters for the aquaculture development. Morphometric measurements include: shell length (PC), and shell width (LC), shell weight (BT). The results showed that PC size 8 - 11.99 cm, LC 8 - 9.99 cm and BT 80 - 99.99 gram dominate the Arakan waters while PC size 6 - 9.99 cm, LC 6 - 7.99 cm and BT 60 - 79.99 gram dominate the Bahoi waters. The results of water quality measurements in both locations are Bahoi waters, namely: 29-32oC, DO 7-8,5 mg/l, pH 7,8-7,9, salinity, 28-32 ppt, 4,5- 7 meters, 11 cm/sec. While in Arakan waters, the temperature was 30-30,8oC, DO 6,3-6,7 mg/l, pH 7,6-7,7 salinity 30 ppt, brightness 4-5 meter, current velocity 4,5 cm / second. Both locations were feasible for the development of pearl aquacultureKeywords: Morphometric, Pinctada margaritifera, Aquaculture

Resolved Structure of the Arp 220 Nuclei at λ ≈ 3 mm

Abstract We analyze the 3 mm emission of the ultraluminous infrared galaxy Arp 220 for the spatially resolved structure and the spectral properties of the merger nuclei. ALMA archival data at ∼0.″05 resolution are used for extensive visibility fitting and deep imaging of the continuum emission. The data are fitted well by two concentric components for each nucleus, such as two Gaussians or one Gaussian plus one exponential disk. The larger components in the individual nuclei are similar in shape and extent, ∼100–150 pc, to the centimeter wave emission due to supernovae. They are therefore identified with the known starburst nuclear disks. The smaller components in both nuclei have about a few 10 pc sizes and peak brightness temperatures ( ) more than twice higher than those in previous single-Gaussian fitting. They correspond to the dust emission that we find centrally concentrated in both nuclei by subtracting the plasma emission measured at 33 GHz. The dust emission in the western nucleus is found to have a peak K and an FWHM of about 20 pc. This component is estimated to have a bolometric luminosity on the order of and a 20 pc scale luminosity surface density . A luminous active galactic nucleus is a plausible energy source for these high values while other explanations remain to be explored. Our continuum image also reveals a third structural component of the western nucleus—a pair of faint spurs perpendicular to the disk major axis. We attribute it to a bipolar outflow from the highly inclined ( ) western nuclear disk.

Formation of stellar clusters

We investigate the triggering of star formation and the formation of stellar clusters in molecular clouds that form as the ISM passes through spiral shocks. The spiral shock compresses gas into $\sim$100 pc long main star formation ridge, where clusters forming every 5-10 pc along the merger ridge. We use a gravitational potential based cluster finding algorithm, which extracts individual clusters, calculates their physical properties and traces cluster evolution over multiple time steps. Final cluster masses at the end of simulation range between 1000 and 30000 M$_{\odot}$ with their characteristic half-mass radii between 0.1 pc and 2 pc. These clusters form by gathering material from 10-20 pc size scales. Clusters also show a mass - specific angular momentum relation, where more massive clusters have larger specific angular momentum due to the larger size scales, and hence angular momentum from which they gather their mass. The evolution shows that more massive clusters experiences hierarchical merging process, which increases stellar age spreads up to 2-3 Myr. Less massive clusters appear to grow by gathering nearby recently formed sinks, while more massive clusters with their large global gravitational potentials are increasing their mass growth from gas accretion.

ORIGINS OF SCATTER IN THE RELATIONSHIP BETWEEN HCN 1-0 AND DENSE GAS MASS IN THE GALACTIC CENTER

ABSTRACT We investigate the correlation of HCN 1-0 with gas mass in the central 300 pc of the Galaxy. We find that on the ∼10 pc size scale of individual cloud cores, HCN 1-0 is well correlated with dense gas mass when plotted as a log–log relationship. There is ∼0.75 dex of scatter in this relationship from clouds like Sgr B2, which has an integrated HCN 1-0 intensity of a cloud less than half its mass, and others that have HCN 1-0 enhanced by a factor of 2–3 relative to clouds of comparable mass. We identify the two primary sources of scatter to be self-absorption and variations in HCN abundance. We also find that the extended HCN 1-0 emission is more intense per unit mass than in individual cloud cores. In fact the majority (80%) of HCN 1-0 emission comes from extended gas with column densities below 7 × 1022 cm−2, accounting for 68% of the total mass. We find variations in the brightness of HCN 1-0 would only yield a ∼10% error in the dense gas mass inferred from this line in the Galactic center. However, the observed order of magnitude HCN abundance variations, and the systematic nature of these variations, warn of potential biases in the use of HCN as dense gas mass tracer in more extreme environments such as an active galactic nucleus and shock-dominated regions. We also investigate other 3 mm tracers, finding that HNCO is better correlated with mass than HCN, and might be a better tracer of cloud mass in this environment.