1,721,058 research outputs found
The Small Scale Physical Evolution of Molecular Gas in Nearby Galaxies
Full text linkWith the term of Interstellar Medium (or ISM) we refer to the gas and the dust that pervade the interstellar space. The ISM consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, dust grains, cosmic rays, and (galactic) magnetic fields. The interstellar medium plays a crucial role in astrophysics because it intermediates
between stellar and galactic scales. Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae,
stellar winds, and supernovae. This interplay between stars and the ISM helps to determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of
active star formation.
The distribution and mass of the different components of the ISM (the atomic gas, the cold dense molecular gas, the hot ionized gas, ...) change for different galaxies, according to their Hubble morphological types, peculia-rities, or properties of the environment. Since differences in stellar contents and light distributions of early- and late-type galaxies can be studied as differences in their ISM, the study of the interstellar medium is fundamental for understanding the nature of the galaxies.
Observations of different energy ranges give us information on different gas components of the ISM: the optical lines trace ionized regions or HII regions, the infrared range gives information on the radiation from interstellar dust grains, the 2.6 mm emission line of the carbon monoxide molecule is a good tracer of the molecular gas, the 21 cm line traces the atomic gas, and in the meter and decimeter range we can detect strong sources of continuous synchrotron radiation.
The study of the interstellar medium in galaxies is currently in a phase of strong technical progress, especially concerning the molecular gas component of the ISM.
Both millimeter-wave single-dish telescopes (FCRAO 14m, IRAM 30m, NRO 45m) and interferometers (IRAM, CARMA, NRO, SMA) already perform high resolution observations (<10") resolving (sub)struc-tures of the molecular clouds present in the galaxies. The immediate future with the construction of new instruments, the Atacama Millimeter Array (ALMA) in particular, will bring a great revolution in millimeter astronomy. ALMA, operating at wavelengths between 0.3 to 9.6 millimeters and with resolutions as fine as 0.005", will play a role similar to HST in the optical domain by opening a completely new range of observational possibilities.
In this PhD Thesis we focus on a important topic of the ALMA science, the molecular gas in external galaxies in order to prepare ourselves to this revolutionary instrument. We investigate the small scale excitation, fragmentation properties, distribution and kinematics of the molecular gas in two nearby galaxies (Messier 81 and NGC~3147) observed both with single-dish and interferometer instruments. We discuss the problem of the H2-CO conversion factor variation in external galaxies. The value of this factor, defined as X=N(H2)/ICO, is matter of discussion because it appears to depend on the particular physical and chemical conditions of the molecular gas
(e.g. temperature, density, and metallicity). The line ratio is also used to derive the physical conditions of the gas, such as excitation temperature and optical depth. In combination and complementary to these two detailed studies on the molecular gas component in specific galaxies, in this Thesis we also present a statistical and interpretative work on the relations existing between different components of the ISM obtained considering a sample of galaxies of all the morphologies, nuclear activities, and type of interaction. Some relations, such as that known between CO and the 100 ?m fluxes for early-type galaxies, are confirmed and extended considering a large and heterogeneous sample of galaxies, other relationships lacking for particular typologies of galaxies are interpreted with the help of chemo-dynamical models.
This double approach to the study of the interstellar medium - a detailed analysis of the molecular component of single galaxies
and a statistical study of the relations between different components of the ISM for a large sample of galaxies - helped us to understand better the complex scenario of the ISM. Major
efforts have been dedicated to the molecular gas component in the galaxies.
The Thesis is organized as follows: in the first chapter we describe the principal characteristics of the different phases of the interstellar medium. The second chapter is completely dedicated to the molecular component of the ISM, describing the properties of the principal molecular gas structures (molecular clouds, molecular clumps, ...) and their link with the star formation process. We also introduce the problem of the H2-CO conversion factor variation, parameter always used in the studies of the molecular gas, and describe the molecular
gas distribution in galaxies as a function of the morphological type, environment, redshift, and activity.
In the third chapter we present a analysis of the relations existing between CO(1-0) line, 100 micron, B, and X-ray fluxes for a sample of 3000 galaxies. The fourth chapter is devoted to a detailed study of the molecular gas component of the M 81 galaxy center. In the fifth chapter we analyze the molecular gas of the Seyfert 2 NGC 3147 galaxy to study the mechanisms for gas fueling of its Active Galactic Nucleus (AGN). In the sixth chapter we summarize our conclusions and describe the role - at the present day - of a Thesis on the molecular gas
The gas content of peculiar galaxies: strongly interacting systems
Full text linkA study of the gas content in 1038 interacting galaxies, essentially selected from Arp, Arp & Madore, Vorontsov-Velyaminov catalogues and some of the published literature, is presented here. The data on the interstellar medium have been extracted from a number of sources in the literature and compared with a sample of 1916 normal galaxies. The mean values for each of the different ISM tracers (FIR, 21 cm, CO lines, X-ray) have been estimated by means of survival analysis techniques, in order to take into account the presence of upper limits. From the data it appears that interacting galaxies have a higher gas content than normal ones. Galaxies classified as ellipticals have both a dust and gas content one order of magnitude higher than normal. Spirals have in most part a normal dust and HI content but an higher molecular gas mass. The X-ray luminosity also appears higher than that of normal galaxies of same morphological type, both including or excluding AGNs. We considered the alternative possibilities that the molecular gas excess may derive from the existence of tidal torques which produce gas infall from the surrounding regions or from a different metallicity which affects the X conversion factor between the observed CO line luminosity and the H_2 calculated mass. According to our tests, it appears that interacting galaxies possess a higher molecular mass than normal galaxies but with a similar star formation efficiency
Molecular Gas and Star Formation in M81
No full textWe present IRAM 30 m observations of the central ̃1.6 kpc of the spiral galaxy M81. The molecular gas appears weak and with an unusual excitation physics. We discuss a possible link between low CO emission and weak FUV surface brightness
Relations between interstellar medium tracers in galaxies
No full textAims:We study the relations existing between fluxes emitted at the CO(1-0) line, 60 and 100 μm wavelengths, and the B and soft X-ray wavebands for galaxies of all morphological types. The large set of data that we created allowed us to revisit some of the already known relations between the different tracers of the interstellar medium (ISM): the link between the FIR flux and the CO line emission, the relation between X-ray emission in non-active galaxies and the blue or FIR luminosity.
Methods: Using both catalogues of galaxies and works in the literature, we collected fluxes in the FIR, 21 cm, and CO(1-0) lines and in the soft X-ray for two samples, consisting of normal and interacting galaxies, respectively. By joining these samples, we have data for a total of 2953 galaxies, not all observed in these four wavebands.
Results: All the relations found are discussed in the frame of the star formation activity that is the link for most of them. We note that when an active star formation is present, it may link the galaxy fluxes at almost all wavelengths, from X to microwaves. In contrast, in early-type galaxies where the current star formation rate has faded, the X-FIR fluxes link disappears. This result for early-type galaxies is discussed and explained in detail in the frame of a suitable theoretical model, obtained by coupling chemo-dynamical N-body simulations with a dusty spectrophotometric code of population synthesis
Study of ISM tracers in galaxies
No full textWe collected data for two samples of normal and interacting galaxies for a total of 2953 galaxies having fluxes in one or more of the following wavebands: FIR, 21 cm line, CO(1-0) lines and soft X-ray. The large set of data obtained allowed us to revisit some of the already known relations between the different tracers of the interstellar medium (ISM), such as the link between the FIR flux and the CO line emission, the relation between X-ray emission and the blue or FIR luminosity. The relation lacking from observations for early-type galaxies has been discussed and explained in detail in the frame of a suitable theoretical model, obtained by coupling chemo-dynamical N-body simulations with a dusty spectrophotometric code of population synthesis
Are truncated stellar disks linked to the molecular gas density?
No full textWe know that the slope of the radial, stellar light distribution in galaxies is well described by an exponential decline and this distribution is often truncated at a break radius (R_br). We don't have a clear understanding for the origin of these outer truncations and several hypotheses have been proposed to explain them. We want to test the various theories with direct observations of the cold molecular gas for a few truncated galaxies in comparison with the non-truncated ones. The answer to the existence of a possible link between truncated stellar disks and the molecular gas density cannot be obtained from CO maps in the literature, because so far there are no galaxies with a clear truncation observed in CO at high resolution
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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