89 research outputs found
Radio emission during the formation of stellar clusters in M33
We investigate thermal and non-thermal radio continuum associated with the
early formation and evolution of Young Stellar Clusters (YSCs) selected by
their MIR emission in M33. For the first time in an external galaxy it has been
possible to identify radio counterparts to more than 300 star forming regions.
We proof the nature of candidate YSCs fully embedded in molecular clouds, by
recovering their associated faint radio continuum luminosities. Using the
Halpha line to identify free-free radio emission at 5 GHz in the more evolved,
partially exposed YSCs, we retrieve information on the relevance of magnetic
fields and cosmic rays across the M33 disk at 25 pc spatial scales. A
cross-correlation of MIR and radio continuum luminosities is established from
bright to very faint YSCs, with MIR-to-radio emission ratio showing a gradual
decline towards the outer disk, while the magnetic field is pervasive at all
radii. We establish and discuss the tight relation between radio continuum and
other star formation indicators, such as Halpha. This relation holds for
individual YSCs over four orders of magnitude as well as for molecular clouds
hosting YSCs. On average about half of radio emission at 5 GHz in YSCs is
non-thermal. For exposed but compact YSCs the non-thermal radio fraction
increases with source brightness, while for large HII regions the fraction is
lower and shows no clear trend. This has been found for YSCs with and without
identified SNRs and underlines the possible role of massive stars in triggering
particle acceleration through winds and shocks: these particles diffuse
throughout the native molecular cloud prior to cloud dispersal.Comment: 14 pages, 10 figures, accepted for publication in A&
Formation of Molecular Gas in the debris of violent Galaxy Interactions
In many gravitational interactions between galaxies, gas and stars that have
been torn from either or both of the precursor galaxies can collect in 'tidal
tails'. Star formation begins anew in these regions to produce 'tidal dwarf
galaxies', giving insight into the process of galaxy formation through the
well-defined timescale of the interaction. But tracking the star formation
process has proved to be difficult: the tidal dwarf galaxies with young stars
showed no evidence of the molecular gas out of which new stars form. Here we
report the discovery of molecular gas (carbon monoxide emission) in two tidal
dwarf galaxies. In both cases, the molecular gas peaks at the same location as
the maximum in atomic-hydrogen density, unlike most gas-rich galaxies. We infer
from this that the molecular gas formed from the HI, rather than being torn in
molecular form from the interacting galaxies. Star formation in the tidal
dwarfs appears to mimic that process in normal spiral galaxies like our own.Comment: 8 pages, 3 figures, Nature in pres
Intergalactic Star Formation
Star formation in interacting systems may take place in various locations,
from the dust--enshrouded core of Ultraluminous Infrared Galaxies to more
unusual places such as the debris of colliding galaxies expelled into the
intergalactic medium. Determining whether star-formation proceeds in the latter
environment, far from the parent galaxies, in a similar way as in spiral disks
has motivated the multi--wavelength study presented here. We collected VLA/HI,
UV/GALEX, optical Halpha and MIR/Spitzer images of a few nearby interacting
systems chosen for their prominent "intergalactic" star formation activity.
Preliminary results on the spectacular collisional HI ring around NGC 5291 are
presented.Comment: 4 pages, 1 fig., tp appear in conference proceedings "Studying Galaxy
Evolution with Spitzer and Herschel", eds. V. Charmandaris, D. Rigopoulou &
N. Kylafi
Molecular hydrogen beyond the optical edge of an isolated spiral galaxy
We know little about the outermost portions of galaxies because there is
little light coming from them. We do know that in many cases atomic hydrogen
(HI) extends well beyond the optical radius \cite{Casertano91}. In the centers
of galaxies, however, molecular hydrogen (H2) usually dominates by a large
factor, raising the question of whether H2 is abundant also in the outer
regions but hitherto unseen.Here we report the detection of emission from
carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius
of the nearby galaxy NGC 4414. The molecular clouds probably formed in the
regions of relatively high HI column density and in the absence of spiral
density waves. The relative strength of the lines from the two lowest
rotational levels indicates that both the temperature and density of the H2 are
quite low compared to conditions closer to the center. The inferred surface
density of the molecular material continues the monotonic decrease from the
inner regions. We conclude that while molecular clouds can form in the outer
region of this galaxy, there is little mass associated with them.Comment: 3 Nature page
The Dependence of the Galactic Star Formation Laws on Metallicity
We describe results from semi-analytical modelling of star formation in
protocluster clumps of different metallicities. In this model, gravitationally
bound cores form uniformly in the clump following a prescribed core formation
efficiency per unit time. After a contraction timescale which is equal to a few
times their free-fall times, the cores collapse into stars and populate the
IMF. Feedback from the newly formed OB stars is taken into account in the form
of stellar winds. When the ratio of the effective energy of the winds to the
gravitational energy of the system reaches unity, gas is removed from the clump
and core and star formation are quenched. The power of the radiation driven
winds has a strong dependence on metallicity and it increases with increasing
metallicity. Thus, winds from stars in the high metallicity models lead to a
rapid evacuation of the gas from the protocluster clump and to a reduced star
formation efficiency, as compared to their low metallicity counterparts. We
derive the metallicity dependent star formation efficiency per unit time in
this model as a function of the gas surface density Sigma_g. This is combined
with the molecular gas fraction in order to derive the dependence of the
surface density of star formation Sigma_SFR on Sigma_g. This feedback regulated
model of star formation reproduces very well the observed star formation laws
in galaxies extending from low gas surface densities up to the starburst
regime. Furthermore, the results show a dependence of Sigma_SFR on metallicity
over the entire range of gas surface densities, and can also explain part of
the scatter in the observations.Comment: In the proceedings of the French Astronomical Society meeting SF2A
2011. 8 pages, 5 figure
Infrared dust emission in the outer disk of M51
We examine faint infrared emission features detected in Spitzer Space
Telescope images of M51, which are associated with atomic hydrogen in the outer
disk and tidal tail at R greater than R_25 (4.9', ~14 kpc at d=9.6 Mpc). The
infrared colors of these features are consistent with the colors of dust
associated with star formation in the bright disk. However, the star formation
efficiency (as a ratio of star formation rate to neutral gas mass) implied in
the outer disk is lower than that in the bright disk of M51 by an order of
magnitude, assuming a similar relationship between infrared emission and star
formation rate in the inner and outer disks.Comment: 13 pages in manuscript form, 2 figures; download PDF of manuscript
with original-resolution Figure 1 at
http://www.eg.bucknell.edu/physics/thornley/thornleym51.pd
HCN and HCO+ emission in the disk of M31
We report observations made with the IRAM 30m radiotelescope in the HCN(1-0)
and HCO+(1-0) lines towards a sample of molecular complexes (GMCs) in the disk
of the Andromeda galaxy (M31). The targets were identified bright CO GMCs
selected from the IRAM 30m CO survey with various morphologies and
environments. The clouds vary in galactocentric distances from 2.4 to 15.5kpc.
The HCN and HCO+ emission is easily detected in almost all observed positions,
with line widths generally similar to the CO ones and there is a good
correlation between the two dense gas tracers. The HCO+ emission is slightly
stronger than the HCN, in particular towards GMCs with a strong star formation
activity. However the HCO+ emission is weaker than the HCN towards a quiescent
cloud in the inner part of M31, which could be due to a lower abundance of
HCO+. We derive I_HCN/I_CO ratios between 0.008 and 0.03 and I_HCO+/I_CO ratios
between less than 0.003 and 0.04. We study the radial distribution of the dense
gas in the disk of M31. Unlike our Galaxy the HCO+/CO ratio is lower in the
center of M31 than in the arms, which can be explained by both a lower
abundance of HCO+ and different conditions of excitation. Furthermore the
HCN/CO and HCO+/CO ratios appear to be higher in the inner spiral arm and
weaker in the outer arm.Comment: Accepted for publication in A&
Etude de l'émission dans l'infrarouge lointain des galaxies spirales proches
Durant ma thèse j'ai exploré les liens morphologiques et physiques entre les phases poussière et stellaire des galaxies spirales proches.J'ai travaillé sur 46 galaxies de l'échantillon KINGFISH à l'aide des données IRAC/MIPS/PACS/SPIRE (de 3.6 à 500 microns).Un biais usuel dans la mesure de l'orientation des galaxies spirales est dû à l'utilisation d'une seule isophote. Pour supprimer ce biais j'ai extrait de nombreuses isophotes des cartes galactiques, j'ai créé un critère pour quantifier la similitude des forme des isophotes. J'ai extrait des zones dans chaque carte où les formes des isophotes se ressemblent. Dans de nombreuses cartes les formes des isophotes sont cohérentes avec l'idée d'un disque sous-jacent et ce malgré des variations de formes des isophotes qui peuvent ponctuellement être notable. De là j'ai obtenu pour chaque galaxie une orientation du disque par carte. En comparant les formes obtenues pour chaque galaxie dans différentes cartes j'ai selectionné 20 galaxies sur 46 dans lesquelles l'accord en terme d'orientation du disque entre les différentes cartes était acceptable. Dans ces galaxies les zones associées au disque galactique ont une taille typique allant jusqu'à 1/3 du rayon galactique visible (R25) que ce soit pour la phase poussière aussi bien que pour la phase stellaire. Ces 20 galaxies sont moins lumineuses dans le visible, moins lumineuses dans l'IR, moins barrées, et de type plus tardifs que la moyenne. Pour ces 20 galaxies, les orientations obtenues par ma méthode sont plus proches des orientations obtenues à partir d'études cinématiques H-alpha que de celles obtenues par une autre étude photometrique utilisant une seule isophote (RC3).A partir des orientations obtenues par ma méthode et par l'étude cinématique H-alpha j'ai moyenné azimuthalement les brillances de surface pour obtenir des profiles radiaux de distribution spectrales d'énergie. Après avoir ajusté dessus un modèle d'émission de la poussière cosmique (Galliano 2011), j'ai trouvé que la densité surfacique d'énergie interceptée par la poussière était proportionnelle au produit de la masse totale de poussière sur la ligne de visée par le champ de radiation interstellaire moyen ressenti par la poussière sur la ligne de visée. Cette densité d'énergie interceptée par la poussière est mieux corrélé à la luminosité bolométrique stellaire totale que la densité surfacique en masse de poussière ou le champ de radiation ressenti par la poussière. Il est donc probable que les étoiles agées à tout le moins soit une importante source de chauffage pour la poussière cosmique. L'énergie interceptée par la poussière est aussi très bien corrélée avec l'énergie totale émise dans l'infrarouge. J'ai également trouvé que la poussière semble intercepter une plus large quantité d'énergie provenant des étoiles dans les galaxies plus actives à former des étoiles.Les profiles radiaux en masse de poussière sont moins bien décrits que les profils en masse stellaire par des profiles de Sersic. Par ailleurs pour les ajustements acceptables par des fonctions de Sersic, les distributions statistiques des indices de Sersic et des rayons de demi masse totale ont des largeurs statistiques plus grandes pour la poussière que pour les étoiles.J'ai également trouvé que le rapport densité surfacique maximum de poussière sur densité surfacique maximum d'étoile est un facteur important à considérer pour expliquer la variation avec le type morphologique du rapport densité surfacique d'énergie interceptée par la poussière sur densité surfacique d'énergie émise par les étoiles. Cette variation pourrait être liée à une variation entre les galaxies de la force de la structure spirale.In my PHD work I explored the links between the physical properties of interstellar dust and other components of nearby spiral galaxies especially their stellar content. I worked on 46 disk galaxies from KINGFISH with IRAC/MIPS/PACS/SPIRE maps (3.6 - 500 microns). A bias is usually introduced in estimating disk orientations by using only a single surface brightness isophote. Thus I devised different surface brightness levels separated by constant steps in surface brightness and extracted isophotes at these levels in all FIR maps as well as in all IRAC 4.5 microns maps. To further assess the coherence of the shapes of isophotes across galactic disks, I built a quantitative indicator of the difference in shape between two ellipses with same center and same semi-major axis.I defined an acceptable level of difference between isophote shapes, by comparing disk orientations found in litterature. Using this level, I found regions inside the galactic disks where the isophotal shapes are similar. From these, I extracted one disk orientation per wavelength band. I found in the vast majority of the disk galaxy maps, be it dominated by stellar or dust emission, that a large fraction of the isophotes I extracted are coherent with the idea of an underlying disk. Comparing, for each galaxy, disk orientations extracted at all wavelengths, I found evidence in 20 galaxies out of 46, that on radial ranges as large as 1/3 of the visible disk (as measured by R25), the shapes of isophotes are morphologically similar. Thus for these 20 galaxies I devised consistent disk orientations both for the stellar and dust content. These 20 galaxies are less luminous, less emitting in the IR w.r.t. the optical, less barred, and characterized by later stage types than average. I also found that the disk orientations devised by my photometric method yield results more similar to H-alpha kinematic orientations than other photometric studies based on a single isophote level.Using the orientations I found and H-alpha dynamics disk orientations, I averaged azimuthally surface brightnesses to produce radial spectral energy distributions (SED) profiles. Once fitted with a cosmic dust emission model, they resulted in radial profiles of dust and stellar content properties. I found the dust intercepted power to be proportionnal to the product of the total dust mass and the average ISRF shining on dust. This former quantity is better correlated with the bolometric stellar luminosity than any of the dust mass or the dust heating ISRF separately. Thus the old stellar populations may be an important heating source for dust. The power intercepted by dust is also very well correlated with the total infrared power. The dust intercepts a larger quantity of power coming from stars in more actively star forming galaxies.Dust exhibit radial mass surface density profiles less well described by Sersic functions than stellar ones. When both profiles are well fitted by Sersic functions, stellar density profiles have smaller half mass radii than the isophotal optical radius (R25) separately in later type galaxies, but also in more quiescent galaxies. Sersic index and half mass radius distributions have larger widths for dust than for stellar surface density profiles.I also found that the ratio of dust over stellar surface density is an important factor to explain the variations with galactic morphological type of the ratio of dust intercepted power over the power emitted by old stellar populations. This later link could be intertwined with spiral structure strength in stage types later than 2.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF
Dense gas and star formation in the Outer Milky Way
We present maps and spectra of the HCN(1-0) and HCO(1-0) lines in the
extreme outer Galaxy, at galactocentric radii between 14 and 22 kpc, with the
13.7 meter Delingha telescope. The 9 molecular clouds were selected from a
CO/CO survey of the outer quadrants. The goal is to better understand
the structure of molecular clouds in these poorly studied subsolar metallicity
regions and the relation with star formation. The lines are all narrow, less
than 2km/s at half power, enabling detection of the HCN hyperfine structure in
the stronger sources and allowing us to observationally test hyperfine
collision rates. The hyperfine line ratios show that the HCN emission is
optically thin with column densities estimated at N(HCN)~\scm. The
HCO emission is approximately twice as strong as the HCN (taken as the sum
of all components), in contrast with the inner Galaxy and nearby galaxies where
they are similarly strong. For an abundance ratio , this requires a relatively low density solution for the dense gas, with
n(H2) \ccm. The CO/CO line ratios are similar
to solar neighborhood values, roughly 7.5, despite the low CO abundance
expected at such large radii. The HCO/CO and HCO/CO integrated
intensity ratios are also standard at about 1/35 and 1/5 respectively. HCN is
weak compared to the CO emission, with HCN/CO even after summing
all hyperfine components. At the parsec scales observed here, the correlation
between star formation, as traced by 24~m emission as is standard in
extragalactic work, and dense gas via the HCN or HCO emission, is poor,
perhaps due to the lack of dynamic range. We find that the lowest dense gas
fractions are in the sources at high galactic latitude (b>2, h>300pc above the
plane), possibly due to lower pressure.Comment: 17 pages, 14 figure
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