66 research outputs found
Gas-phase oxygen abundances and radial metallicity gradients in the two nearby spiral galaxies NGC7793 and NGC4945
Gas-phase abundances in HII regions of two spiral galaxies, NGC7793 and
NGC4945, have been studied to determine their radial metallicity gradients. We
used the strong-line method to derive oxygen abundances from spectra acquired
with GMOS-S, the multi-object spectrograph on the 8m- Gemini South telescope.
We found that NGC7793 has a well-defined gas-phase radial oxygen gradient of
-0.321 0.112 dex R (or -0.054 0.019 dex kpc)
in the galactocentric range 0.17R/R 0.82, not
dissimilar from gradients calculated with direct abundance methods in galaxies
of similar mass and morphology. We also determined a shallow radial oxygen
gradient in NGC4945, -0.253 0.149 dex R (or -0.019
0.011 dex kpc) for 0.04R/R 0.51, where the
larger relative uncertainty derives mostly from the larger inclination of this
galaxy. NGC7793 and NGC4945 have been selected for this study because they are
similar, in mass and morphology, to M33 and the Milky Way, respectively. Since
at zeroth order we expect the radial metallicity gradients to depend on mass
and galaxy type, we compared our galaxies in the framework of radial
metallicity models best suited for M33 and the Galaxy. We found a good
agreement between M33 and NGC7793, pointing toward similar evolution for the
two galaxies. We notice instead differences between NGC4945 and the radial
metallicity gradient model that best fits the Milky Way. We found that these
differences are likely related to the presence of an AGN combined with a bar in
the central regions of NGC4945, and to its interacting environment.Comment: ApJ, in pres
The radial metallicity gradient and the history of elemental enrichment in M81 through emission-line probes
We present a new set of weak-line abundances of HII regions in M81, based on
Gemini Multi-Object Spectrograph (GMOS) observations. The aim is to derive
plasma and abundance analysis for a sizable set of emission-line targets to
study the galactic chemical contents in the framework of galactic metallicity
gradients. We used the weak-line abundance approach by deriving electron
density and temperatures for several HII regions in M81. Gradient analysis is
based on oxygen abundances.Together with a set of HII region abundances
determined similarly by us with Multi-Mirror Telescope (MMT) spectra, the new
data yield to a radial oxygen gradient of -0.0880.013 dex kpc,
which is steeper than the metallicity gradient obtained for planetary nebulae
(-0.0440.007 dex kpc). This result could be interpreted as gradient
evolution with time: Models of galactic evolution with inside-out disk
formation associated to pre-enriched gas infall would produce such difference
of gradients, although stellar migration effects would also induce a difference
in the metallicity gradients between the old and young populations. By
comparing the M81 metallicity gradients with those of other spiral galaxies,
all consistently derived from weak-line analysis, we can infer that similar
gradient difference is common among spirals. The metallicity gradient slopes
for HII regions and PNe seem to be steeper in M81 than in other galactic disks,
which is likely due to the fact that M81 belongs to a galaxy group. We also
found that M81 has experienced an average oxygen enrichment of 0.140.08
dex in the spatial domain defined by the observations. Our data are compatible
with a break in the radial oxygen gradient slope around R as inferred by
other authors both in M81 and in other galaxies.Comment: Astronomy and Astrophysics, in pres
The resolved star-formation relation in nearby active galactic nuclei
We present an analysis of the relation between star formation rate (SFR)
surface density (sigmasfr) and mass surface density of molecular gas
(sigmahtwo), commonly referred to as the Kennicutt-Schmidt (K-S) relation, at
its intrinsic spatial scale, i.e. the size of giant molecular clouds (10-150
pc), in the central, high-density regions of four nearby low-luminosity active
galactic nuclei (AGN). We used interferometric IRAM CO(1-0) and CO(2-1), and
SMA CO(3-2) emission line maps to derive sigmahtwo and HST-Halpha images to
estimate sigmasfr. Each galaxy is characterized by a distinct molecular SF
relation at spatial scales between 20 to 200 pc. The K-S relations can be
sub-linear, but also super-linear, with slopes ranging from 0.5 to 1.3.
Depletion times range from 1 and 2Gyr, compatible with results for nearby
normal galaxies. These findings are valid independently of which transition,
CO(1-0), CO(2-1), or CO(3-2), is used to derive sigmahtwo. Because of
star-formation feedback, life-time of clouds, turbulent cascade, or magnetic
fields, the K-S relation might be expected to degrade on small spatial scales
(<100 pc). However, we find no clear evidence for this, even on scales as small
as 20 pc, and this might be because of the higher density of GMCs in galaxy
centers which have to resist higher shear forces. The proportionality between
sigmahtwo and sigmasfr found between 10 and 100 Msun/pc2 is valid even at high
densities, 10^3 Msun/pc2. However, by adopting a common CO-to-H2 conversion
factor (alpha_CO), the central regions of the galaxies have higher sigmasfr for
a given gas column than those expected from the models, with a behavior that
lies between the mergers/high-redshift starburst systems and the more quiescent
star-forming galaxies, assuming that the first ones require a lower value of
alpha_CO.Comment: 22 pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
Molecular gas and nuclear activity in early-type galaxies: any link with radio-loudness?
Aims. We want to study the amount of molecular gas in a sample of nearby
early-type galaxies (ETGs) which host low-luminosity Active Galactic Nuclei
(AGN). We look for possible differences between the radio-loud (RL) and
radio-quiet (RQ) AGN. Methods. We observed the CO(1-0) and CO(2-1) spectral
lines with the IRAM 30m and NRO 45m telescopes for eight galaxies. They belong
to a large sample of 37 local ETGs which host both RQ and RL AGN. We gather
data from the literature for the entire sample. Results. We report the new
detection of CO(1-0) emission in four galaxies (UGC0968, UGC5617, UGC6946, and
UGC8355) and CO(2-1) emission in two of them (UGC0968 and UGC5617). The
CO(2-1)/CO(1-0) ratio in these sources is . Considering both the
new observations and the literature, the detection rate of CO in our sample is
55 9%, with no statistically significant difference between the hosts of
RL and RQ AGNs. For all the detected galaxies we converted the CO luminosities
into the molecular masses, , that range from 10 to 10
M, without any statistically significant differences between RL and
RQ galaxies. This suggests that the amount of molecular gas does not likely set
the radio-loudness of the AGN. Furthermore, despite the low statistical
significance, the presence of a weak trend between the H mass with
various tracers of nuclear activity (mainly [O III] emission line nuclear
power) cannot be excluded.Comment: Accepted for publication on A&A, 9 pages, 5 figure
Ancient stellar populations in the outskirts of nearby grand-design spirals: Investigation of their star formation histories
The main sequence (MS) of star-forming galaxies (SFGs) is the tight relation
between the galaxy stellar mass and its star formation rate (SFR) and was
observed up to z ~ 6. The MS relation can be used as a reference for
understanding the differences among galaxies, characterised by different rates
of stellar production (starbursts, SFGs, and passive galaxies), and those
inside a galaxy made up of different components (bulge, disk, and halo). To
investigate peculiar features found in our sample galaxies, we focus here on
their star formation history (SFH). We performed a spectral energy distribution
fitting procedure that accounted for the energetic balance between UV and
far-IR radiation on a sample of eight nearby face-on spiral galaxies from the
DustPedia sample. This approach allowed us to study the spatially resolved MS
of the sample and to recover the past SFH. By exploiting the BAGPIPES code, we
constrained the SFHs for each galaxy with a delayed exponentially declining
model to derive their mass-weighted age (tMW). A central old region (tMW up
to~7Gyr, consistent with the presence of a bulge for various systems) is
followed by younger regions in which the disks are still forming stars
(tMW~4Gyr). At larger distances, tMW increases mildly in general. Strikingly,
in two galaxies (NGC4321 and NGC5194), we found a steep increase in tMW that
reached levels similar to those of the bulge. These old stellar populations in
the very galaxy outskirts are unexpected. We discuss their potential origin by
considering the different gas phases of the source with the most prominent
quenched ring, NGC4321, and argue for two main possibilities: 1) some
environmental effect (e.g. starvation) or 2) the circumgalactic medium of
sources outside of high-density clusters might have stopped to supply pristine
gas to the galaxy (e.g. if its specific angular moment is too high for being
accreted).Comment: Accepted for publication on Astronomy and Astrophysic
AGN impact on the molecular gas in galactic centers as probed by CO lines
We present a detailed analysis of the X-ray, infrared, and carbon monoxide
(CO) emission for a sample of 35 local (), active ( erg s) galaxies. Our goal is to infer the contribution of
far-ultraviolet (FUV) radiation from star formation (SF), and X-ray radiation
from the active galactic nuclei (AGN), respectively producing photodissociation
regions (PDRs) and X-ray dominated regions (XDRs), to the molecular gas
heating. To this aim, we exploit the CO spectral line energy distribution (CO
SLED) as traced by Herschel, complemented with data from single-dish telescopes
for the low-J lines, and high-resolution ALMA images of the mid-J CO emitting
region. By comparing our results to the Schmidt-Kennicutt relation, we find no
evidence for AGN influence on the cold and low-density gas on kpc-scales. On
nuclear (r = 250 pc) scales, we find weak correlations between the CO line
ratios and either the FUV or X-ray fluxes: this may indicate that neither SF
nor AGN radiation dominates the gas excitation, at least at r = 250 pc. From a
comparison of the CO line ratios with PDR and XDR models, we find that PDRs can
reproduce observations only in presence of extremely high gas densities ( cm). In the XDR case, instead, the models suggest moderate
densities ( cm). We conclude that a mix of the two
mechanisms (PDR for the mid-J, XDR or possibly shocks for the high-J) is
necessary to explain the observed CO excitation in active galaxies
Synergies between SKA and ALMA: observations of Nearby Galaxies
The past decade has seen amazing advances in radioastronomy, which led to the construction of brand-new instruments such as LOFAR and ALMA, and the updating of existing ones, e. g. JVLA and e-MERLIN. The SKA will be the spearhead of a future technological development and it will change the way astrophysical topics have been studied so far by opening up new frequency windows with unprecedented spatial resolution and sensitivity. The SKA location in the southern hemisphere makes it particularly suitable to complement ALMA, which is already giving exciting results both on the local and the more distant Universe.
Among the possible synergies between SKA and ALMA, we focus on the observations of nearby star forming galaxies. Star formation processes in galaxies involve all the components of the interstellar medium, so the only way to have a complete picture of them is through multifrequency observations.
ALMA observes gas and dust emission, while the SKA will trace both the free-free thermal and the non-thermal synchrotron emission. The spatial comparison between these components gives information about the contribution to star formation processes provided by magnetic fields and
cosmic rays. The high spatial resolution achievable with ALMA and SKA will make it possible to compare these emissions on very small spatial scales, by resolving single molecular clouds in nearby galaxies.
By the time the SKA will start observing, ALMA will have already imaged many nearby galaxies in the southern hemisphere, for which no low frequency data at comparably high spatial resolution will be available. The SKA will fill this gap, and have a profound impact on the studies of nearby galaxies, making valuable contributions to our understanding of star formation processes, and of the role of magnetic fields and cosmic rays in them
Probing the spectral shape of dust emission with the DustPedia galaxy sample
The objective of this paper is to understand the variance of the far-infrared
(FIR) spectral energy distribution (SED) of the DustPedia galaxies, and its
link with the stellar and dust properties. An interesting aspect of the dust
emission is the inferred FIR colours which could inform us about the dust
content of galaxies, and how it varies with the physical conditions within
galaxies. However, the inherent complexity of dust grains as well as the
variety of physical properties depending on dust, hinder our ability to utilise
their maximum potential. We use principal component analysis (PCA) to explore
new hidden correlations with many relevant physical properties such as the dust
luminosity, dust temperature, dust mass, bolometric luminosity, star-formation
rate (SFR), stellar mass, specific SFR, dust-to-stellar mass ratio, the
fraction of absorbed stellar luminosity by dust (f_abs), and metallicity. We
find that 95% of the variance in our sample can be described by two principal
components (PCs). The first component controls the wavelength of the peak of
the SED, while the second characterises the width. The physical quantities that
correlate better with the coefficients of the first two PCs, and thus control
the shape of the FIR SED are: the dust temperature, the dust luminosity, the
SFR, and f_abs. Finally, we find a weak tendency for low-metallicity galaxies
to have warmer and broader SEDs, while on the other hand high-metallicity
galaxies have FIR SEDs that are colder and narrower
The AGN fuelling/feedback cycle in nearby radio galaxies - IV. Molecular gas conditions and jet-ISM interaction in NGC 3100
This is the fourth paper of a series investigating the AGN fuelling/feedback processes in a sample of 11 nearby low-excitation radio galaxies (LERGs). In this paper, we present follow-up Atacama Large Millimeter/submillimeter Array (ALMA) observations of one source, NGC 3100, targeting the 12CO(1-0), 12CO(3-2), HCO+(4-3), SiO(3-2), and HNCO(6-5) molecular transitions. 12CO(1-0) and 12CO(3-2) lines are nicely detected and complement our previous 12CO(2-1) data. By comparing the relative strength of these three CO transitions, we find extreme gas excitation conditions (i.e. Tex ≳ 50 K) in regions that are spatially correlated with the radio lobes, supporting the case for a jet–ISM interaction. An accurate study of the CO kinematics demonstrates that although the bulk of the gas is regularly rotating, two distinct non-rotational kinematic components can be identified in the inner gas regions: one can be associated to inflow/outflow streaming motions induced by a two-armed spiral perturbation; the second one is consistent with a jet-induced outflow with vmax ≈ 200 km s−1 and M˙≲0.12 M⊙ yr−1. These values indicate that the jet-CO coupling ongoing in NGC 3100 is only mildly affecting the gas kinematics, as opposed to what expected from existing simulations and other observational studies of (sub-)kpc scale jet–cold gas interactions. HCO+(4-3) emission is tentatively detected in a small area adjacent to the base of the northern radio lobe, possibly tracing a region of jet-induced gas compression. The SiO(3-2) and HNCO(6-5) shock tracers are undetected: this – along with the tentative HCO+(4-3) detection – may be consistent with a deficiency of very dense (i.e. ncrit > 106 cm−3) cold gas in the central regions of NGC 3100
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