We investigated the radio continuum spectra of 14 star-forming galaxies by fitting nonthermal (synchrotron) and thermal (free-free) radiation laws. The underlying radio continuum measurements cover a ...frequency range of ~325 MHz to 24.5 GHz (32 GHz in case of M 82). It turns out that most of these synchrotron spectra are not simple power-laws, but are best represented by a low-frequency spectrum with a mean slope αnth = 0.59 ± 0.20 (Sν ∝ ν−α), and by a break or an exponential decline in the frequency range of 1–12 GHz. Simple power-laws or mildly curved synchrotron spectra lead to unrealistically low thermal flux densities, and/or to strong deviations from the expected optically thin free-free spectra with slope αth = 0.10 in the fits. The break or cutoff energies are in the range of 1.5–7 GeV. We briefly discuss the possible origin of such a cutoff or break. If the low-frequency spectra obtained here reflect the injection spectrum of cosmic-ray electrons, they comply with the mean spectral index of Galactic supernova remnants. A comparison of the fitted thermal flux densities with the (foreground-corrected) Hα fluxes yields the extinction, which increases with metallicity. The fraction of thermal emission is higher than believed hitherto, especially at high frequencies, and is highest in the dwarf galaxies of our sample, which we interpret in terms of a lack of containment in these low-mass systems, or a time effect caused by a very young starburst.
Context. The relative abundance of the dust grain types in the interstellar medium is directly linked to physical quantities that trace the evolution of galaxies. Because of the poor spatial ...resolution of the infrared and submillimetre data, we are able to study the dependence of the resolved infrared spectral energy distribution (SED) across regions of the interstellar medium (ISM) with different physical properties in just a few objects. Aims. We aim to study the dust properties of the whole disc of M 33 at spatial scales of ~170 pc. This analysis allows us to infer how the relative dust grain abundance changes with the conditions of the ISM, study the existence of a submillimetre excess and look for trends of the gas-to-dust mass ratio (GDR) with other physical properties of the galaxy. Methods. For each pixel in the disc of M 33 we have fitted the infrared SED using a physically motivated dust model that assumes an emissivity index β close to two. We applied a Bayesian statistical method to fit the individual SEDs and derived the best output values from the study of the probability density function of each parameter. We derived the relative amount of the different dust grains in the model, the total dust mass, and the strength of the interstellar radiation field (ISRF) heating the dust at each spatial location. Results. The relative abundance of very small grains tends to increase, and for big grains to decrease, at high values of Hα luminosity. This shows that the dust grains are modified inside the star-forming regions, in agreement with a theoretical framework of dust evolution under different physical conditions. The radial dependence of the GDR is consistent with the shallow metallicity gradient observed in this galaxy. The strength of the ISRF derived in our model correlates with the star formation rate in the galaxy in a pixel by pixel basis. Although this is expected, it is the first time that a correlation between the two quantities has been reported. We have produced a map of submillimetre excess in the 500 μm SPIRE band for the disc of M 33. The excess can be as high as 50% and increases at large galactocentric distances. We further studied the relation of the excess with other physical properties of the galaxy and find that the excess is prominent in zones of diffuse ISM outside the main star-forming regions, where the molecular gas and dust surface density are low.
Context. The construction of catalogues of galaxies and the a posteriori study of galaxy properties in relation to their environment have been hampered by scarce redshift information. The new ...3-dimensional (3D) surveys permit small, faint, physically bound satellites to be distinguished from a background-projected galaxy population, giving a more comprehensive 3D picture of the surroundings. Aims. We aim to provide representative samples of isolated galaxies, isolated pairs, and isolated triplets for testing galaxy evolution and secular processes in low density regions of the local Universe, as well as to characterise their local and large-scale environments. Methods. We used spectroscopic data from the tenth data release of the Sloan Digital Sky Survey (SDSS-DR10) to automatically and homogeneously compile catalogues of 3702 isolated galaxies, 1240 isolated pairs, and 315 isolated triplets in the local Universe (z ≤ 0.080). To quantify the effects of their local and large-scale environments, we computed the projected density and the tidal strength for the brightest galaxy in each sample. Results. We find evidence of isolated pairs and isolated triplets that are physically bound at projected separations up to d ≤ 450 kpc with radial velocity difference Δν ≤ 160 km s-1, where the effect of the companion typically accounts for more than 98% of the total tidal strength affecting the central galaxy. For galaxies in the catalogues, we provide their positions, redshifts, and degrees of relation with their physical and large-scale environments. The catalogues are publicly available to the scientific community. Conclusions. For isolated galaxies, isolated pairs, and isolated triplets, there is no difference in their degree of interaction with the large-scale structure (up to 5 Mpc), which may suggest that they have a common origin in their formation and evolution. We find that most of them belong to the outer parts of filaments, walls, and clusters, and generally differ from the void population of galaxies.
Aims. We use different tracers, such as Hα, ultraviolet (UV), and infrared (IR) emissions at various wavelengths, to study the dust and star formation (SF) conditions throughout the disk of M 33. ...Methods. We derive the radial distribution of dust, of the old and young stellar population using Spitzer and GALEX data, complemented by ground-based optical data and available surveys of atomic and molecular gas. We separate the contribution of discrete sources to the IR brightness from the diffuse emission. Results. At 8 and 24 μm, discrete sources account for $\ga$40% of the IR emission in the innermost 3 kpc, and for $\la$20% further out. We find that stochastic emission from very small grains in the diffuse interstellar medium accounts for only ~10% of the diffuse 24 μm emission, and that dusty circumstellar shells of unresolved, evolved AGB stars (carbon stars) are a viable alternative. The 8 μm profile suggests that PAH emission declines faster with radius than the dust continuum. In annular regions 0.24 kpc wide, we find a mean extinction value for the stellar continuum $A_{\rm V} \sim 0.25$ mag with a weak dependence on radius, consistent with the shallow metallicity gradient observed. Dust opacity derived from the 160 μm emission decreases instead by a factor of 10 from the center to edge of the star forming disk. Conclusions. Using extinction corrected UV and Hα maps we find the global SF rate in M 33 over the last 100 Myr to be $0.45\pm 0.10~M_\odot$ yr-1. Far-IR and total-IR luminosities can trace SF even though a high conversion factor is required to recover the effective rate. If carbon stars are powering the diffuse 24 μm emission in M 33, this can trace star formation 1 Gyr ago and provide a more complete view of the SF history of the galaxy. Today the SF rate declines radially with a scale length of ~2 kpc, longer than for the old stellar population, suggesting an inside-out growth of the disk.
Context. Active galactic nuclei (AGN) are one of the main drivers for the transition from star-forming disk to passive spheroidal galaxies, however, the role of large-scale environment versus ...one-on-one interactions in triggering different types of AGN is still uncertain. We present a statistical study of the prevalence of the nuclear activity in isolated galaxies and physically bound isolated pairs. Aims. For the purpose of this study we considered optically and radio selected nuclear activity types. We aim to assess the effect of one-on-one interaction on the fraction of AGN and the role of their large-scale environment. Methods. To study the effect of one-on-one interaction on the fraction of AGN in isolated galaxy pairs, we compare these AGN with a sample of isolated galaxies homogeneously selected under the same isolation criterion. We examine the effect of the large-scale environment by comparing isolated systems with control samples of single galaxies and galaxy pairs. We use the tidal strength parameter to quantify the effects of local and large-scale environments. Results. In general we found no difference in the prevalence of optical AGN for the considered samples. For massive galaxies, the fraction of optical AGN in isolated galaxies is slightly higher than that in the control samples. Also, the fraction of passives in high mass isolated galaxies is smaller than in any other sample. Generally, there is no dependence on optical nuclear activity with local environment. On the other hand, we found evidence that radio AGN are strongly affected by the local environment. Conclusions. The optical AGN phenomenon is related to cold gas accretion, while radio AGN are related to hot gas accretion. In this context, there is more cold gas, fuelling the central optical AGN, in isolated systems. Our results are in agreement with a scenario where cold gas accretion by secular evolution is the main driver of optical AGN, while hot gas accretion and one-on-one interactions are the main drivers of radio AGN activity.
Context.
While some galactic bars show recent massive star formation (SF) along them, some others do not. Whether bars with low level of SF are a consequence of low star formation efficiency, low gas ...inflow rate, or dynamical effects remains a matter of debate.
Aims.
In order to study the physical conditions that enable or prevent SF, we perform a multi-wavelength analysis of 12 strongly barred galaxies with total stellar masses log
10
(
M
⋆
/
M
⊙
)∈10.2, 11, chosen to host different degrees of SF along the bar major axis without any prior condition on gas content. We observe the CO(1–0) and CO(2–1) emission within bars with the IRAM-30 m telescope (beam sizes of 1.7–3.9 kpc and 0.9–2.0 kpc, respectively; 7–8 pointings per galaxy on average).
Methods.
We estimated molecular gas masses (
M
mol
) from the CO(1–0) and CO(2–1) emissions. SF rates (SFRs) were calculated from GALEX near-ultraviolet (UV) and WISE 12 μm images within the beam-pointings, covering the full bar extent (SFRs were also derived from far-UV and 22 μm).
Results.
We detect molecular gas along the bars of all probed galaxies. Molecular gas and SFR surface densities span the ranges log
10
(Σ
mol
/
M
⊙
pc
−2
) ∈ 0.4,2.4 and log
10
(Σ
SFR
/
M
⊙
pc
−1
kpc
−2
) ∈ −3.25, −0.75, respectively. The star formation efficiency (SFE; i.e., SFR/
M
mol
) in bars varies between galaxies by up to an order of magnitude (SFE ∈0.1, 1.8 Gyr
−1
). On average, SFEs are roughly constant along bars. SFEs are not significantly different from the mean value in spiral galaxies reported in the literature (∼0.43 Gyr
−1
), regardless of whether we estimate
M
mol
from CO(1–0) or CO(2–1). Interestingly, the higher the total stellar mass of the host galaxy, the lower the SFE within their bars. In particular, the two galaxies in our sample with the lowest SFE and Σ
SFR
(NGC 4548 and NGC 5850, SFE ≲ 0.25 Gyr
−1
, Σ
SFR
≲ 10
−2.25
M
⊙
yr
−1
kpc
−2
,
M
⋆
≳ 10
10.7
M
⊙
) are also those hosting massive bulges and signs of past interactions with nearby companions.
Conclusions.
We present a statistical analysis of the SFE in bars for a sample of 12 galaxies. The SFE in strong bars is not systematically inhibited (either in the central, middle, or end parts of the bar). Both environmental and internal quenching are likely responsible for the lowest SFEs reported in this work.
Power spectra of deprojected images of late-type galaxies in gas or dust emission are very useful diagnostics of the dynamics and stability of their interstellar medium. Previous studies have shown ...that the power spectra can be approximated as two power laws, a shallow one on large scales (larger than 500 pc) and a steeper one on small scales, with the break between the two corresponding to the line-of-sight thickness of the galaxy disk. The break separates the 3D behavior of the interstellar medium on small scales, controlled by star formation and feedback, from the 2D behavior on large scales, driven by density waves in the disk. The break between these two regimes depends on the thickness of the plane, which is determined by the natural self-gravitating scale of the interstellar medium. We present a thorough analysis of the power spectra of the dust and gas emission at several wavelengths in the nearby galaxy M 33. In particular, we use the recently obtained images at five wavelengths by PACS and SPIRE onboard Herschel. The wide dynamical range (2–3 dex in scale) of most images allows us to clearly determine the change in slopes from −1.5 to −4, with some variations with wavelength. The break scale increases with wavelength from 100 pc at 24 and 100 μm to 350 pc at 500 μm, suggesting that the cool dust lies in a thicker disk than the warm dust, perhaps because of star formation that is more confined to the plane. The slope on small scales tends to be steeper at longer wavelength, meaning that the warmer dust is more concentrated in clumps. Numerical simulations of an isolated late-type galaxy, rich in gas and with no bulge, such as M 33, are carried out to better interpret these observed results. Varying the star formation and feedback parameters, it is possible to obtain a range of power spectra, with two power-law slopes and breaks, that nicelybracket the data. The small-scale power-law does indeed reflect the 3D behavior of the gas layer, steepening strongly while the feedback smoothes the structures by increasing the gas turbulence. M 33 appears to correspond to a fiducial model with an SFR of ~ 0.7 M⊙/yr, with 10% supernovae energy coupled to the gas kinematics.
Context.
The underlying scenario of the formation and evolution of galaxy triplets is still uncertain. Mergers of galaxies in isolated triplets give us the opportunity to study the already complex ...merging process, with minimal contamination of other environmental effects that potentially allow and accelerate galaxy transitions from active star-forming to passive galaxies.
Aims.
The merging system SIT 45 (UGC 12589) is one of 315 systems in the SDSS-based catalogue of Isolated Triplets (SIT); it is an unusual isolated galaxy triplet, consisting of three merging late-type galaxies. The main aims of this work are to study the dynamical evolution and star formation history (SFH) of SIT 45, as well as its dependence on its local and large-scale environment.
Methods.
To study its dynamics, parameters such as the velocity dispersion (
σ
v
), the harmonic radius (
R
H
), the crossing time (
H
0
t
c
), and the virial mass (
M
vir
), along with the compactness of the triplet (
S
) were considered. To investigate the possible dependence of these dynamical parameters on the environment, the tidal force
Q
parameters (both local and large-scale) and the projected local density (
η
k
) were used. To constrain the SFH, we used CIGALE to fit its observed spectral energy distribution using multiwavelength data from the ultraviolet to the infrared.
Results.
SIT 45 is one of the most compact triplets in the SIT, and it is also more compact than triplets in other samples. According to its SFH, SIT 45 presents star formation, where the galaxies also present recent (∼200 Myr) star formation increase, indicating that this activity may have been triggered by the interaction. Its dynamical configuration suggests that the system is highly evolved in comparison to the SIT. However, this is not expected for systems composed of star-forming late-type galaxies, based on observations in compact groups.
Conclusions.
We conclude that SIT 45 is a system of three interacting galaxies that are evolving within the same dark matter halo; its compact configuration is a consequence of the ongoing interaction rather than being due to a long-term evolution (as suggested from its
H
0
t
c
value). We consider two scenarios for the present configuration of the triplet, one where one of the members is a tidal galaxy, and another where this galaxy arrives to the system after the interaction. Both scenarios need further exploration. The isolated triplet SIT 45 is therefore an ideal system to study short timescale mechanisms (∼10
8
yr), such as starbursts triggered by interactions which are more frequent at higher redshift.
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