Mergers of galaxies are an important mode for galaxy evolution because they serve as an efficient trigger of powerful starbursts. However, observational studies of molecular gas properties during ...their early stages are scarce. We present interferometric CO(2-1) maps of two luminous infrared galaxies, NGC 3110 and NGC 232, obtained with the Submillimeter Array with ∼1 kpc resolution. While NGC 3110 is a spiral galaxy interacting with a minor (14:1 stellar mass) companion, NGC 232 is interacting with a similarly sized object. We find that such interactions in these galaxies have likely induced enhancements in the molecular gas content and central concentrations, partly at the expense of atomic gas. The obtained molecular gas surface densities in their circumnuclear regions are mol 102.5 M pc−2, higher than in noninteracting objects by an order of magnitude. Gas depletion times of 0.5-1 Gyr are found for the different regions, lying in between noninteracting disk galaxies and the starburst sequence. In the case of NGC 3110, the spiral arms show on average 0.5 dex shorter depletion times than in the circumnuclear regions if we assume a similar H2-CO conversion factor. We show that even in the early stages of the interaction with a minor companion, a starburst is formed along the circumnuclear region and spiral arms, where a large population of SSCs is found (∼350), and at the same time a large central gas concentration is building up that might be the fuel for an active galactic nucleus. The main morphological properties of the NGC 3110 system are reproduced by our numerical simulations and allow us to estimate that the current epoch of the interaction is at ∼150 Myr after closest approach.
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.
Galaxies in the Universe are distributed in a web-like structure characterized by different large-scale environments: dense clusters, elongated filaments, sheetlike walls and under-dense regions, ...called voids
. The low density in voids is expected to affect the properties of their galaxies. Indeed, previous studies
have shown that galaxies in voids are, on average, bluer and less massive, and have later morphologies and higher current star formation rates than galaxies in denser large-scale environments. However, it has never been observationally proved that the star formation histories (SFHs) in voids are substantially different from those in filaments, walls and clusters. Here we show that void galaxies have had, on average, slower SFHs than galaxies in denser large-scale environments. We also find two main SFH types present in all the environments: 'short-timescale' galaxies are not affected by their large-scale environment at early times but only later in their lives; 'long-timescale' galaxies have been continuously affected by their environment and stellar mass. Both types have evolved more slowly in voids than in filaments, walls and clusters.
Context. The AMIGA project aims to build a well defined and statistically significant reference sample of isolated galaxies in order to estimate the environmental effects on the formation and ...evolution of galaxies. Aims. The goal of this paper is to provide a measure of the environment of the isolated galaxies in the AMIGA sample, quantifying the influence of the candidate neighbours identified in our previous work and their potential effects on the evolution of the primary galaxies. Here we provide a quantification of the isolation degree of the galaxies in this sample. Methods. Our starting sample is the Catalogue of Isolated Galaxies (CIG). We used two parameters to estimate the influence exerted by the neighbour galaxies on the CIG galaxy: the local number density of neighbour galaxies and the tidal strength affecting the CIG galaxy. We show that both parameters together provide a comprehensive picture of the environment. For comparison, those parameters have also been derived for galaxies in denser environments such as triplets, groups and clusters. Results. The CIG galaxies show a continuous spectrum of isolation, as quantified by the two parameters, from very isolated to interacting. The fraction of CIG galaxies whose properties are expected to be influenced by the environment is however low (159 out of 950 galaxies). The isolated parameters derived for the comparison samples gave higher values than for the CIG and we found clear differences for the average values of the 4 samples considered, proving the sensitivity of these parameters. Conclusions. The environment of the galaxies in the CIG has been characterised, using two complementary parameters quantifying the isolation degree, the local number density of the neighbour galaxies and the tidal forces affecting the isolated galaxies. A final catalogue of galaxies has been produced and the most isolated of these galaxies are consequently appropriate to serve as a reference sample for the AMIGA project.
Context.
Void galaxies are essential for understanding the physical processes that drive galaxy evolution because they are less affected by external factors than galaxies in denser environments, that ...is, in filaments, walls, and clusters. The stellar metallicity of a galaxy traces the accumulated fossil record of the star formation through the entire life of the galaxy. A comparison of the stellar metallicity of galaxies in various environments, including voids, filaments, walls, and clusters can provide valuable insights into how the large-scale environment affects the chemical evolution of the galaxy.
Aims.
We present the first comparison of the relation of the total stellar mass versus central stellar metallicity between galaxies in voids, filaments, walls, and clusters with different star formation history (SFH) types, morphologies, and colours for stellar masses between 10
8.0
to 10
11.5
solar masses and redshift 0.01 <
z
< 0.05. We aim to better understand how the large-scale structure affects galaxy evolution by studying the stellar mass-metallicity relation of thousands of galaxies, which allows us to make a statistically sound comparison between galaxies in voids, filaments, walls, and clusters.
Methods.
We applied non-parametric full spectral fitting techniques (pPXF and STECKMAP) to 10 807 spectra from the SDSS-DR7 (987 in voids, 6463 in filaments and walls, and 3357 in clusters) and derived their central mass-weighted average stellar metallicity (M/H
M
).
Results.
We find that galaxies in voids have slightly lower stellar metallicities on average than galaxies in filaments and walls (by ∼0.1 dex), and they are much lower than those of galaxies in clusters (by ∼0.4 dex). These differences are more significant for low-mass (∼10
9.25
M
⊙
) than for high-mass galaxies, for long-timescale SFH (extended along time) galaxies than for short-timescale SFHs (concentrated at early times) galaxies, for spiral than for elliptical galaxies, and for blue than for red galaxies.
Context.
M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular ...clouds.
Aims.
We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum.
Methods.
We mapped the emission of gas and dust in M 33 using the far-infrared lines of C
II
and O
I
(63
μ
m) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the
Herschel
Space Observatory. These maps have 50 pc resolution and form a ∼370 pc wide stripe along its major axis covering the sites of bright H
II
regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24
μ
m from
Spitzer
/MIPS, and at 70
μ
m, 100
μ
m, and 160
μ
m from
Herschel
/PACS were combined to obtain a map of the TIR.
Results.
TIR and C
II
intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of
G
0, obs
∼ 2 to 200 in units of the average Galactic radiation field. The binned C
II
/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the C
II
emission, as estimated from VLA H
I
data, is on average only 10%. Fits of modified black bodies to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the O
I
data before comparing it with models of photon dominated regions. Most of the ratios of C
II
/O
I
and (C
II
+O
I
)/TIR are consistent with two model solutions. The median ratios are consistent with one solution at
n
∼ 2 × 10
2
cm
−3
,
G
0
∼ 60, and a second low-FUV solution at
n
∼ 10
4
cm
−3
,
G
0
∼ 1.5.
Conclusions.
The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ∼1. A fraction of the gas may, however, be represented by the second solution.
The Universe is shaped as a web-like structure, formed by clusters, filaments, and walls that leave large low number-density volumes in between named voids. Galaxies in voids have been found to be of ...a later type, bluer, less massive, and to have a slower evolution than galaxies in denser environments (filaments and walls). However, the effect of the void environment on their stellar population properties is still unclear. We aim to address this question using 118 optical integral field unit datacubes from the Calar Alto Void Integral-field Treasury surveY (CAVITY), observed with the PMAS/PPaK spectrograph at the 3.5 m telescope at the Calar Alto Observatory (Almería, Spain). We fitted their spectra from 3750 Å to 7000 Å with the non-parametric full spectral fitting code STARLIGHT to estimate their stellar population properties: stellar mass, stellar mass surface density, age, star formation rate (SFR), and specific star formation rate (sSFR). We analysed the results through the global properties, assessing the behaviour of the whole galaxy, and the spatially resolved information, by obtaining the radial profiles from the 2D maps up to the 2 half-light radius of each stellar population property. The results were examined with respect to their morphological type and stellar mass. Then, we compared them with a control sample of galaxies in filaments and walls, selected from the CALIFA survey and analysed following the same procedure. To make a fair comparison between the samples, we selected a twin filament galaxy for each void galaxy of the same morphological type and closest stellar mass, to match the void galaxy sample as much as possible in morphology and mass. Key findings from our global and spatially resolved analysis include void galaxies having a slightly higher half-light radius (HLR), lower stellar mass surface density, and younger ages across all morphological types, and slightly elevated SFR and sSFR (only significant enough for Sas). Many of these differences appear in the outer parts of spiral galaxies (HLR > 1), where discs are younger and exhibit a higher sSFR compared to galaxies in filaments and walls, indicative of less evolved discs. This trend is also found for early-type spirals, suggesting a slower transition from star-forming to quiescent states in voids. Our analysis indicates that void galaxies, influenced by their surroundings, undergo a more gradual evolution, especially in their outer regions, with a more pronounced effect for low-mass galaxies. We find that below a certain mass threshold, environmental processes play a more influential role in galactic evolution.
The AMIGA project (Analysis of the Interstellar Medium of Isolated Galaxies) is compiling a multiwavelength database of isolated galaxies that includes optical (B and Ha), infrared (FIR and NIR) and ...radio (continuum plus HI and CO lines) properties. It involves a refinement of the pioneering Catalog of Isolated Galaxies. This paper is the first in a series and begins with analysis of the global properties of the nearly redshift-complete CIG with emphasis on the Optical Luminosity Function (OLF) which we compare with other recent estimates of the OLF for a variety of environments. The CIG redshift distribution for n = 956 galaxies re-enforces the evidence for a bimodal structure seen earlier in smaller samples. The peaks at redshift near 1500 and 6000 km s-1, corresponding respectively to galaxies in the local supercluster and those in more distant large-scale components (particularly Perseus-Pisces). The two peaks in the redshift distribution are superimposed on 50% or more of the sample that is distributed in a much more homogeneous way. The CIG probably represents the most homogeneous local field example that has ever been compiled. Our derivation of the CIG OLF is consistent with other studies of the OLF for lower density environments. This comparison via the Schechter parameter formalization shows that: 1) M* increases with galaxy surface density on the sky and 2) a shows a weaker tendency to do the same. The CIG represents the largest and most complete foundation for studies of isolated galaxies and is likely as close as we can come to a field sample.
Infrared emission is an invaluable tool for quantifying star formation in galaxies. Because the 8 mum polycyclic aromatic hydrocarbon (PAH) emission has been found to correlate with other well-known ...star formation tracers, it has widely been used as a star formation rate (SFR) tracer. There are, however, studies that challenge the accuracy and reliability of the 8 mum emission as a SFR tracer. Our study, part of the Herschel (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA) M33 Extended Survey (HERM33ES) open time key program, aims at addressing this issue by analyzing the infrared emission from the nearby spiral galaxy M33 at die high spatial scale of ~75 pc. We find that the majority of the 8 mum emission is fully consistent with heating by the diffuse interstellar medium, similar to what recently determined for the dust emission in M31 by Draine et al.
Aims. We describe the mid- (MIR) and far- (FIR) infrared properties of a large ( similar to 1000) sample of the most isolated galaxies in the local Universe. This sample is intended as a ..."nurture-free" zero point against which more environmentally influenced samples can be compared. Methods. We reprocess IRAS MIR/FIR survey data using the ADDSCAN/SCANPI utility for 1030 out of 1050 galaxies from the Catalogue of Isolated Galaxies (CIG) as part of the AMIGA project. We focus on diagnostics (FIR luminosity L_{\rm FIR}, R=\log(L_{\rm FIR}/L_{B}), and IRAS colours) thought to be sensitive to effects of environment or interaction. Results. The distribution of \log(L_{\rm FIR}) sharply peaks from 9.0-10.5, with very few (<2%) galaxies above 10.5. Review of available optical images of the most FIR luminous galaxies finds the majority likely to be interacting systems missed in our earlier morphological reevaluation. The optically normalised luminosity diagnostic R=\log(L_{\rm FIR}/L_{B}) shows a sharply peaked distribution between 0.0 and-1.0. These results were compared to the magnitude limited sample of the Center for Astrophysics that was selected without environmental discrimination. This modestly (e.g., compared to cluster, binary galaxy, and compact group samples) environmentally affected sample shows a significantly higher mean \log(L_{\rm FIR}), and R, whereas the mean \log(L_{B}) is the same. Our sample shows a strong L_{\rm FIR} vs. L sub( )Bcorrelation, with a slope steeper than one ( L_{\rm FIR} \propto L_). Interacting galaxies were found above this correlation, showing an enhancement in L_{\rm FIR}. With respect to the IRAS colours, we found higher F_{\rm 60}/F_{\rm 100} values for ellipticals and late-type galaxies than for spirals, indicating a higher dust temperature. The mean value of F_{\rm 60}/F_{\rm 100} was found to be lower than for interacting samples from the literature. Conclusions. The results indicate that the FIR emission is a variable enhanced by interaction, and that our sample probably shows the lowest possible mean value. This attests to the utility of our sample for defining a nurture-free zero point.
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