Close encounters between galaxies are expected to be a viable mechanism, as predicted by numerical simulations, by which accretion onto supermassive black holes can be initiated. To test this ...scenario, we construct a sample of 562 galaxies (M * > 2.5 X 1010 M ) in kinematic pairs over the redshift range 0.25 < z < 1.05 that are more likely to be interacting than a well-matched control sample of 2726 galaxies not identified as being in a pair, both from the zCOSMOS 20k spectroscopic catalog. Galaxies that harbor an active galactic nucleus (AGN) are identified on the basis of their X-ray emission (L 0.5-10 keV > 2 X 1042 erg s--1) detected by Chandra. We find a higher fraction of an AGN in galaxies in pairs relative to isolated galaxies of similar stellar mass. Our result is primarily due to an enhancement of AGN activity, by a factor of 1.9 (observed) and 2.6 (intrinsic), for galaxies in pairs of projected separation less than 75 kpc and line-of-sight velocity offset less than 500 km s--1. This study demonstrates that close kinematic pairs are conducive environments for black hole growth, either indicating a causal physical connection or an inherent relation, such as, to enhanced star formation. In the Appendix, we describe a method for estimating the intrinsic fractions of galaxies (either in pairs or the field) hosting an AGN with confidence intervals, and an excess fraction in pairs. We estimate that 17.8+8.4 --7.4% of all moderate-luminosity AGN activity takes place within galaxies undergoing early stages of interaction that leaves open the question as to what physical processes are responsible for fueling the remaining ~80% that may include late-stage mergers.
ABSTRACT Models of galaxy formation predict that gas accretion from the cosmic web is a primary driver of star formation over cosmic history. Except in very dense environments where galaxy mergers ...are also important, model galaxies feed from cold streams of gas from the web that penetrate their dark matter halos. Although these predictions are unambiguous, the observational support has been indirect so far. Here, we report spectroscopic evidence for this process in extremely metal-poor galaxies (XMPs) of the local universe, taking the form of localized starbursts associated with gas having low metallicity. Detailed abundance analyses based on Gran Telescopio Canarias optical spectra of 10 XMPs show that the galaxy hosts have metallicities around 60% solar, on average, while the large star-forming regions that dominate their integrated light have low metallicities of some 6% solar. Because gas mixes azimuthally in a rotation timescale (a few hundred Myr), the observed metallicity inhomogeneities are only possible if the metal-poor gas fell onto the disk recently. We analyze several possibilities for the origin of the metal-poor gas, favoring the metal-poor gas infall predicted by numerical models. If this interpretation is correct, XMPs trace the cosmic web gas in their surroundings, making them probes to examine its properties.
We derived the oxygen abundance (O/H), the nitrogen-to-oxygen (N/O) abundance ratio, and their corresponding radial gradients for a sample of 1431 galaxies from the MaNGA DR15 survey using two ...different realisations of the strong line method: empirical R calibration and the Bayesian model-based HII-CHI-
MISTRY
(HC
M
) code. We find that both abundance calculation methods reveal a correlation between the O/H gradient and the stellar mass of a galaxy. This relation is non-linear, with the steepest average gradients in the intermediate mass range and flatter average gradients for high- and low-mass galaxies. The relation between the N/O gradient and the stellar mass is, on average, non-linear with the steepest gradients in the intermediate mass range (log(
M
/
M
⊙
)∼10), flatter gradients for high-mass galaxies, and the flattest gradients for low-mass galaxies. However, the general trend of steepening N/O gradients for higher masses, as reported in previous studies, remains evident. We find a dependence between the O/H and N/O gradients and the galaxy mean stellar age traced by the
D
(4000) index. For galaxies of lower masses, both gradients are, generally, steeper for intermediate values of
D
(4000) and flatter for low and high values of
D
(4000). Only the most massive galaxies do not show this correlation. We interpret this behaviour as an evolution of the metallicity gradients with the age of stellar population. Though the galaxies with a positive slope of the
D
(4000) radial gradient tend to have flatter O/H and N/O gradients, as compared to those with a negative
D
(4000) gradient.
Context . The physics and demographics of high-redshift obscured active galactic nuclei (AGN) is still scarcely investigated. New samples of such objects, selected with different techniques, can ...provide useful insights into their physical properties. Aims . With the goal to determine the properties of the gas in the emitting region of type 2 AGN, in particular, the gas metal content, we exploit predictions from photoionization models, including new parameterizations for the distance of gas distribution from the central source and internal microturbulence in the emitting clouds, to interpret rest-frame UV spectral data. Methods . We selected a sample of 90 obscured (type 2) AGN with 1.45 ≤ z ≤ 3.05 from the zCOSMOS-deep galaxy sample by 5 σ detection of the high-ionization C IV λ 1549 narrow emission line. This feature in a galaxy spectrum is often associated with nuclear activity, and the selection effectiveness has also been confirmed by diagnostic diagrams based on utraviolet (UV) emission-line ratios. We applied the same selection technique and collected a sample of 102 unobscured (type 1) AGN. Taking advantage of the large amount of multiband data available in the COSMOS field, we investigated the properties of the C IV -selected type 2 AGN, focusing on their host galaxies, X-ray emission, and UV emission lines. Finally, we investigated the physical properties of the ionized gas in the narrow-line region (NLR) of this type 2 AGN sample by combining the analysis of strong UV emission lines with predictions from photoionization models. Results . We find that in order to successfully reproduce the relative intensity of UV emission lines of the selected high- z type 2 AGN, two new ingredients in the photoionization models are fundamental: small inner radii of the NLR (≈90 pc for L AGN = 10 45 erg s −1 ), and the internal dissipative microturbulence of the gas-emitting clouds (with v micr ≈ 100 km s −1 ). With these modified models, we compute the gas-phase metallicity of the NLR, and our measurements indicate a statistically significant evolution of the metal content with redshift. Finally, we do not observe a strong relationship between the NLR gas metallicity and the stellar mass of the host galaxy in our C IV -selected type 2 AGN sample.
Large area surveys with a high number of galaxies observed have undoubtedly marked a milestone in the understanding of several properties of galaxies, such as star-formation history, morphology, and ...metallicity. However, in many cases, these surveys provide fluxes from fixed small apertures (e.g. fibre), which cover a scant fraction of the galaxy, compelling us to use aperture corrections to study the global properties of galaxies. In this work, we derive the current total star formation rate (SFR) of Sloan Digital Sky Survey (SDSS) star-forming galaxies, using an empirically based aperture correction of the measured Hα flux for the first time, thus minimising the uncertainties associated with reduced apertures. All the Hα fluxes have been extinction-corrected using the Hα/ Hβ ratio free from aperture effects. The total SFR for ~210 000 SDSS star-forming galaxies has been derived applying pure empirical Hα and Hα/ Hβ aperture corrections based on the Calar Alto Legacy Integral Field Area (CALIFA) survey. We find that, on average, the aperture-corrected SFR is ~0.65 dex higher than the SDSS fibre-based SFR. The relation between the SFR and stellar mass for SDSS star-forming galaxies (SFR-M⋆) has been obtained, together with its dependence on extinction and Hα equivalent width. We compare our results with those obtained in previous works and examine the behaviour of the derived SFR in six redshift bins, over the redshift range 0.005 ≤ z ≤ 0.22. The SFR-M⋆ sequence derived here is in agreement with selected observational studies based on integral field spectroscopy of individual galaxies as well as with the predictions of recent theoretical models of disc galaxies.
Aims. We explored the softness parameter in the infrared, η IR ′ , whose main purpose is the characterisation of the hardness of the incident ionising radiation in emission-line nebulae. This ...parameter is obtained from the combination of mid-infrared wavelength range transitions corresponding to consecutive ionisation stages in star-forming regions. Methods. We compiled observational data from a sample of star-forming galaxies (SFGs), including luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs), to study the η IR ′ and its equivalent expression in two dimensions, the softness diagram. We compared them with predictions from photoionisation models to determine the shape of the ionising continuum energy distribution in each case. We also used the measured emission-line ratios as input for HC MISTRY -T EFF - IR , a code that performs a Bayesian-like comparison with photoionisation model predictions in order to quantify the equivalent effective temperature ( T * ) and the ionisation parameter. Results. We found similar average values within the errors of η IR ′ in (U)LIRGs (−0.57) in the rest of the SFGs (−0.51), which could be interpreted as indicative of a similar incident radiation field. This result is confirmed from the analysis using HC M -T EFF - IR , which simultaneously points to a slightly lower, although similar within the errors, T * scale for (U)LIRGs, even when a higher dust-to-gas mass ratio is considered in the models for these objects. These derived T * values are compatible with the ionisation from massive stars, without any need of harder ionising sources, both for (U)LIRGs and the rest of the SFGs. However, the derived T * in (U)LIRGs do not show any correlation with metallicity. This could be interpreted as a sign that their similar average T * values are due to the attenuation of the energetic incident flux from massive stars by the heated dust mixed with the gas. This is supported by the known very large amounts of small grains associated with the very high star formation rates measured in galaxies of this type.
We present a detailed auto-consistent study of the nearest blue compact dwarf galaxy NGC 6789 by means of optical and ultraviolet (UV) archive photometry data and optical long-slit Intermediate ...dispersion Spectrograph and Imaging System-William Herschel Telescope (WHT) spectroscopy observations of the five brightest star-forming knots. The analysis of the spectra in all knots allowed the derivation of ionic chemical abundances of oxygen, nitrogen, sulphur, argon and neon using measures of both the high- and low-excitation electron temperatures, leading to the conclusion that NGC 6789 is chemically homogeneous with low values of the abundance of oxygen in the range 12+log(O/H) = 7.80-7.93, but presenting at the same time higher values of the nitrogen-to-oxygen ratio than expected for its metal regime.
We used archival Hubble Space Telescope/Wide-Field Planetary Camera 2 (HST/WFPC2) F555W and F814W observations of NGC 6789 to perform a photometric study of the colour-magnitude diagram (CMD) of the resolved stellar populations and derive its star formation history (SFH), which is compatible with the presence of different young and old stellar populations whose metallicities do not necessarily increase with age. We fit the observed optical spectrum in all the five knots using the starlight code and a combination of single stellar populations following the SFH obtained from the CMD. We compare the resulting stellar masses and the relative fractions of the ionizing populations with a non-constrained SFH case. The properties of the younger populations were obtained using cloudy photoionization models, giving similar ages in all the knots in the range 3-6 Myr and the estimation of the dust absorption factor, which correlates with the observed GALEX far-ultraviolet-near-ultraviolet colour indices. The total photometric extinction and dust-absorption corrected Hα fluxes were finally used to derive the star formation rates.
Green Pea galaxies (GPs) present among the most intense starbursts known in the nearby Universe. These galaxies are regarded as local analogs of high-redshift galaxies, making them a benchmark in the ...understanding of the star formation processes and the galactic evolution in the early Universe. In this work, we performed an integral field spectroscopic (IFS) study for a set of 24 GPs to investigate the interplay between its ionized interstellar medium (ISM) and the massive star formation that these galaxies present. Observations were taken in the optical spectral range (
λ
4750 Å–
λ
9350 Å) with the MUSE spectrograph attached to the 8.2 m telescope VLT. Spatial extension criteria were employed to verify which GPs are spatially resolved in the MUSE data cubes. We created and analyzed maps of spatially distributed emission lines (at different stages of excitation), continuum emission, and properties of the ionized ISM (e.g., ionization structure indicators, physical-chemical conditions, dust extinction). We also took advantage of our IFS data to produce integrated spectra of selected galactic regions in order to study their physical-chemical conditions. Maps of relevant emission lines and emission line ratios show that higher-excitation gas is preferentially located in the center of the galaxy, where the starburst is present. The continuum maps, with an average angular extent of 4″, exhibit more complex structures than the emission line maps. However, the O
III
λ
5007 Å emission line maps tend to extend beyond the continuum images (the average angular extent is 5.5″), indicating the presence of low surface brightness ionized gas in the outer parts of the galaxies. H
α
/H
β
, S
II
/H
α
, and O
I
/H
α
maps trace low-extinction, optically thin regions. The line ratios O
III
/H
β
and N
II
/H
α
span extensive ranges, with values varying from 0.5 dex to 0.9 dex and from −1.7 dex to −0.8 dex, respectively. Regarding the integrated spectra, the line ratios were fit to derive physical properties including the electron densities
n
e
= 30 − 530 cm
−3
, and, in six GPs with a measurable O
III
λ
4363 Å line, electron temperatures of
T
e
= 11 500 K–15 500 K, so the direct method was applied in these objects to retrieve metallicities 12 + log(O/H)≃8. We found the presence of the high-ionizing nebular He
II
λ
4686 Å line in three GPs, where two of them present among the highest sSFR values (> 8 × 10
8
yr
−1
) in this sample. Non-Wolf-Rayet (WR) features are detected in these galaxy spectra.
Context.
Feedback from massive stars plays a crucial role in regulating the growth of young star-forming galaxies (SFGs) and in shaping their interstellar medium (ISM). This feedback contributes to ...the removal and mixing of metals via galactic outflows and to the clearance of neutral gas, which facilitates the escape of ionizing photons.
Aims.
Our goal is to study the impact of stellar feedback on the chemical abundances of the ISM in a sample of SFGs with strong emission lines at
z
∼ 3.
Methods.
We selected 35 low-mass SFGs (7.9 < log(
M
⋆
/
M
⊙
) < 10.3) from deep spectroscopic surveys based on their CIII
λ
1908 emission. We used new follow-up near-infrared (NIR) observations to examine their rest-optical emission lines and to identify ionized outflow signatures through broad emission line wings detected after Gaussian modeling of OIII
λλ
4959,5007 profiles. We characterized the gas-phase metallicity and carbon-to-oxygen (C/O) abundance of the galaxies using a
T
e
-based method via the OIII
λ
1666/OIII
λ
5007 ratio and photoionization models.
Results.
We find line ratios and rest-frame equivalent widths (EWs) characteristic of high-ionization conditions powered by massive stars. Our sample displays a mean rest-frame EW(OIII
λ
5007) of ∼560 Å, while about 15% of the SFGs show EW(OIII
λλ
4959,5007) > 1000 Å and EW(CIII) > 5 Å, closely resembling those now seen in epoch of reionization (EoR) galaxies with the
James Webb
Space Telescope. We find high
T
e
values, which imply low gas-phase metallicities 12+log(O/H) ∼ 7.5–8.5 (mean of 17% solar) and C/O abundances from 23% to 128% solar, with no apparent increasing trend with metallicity. Our sample follows the mass-metallicity relation at
z
∼ 3, with some galaxies showing lower gas-phase metallicities. This results in significant deviations from the fundamental metallicity relation. From our OIII
λλ
4959,5007 line profile modeling, we find that 65% of our sample shows an outflow component, which is found both blue- or redshifted relative to the ionized gas systemic velocity, and the mean maximum velocities are
v
max
∼ 280 km s
−1
. We find a weak correlation between
v
max
and the star formation rate surface density (Σ
SFR
) of
v
max
= (2.41 ± 0.03) × Σ
SFR
(0.06 ± 0.03)
. Moreover, we find that the mass-loading factor
μ
of our galaxy sample is typically lower than in more massive galaxies from the literature, but it is higher than in typical local dwarf galaxies. In the stellar mass range covered by our sample, we find that
μ
increases with Σ
SFR
. This suggests that for a given stellar mass, denser starbursts in low-mass galaxies produce stronger outflows. Our results complement the picture drawn by similar studies at lower redshift, suggesting that the removal of ionized gas in low-mass SFGs driven by stellar feedback is regulated by their stellar mass and by the strength and concentration of their star formation, that is, Σ
SFR
.
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