Metallicity Of Galaxies Research Articles

Measuring the Metallicity of Early-type Galaxies. I. Composite Region

Abstract We present the data of 9739 early-type galaxies (ETGs), cross-matching Galaxy Zoo 1 with our sample selected from the catalog of the Sloan Digital Sky Survey Data Release 7 of MPA-JHU emission-line measurements. We first investigate the divisor between ETGs with and without star formation (SF), and find the best separator of W2–W3 = 2.0 is added. We explore the ETG sample by refusing a variety of ionization sources, and derive 5376 ETGs with SF by utilizing a diagnostic tool of the division line of W2–W3 = 2.0. We measure their metallicities with four abundance calibrators. We find that our composite ETG sample has similar distributions of M * and star formation rate as star-forming galaxies (SFGs) do, that most of them lie on the “main sequence,” and that our fit is a slightly steeper slope than that derived in Renzini & Peng. Compared with the distributions between different metallicities calibrated by four abundance indicators, we find that the Curti17 method is the most accurate calibrator for composite ETGs among the four abundance indicators. We present a weak positive correlation of SFR and metallicity only when the metallicity is calibrated by the PP04, Curti17, and T04 indicators. The correlation is not consistent with the negative correlation of both parameters in SFGs. We suggest that the weak correlation is due to the dilution effect of gas inflow driven by minor mergers.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

Aims. We measure far-infrared (FIR) emission from tails of stripped dust following the ionised and atomic gas components in galaxies undergoing ram pressure stripping. We study the dust-to-gas relative distribution and mass ratio in the stripped interstellar medium and relate them to those of the intra-cluster medium (ICM), thus linking the cluster-ICM-galaxy evolution at small-scales. The galaxy sample consists of three Scd Virgo galaxies with stellar masses in the range of 109 ≲ M* ≲ 1010 M⊙ and within 1 Mpc from the cluster centre, namely NGC 4330, NGC 4522, and NGC 4654. Methods. Through the analysis of Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) Hα, Herschel SPIRE FIR, and VLA Imaging of Virgo in Atomic gas HI data, we trace the spatial distribution of the tails and infer the dust and gas masses from the measured FIR 250 μm and HI flux densities. Dust-to-gas mass ratios in the tails are analysed as a function of the galaxy mass, metallicity, and dust temperature. Results. Along the stripped component, the dust distribution closely follows the HI and Hα emitting gas, which extend beyond the optical disc (defined by the B-band 25th magnitude isophote). In these regions, the dust-to-gas mass ratios are 2.0 ± 0.6 × 10−3, 0.7 ± 0.1 × 10−3, and 0.4 ± 0.03 × 10−3 for NGC 4330, NGC 4522, and NGC 4654, respectively. Thus, dust is widespread in the stripped material with a lower dust-to-gas mass ratio (up to a factor of 15) than the one measured in the main body of nearby galaxies. We also find a negative trend in the dust-to-gas mass ratio as a function of the metallicity that can be explained in terms of a dust component more centrally concentrated in more metal-rich systems. Together with the finding that the stripped dust is cold, Td ≲ 25 K, our results can be interpreted as a consequence of an outside-in stripping of the galaxy interstellar medium. Conclusions. Gas and dust in galaxies are perturbed in a similar fashion by the cluster environment, although their relative contribution differs from the one measured in the main body of the galaxies. When this value is considered, ram pressure stripping is consistent with being one of the key mechanisms in building up the Virgo intra-cluster component, injecting dust grains into the ICM, thus contributing to its metal enrichment.

Dependence of the IRX-β Dust Attenuation Relation on Metallicity and Environment *

Abstract We use a sample of star-forming field and protocluster galaxies at z = 2.0–2.5 with Keck/MOSFIRE K-band spectra, a wealth of rest-frame ultraviolet (UV) photometry, and Spitzer/MIPS and Herschel/PACS observations, to dissect the relation between the ratio of infrared (IR) to UV luminosity (IRX) versus UV slope (β) as a function of gas-phase metallicity ( ∼ 8.2–8.7). We find no significant dependence of the IRX-β trend on environment. However, we find that at a given β, IRX is highly correlated with metallicity, and less correlated with mass, age, and specific star formation rate (sSFR). We conclude that, of the physical properties tested here, metallicity is the primary physical cause of the IRX-β scatter, and the IRX correlation with mass is presumably due to the mass dependence on metallicity. Our results indicate that the UV attenuation curve steepens with decreasing metallicity, and spans the full range of slope possibilities from a shallow Calzetti-type curve for galaxies with the highest metallicity in our sample ( ∼ 8.6) to a steep Small Magellanic Cloud (SMC)-like curve for those with ∼ 8.3. Using a Calzetti (SMC) curve for the low (high) metallicity galaxies can lead to up to a factor of 3 overestimation (underestimation) of the UV attenuation and obscured star formation rate. We speculate that this change is due to different properties of dust grains present in the interstellar medium of low- and high-metallicity galaxies.

The First Robust Constraints on the Relationship between Dust-to-gas Ratio and Metallicity in Luminous Star-forming Galaxies at High Redshift*

Abstract We present rest-optical spectroscopic properties of a sample of four galaxies in the Atacama Large Millimeter/submillimeter Array Hubble Ultra Deep Field (ALMA HUDF). These galaxies span the redshift range and the stellar mass range . They have existing far-infrared and radio measurements of dust-continuum and molecular gas emission from which bolometric star formation rates (SFRs), dust masses, and molecular gas masses have been estimated. We use new H- and K-band near-infrared spectra from the Keck/Multi-object Spectrometer for Infrared Exploration (MOSFIRE) to estimate SFRs from dust-corrected Hα emission (SFR(Hα)) and gas-phase oxygen abundances from the ratio [N ii] /Hα. We find that the dust-corrected SFR(Hα) is systematically lower than the bolometric SFR by a factor of several, and measure gas-phase oxygen abundances in a narrow range, ( ). Relative to a large z ∼ 2 comparison sample from the MOSFIRE Deep Evolution Field (MOSDEF) survey, the ALMA HUDF galaxies scatter roughly symmetrically around the best-fit linear mass–metallicity relation, providing tentative evidence for a flattening in the SFR dependence of metallicity at high stellar mass. Combining oxygen abundances with estimates of dust and molecular gas masses, we show that there is no significant evolution in the normalization of the dust-to-gas ratio (DGR) versus metallicity relation from z ∼ 0 to z ∼ 2. This result is consistent with some semi-analytic models and cosmological simulations describing the evolution of dust in galaxies. Tracing the actual form of the DGR versus metallicity relation at high redshift now requires combined measurements of dust, gas, and metallicity over a significantly wider range in metallicity.

Effects of environment on stellar metallicity profiles of late-type galaxies in the CALIFA survey

Aims. We explore the effects of environment in the evolution of late-type galaxies by studying the radial profiles of light- and mass-weighted metallicities of galaxies in two discrete environments: field and groups. Methods. We used a sample of 167 late-type galaxies with stellar masses of 9 ≤ log(M⋆/M⊙) ≤ 12 drawn from the Calar Alto Legacy Integral Field Area (CALIFA) survey. Firstly, we obtained light- and mass-weighted stellar metallicity profiles and stellar mass density profiles of these galaxies using publicly available data. We then classified them according to their environment into field and group galaxies. Finally, we studied the metallicity of galaxies in these two environments, including a comparison of the metallicity as a function of radius, at a characteristic scale, and as a function of stellar mass surface density. As metallicity depends on galaxy mass, we took special care throughout the study to compare, in all cases, subsamples of galaxies in groups and in the field that have similar masses. Results. We find significant differences between group and field late-type galaxies in terms of their metallicity: group galaxies are systematically higher in metallicity than their field counterparts. We find that field galaxies, in general, have metallicity profiles that show a negative gradient in their inner regions and a shallower profile at larger radii. This is in contrast to the metallicity profiles of galaxies in groups, which tend to be flat in the inner regions and to have a negative gradient in the outer parts. Regarding the metallicity at the characteristic radius of the luminosity profiles, we consistently find that it is higher for group galaxies irrespective of galaxy mass. At fixed local stellar surface mass density, group galaxies are again higher in metallicity, also the dependence of metallicity on surface density is less important for group galaxies. Conclusions. The evidence of a clear difference in metallicity between group and field galaxies as a function of mass, spatial scale, and local stellar mass density is indicative of the different evolutionary paths followed by galaxies in groups and in the field. We discuss some possible implications of the observed differences.

Box/peanut-shaped bulges in action space

ABSTRACT We introduce the study of box/peanut (B/P) bulges in the action space of the initial axisymmetric system. We explore where populations with different actions end up once a bar forms and a B/P bulge develops. We find that the density bimodality due to the B/P bulge (the X-shape) is better traced by populations with low radial, ${\it J}_{R,0}$, or vertical, ${\it J}_{z,0}$, actions, or high azimuthal action, ${\it J}_{\phi ,0}$. Generally, populations separated by ${\it J}_{R,0}$ have a greater variation in bar strength and vertical heating than those separated by ${\it J}_{z,0}$. While the bar substantially weakens the initial vertical gradient of ${\it J}_{z,0}$, it also drives a strikingly monotonic vertical profile of ${\it J}_{R,0}$. We then use these results to guide us in assigning metallicity to star particles in a pure N-body model. Because stellar metallicity in unbarred galaxies depends on age as well as radial and vertical positions, the initial actions are particularly well suited for assigning metallicities. We argue that assigning metallicities based on single actions, or on positions, results in metallicity distributions inconsistent with those observed in real galaxies. We therefore use all three actions to assign metallicity to an N-body model by comparing with the actions of a star-forming, unbarred simulation. The resulting metallicity distribution is pinched on the vertical axis, has a realistic vertical gradient, and has a stronger X-shape in metal-rich populations, as found in real galaxies.

Dissecting the Global Cold Dust Properties and Possible Submillimeter Excess of 13 Nearby Spiral Galaxies from the NGLS

Abstract We select 13 nearby spiral galaxies from the Nearby Galaxies Legacy Survey (NGLS) project and perform spectral energy distribution fitting for each galaxy applying two-component modified blackbody models on a global scale aim to probe the potential submillimeter (submm) excess. We find that NGC 2976, NGC 3351, and NGC 4631 show excess emission at 850 μm when using β c = 2.0. The contributions of CO(3–2), free–free emission or synchrotron radiation cannot explain their 850 μm excess. Our results suggest that a submm excess at 850 μm may be more easily detected for galaxies with faint total infrared luminosity and low cold dust mass. The colder temperature of cold dust, the more radiation of dust there is at 850 μm. The submm excess are prone to be detected in spiral galaxies with low stellar mass. As the metallicity of galaxies become poor, the submm excess is more obvious.

The Cosmic Baryon and Metal Cycles

Characterizing the relationship between stars, gas, and metals in galaxies is a critical component of understanding the cosmic baryon cycle. We compile contemporary censuses of the baryons in collapsed structures and their chemical makeup and dust content. We show the following: ▪ The [Formula: see text] mass density of the Universe is well determined to redshifts [Formula: see text] and shows minor evolution with time. New observations of molecular hydrogen reveal its evolution mirrors that of the global star-formation rate density, implying a universal cosmic molecular gas depletion timescale. The low-redshift decline of the star-formation history is thus driven by the lack of molecular gas supply due to a drop in net accretion rate related to the decreased growth of dark matter halos. ▪ The metal mass density in cold gas ([Formula: see text] K) contains virtually all the metals produced by stars for [Formula: see text]. At lower redshifts, the contributors to the total amount of metals are more diverse; at [Formula: see text], most of the observed metals are bound in stars. Overall, there is little evidence for a “missing metals problem” in modern censuses. ▪ We characterize the dust content of neutral gas over cosmic time, finding the dust-to-gas and dust-to-metals ratios fall with decreasing metallicity. We calculate the cosmological dust mass density in the neutral gas up to [Formula: see text]. There is good agreement between multiple tracers of the dust content of the Universe.

The role of dust destruction and dust growth in the evolution of the interstellar medium

ABSTRACT We use Milky Way-like chemodynamical simulations with a new treatment for dust destruction and growth to investigate how these two processes affect the properties of the interstellar medium in galaxies. We focus on the role of two specific parameters, namely fdes (a new parameter that determines the fraction of dust destroyed in a single gas particle vicinity of a supernova) and Cs (the probability that a metal atom or ion sticks to the dust grain after colliding, i.e. the sticking coefficient), in regulating the amount and distribution of dust, cold gas and metals in galaxies. We find that simulated galaxies with low fdes and/or high Cs values not only produce more dust, but they also have a shallower correlation between the dust surface density and the total gas surface density, and a steeper correlation between the dust-to-gas ratio and the metallicity. Only for values of fdes between 0.01 and 0.02, and of Cs between 0.5 and 1 do our simulations produce an average slope of the dust-to-gas ratio versus metallicity relationship that is consistent with observations. fdes values correspond to a total fraction of dust destroyed by a single supernova ranging between 0.42 and 0.44. Finally, we compare predictions of several simulations (with different star formation recipes, gas fractions, central metallicities, and metallicity gradients) with the spatially resolved M101 galaxy, and conclude that metallicity is the primary driver of the spatial distribution of dust, while the dust-to-gas ratio controls the cold gas distribution, as it regulates the atomc-to-molecular hydrogen conversion rate.

Absorption spectra from galactic wind models: a framework to link PLUTO simulations to TRIDENT

AbstractGalactic winds probe how feedback regulates the mass and metallicity of galaxies. Galactic winds have cold gas, which is mainly observable with absorption and emission lines. Theoretically studying how absorption lines are produced requires numerical simulations and realistic starburst UV backgrounds. We use outputs from a suite of 3D PLUTO simulations of wind-cloud interactions to first estimate column densities and temperatures. Then, to create synthetic spectra, we developed a python interface to link our PLUTO simulations to TRIDENT via the YT-package infrastructure. We produce UV backgrounds accounting for the star formation rate of starbursts. For this purpose, we use fluxes generated by STARBURST99, which are then processed through CLOUDY to create customised ion tables. Such tables are subsequently read into TRIDENT to generate absorption spectra. We explain how the various packages and tools communicate with each other to create ion spectra consistent with spectral energy distributions of starburst systems.

Using realistic host galaxy metallicities to improve the GRB X-ray equivalent total hydrogen column density and constrain the intergalactic medium density

ABSTRACT It is known that the GRB equivalent hydrogen column density (NHX) changes with redshift and that, typically, NHX is greater than the GRB host neutral hydrogen column density. We have compiled a large sample of data for GRB NHX and metallicity [X/H]. The main aims of this paper are to generate improved NHX for our sample by using actual metallicities, dust corrected where available for detections, and for the remaining GRB, a more realistic average intrinsic metallicity using a standard adjustment from solar. Then, by approximating the GRB host intrinsic hydrogen column density using the measured neutral column (NHI, IC) adjusted for the ionization fraction, we isolate a more accurate estimate for the intergalactic medium (IGM) contribution. The GRB sample mean metallicity is = −1.17 ± 0.09 rms (or 0.07 ± 0.05 Z/Zsol) from a sample of 36 GRB with a redshift 1.76 ≤ z ≤ 5.91, substantially lower than the assumption of solar metallicity used as standard for many fitted NHX. Lower GRB host mean metallicity results in increased estimated NHX with the correction scaling with redshift as Δlog (NHX cm−2) = (0.59 ± 0.04)log(1 + z) + 0.18 ± 0.02. Of the 128 GRB with data for both NHX and NHI, IC in our sample, only six have NHI, IC > NHX when revised for realistic metallicity, compared to 32 when solar metallicity is assumed. The lower envelope of the revised NHX – NHI, IC, plotted against redshift can be fit by log(NHX – NHI, IC cm−2) = 20.3 + 2.4 log(1 + z). This is taken to be an estimate for the maximum IGM hydrogen column density as a function of redshift. Using this approach, we estimate an upper limit to the hydrogen density at redshift zero (n0) to be consistent with n0 = 0.17 × 10−7cm−3.

Age and metallicity of galaxies in different environments of the Coma supercluster

We analyse luminosity-weighted ages and metallicity (Z) of galaxies in a continuous range of environments, i.e. clusters, filaments and voids prevalent in the Coma supercluster (∼100h−1 Mpc). Specifically, we employ two absorption line indices, Hβ and ⟨Fe⟩ as tracers of age and metallicity of galaxies. We find that the stellar-phase metallicity of galaxies declines with increasing age as a function of stellar mass (M*) as well as environment. On the filaments, metallicity of galaxies varies as a function of their distance from the spine of the filament, such that galaxies closer to the centre of the filaments have lower metallicity relative to their counterparts 1 Mpc away from it. The mean age of intermediate mass galaxies (1010 < M*/M⊙ < 1010.5) galaxies is statistically significantly different in different environments such that, the galaxies in clusters are older than the filament galaxies by 1-1.5 Gyr, while their counterparts in the voids are younger than filament galaxies by ~ 1 Gyr. The massive galaxies (M*/M⊙ > 1010.5), on the other hand show no such difference for the galaxies in clusters and filaments, but their counterparts in voids are found to be younger by ~ 0.5 Gyr. At fixed age however, Z of galaxies is independent of their M* in all environments, except the most massive (M*/M⊙ ≳ 1010.7), oldest galaxies ( ≳ 9 Gyr) which show a sharp decline in their Z with M*. Our results support a scenario where galaxies in the nearby Universe have grown by accreting smaller galaxies or primordial gas from the large-scale cosmic web.

Quantifying the effects of spatial resolution and noise on galaxy metallicity gradients

ABSTRACT Metallicity gradients are important diagnostics of galaxy evolution, because they record the history of events such as mergers, gas inflow, and star formation. However, the accuracy with which gradients can be measured is limited by spatial resolution and noise, and hence, measurements need to be corrected for such effects. We use high-resolution (∼20 pc) simulation of a face-on Milky Way mass galaxy, coupled with photoionization models, to produce a suite of synthetic high-resolution integral field spectroscopy (IFS) datacubes. We then degrade the datacubes, with a range of realistic models for spatial resolution (2−16 beams per galaxy scale length) and noise, to investigate and quantify how well the input metallicity gradient can be recovered as a function of resolution and signal-to-noise ratio (SNR) with the intention to compare with modern IFS surveys like MaNGA and SAMI. Given appropriate propagation of uncertainties and pruning of low SNR pixels, we show that a resolution of 3–4 telescope beams per galaxy scale length is sufficient to recover the gradient to ∼10–20 per cent uncertainty. The uncertainty escalates to ∼60 per cent for lower resolution. Inclusion of the low SNR pixels causes the uncertainty in the inferred gradient to deteriorate. Our results can potentially inform future IFS surveys regarding the resolution and SNR required to achieve a desired accuracy in metallicity gradient measurements.

Photometric properties of reionization-epoch galaxies in the simba simulations

ABSTRACT We study the photometric properties and sizes of the reionization-epoch galaxies in high-resolution simba cosmological hydrodynamical simulations with box sizes of $[25,50]\, h^{-1}\, {\rm Mpc}$. Assuming various attenuation laws, we compute photometry by extincting each star particle’s spectrum using the line-of-sight gas metal column density. The predicted ultraviolet luminosity function (UVLF) generally agrees with observations at z = 6, owing to a partial cancellation between the high metallicities of the simulated galaxies and lower dust-to-metal ratios. The simulated z = 8 UVLF is low compared to observations, likely owing to excessive dust extinction. simba predicts UV continuum slopes (β) in agreement with the z = 6 observations, with the best agreement obtained using a Calzetti extinction law. Interestingly, the gas-phase mass–metallicity relation in simba is higher at z ∼ 6 than at z ∼ 2, suggesting that rapid early enrichment (and dust growth) might be necessary to match the observed β. We find that β is more sensitive to the dust extinction law than the UVLF. By generating mock James Webb Space Telescope (JWST) images and analysing in a manner similar to observations, we show that simba’s galaxy size–luminosity relation well reproduces the current z = 6 Hubble observations. Unlike observations at lower redshifts, simba predicts similar rest-UV and rest-optical sizes of z = 6 galaxies, owing to weak age gradients and dust extinction in star-forming regions counteract each other to weaken the colour gradients within galaxies. These predictions will be testable with JWST.

The habitability of large elliptical galaxies

ABSTRACT Based on numbers of stars, supernova rates, and metallicity, a prior study concluded that large elliptical galaxies contain up to 10 000 times more habitable planets than the Milky Way and are thus the ‘cradles of life’. Using the results of their model and taking into account galactic number distributions and supernova rates, I argue here that this result constitutes a violation of the principle of mediocrity as applied to the reference class of all extant technological species. Assuming that we are a typical technological species in the attribute of inhabiting a relatively large disc-dominated galaxy, I outline two hypotheses that could significantly limit the habitability of large elliptical galaxies: (1) massive galactic sterilization events associated with quasar/active galactic nucleus activity and starburst supernovae that occurred when the antecedents of today’s large elliptical galaxies were much more compact; and (2) the probability of habitable planet formation in large elliptical galaxies may be small since a disproportionately larger number of gaseous planets are expected to form as a result of the generally higher metallicity in large elliptical galaxies. Consequently, fewer habitable planets will accrete if the gaseous planets' inward migrations are sufficiently slow. The sterilization events of hypothesis (1) occurred at earlier epochs ($z$ ≥ 1) and so they must be effectively permanent, implying two possible scenarios regarding the origin and evolution of life. In connection with one of these scenarios, independent applications of the principle of mediocrity suggest that M-dwarf stars are not significant hosts of technological life.

A Comparison of UV and Optical Metallicities in Star-forming Galaxies

Abstract Our ability to study the properties of the interstellar medium in the earliest galaxies will rely on emission-line diagnostics at rest-frame ultraviolet (UV) wavelengths. In this work, we identify metallicity-sensitive diagnostics using UV emission lines. We compare UV-derived metallicities with standard, well-established optical metallicities using a sample of galaxies with rest-frame UV and optical spectroscopy. We find that the He2–O3C3 diagnostic (He ii λ1640 /C iii] λ1906,1909 versus [O iii] λ1666 /C iii] λ1906,9 ) is a reliable metallicity tracer, particularly at low metallicity ( ), where stellar contributions are minimal. We find that the Si3–O3C3 diagnostic ([Si iii] λ1883 /C iii] λ1906 versus [O iii] λ1666 /C iii] λ1906,9 ) is a reliable metallicity tracer, though with large scatter (0.2–0.3 dex), which we suggest is driven by variations in gas-phase abundances. We find that the C4–O3C3 diagnostic (C iv λ 1548,50 /[O iii] λ 1666 versus [O iii] λ 1666 /C iii] λ 1906,9 ) correlates poorly with optically derived metallicities. We discuss possible explanations for these discrepant metallicity determinations, including the hardness of the ionizing spectrum, contribution from stellar wind emission, and non-solar-scaled gas-phase abundances. Finally, we provide two new UV oxygen abundance diagnostics, calculated from polynomial fits to the model grid surface in the He2–O3C3 and Si3–O3C3 diagrams.

Weak evolution of the mass–metallicity relation at cosmic dawn in the FirstLight simulations

ABSTRACT Little is known about the mass–metallicity relation (MZR) in galaxies at cosmic dawn. Studying the first appearance of the MZR is one of the keys to understand the formation and evolution of the first galaxies. In order to lay the groundwork for upcoming observational campaigns, we analyse 290 galaxies in haloes spanning Mh = 109–1011 M⊙ selected from the FirstLight cosmological zoom simulations to predict the MZR at z = 5–8. Over this interval, the metallicity of FirstLight galaxies with stellar mass M* = 108 M⊙ declines by ≤0.2 dex. This contrasts with the observed tendency for metallicities to increase at lower redshifts, and reflects weakly evolving or even increasing gas fractions. We assess the use of the R3 strong-line diagnostic as a metallicity indicator, finding that it is informative for 12 + log (O/H) &amp;lt; 8 but saturates to R3 ≈ 3 at higher metallicities owing to a cancellation between enrichment and spectral softening. None the less, campaigns with JWST should be able to detect a clear trend between R3 and stellar mass for M* &amp;gt; 107.5 M⊙. We caution that, at fixed metallicity, galaxies with higher specific star formation show higher R3 owing to their more intense radiation fields, indicating a potential for selection biases.

Measuring the Metallicity of Early-type Galaxies

Abstract We use data for 6048 early-type galaxies (ETGs) from Galaxy Zoo 1 that have been cross-matched with the catalog of the MPA-JHU emission-line measurements for the Sloan Digital Sky Survey Data Release 7. We measure the metallicity of these ETGs by excluding various ionization sources, and study other properties as well. We use the optimal division line of W2–W3 = 2.5 as a diagnostic tool, and for the first time derive metallicity measurements for 2218 ETGs. We find that these ETGs actually are closer to H ii regions as defined by Kauffmann et al. in the Baldwin–Philips–Terevich diagram, and they display younger stellar populations. We present a full mass–metallicity relation and find that most ETGs have lower metallicities than star-forming galaxies (SFGs) at a given galaxy stellar mass. We use five metallicity calibrators to check our results. We find that these metallicity indicators (R23, O32, and O3S2) give consistent results. We suggest that the remaining two metallicity calibrators, which increase metallicity by N-enrichment, can be used to calibrate metallicities for SFGs, but not to estimate the metallicities of ETGs.

Absorption-line Abundances in the SMC-like Galaxy UGC 5282: Evidence of ISM Dilution from Inflows on Kiloparsec Scales*

Abstract We present a Hubble Space Telescope Cosmic Origins Spectrograph spectrum of the QSO SDSS J095109.12+330745.8 ( ) whose sightline passes through the SMC-like dwarf galaxy UGC 5282 ( , cz = 1577 km s−1), 1.2 kpc in projection from the central H ii region of the galaxy. Damped Lyα (DLA) absorption is detected at the redshift of UGC 5282 with log [N(H i) cm . Analysis of the accompanying S ii, P ii, and O i metal lines yields a neutral gas metallicity, , of [S/H] ≃ [P/H] = −0.80 ± 0.24. The metallicity of ionized gas from the central H ii region measured from its emission lines is [O/H] = −0.37 ± 0.10, a difference of +0.43 ± 0.26 from . This difference δ is consistent with that seen toward H ii regions in other star-forming galaxies and supports the idea that ionized gas near star-forming regions shows systematically higher metallicities than exist in the rest of a galaxy’s neutral interstellar medium (ISM). The positive values of δ found in UGC 5282 (and the other star-forming galaxies) is likely due to infalling low-metallicity gas from the intergalactic medium that mixes with the galaxy’s ISM on kiloparsec scales. This model is also consistent with broad Lyα emission detected at the bottom of the DLA absorption, offset by ∼125 km s−1 from the absorption velocity. Models of galaxy evolution that attempt to replicate population characteristics, such as the mass–metallicity relation, may need to start with a galaxy metallicity represented by rather than that measured traditionally from .

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread throughout the Universe, as revealed by their distinctive set of emission bands at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 μm, which are characteristic of their vibrational modes. They are ubiquitously seen in a wide variety of astrophysical regions, ranging from planet-forming disks around young stars to the interstellar medium of the Milky Way and other galaxies out to high redshifts at z ≳ 4. PAHs profoundly influence the thermal budget and chemistry of the interstellar medium by dominating the photoelectric heating of the gas and controlling the ionization balance. Here I review the current state of knowledge of the astrophysics of PAHs, focusing on their observational characteristics obtained from the Spitzer Space Telescope and their diagnostic power for probing the local physical and chemical conditions and processes. Special attention is paid to the spectral properties of PAHs and their variations revealed by the Infrared Spectrograph onboard Spitzer across a much broader range of extragalactic environments (for example, distant galaxies, early-type galaxies, galactic halos, active galactic nuclei and low-metallicity galaxies) than was previously possible with the Infrared Space Observatory or any other telescope facilities. Also highlighted is the relation between the PAH abundance and the galaxy metallicity established for the first time by Spitzer. Spitzer revealed the power of astrophysical polycyclic aromatic hydrocarbon molecules to probe the local physical and chemical conditions and processes, for example, establishing the relation between their abundance and galaxy metallicity for the first time.