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Register a new userThe gas, metal and dust evolution in low-metallicity local and high-redshift galaxies
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The chemical enrichment in the interstellar medium (ISM) of galaxies is regulated by several physical processes: stellar evolution, grain formation and destruction, galactic inflows and outflows. Understanding such processes is essential to follow the chemical enrichment of galaxies through the cosmic epochs, and to interpret the observations. Despite the importance of such topics, the efficiency of the different processes driving the evolution of baryons in galaxies, remain controversial. We revise the current description of metal and dust evolution in local low-metallicity dwarf galaxies and we develop a description for Lyman Break Galaxies. Our main goal is to reproduce i) the peak in the mass of dust over the mass of stars (sMdust) observed within few hundred Myrs; ii) the decrease of the sMdust at later time. The spectral energy distribution of the galaxies is fitted with the Code Investigating GALaxies Emission (CIGALE), through which the stellar and dust masses, and the star formation rate are estimated. For some of the dwarf galaxies, the metal and gas content are also available. We run different calculations of chemical evolution in galaxies, and we fit the observed properties through the model predictions. We show that i) a top-heavy initial mass function that favours massive stars and a dust condensation fraction for Type II Supernovae (SNe II) of 50% or more help to reproduce the peak of sMdust observed after 100 Myrs since the beginning of the cycle; ii) galactic outflows play a crucial role in reproducing the decline in sMdust with age, and they are more efficient than grain destruction from SNe II; iii) a star formation efficiency (mass of gas converted into stars) of few per cent is required to explain the metallicity of local dwarf galaxies; iv) dust growth in the ISM is not necessary to reproduce the sMdust and, if present, its effect is erased by galactic outflows.
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Ultraviolet (UV) observations of local star-forming galaxies have begun to establish an empirical baseline for interpreting the rest-UV spectra of reionization-era galaxies. However, existing high-ionization emission line measurements at $z>6$ ($mathrm{W_{C IV, 0}} gtrsim 20$ {AA}) are uniformly stronger than observed locally ($mathrm{W_{C IV, 0}} lesssim 2$ {AA}), likely due to the relatively high metallicities ($Z/Z_odot > 0.1$) typically probed by UV surveys of nearby galaxies. We present new HST/COS spectra of six nearby ($z<0.01$) extremely metal-poor galaxies (XMPs, $Z/Z_odot lesssim 0.1$) targeted to address this limitation and provide constraints on the highly-uncertain ionizing spectra powered by low-metallicity massive stars. Our data reveal a range of spectral features, including one of the most prominent nebular C IV doublets yet observed in local star-forming systems and strong He II emission. Using all published UV observations of local XMPs to-date, we find that nebular C IV emission is ubiquitous in very high specific star formation rate systems at low metallicity, but still find equivalent widths smaller than those measured in individual lensed systems at $z>6$. Our moderate-resolution HST/COS data allow us to conduct an analysis of the stellar winds in a local nebular C IV emitter, which suggests that some of the tension with $z>6$ data may be due to existing local samples not yet probing sufficiently high $mathrm{alpha/Fe}$ abundance ratios. Our results indicate that C IV emission can play a crucial role in the JWST and ELT era by acting as an accessible signpost of very low metallicity ($Z/Z_odot < 0.1$) massive stars in assembling reionization-era systems.
We combine samples of nearby galaxies with Herschel photometry selected on their dust, metal, HI, and stellar mass content, and compare these to chemical evolution models in order to discriminate between different dust sources. In a companion paper, we used a HI-selected sample of nearby galaxies to reveal a sub-sample of very gas rich (gas fraction > 80 per cent) sources with dust masses significantly below predictions from simple chemical evolution models, and well below $M_d/M_*$ and $M_d/M_{gas}$ scaling relations seen in dust and stellar-selected samples of local galaxies. We use a chemical evolution model to explain these dust-poor, but gas-rich, sources as well as the observed star formation rates (SFRs) and dust-to-gas ratios. We find that (i) a delayed star formation history is required to model the observed SFRs; (ii) inflows and outflows are required to model the observed metallicities at low gas fractions; (iii) a reduced contribution of dust from supernovae (SNe) is needed to explain the dust-poor sources with high gas fractions. These dust-poor, low stellar mass galaxies require a typical core-collapse SN to produce 0.01 - 0.16 $M_{odot}$ of dust. To match the observed dust masses at lower gas fractions, significant grain growth is required to counteract the reduced contribution from dust in SNe and dust destruction from SN shocks. These findings are statistically robust, though due to intrinsic scatter it is not always possible to find one single model that successfully describes all the data. We also show that the dust-to-metals ratio decreases towards lower metallicity.
Stars form out of the densest parts of molecular clouds. Far-IR emission can be used to estimate the Star Formation Rate (SFR) and high dipole moment molecules, typically HCN, trace the dense gas. A strong correlation exists between HCN and Far-IR emission, with the ratio being nearly constant, over a large range of physical scales. A few recent observations have found HCN to be weak with respect to the Far-IR and CO in subsolar metallicity (low-Z) objects. We present observations of the Local Group galaxies M33, IC10, and NGC6822 with the IRAM 30meter and NRO 45m telescopes, greatly improving the sample of low-Z galaxies observed. HCN, HCO$^+$, CS, C$_2$H, and HNC have been detected. Compared to solar metallicity galaxies, the Nitrogen-bearing species are weak (HCN, HNC) or not detected (CN, HNCO, N$_2$H$^+$) relative to Far-IR or CO emission. HCO$^+$ and C$_2$H emission is normal with respect to CO and Far-IR. While $^{13}$CO is the usual factor 10 weaker than $^{12}$CO, C$^{18}$O emission was not detected down to very low levels. Including earlier data, we find that the HCN/HCO$^+$ ratio varies with metallicity (O/H) and attribute this to the sharply decreasing Nitrogen abundance. The dense gas fraction, traced by the HCN/CO and HCO$^+$/CO ratios, follows the SFR but in the low-Z objects the HCO$^+$ is much easier to measure. Combined with larger and smaller scale measurements, the HCO$^+$ line appears to be an excellent tracer of dense gas and varies linearly with the SFR for both low and high metallicities.
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 $1.41 leq z leq 2.54$ and the stellar mass range $10.36leqlog(M_*/{rm M}_{odot})leq10.91$. 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/MOSFIRE spectrograph to estimate SFRs from dust-corrected H$alpha$ emission (SFR(H$alpha$)) and gas-phase oxygen abundances from the ratio [NII]$lambda 6584$/H$alpha$. We find that the dust-corrected SFR(H$alpha$) is systematically lower than the bolometric SFR by a factor of several, and measure gas-phase oxygen abundances in a narrow range, $12+log(mbox{O/H})=8.59-8.69$ ($0.8-1.0: (mbox{O/H})_{odot}$). Relative to a large $zsim 2$ comparison sample from the 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 vs. metallicity relation from $zsim0$ to $zsim2$. 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 vs. metallicity relation at high redshift now requires combined measurements of dust, gas, and metallicity over a significantly wider range in metallicity.
Obscured AGN represent a significant fraction of the entire AGN population, especially at high redshift (~70% at z=3--5). They are often characterized by the presence of large gas and dust reservoirs that are thought to sustain and possibly obscure vigorous star formation processes that make these objects shine at far-IR and sub-mm wavelengths. We exploit ALMA Cycle 4 observations of the continuum (~2.1mm) and high-J CO emission of a sample of six X-ray selected SMGs hosting an obscured AGN at z_spec>2.5 in the 7 Ms CDF-S. We measured the masses and sizes of the dust and molecular gas and we derived the gas density and column density on the basis of a uniform sphere geometry. Finally, we compared the measured column densities with those derived from the Chandra X-ray spectra. We detected both the continuum and line emission for three sources for which we measured both the flux density and size. For the undetected sources, we derived an upper limit on the flux density. We found that the detected galaxies are rich in gas and dust (molecular gas mass in the range <0.5 - 2.7 x 10^10 M_sun for {alpha}_CO=0.8 and up to ~2 x 10^11~M_sun for {alpha}_CO=6.5, and dust mass <0.9 - 4.9 x 10^8 M_sun) and compact (gas major axis 2.1-3.0 kpc, dust major axis 1.4-2.7 kpc). The column densities associated with the ISM are on the order of 10^(23-24) cm-2, which is comparable with those derived from the X-ray spectra. For the detected sources we also derived dynamical masses in the range 0.8 - 3.7 x 10^10 M_sun. We conclude that the ISM of high redshift galaxies can substantially contribute to nuclear obscuration up to the Compton-thick (>10^24 cm-2) regime. In addition, we found that all the detected sources show a velocity gradient reminding one rotating system, even though two of them show peculiar features in their morphology that can be associated with a chaotic, possibly merging, structure.
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