AGB populations in post-starburst galaxies

In a previous paper we compared the SEDs of a sample of 808 K+A galaxies from the FUV to the MIR to the predictions of the spectrum synthesis models explicitly using AGB components. Here we use the new AGB-light models from C. Maraston (including less fuel for the later stages of stellar evolution and improved calibrations) to address the discrepancies between our observations and the AGB-heavy models used in our previous paper, which over-predict the infrared fluxes of post-starburst galaxies by an order of magnitude. The new models yield a much better fit to the data, especially in the near-IR, compared to previous realizations where AGB stars caused a large excess in the H and K bands. We { also compare the predictions of the M2013 models to those with BC03 and find that both reproduce the observations equally well. } We still find a significant discrepancy with { both sets of models} in the Y and J bands, which however is probably due to the spectral features of AGB stars. We also find that { both the M2013 and the BC03 models} still over-predict the observed fluxes in the UV bands, even invoking extinction laws that are stronger in these bands. While there may be some simple explanations for this discrepancy, we find that further progress requires new observations and better modelling. Excess mid-infrared emission longward of 5$\mu$m is well modelled by a $T_{dust}=300^oK$ Black-Body, which may arise from dust emission from the circumstellar envelopes of Oxygen rich M stars (expected for a metal-rich population of AGB stars).Comment: A&A accepte

The central density of R136 in 30 Doradus

The central density rho_0 of a stellar cluster is an important physical parameter for determining its evolutionary and dynamical state. How much mass segregation there is, or whether the cluster has undergone core collapse both depends on rho_0. We reanalyze the results of a previous paper that gives the mass density profile of R136 and combine them with both a conservative upper limit for the core parameter and a more uncertain recent measurement. We thus place a lower limit on rho_0 under reasonable and defensible assumptions about the IMF, finding rho_0 >~ 1.5x10^4 Msun/pc^3 for the conservative assumption a < 0.4 pc for the cluster core parameter. If we use the lower, but more uncertain value a = 0.025 pc, the central density estimate becomes greater than 10^7 Msun/pc^3. A mechanism based on the destruction of a large number of circumstellar disks is posited to explain the hitherto unexplained increase in reddening presented in that same work.Comment: 6 pages, 2 figure

HII Galaxies as deep cosmological probes

We re-investigated the use of the Hubble diagram to measure the cosmological constant ($\Lambda$) and the mass density of the Universe ($\Omega_M$). We find an important focusing effect in $\Lambda$ for redshifts about 3. This effect implies that the apparent magnitude of a standard candle at redshifts z=2-3 has almost no dependence on $\Lambda$ for $\Omega_M>0.2$. This means that $\Omega_M$ can be measured independently of $\Omega_{\Lambda}$ by targeting the redshift range according to an estimate of the value of $\Omega_M$. We explore the evidence in support of the suggestion that extreme starburst galaxies also known as HII galaxies can be used as distance estimators over a wide range of redshifts and reaching very high values. We have compiled literature data of HII galaxies up to $z\sim 3$ and found a good correlation between their luminosity and velocity dispersion measured from their strong emission lines, thus confirming the correlation already known to exist for HII galaxies in the nearby Universe. Several systematic effects such as age, extinction, kinematics, and metallicity are discussed as well as the effects of different cosmologies.Comment: To appear in MNRAS, 7 pages, 6 figure

The Stellar Populations of HII galaxies: A tale of three bursts

We present a UV to mid-IR spectral energy distribution study of a large sample of SDSS DR13 HII galaxies. These are selected as starburst (EW(H$\alpha) > 50$\AA) and for their high excitation locus in the upper-left region of the BPT diagram. Their photometry was derived from the cross-matched GALEX, SDSS, UKDISS and WISE catalogues. We have used CIGALE modelling and SED fitting routine with the parametrization of a three burst star formation history, and a comprehensive analysis of all other model parameters. We have been able to estimate the contribution of the underlying old stellar population to the observed equivalent width of H$\beta$ and allow for more accurate burst age determination. We found that the star formation histories of HII Galaxies can be reproduced remarkably well by three major eras of star formation. In addition, the SED fitting results indicate that: i) in all cases the current burst produces less than a few percent of the total stellar mass: the bulk of stellar mass in HII galaxies have been produced by the past episodes of star formation; ii) at a given age the H$\beta$ luminosity depends only on the mass of young stars favouring a universal IMF for massive stars; iii) the current star formation episodes are {\it maximal} starbursts, producing stars at the highest possible rate.Comment: 12 pages, 14 Figures, Accepted for publication in Astronomy & Astrophysic

Tracking the Biden Agenda on Immigration Enforcement

This report analyzes some of the most consequential changes to immigration enforcement implemented by the Trump administration, the changes that President Biden committed to making during his campaign and transition, and the progress that his administration has achieved in its first 100 days. The report concludes with recommendations for additional changes that the administration should prioritize in working to create a fairer and more humane system of immigration enforcement

Tracking the Biden Agenda on Legal Immigration in the First 100 Days

President Biden assumed office after making considerable commitments to implement changes to legal immigration in the United States, both to reverse harmful changes by former President Trump, but also in reforming and updating the system more broadly. Trump executed prolonged attacks on many categories of immigrants in thinly veiled attempts to limit the number of noncitizens entering the United States temporarily and permanently. These changes created a series of often duplicative barriers impacting the same populations and limiting the ability of many noncitizens to obtain or maintain immigration status in the United States. While the Biden administration has made significant progress in meeting many of its commitments in restoring and reforming legal immigration in the United States, significant barriers to access remain that will need to be addressed for the system to function in a meaningful manner.This special report analyzes some of the most significant changes to immigration policy made by the Trump administration, as well as the subsequent commitments and accomplishments made by the Biden administration on these issues during its first 100 days. The report also provides recommendations for action throughout the remainder of the Biden presidency to foster a fair and efficient system of legal immigration

Towards a Precision Cosmology from Starburst Galaxies at z>2

This work investigates the use of a well-known empirical correlation between the velocity dispersion, metallicity, and luminosity in H beta of nearby HII galaxies to measure the distances to HII-like starburst galaxies at high redshifts. This correlation is applied to a sample of 15 starburst galaxies with redshifts between z=2.17 and z=3.39 to constrain Omega_m, using data available from the literature. A best-fit value of Omega_m = 0.21 +0.30 -0.12 in a Lambda-dominated universe and of Omega_m = 0.11 +0.37 -0.19 in an open universe is obtained. A detailed analysis of systematic errors, their causes, and their effects on the values derived for the distance moduli and Omega_m is carried out. A discussion of how future work will improve constraints on Omega_m by reducing the errors is also presented.Comment: 7 pages, 3 figures, accepted for publication in MNRA

Emission Line Properties of Seyfert 2 Nuclei

We present a study of the global properties of Joguet's sample of 65 Seyfert 2 galaxies. We use the population synthesis models of our Paper II to derive `pure' emission-line spectra, and thus explore the statistical properties of the nuclear nebular components and their relation to the stellar populations. We find that the emission line clouds suffer substantially more extinction than the starlight, and confirm the correlations between stellar and nebular velocity dispersions and between emission line luminosity and velocity dispersions. Nuclear luminosities correlate with stellar velocity dispersions. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation, $L \propto \sigma_\star^4$, consistent with the Faber-Jackson law. Emission line ratios indicative of the gas excitation such as [OIII]/H$\beta$ and [OIII]/[OII] are statistically smaller for Seyferts with significant star-formation, implying that ionization by massive stars is responsible for a substantial, and sometimes even a dominant, fraction of the H$\beta$ and [OII] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden AGN. We correlate this fraction with classical indicators of AGN photo-ionization: X-ray luminosity and nebular excitation, but find no significant correlations. Thus, while there is a strong contribution of starbursts to the excitation of the nuclear nebular emission in low-luminosity Seyferts, the contribution of the hidden AGN remains elusive even in hard X-rays. (abridged)Comment: 12 pages, 13 figures. Final version accepted for publication in MNRA