23 research outputs found
Self-consistent predictions for LIER-like emission lines from post-AGB stars
Early type galaxies (ETGs) frequently show emission from warm ionized gas.
These Low Ionization Emission Regions (LIERs) were originally attributed to a
central, low-luminosity active galactic nuclei. However, the recent discovery
of spatially-extended LIER emission suggests ionization by both a central
source and an extended component that follows a stellar-like radial
distribution. For passively-evolving galaxies with old stellar populations, hot
post-Asymptotic Giant Branch (AGB) stars are the only viable extended source of
ionizing photons. In this work, we present the first prediction of LIER-like
emission from post-AGB stars that is based on fully self-consistent stellar
evolution and photoionization models. We show that models where post-AGB stars
are the dominant source of ionizing photons reproduce the nebular emission
signatures observed in ETGs, including LIER-like emission line ratios in
standard optical diagnostic diagrams and H equivalent widths of order
0.1-3 angstroms. We test the sensitivity of LIER-like emission to the details
of post-AGB models, including the mass loss efficiency and convective mixing
efficiency, and show that line strengths are relatively insensitive to post-AGB
timescale variations. Finally, we examine the UV-optical colors of the models
and the stellar populations responsible for the UV-excess observed in some
ETGs. We find that allowing as little as 3% of the HB population to be
uniformly distributed to very hot temperatures (30,000 K) produces realistic UV
colors for old, quiescent ETGs.Comment: ApJ accepted. 20 pages, 8 figure
Physical Properties of II Zw 40's Super Star Cluster and Nebula: New Insights and Puzzles from UV Spectroscopy
We analyze far-ultraviolet spectra and ancillary data of the super star
cluster SSC-N and its surrounding H II region in the nearby dwarf galaxy II Zw
40. From the ultraviolet spectrum, we derive a low internal reddening of E(B-V)
= 0.07 +/- 0.03, a mass of (9.1 +/- 1.0) x 10^5 Lsol, a bolometric luminosity
of (1.1 +/- 0.1) x 10^9 Lsol, a number of ionizing photons of (6 +/- 2) x 10^52
s^-1, and an age of (2.8 +/- 0.1) Myr. These parameters agree with the values
derived from optical and radio data, indicating no significant obscured star
formation, absorption of photons by dust, or photon leakage. SSC-N and its
nebulosity are an order of magnitude more massive and luminous than 30 Doradus
and its ionizing cluster. Photoionization modeling suggests a high ionization
parameter and a C/O ratio where C is between primary and secondary. We
calculate diagnostic emission-line ratios and compare SSC-N to local
star-forming galaxies. The SSC-N nebula does not coincide with the locus
defined by local galaxies. Rather, it coincides with the location of "Green
Pea" galaxies, objects which are often considered nearby analogs of the
galaxies reionizing the universe. Most stellar features are well-reproduced by
synthetic spectra. However, the SSC-N cluster has strong, broad, stellar He II
1640 emission that cannot be reproduced, suggesting a deficit of He-enhanced
stars with massive winds in the models. We discuss possible sources for the
broad He II emission, including very massive stars and/or enhanced mixing
processes.Comment: The Astrophysical Journal, accepte
Comparison of Theoretical Starburst Photoionisation Models for Optical Diagnostics
We study and compare different examples of stellar evolutionary synthesis
input parameters used to produce photoionisation model grids using the MAPPINGS
V modelling code. The aim of this study is to (a) explore the systematic
effects of various stellar evolutionary synthesis model parameters on the
interpretation of emission lines in optical strong-line diagnostic diagrams,
(b) characterise the combination of parameters able to reproduce the spread of
local galaxies located in the star-forming region in the Sloan Digital Sky
Survey, and (c) investigate the emission from extremely metal-poor galaxies
using photoionisation models. We explore and compare the stellar input ionising
spectrum (stellar population synthesis code [Starburst99, SLUG, BPASS], stellar
evolutionary tracks, stellar atmospheres, star-formation history, sampling of
the initial mass function) as well as parameters intrinsic to the H II region
(metallicity, ionisation parameter, pressure, H II region boundedness). We also
perform a comparison of the photoionisation codes MAPPINGS and CLOUDY. On the
variations in the ionising spectrum model parameters, we find that the
differences in strong emission-line ratios between varying models for a given
input model parameter are small, on average ~0.1 dex. An average difference of
~0.1 dex in emission-line ratio is also found between models produced with
MAPPINGS and CLOUDY. Large differences between the emission-line ratios are
found when comparing intrinsic H II region parameters. We find that
low-metallicity galaxies are better explained by a density-bounded H II region
and higher pressures better encompass the spread of galaxies at high redshift.Comment: 33 pages, 26 figures, accepted for publication in Ap
Physical Properties of II Zw 40's Super Star Cluster and Nebula: New Insights and Puzzles from UV Spectroscopy
We analyze far-ultraviolet spectra and ancillary data of the super star cluster SSC-N and its surrounding H ii region in the nearby dwarf galaxy II Zw 40. From the ultraviolet spectrum, we derive a low internal reddening of E(B − V) = 0.07 ± 0.03, a mass of (9.1 ± 1.0) × 10^5 M_⊙, a bolometric luminosity of (1.1 ± 0.1) × 10^9 L_⊙, a number of ionizing photons of (6 ± 2) × 10^(52) s^(−1), and an age of (2.8 ± 0.1) Myr. These parameters agree with the values derived from optical and radio data, indicating no significant obscured star formation, absorption of photons by dust, or photon leakage. SSC-N and its nebulosity are an order of magnitude more massive and luminous than 30 Doradus and its ionizing cluster. Photoionization modeling suggests a high ionization parameter and a C/O ratio where C is between primary and secondary. We calculate diagnostic emission-line ratios and compare SSC-N to local star-forming galaxies. The SSC-N nebula does not coincide with the locus defined by local galaxies. Rather, it coincides with the location of "Green Pea" galaxies, objects that are often considered nearby analogs of the galaxies reionizing the universe. Most stellar features are well reproduced by synthetic spectra. However, the SSC-N cluster has strong, broad, stellar He ii λ1640 emission that cannot be reproduced, suggesting a deficit of He-enhanced stars with massive winds in the models. We discuss possible sources for the broad He ii emission, including very massive stars and/or enhanced mixing processes
A Comparison of UV and Optical Metallicities in Star-forming Galaxies
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 Å/C III] λ1906,1909 Å versus [O III] λ1666 Å/C III] λ1906,9 Å) is a reliable metallicity tracer, particularly at low metallicity (12 log O H 8 + 10( ), where stellar contributions are minimal. We find that the Si3-O3C3 diagnostic ([Si III] λ1883 Å/C III] λ1906 Å versus [O III] λ1666 Å/C III] λ1906,9 Å) 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 Å/[O III] λ 1666 Å versus [O III] λ 1666 Å/C III] λ 1906,9 Å) 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
The BPT Diagram in Cosmological Galaxy Formation Simulations: Understanding the Physics Driving Offsets at High Redshift
© 2022. The Author(s). Published by the American Astronomical Society This work is licenced under Creative Commons Attribution 4.0 licence. https://creativecommons.org/licenses/by/4.0/The Baldwin, Philips, & Terlevich diagram of [O iii]/Hβ versus [N ii]/Hα (hereafter N2-BPT) has long been used as a tool for classifying galaxies based on the dominant source of ionizing radiation. Recent observations have demonstrated that galaxies at z ∼ 2 reside offset from local galaxies in the N2-BPT space. In this paper, we conduct a series of controlled numerical experiments to understand the potential physical processes driving this offset. We model nebular line emission in a large sample of galaxies, taken from the simba cosmological hydrodynamic galaxy formation simulation, using the cloudy photoionization code to compute the nebular line luminosities from H ii regions. We find that the observed shift toward higher [O iii]/Hβ and [N ii]/Hα values at high redshift arises from sample selection: when we consider only the most massive galaxies M * ∼ 1010–11 M ⊙, the offset naturally appears, due to their high metallicities. We predict that deeper observations that probe lower-mass galaxies will reveal galaxies that lie on a locus comparable to z ∼ 0 observations. Even when accounting for samples-selection effects, we find that there is a subtle mismatch between simulations and observations. To resolve this discrepancy, we investigate the impact of varying ionization parameters, H ii region densities, gas-phase abundance patterns, and increasing radiation field hardness on N2-BPT diagrams. We find that either decreasing the ionization parameter or increasing the N/O ratio of galaxies at fixed O/H can move galaxies along a self-similar arc in N2-BPT space that is occupied by high-redshift galaxies.Peer reviewedFinal Published versio
Overview of the SDSS-IV MaNGA Survey: Mapping Nearby Galaxies at Apache Point Observatory
We present an overview of a new integral field spectroscopic survey called MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV) that began on 2014 July 1. MaNGA will investigate the internal kinematic structure and composition of gas and stars in an unprecedented sample of 10,000 nearby galaxies. We summarize essential characteristics of the instrument and survey design in the context of MaNGA\u27s key science goals and present prototype observations to demonstrate MaNGA\u27s scientific potential. MaNGA employs dithered observations with 17 fiber-bundle integral field units that vary in diameter from 12\u27\u27 (19 fibers) to 32\u27\u27 (127 fibers). Two dual-channel spectrographs provide simultaneous wavelength coverage over 3600-10300 Å at R ~ 2000. With a typical integration time of 3 hr, MaNGA reaches a target r-band signal-to-noise ratio of 4-8 (Å–1 per 2\u27\u27 fiber) at 23 AB mag arcsec–2, which is typical for the outskirts of MaNGA galaxies. Targets are selected with M * ≳ 109 M ☉ using SDSS-I redshifts and i-band luminosity to achieve uniform radial coverage in terms of the effective radius, an approximately flat distribution in stellar mass, and a sample spanning a wide range of environments. Analysis of our prototype observations demonstrates MaNGA\u27s ability to probe gas ionization, shed light on recent star formation and quenching, enable dynamical modeling, decompose constituent components, and map the composition of stellar populations. MaNGA\u27s spatially resolved spectra will enable an unprecedented study of the astrophysics of nearby galaxies in the coming 6 yr