To evaluate the impact of stellar feedback, it is critical to estimate the mass outflow rates of galaxies. Past estimates have been plagued by uncertain assumptions about the outflow geometry, ...metallicity, and ionization fraction. Here we use Hubble Space Telescope ultraviolet spectroscopic observations of the nearby starburst NGC 6090 to demonstrate that many of these quantities can be constrained by the data. We use the Si iv absorption lines to calculate the scaling of velocity (v), covering fraction (C
f), and density with distance from the starburst (r), assuming the Sobolev optical depth and a velocity law of the form: v ∝ (1 − R
i/r)β (where R
i is the inner outflow radius). We find that the velocity (β = 0.43) is consistent with an outflow driven by an r
−2 force with the outflow radially accelerated, while the scaling of the covering fraction (C
f ∝ r
−0.82) suggests that cool clouds in the outflow are in pressure equilibrium with an adiabatically expanding medium. We use the column densities of four weak metal lines and cloudy photoionization models to determine the outflow metallicity, the ionization correction, and the initial density of the outflow. Combining these values with the profile fitting, we find R
i = 63 pc, with most of the mass within 300 pc of the starburst. Finally, we find that the maximum mass outflow rate is 2.3 M⊙ yr−1 and the mass-loading factor (outflow divided by the star formation rate) is 0.09, a factor of 10 lower than the value calculated using common assumptions for the geometry, metallicity, and ionization structure of the outflow.
Abstract
Galactic outflows are believed to play an important role in regulating star formation in galaxies, but estimates of the outflowing mass and momentum have historically been based on uncertain ...assumptions. Here, we measure the mass, momentum and energy outflow rates of seven nearby star-forming galaxies using ultraviolet absorption lines and observationally motivated estimates for the density, metallicity, and radius of the outflow. Low-mass galaxies generate outflows faster than their escape velocities with mass outflow rates up to twenty times larger than their star formation rates. These outflows from low-mass galaxies also have momenta larger than provided from supernovae alone, indicating that multiple momentum sources drive these outflows. Only 1–20 per cent of the supernovae energy is converted into kinetic energy, and this fraction decreases with increasing stellar mass, such that low-mass galaxies drive more efficient outflows. We find scaling relations between the outflows and the stellar mass of their host galaxies (M*) at the 2–3σ significance level. The mass-loading factor, or the mass outflow rate divided by the star formation rate, scales as M
$_\ast ^{-0.4}$
and with the circular velocity as v
$_\mathrm{circ}^{-1.6}$
. The scaling of the mass-loading factor is similar to recent simulations, but the observations are a factor of 5 smaller, possibly indicating that there is a substantial amount of unprobed gas in a different ionization phase. The outflow momenta are consistent with a model where star formation drives the outflow while gravity counteracts this acceleration.
We study the ionization structure of galactic outflows in 37 nearby, star-forming galaxies with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We use the O i, Si ii, Si iii, and Si iv ...ultraviolet absorption lines to characterize the different ionization states of outflowing gas. We measure the equivalent widths, line widths, and outflow velocities of the four transitions, and find shallow scaling relations between them and galactic stellar mass and star formation rate. Regardless of the ionization potential, lines of similar strength have similar velocities and line widths, indicating that the four transitions can be modelled as a comoving phase. The Si equivalent width ratios (e.g. Si iv/Si ii) have low dispersion, and little variation with stellar mass; while ratios with O i and Si vary by a factor of 2 for a given stellar mass. Photoionization models reproduce these equivalent width ratios, while shock models under predict the relative amount of high ionization gas. The photoionization models constrain the ionization parameter (U) between −2.25 < log (U) < −1.5, and require that the outflow metallicities are greater than 0.5 Z⊙. We derive ionization fractions for the transitions, and show that the range of ionization parameters and stellar metallicities leads to a factor of 1.15–10 variation in the ionization fractions. Historically, mass outflow rates are calculated by converting a column density measurement from a single metal ion into a total hydrogen column density using an ionization fraction, thus mass outflow rates are sensitive to the assumed ionization structure of the outflow.
Detailed modeling of the recent star formation histories (SFHs) of post-starburst (or "E+A") galaxies is impeded by the degeneracy between the time elapsed since the starburst ended (post-burst age), ...the fraction of stellar mass produced in the burst (burst strength), and the burst duration. To resolve this issue, we combine GALEX ultraviolet photometry, SDSS photometry and spectra, and new stellar population synthesis models to fit the SFHs of 532 post-starburst galaxies. In addition to an old stellar population and a recent starburst, 48% of the galaxies are best fit with a second recent burst. Lower stellar mass galaxies (log M /M☉ < 10.5) are more likely to experience two recent bursts, and the fraction of their young stellar mass is more strongly anticorrelated with their total stellar mass. Applying our methodology to other, younger post-starburst samples, we identify likely progenitors to our sample and examine the evolutionary trends of molecular gas and dust content with post-burst age. We discover a significant (4 ) decline, with a 117-230 Myr characteristic depletion time, in the molecular gas to stellar mass fraction with the post-burst age. The implied rapid gas depletion rate of 2-150 M☉ yr−1 cannot be due to current star formation, given the upper limits on the current star formation rates in these post-starbursts. Nor are stellar winds or supernova feedback likely to explain this decline. Instead, the decline points to the expulsion or destruction of molecular gas in outflows, a possible smoking gun for active galactic nucleus feedback.
Abstract
We study the gas phase metallicity (O/H) and nitrogen abundance gradients traced by star-forming regions in a representative sample of 550 nearby galaxies in the stellar mass range ...109–1011.5 M⊙ with resolved spectroscopic data from the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory survey. Using strong-line ratio diagnostics (R23 and O3N2 for metallicity and N2O2 for N/O) and referencing to the effective (half-light) radius (Re), we find that the metallicity gradient steepens with stellar mass, lying roughly flat among galaxies with log (M⋆/M⊙) = 9.0 but exhibiting slopes as steep as −0.14 dex $R_{\rm e}^{-1}$ at log (M⋆/M⊙) = 10.5 (using R23, but equivalent results are obtained using O3N2). At higher masses, these slopes remain typical in the outer regions of our sample (R > 1.5Re), but a flattening is observed in the central regions (R < 1Re). In the outer regions (R > 2.0Re), we detect a mild flattening of the metallicity gradient in stacked profiles, although with low significance. The N/O ratio gradient provides complementary constraints on the average chemical enrichment history. Unlike the oxygen abundance, the average N/O profiles do not flatten out in the central regions of massive galaxies. The metallicity and N/O profiles both depart significantly from an exponential form, suggesting a disconnect between chemical enrichment and stellar mass surface density on local scales. In the context of inside-out growth of discs, our findings suggest that central regions of massive galaxies today have evolved to an equilibrium metallicity, while the nitrogen abundance continues to increase as a consequence of delayed secondary nucleosynthetic production.
We utilize Sloan Digital Sky Survey imaging and spectroscopy of 653,000 star-forming galaxies at z60.1 to study the relation between stellar mass and gas-phase metallicity. We derive gas-phase oxygen ...abundances and stellar masses using new techniques that make use of the latest stellar evolutionary synthesis and photoionization models. We find a tight (c0.1 dex) correlation between stellar mass and metallicity spanning over 3 orders of magnitude in stellar mass and a factor of 10 in metallicity. The relation is relatively steep from 10 super(8.5) to 10 super(10.5) M sub( )h70 super(-2), in good accord with known trends between luminosity and metallicity, but flattens above 10 super(10.5) M sub( ). We use indirect estimates of the gas mass based on the Ha luminosity to compare our data to predictions from simple closed box chemical evolution models. We show that metal loss is strongly anticorrelated with baryonic mass, with low-mass dwarf galaxies being 5 times more metal depleted than L* galaxies at z60.1. Evidence for metal depletion is not confined to dwarf galaxies but is found in galaxies with masses as high as 10 super(10) M sub( ). We interpret this as strong evidence of both the ubiquity of galactic winds and their effectiveness in removing metals from galaxy potential wells.
SDSS-IV MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) is the largest integral-field unit (IFU) spectroscopy survey to date, aiming to observe a statistically representative sample of ...10,000 low-redshift galaxies. In this paper, we study the reliability of the emission-line fluxes and kinematic properties derived by the MaNGA Data Analysis Pipeline (DAP). We describe the algorithmic choices made in the DAP with regards to measuring emission-line properties, and the effect of our adopted strategy of simultaneously fitting the continuum and line emission. The effects of random errors are quantified by studying various fit-quality metrics, idealized recovery simulations, and repeat observations. This analysis demonstrates that the emission lines are well fit in the vast majority of the MaNGA data set and the derived fluxes and errors are statistically robust. The systematic uncertainty on emission-line properties introduced by the choice of continuum templates is also discussed. In particular, we test the effect of using different stellar libraries and simple stellar-population models on the derived emission-line fluxes and the effect of introducing different tying prescriptions for the emission-line kinematics. We show that these effects can generate large (>0.2 dex) discrepancies at low signal-to-noise ratio and for lines with low equivalent width (EW); however, the combined effect is noticeable even for H EW > 6 . We provide suggestions for optimal use of the data provided by SDSS data release 15 and propose refinements on the DAP for future MaNGA data releases.
We present intermediate-resolution optical spectrophotometry of 65 galaxies obtained in support of the Spitzer Infrared Nearby Galaxies Survey (SINGS). For each galaxy we obtain a nuclear, ...circumnuclear, and semi-integrated optical spectrum designed to coincide spatially with mid- and far-infrared spectroscopy from the Spitzer Space Telescope. We make the reduced, spectrophotometrically calibrated one-dimensional spectra, as well as measurements of the fluxes and equivalent widths of the strong nebular emission lines, publically available. We use optical emission-line ratios measured on all three spatial scales to classify the sample into star-forming, active galactic nuclei (AGNs), and galaxies with a mixture of star formation and nuclear activity. We find that the relative fraction of the sample classified as star forming versus AGN is a strong function of the integrated light enclosed by the spectroscopic aperture. We supplement our observations with a large database of nebular emission-line measurements of individual H II regions in the SINGS galaxies culled from the literature. We use these ancillary data to conduct a detailed analysis of the radial abundance gradients and average H II-region abundances of a large fraction of the sample. We combine these results with our new integrated spectra to estimate the central and characteristic (globally averaged) gas-phase oxygen abundances of all 75 SINGS galaxies. We conclude with an in-depth discussion of the absolute uncertainty in the nebular oxygen abundance scale.
Abstract
We investigate galactic winds in the HizEA galaxies, a collection of 46 late-stage galaxy mergers at
z
= 0.4–0.8, with stellar masses of
log
(
M
*
/
M
⊙
)
=
10.4
–
11.5
, star formation ...rates (SFRs) of 20–500
M
⊙
yr
−1
, and ultra-compact (a few 100 pc) central star-forming regions. We measure their gas kinematics using the Mg
ii
λ
λ
2796,2803 absorption lines in optical spectra from MMT, Magellan, and Keck. We find evidence of outflows in 90% of targets, with maximum outflow velocities of 550–3200 km s
−1
. We combine these data with ten samples from the literature to construct scaling relations for outflow velocity versus SFR, star formation surface density (Σ
SFR
),
M
*
, and SFR/
M
*
. The HizEA galaxies extend the dynamic range of the scaling relations by a factor of ∼2–4 in outflow velocity and an order of magnitude in SFR and Σ
SFR
. The ensemble scaling relations exhibit strong correlations between outflow velocity, SFR, SFR/
R
, and Σ
SFR
, and weaker correlations with
M
*
and SFR/
M
*
. The HizEA galaxies are mild outliers on the SFR and
M
*
scaling relations, but they connect smoothly with more typical star-forming galaxies on plots of outflow velocity versus SFR/
R
and Σ
SFR
. These results provide further evidence that the HizEA galaxies’ exceptional outflow velocities are a consequence of their extreme star formation conditions rather than hidden black hole activity, and they strengthen previous claims that Σ
SFR
is one of the most important properties governing the velocities of galactic winds.
The Intrinsic Sizes of Odd Radio Circles Rupke, David S. N.; Coil, Alison L.; Whalen, Kelly E. ...
The Astrophysical journal,
05/2024, Volume:
967, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract A new class of sources, the so-called odd radio circles (ORCs), have been discovered by recent sensitive, large-area radio continuum surveys. The distances of these sources have so far ...relied on photometric redshifts of optical galaxies found at the centers of or near ORCs. Here we present Gemini rest-frame optical spectroscopy of six galaxies at the centers of, or potentially associated with, the first five ORC discoveries. We supplement this with Legacy Survey imaging and Prospector fits to their griz +W1/W2 photometry. Of the three ORCs with central galaxies, all lie at distances ( z = 0.27–0.55) that confirm the large intrinsic diameters of the radio circles (300–500 kpc). The central galaxies are massive ( M * ∼ 10 11 M ☉ ), red, unobscured ellipticals with old (≳1 Gyr) stellar populations. They have LINER spectral types that are shock-powered or active galactic nucleus (AGN)-powered. All three host low-luminosity, radio-quiet AGN. The similarity of their central galaxies is consistent with a common origin, perhaps as a blast wave from an ancient starburst. The other two ORCs are adjacent and have no prominent central galaxies. However, the z = 0.25 disk galaxy that lies between them hosts a Type 2, moderate-luminosity AGN. They may instead be the lobes of a radio jet from this AGN.
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