Abstract
Galaxy formation and evolution are regulated by the feedback from galactic winds. Absorption lines provide the most widely available probe of winds. However, since most data only provide ...information integrated along the line of sight, they do not directly constrain the radial structure of the outflows. In this paper, we present a method to directly measure the gas electron density in outflows (
n
e
), which in turn yields estimates of outflow cloud properties (e.g., density, volume filling factor, and sizes/masses). We also estimate the distance (
r
n
) from the starburst at which the observed densities are found. We focus on 22 local star-forming galaxies primarily from the COS Legacy Archive Spectroscopic SurveY (CLASSY). In half of them, we detect absorption lines from fine-structure excited transitions of Si
ii
(i.e., Si
ii
*). We determine
n
e
from relative column densities of Si
ii
and Si
ii
*, given Si
ii
* originates from collisional excitation by free electrons. We find that the derived
n
e
correlates well with the galaxy’s star formation rate per unit area. From photoionization models or assuming the outflow is in pressure equilibrium with the wind fluid, we get
r
n
∼ 1–2
r
*
or ∼5
r
*
, respectively, where
r
*
is the starburst radius. Based on comparisons to theoretical models of multiphase outflows, nearly all of the outflows have cloud sizes large enough for the clouds to survive their interaction with the hot wind fluid. Most of these measurements are the first ever for galactic winds detected in absorption lines and, thus, will provide important constraints for future models of galactic winds.
Abstract
We present a new selection of 358 blue compact dwarf galaxies (BCDs) from 5000 square degrees in the Dark Energy Survey, and the spectroscopic follow-up of a subsample of 68 objects. For the ...subsample of 34 objects with deep spectra, we measure the metallicity via the direct
T
e
method using the auroral O
iii
λ
4363 emission line. These BCDs have an average oxygen abundance of 12+log(O/H) = 7.8, with stellar masses between 10
7
and 10
8
M
⊙
and specific star-formation rates between ∼10
−9
and 10
−7
yr
−1
. We compare the position of our BCDs with the mass–metallicity (
M
–
Z
) and luminosity–metallicity (
L
–
Z
) relation derived from the Local Volume Legacy sample. We find the scatter about the
M
–
Z
relation is smaller than the scatter about the
L
–
Z
relation. We identify a correlation between the offsets from the
M
–
Z
and
L
–
Z
relation that we suggest is due to the contribution of metal-poor inflows. Finally, we explore the validity of the mass–metallicity–SFR fundamental plane in the mass range probed by our galaxies. We find that BCDs with stellar masses smaller than 10
8
M
⊙
do not follow the extrapolation of the fundamental plane. This result suggests that mechanisms other than the balance between inflows and outflows may be at play in regulating the position of low-mass galaxies in the
M
–
Z
–SFR space.
We present the rest-1500 UV luminosity functions (LF) for star-forming galaxies during the cosmic high noon-the peak of cosmic star formation rate at . We use deep NUV imaging data obtained as part ...of the Hubble Ultra-Violet Ultra Deep Field (UVUDF) program, along with existing deep optical and NIR coverage on the HUDF. We select F225W, F275W, and F336W dropout samples using the Lyman break technique, along with samples in the corresponding redshift ranges selected using photometric redshifts, and measure the rest-frame UV LF at , respectively, using the modified maximum likelihood estimator. We perform simulations to quantify the survey and sample incompleteness for the UVUDF samples to correct the effective volume calculations for the LF. We select galaxies down to and fit a faint-end slope of at , , and , respectively. We compare the star formation properties of galaxies from these UV observations with results from H and UV+IR observations. We find a lack of high-SFR sources in the UV LF compared to the H and UV+IR, likely due to dusty SFGs not being properly accounted for by the generic relation used to correct for dust. We compute a volume-averaged UV-to-H ratio by abundance matching the rest-frame UV LF and H LF. We find an increasing UV-to-H ratio toward low-mass galaxies ( ). We conclude that this could be due to a larger contribution from starbursting galaxies compared to the high-mass end.
Abstract
We present a new constraint on the Lyman continuum (LyC) escape fraction at
. We obtain deep, high sensitivity far-UV imaging with the Advanced Camera for Surveys Solar Blind Channel on the ...Hubble Space Telescope, targeting 11 star-forming galaxies at 1.2 <
z
< 1.4. The galaxies are selected from the 3D-HST survey to have high H
α
equivalent width (EW) with an EW > 190 Å, low stellar mass (
M
*
< 10
10
M
⊙
), and
U
-band magnitude of
U
< 24.2. These criteria identify young, low metallicity star-bursting populations similar to the early star-forming galaxies believed to have reionized the universe. We do not detect any LyC signal (with a signal-to-noise ratio > 3) in the individual galaxies or in the stack in the far-UV images. We place 3
σ
limits on the relative escape fraction of individual galaxies to be
and a stacked 3
σ
limit of
. Measuring various galaxy properties, including stellar mass, dust attenuation, and star formation rate, we show that our measured values fall within the broad range of values covered by the confirmed LyC emitters from the literature. In particular, we compare the distribution of H
α
and O
III
EWs of confirmed LyC emitters and non-detections, including the galaxies in this study. Finally, we discuss if a dichotomy seen in the distribution of H
α
EWs can perhaps distinguish the LyC emitters from the non-detections.
Abstract
Star-forming, H
α
-emitting clumps are found embedded in the gaseous tails of galaxies undergoing intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These ...clumps offer a unique opportunity to study star formation under extreme conditions, in the absence of an underlying disk and embedded within the hot intracluster medium. Yet, a comprehensive, high-spatial-resolution study of these systems is missing. We obtained UVIS/Hubble Space Telescope (HST) data to observe the first statistical sample of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination of broadband (UV to I) filters and a narrowband H
α
filter. HST observations are needed to study the sizes, stellar masses, and ages of the clumps and their clustering hierarchy. These observations will be used to study the clump scaling relations and the universality of the star formation process, and to verify whether a disk is irrelevant, as hinted at by results from jellyfish galaxies. This paper presents the observations, data reduction strategy, and some general results based on the preliminary data analysis. The high spatial resolution of UVIS gives an unprecedentedly sharp view of the complex structure of the inner regions of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails are extremely bright and compact and we did not detect a significant number of star-forming clumps in regions where MUSE did not detect any. The paper finally presents the development plan for the project.
Modern large-scale cosmological simulations model the universe with increasing sophistication and at higher spatial and temporal resolutions. These ongoing enhancements permit increasingly detailed ...comparisons between the simulation outputs and real observational data. Recent projects such as Illustris are capable of producing simulated images that are designed to be comparable to those obtained from local surveys. This paper tests the degree to which Illustris achieves this goal across a diverse population of galaxies using visual morphologies derived from Galaxy Zoo citizen scientists. Morphological classifications provided by these volunteers for simulated galaxies are compared with similar data for a compatible sample of images drawn from the Sloan Digital Sky Survey (SDSS) Legacy Survey. This paper investigates how simple morphological characterization by human volunteers asked to distinguish smooth from featured systems differs between simulated and real galaxy images. Significant differences are identified, which are most likely due to the limited resolution of the simulation, but which could be revealing real differences in the dynamical evolution of populations of galaxies in the real and model universes. Specifically, for stellar masses , a substantially larger proportion of Illustris galaxies that exhibit disk-like morphology or visible substructure, relative to their SDSS counterparts. Toward higher masses, the visual morphologies for simulated and observed galaxies converge and exhibit similar distributions. The stellar mass threshold indicated by this divergent behavior confirms recent works using parametric measures of morphology from Illustris simulated images. When , the Illustris data set contains substantially fewer galaxies that classifiers regard as unambiguously featured. In combination, these results suggest that comparison between the detailed properties of observed and simulated galaxies, even when limited to reasonably massive systems, may be misleading.
The increasing abundance of passive “red-sequence” galaxies since z similar to 1-2 is mirrored by a coincident rise in the number of galaxies with spheroidal morphologies. In this paper, however, we ...show in detail, that, the correspondence between galaxy morphology and color is not perfect, providing insight into the physical origin of this evolution. Using the COSMOS survey, we study a significant population of red-sequence galaxies with disk-like morphologies. These passive disks typically have Sa-Sb morphological types with large bulges, but they are not confined to dense environments. They represent nearly one-half of all red-sequence galaxies and dominate at lower masses (less than or similar to 10(10) M(circle dot)) where they are increasingly disk-dominated. As a function of time, the abundance of passive disks with M(*) less than or similar to 10(11) M(circle dot) increases, but not as fast as red-sequence spheroidals in the same mass range. At higher mass, the passive disk population has declined since z similar to 1, likely because they transform into spheroidals. Based on these trends, we estimate that as much as 60% of galaxies transitioning onto the red sequence evolve through a passive disk phase. The origin of passive disks therefore has broad implications for our understanding of how star formation shuts down. Because passive disks tend to be more bulge-dominated than their star-forming counterparts, a simple fading of blue disks does not fully explain their origin. We explore the strengths and weaknesses of several more sophisticated explanations, including environmental effects, internal stabilization, and disk regrowth during gas-rich mergers. While previous work has sought to explain color and morphological transformations with a single process, these observations open the way to new insight by highlighting the fact that galaxy evolution may actually proceed through several separate stages.
Abstract
Ly
α
line profiles are a powerful probe of interstellar medium (ISM) structure, outflow speed, and Lyman-continuum escape fraction. In this paper, we present the Ly
α
line profiles of the ...Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Ly
α
emission profile in the bottom of a damped, Ly
α
absorption trough. Such profiles reveal an inhomogeneous ISM. We successfully fit the damped Ly
α
absorption and the Ly
α
emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Ly
α
exchange between high-
N
H
i
and low-
N
H
i
paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Ly
α
peak separation and the O
iii
/O
ii
flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Ly
α
peak separation decreases. We combine measurements of Ly
α
peak separation and Ly
α
red peak asymmetry in a diagnostic diagram, which identifies six Lyman-continuum leakers in the COS Legacy Archive Spectrocopy SurveY (CLASSY) sample. We find a strong correlation between the Ly
α
trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Ly
α
peak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Ly
α
photons outside the spectroscopic aperture reshapes Ly
α
profiles because the distances to these compact starbursts span a large range.
Abstract
Observations of low-ionization state metal lines provide crucial insights into the interstellar medium (ISM) of galaxies, yet, disentangling the physical processes responsible for the ...emerging line profiles is difficult. This work investigates how mock spectra generated using a single galaxy in a radiation-hydrodynamical simulation can help us interpret observations of a real galaxy. We create 22,500 C
ii
and Si
ii
spectra from the virtual galaxy at different times and through multiple lines of sight and compare them with the 45 observations of low-redshift star-forming galaxies from the COS Legacy Spectroscopic SurveY (
classy
). We find that the mock profiles provide accurate replicates of the observations of 38 galaxies with a broad range of stellar masses (10
6
–10
9
M
⊙
) and metallicities (0.02–0.55
Z
⊙
). Additionally, we highlight that aperture losses explain the weakness of the fluorescent emission in several
classy
spectra and must be accounted for when comparing simulations to observations. Overall, we show that the evolution of a single simulated galaxy can produce a large diversity of spectra whose properties are representative of galaxies of comparable or smaller masses. Building upon these results, we explore the origin of the continuum, residual flux, and fluorescent emission in the simulation. We find that these different spectral features all emerge from distinct regions in the galaxy’s ISM, and their characteristics can vary as a function of the viewing angle. While these outcomes challenge simplified interpretations of down-the-barrel spectra, our results indicate that high-resolution simulations provide an optimal framework to interpret these observations.
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