58 research outputs found
Fine-Structure FeII* Emission and Resonant MgII Emission in z = 1 Star-Forming Galaxies
We present a study of the prevalence, strength, and kinematics of ultraviolet
FeII and MgII emission lines in 212 star-forming galaxies at z = 1 selected
from the DEEP2 survey. We find FeII* emission in composite spectra assembled on
the basis of different galaxy properties, indicating that FeII* emission is
prevalent at z = 1. In these composites, FeII* emission is observed at roughly
the systemic velocity. At z = 1, we find that the strength of FeII* emission is
most strongly modulated by dust attenuation, and is additionally correlated
with redshift, star-formation rate, and [OII] equivalent width, such that
systems at higher redshifts with lower dust levels, lower star-formation rates,
and larger [OII] equivalent widths show stronger FeII* emission. We detect MgII
emission in at least 15% of the individual spectra and we find that objects
showing stronger MgII emission have higher specific star-formation rates,
smaller [OII] linewidths, larger [OII] equivalent widths, lower dust
attenuations, and lower stellar masses than the sample as a whole. MgII
emission strength exhibits the strongest correlation with specific
star-formation rate, although we find evidence that dust attenuation and
stellar mass also play roles in the regulation of MgII emission. Future
integral field unit observations of the spatial extent of FeII* and MgII
emission in galaxies with high specific star-formation rates, low dust
attenuations, and low stellar masses will be important for probing the
morphology of circumgalactic gas.Comment: 29 pages, 22 figures, 2 tables; accepted to Ap
The Relationship between Stellar Populations and Lyα Emission in Lyman Break Galaxies
We present the results of a photometric and spectroscopic survey of 321 Lyman break galaxies (LBGs) at z ~ 3 to investigate systematically the relationship between Lyα emission and stellar populations. Lyα equivalent widths (W_(Lyα)) were calculated from rest-frame UV spectroscopy and optical/near-infrared/Spitzer photometry was used in population synthesis modeling to derive the key properties of age, dust extinction, star formation rate (SFR), and stellar mass. We directly compare the stellar populations of LBGs with and without strong Lyα emission, where we designate the former group (W_(Lyα) ≥ 20 Å) as Lyα emitters (LAEs) and the latter group (W_(Lyα) < 20 Å) as non-LAEs. This controlled method of comparing objects from the same UV luminosity distribution represents an improvement over previous studies in which the stellar populations of LBGs and narrowband-selected LAEs were contrasted, where the latter were often intrinsically fainter in broadband filters by an order of magnitude simply due to different selection criteria. Using a variety of statistical tests, we find that Lyα equivalent width and age, SFR, and dust extinction, respectively, are significantly correlated in the sense that objects with strong Lyα emission also tend to be older, lower in SFR, and less dusty than objects with weak Lyα emission, or the line in absorption. We accordingly conclude that, within the LBG sample, objects with strong Lyα emission represent a later stage of galaxy evolution in which supernovae-induced outflows have reduced the dust covering fraction. We also examined the hypothesis that the attenuation of Lyα photons is lower than that of the continuum, as proposed by some, but found no evidence to support this picture
An HST/WFC3-IR Morphological Survey of Galaxies at z = 1.5-3.6: II. The Relation between Morphology and Gas-Phase Kinematics
We analyze rest-frame optical morphologies and gas-phase kinematics as traced
by rest-frame far-UV and optical spectra for a sample of 204 star forming
galaxies in the redshift range z ~ 2-3 drawn from the Keck Baryonic Structure
Survey (KBSS). We find that spectroscopic properties and gas-phase kinematics
are closely linked to morphology: compact galaxies with semi-major axis radii r
<~ 2 kpc are substantially more likely than their larger counterparts to
exhibit LyA in emission. Although LyA emission strength varies widely within
galaxies of a given morphological type, all but one of 19 galaxies with LyA
equivalent width W_LyA > 20 Angstroms have compact and/or multiple-component
morphologies with r <= 2.5 kpc. The velocity structure of absorption lines in
the galactic continuum spectra also varies as a function of morphology.
Galaxies of all morphological types drive similarly strong outflows (as traced
by the blue wing of interstellar absorption line features), but the outflows of
larger galaxies are less highly ionized and exhibit larger optical depth at the
systemic redshift that may correspond to a decreasing efficiency of feedback in
evacuating gas from the galaxy. This v ~ 0 km/s gas is responsible both for
shifting the mean absorption line redshift and attenuating W_LyA (via a longer
resonant scattering path) in galaxies with larger rest-optical half light
radii. In contrast to galaxies at lower redshifts, there is no evidence for a
correlation between outflow velocity and inclination, suggesting that outflows
from these puffy and irregular systems may be poorly collimated. (Abbrev.)Comment: 18 pages, 11 figures. Revised version accepted for publication in
ApJ. Version with full-resolution figures is available at
http://di.utoronto.ca/~drlaw/Papers/wfc3_uvspec.pd
Gas Accretion Traced in Absorption in Galaxy Spectroscopy
The positive velocity shift of absorption transitions tracing diffuse
material observed in a galaxy spectrum is an unambiguous signature of gas flow
toward the host system. Spectroscopy probing, e.g., NaI D resonance lines in
the rest-frame optical or MgII and FeII in the near-ultraviolet is in principle
sensitive to the infall of cool material at temperatures ~ 100-10,000 K
anywhere along the line of sight to a galaxy's stellar component. However,
secure detections of this redshifted absorption signature have proved
challenging to obtain due to the ubiquity of cool gas outflows giving rise to
blueshifted absorption along the same sightlines. In this chapter, we review
the bona fide detections of this phenomenon. Analysis of NaI D line profiles
has revealed numerous instances of redshifted absorption observed toward
early-type and/or AGN-host galaxies, while spectroscopy of MgII and FeII has
provided evidence for ongoing gas accretion onto >5% of luminous, star-forming
galaxies at z ~ 0.5-1. We then discuss the potentially ground-breaking benefits
of future efforts to improve the spectral resolution of such studies, and to
leverage spatially-resolved spectroscopy for new constraints on inflowing gas
morphology.Comment: 21 pages, 7 figures. Invited review to appear in Gas Accretion onto
Galaxies, Astrophysics and Space Science Library, eds. A. J. Fox & R. Dav\'e,
to be published by Springe
Diffuse Lyman Alpha Emitting Halos: A Generic Property of High Redshift Star Forming Galaxies
Using a sample of 92 UV continuum-selected, spectroscopically identified
galaxies with = 2.65, all of which have been imaged in the Ly-a line with
extremely deep narrow-band imaging, we examine galaxy Ly-a emission profiles to
very faint surface brightness limits. The galaxies are representative of
spectroscopic samples of LBGs at similar redshifts in terms of apparent
magnitude, UV luminosity, inferred extinction, and star formation rate, and
were selected without regard to Ly-a emission properties. We use extremely deep
stacks of UV continuum and Ly-a emission line images to show that all
sub-samples exhibit diffuse Ly-a emission to radii of at least 10" (80 physical
kpc), including galaxies whose spectra exhibit Ly-a in net absorption. The
intensity scaling, but not the surface brightness distribution, is strongly
correlated with the emission observed in the central ~1". The characteristic
scale length for Ly-a line emission exceeds that of the UV continuum light for
the same galaxies by factors of at least 5-10, regardless of the spectral
morphology of Ly-a. Including the extended Ly-a halos increases the total Ly-a
flux [and rest equivalent width W_0(Lya)] by an average factor of 5. We argue
that most, if not all, of the observed Ly-a emission in the diffuse halos
originates in the galaxy H II regions and is scattered in our direction by H I
gas in the galaxy's circum-galactic medium (CGM). We show that whether or not a
galaxy is classified as a giant "Lyman-a Blob" (LAB) depends sensitively on the
Ly-a surface brightness threshold reached by an observation. Accounting for
diffuse Ly-a halos, all LBGs would be LABs if surveys were routinely sensitive
to 10 times lower surface brightness thresholds; also, essentially all LBGs
would qualify as LAEs (W_0(Lya) > 20 A).Comment: Updated to match final version to appear in ApJ; 20 pages, 14 figure
Physical Conditions in a Young, Unreddened, Low Metallicity Galaxy at High Redshift
Increasingly large samples of galaxies are now being discovered at redshifts
z~5-6 and higher. Many of these objects are inferred to be young, low in mass,
and relatively unreddened, but detailed analysis of their high quality spectra
will not be possible until the advent of future facilities. In this paper we
shed light on the physical conditions in a plausibly similar low mass galaxy by
presenting the analysis of the rest-frame optical and UV spectra of
Q2343-BX418, an L* galaxy at z=2.3 with a very low mass-to-light ratio and
unusual properties: BX418 is young (<100 Myr), low mass (M_star ~ 10^9 Msun),
low in metallicity (Z ~ 1/6 Zsun), and unreddened (E(B-V)~0.02, UV continuum
slope beta=-2.1). We infer a metallicity 12+log(O/H)=7.9 +/- 0.2 from the
rest-frame optical emission lines. We also determine the metallicity via the
direct, electron temperature method, using the ratio O III] 1661, 1666/[O III]
5007 to determine the electron temperature and finding 12+ log(O/H)=7.8 +/-
0.1. These measurements place BX418 among the most metal-poor galaxies observed
in emission at high redshift. The rest-frame UV spectrum contains strong
emission from Lya (with rest-frame equivalent width 54 A), He II 1640 (both
stellar and nebular), C III] 1907, 1909 and O III] 1661, 1666. The C IV/C III]
ratio indicates that the source of ionization is unlikely to be an AGN.
Analysis of the He II, O III] and C III] line strengths indicates a very high
ionization parameter log U ~ -1, while Lya and the interstellar absorption
lines indicate that outflowing gas is highly ionized over a wide range of
velocities. It remains to be determined how many of BX418's unique spectral
features are due to its global properties, such as low metallicity and dust
extinction, and how many are indicative of a short-lived phase in the early
evolution of an otherwise normal star-forming galaxy.Comment: Accepted for publication in ApJ. 28 pages, 14 figure
Observational Diagnostics of Gas Flows: Insights from Cosmological Simulations
Galactic accretion interacts in complex ways with gaseous halos, including
galactic winds. As a result, observational diagnostics typically probe a range
of intertwined physical phenomena. Because of this complexity, cosmological
hydrodynamic simulations have played a key role in developing observational
diagnostics of galactic accretion. In this chapter, we review the status of
different observational diagnostics of circumgalactic gas flows, in both
absorption (galaxy pair and down-the-barrel observations in neutral hydrogen
and metals; kinematic and azimuthal angle diagnostics; the cosmological column
density distribution; and metallicity) and emission (Lya; UV metal lines; and
diffuse X-rays). We conclude that there is no simple and robust way to identify
galactic accretion in individual measurements. Rather, progress in testing
galactic accretion models is likely to come from systematic, statistical
comparisons of simulation predictions with observations. We discuss specific
areas where progress is likely to be particularly fruitful over the next few
years.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dave, to be published by
Springer. Typos correcte
Ice Chemistry Through Quiescent Dust In IC5146
IC 5146 is a nearby (200pc) dark cloud complex in Cygnus. The lack of star formation activity makes it an excellent laboratory for the study of the chemical complexity in the earliest stages of dense molecular cloud evolution. We have used the Spitzer Infrared Spectrometer (IRS) to probe dust along 10 sight-lines toward K-Giant background stars, sampling a range of visual extinction from 2-20mag. Here we present 5-20micron spectra and correlation studies of the 6.0micron water-ice band and 9.7micron silicate absorption band with Av for a sample of our Spitzer program sources. Our IC5146 Spitzer data indicate grain growth and ice formation occurs early in the history of dense cloud formation. Each sight-line observed reveals the 9.7micron amorphous silicate absorption band. The highest Av sightlines show clear detections of ices at 6.0micron (water-ice mixture), 6.85micron (processed ice) and 15.2micron (CO2). However, sight-lines in the low-to-mid Av range provide intriguing variations. We may have the first example of two objects, one with ice features and one without, seen through the same cloud sightline with similar Av approx. 6 and similar silicate band optical depths. Also, the nominally expected linear correlation of Av with silicate band depth does not appear to hold for this cloud (turnover at Av approx.10-12). Both trends imply complexities in the grain growth at one of the earliest stages of dust and ice interaction ever observed. The highest extinction source in our sample, Av=20, reveals the 6.0micron (water), 6.85micron (processed ice), 9.7micron silicate and 15.2micron CO2 ice bands. Until recently, the 6.85micron band had only been detected towards embedded protostellar objects. Two additional quiescent dust sightlines from the C2D results now also indicate the 6.85micron band (Taurus and Serpens) by Knez et al. 2005, demonstrating the role of energetic processing within pristine ices prior to the onset of star formation
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