Using the cosmological baryonic accretion rate and normal star formation (SF) efficiencies, we present a very simple model for star-forming galaxies that accounts for the mass and redshift ...dependences of the star formation rate (SFR)-mass and Tully-Fisher (TF) relations from z ∼ 2 to the present. The time evolution follows from the fact that each modeled galaxy approaches a steady state where the SFR follows the (net) cold gas accretion rate. The key feature of the model is a halo mass floor M min 10 11 M below which accretion is quenched in order to simultaneously account for the observed slopes of the SFR-mass and TF relations. The same successes cannot be achieved via an SF threshold (or delay) nor by varying the SF efficiency or the feedback efficiency. Combined with the mass ceiling for cold accretion due to virial shock heating, the mass floor M min explains galaxy "downsizing," where more massive galaxies formed earlier and over a shorter period of time. It turns out that the model also accounts for the observed galactic baryon and gas fractions as a function of mass and time, and the cosmic SFR density, which are all resulting from the mass floor M min. The model helps us to understand that it is the cosmological decline of accretion rate that drives the decrease of cosmic SFR density between z ∼ 2 and z = 0 and the rise of the cosmic SFR density from z ∼ 6 to z ∼ 2 that allows us to put a constraint on our main parameter M min 10 11 M. Among the physical mechanisms that could be responsible for the mass floor, our view is that photoionization feedback (from first in situ hot stars) lowering the cooling efficiency is likely to play a large role.
Galaxies are thought to be fed by the continuous accretion of intergalactic gas, but direct observational evidence has been elusive. The accreted gas is expected to orbit about the galaxy's halo, ...delivering not just fuel for star formation but also angular momentum to the galaxy, leading to distinct kinematic signatures. We report observations showing these distinct signatures near a typical distant star-forming galaxy, where the gas is detected using a background quasar passing 26 kiloparsecs from the host. Our observations indicate that gas accretion plays a major role in galaxy growth because the estimated accretion rate is comparable to the star-formation rate.
Abstract
Background quasars are potentially sensitive probes of galactic outflows provided that one can determine the origin of the absorbing material since both gaseous discs and strong bipolar ...outflows can contribute to the absorption cross-section. Using a dozen quasars passing near spectroscopically identified galaxies at z ∼ 0.1, we find that the azimuthal orientation of the quasar sightlines with strong Mg ii absorption (with ) is bi-modal: about half the Mg ii sightlines are aligned with the major axis and the other half are within α = 30° of the minor axis, suggesting that bipolar outflows can contribute to the Mg ii cross-section. This bi-modality is also present in the instantaneous star formation rates (SFRs) of the hosts. For the sightlines aligned along the minor axis, a simple bi-conical wind model is indeed able to reproduce the observed Mg ii kinematics and the Mg ii dependence with impact parameter b, (). Using our wind model, we can directly extract key wind properties such as the de-projected outflow speed V
out of the cool material traced by Mg ii and the outflow rates . The outflow speeds V
out are found to be 150-300 km s−1, i.e. of the order of the circular velocity, and smaller than the escape velocity by a factor of ∼2. The outflow rates are typically two to three times the instantaneous SFRs. Our results demonstrate how background quasars can be used to measure wind properties with high precision.
Abstract
This paper provides an update of our previous scaling relations between galaxy-integrated molecular gas masses, stellar masses, and star formation rates (SFRs), in the framework of the star ...formation main sequence (MS), with the main goal of testing for possible systematic effects. For this purpose our new study combines three independent methods of determining molecular gas masses from CO line fluxes, far-infrared dust spectral energy distributions, and ∼1 mm dust photometry, in a large sample of 1444 star-forming galaxies between
z
= 0 and 4. The sample covers the stellar mass range log(
M
*
/
M
⊙
) = 9.0–11.8, and SFRs relative to that on the MS,
δ
MS = SFR/SFR(MS), from 10
−1.3
to 10
2.2
. Our most important finding is that all data sets, despite the different techniques and analysis methods used, follow the same scaling trends, once method-to-method zero-point offsets are minimized and uncertainties are properly taken into account. The molecular gas depletion time
t
depl
, defined as the ratio of molecular gas mass to SFR, scales as (1 +
z
)
−0.6
× (
δ
MS)
−0.44
and is only weakly dependent on stellar mass. The ratio of molecular to stellar mass
μ
gas
depends on (
1
+
z
)
2.5
×
(
δ
MS
)
0.52
×
(
M
*
)
−
0.36
, which tracks the evolution of the specific SFR. The redshift dependence of
μ
gas
requires a curvature term, as may the mass dependences of
t
depl
and
μ
gas
. We find no or only weak correlations of
t
depl
and
μ
gas
with optical size
R
or surface density once one removes the above scalings, but we caution that optical sizes may not be appropriate for the high gas and dust columns at high
z
.
We report the discovery of diffuse extended Ly
α
emission from redshift 3.1 to 4.5, tracing cosmic web filaments on scales of 2.5−4 cMpc. These structures have been observed in overdensities of Ly
α
...emitters in the MUSE Extremely Deep Field, a 140 h deep MUSE observation located in the
Hubble
Ultra-Deep Field. Among the 22 overdense regions identified, five are likely to harbor very extended Ly
α
emission at high significance with an average surface brightness of 5 × 10
−20
erg s
−1
cm
−2
arcsec
−2
. Remarkably, 70% of the total Ly
α
luminosity from these filaments comes from beyond the circumgalactic medium of any identified Ly
α
emitter. Fluorescent Ly
α
emission powered by the cosmic UV background can only account for less than 34% of this emission at
z
≈ 3 and for not more than 10% at higher redshift. We find that the bulk of this diffuse emission can be reproduced by the unresolved Ly
α
emission of a large population of ultra low-luminosity Ly
α
emitters (< 10
40
erg s
−1
), provided that the faint end of the Ly
α
luminosity function is steep (
α
⪅ −1.8), it extends down to luminosities lower than 10
38
− 10
37
erg s
−1
, and the clustering of these Ly
α
emitters is significant (filling factor < 1/6). If these Ly
α
emitters are powered by star formation, then this implies their luminosity function needs to extend down to star formation rates < 10
−4
M
⊙
yr
−1
. These observations provide the first detection of the cosmic web in Ly
α
emission in typical filamentary environments and the first observational clue indicating the existence of a large population of ultra low-luminosity Ly
α
emitters at high redshift.
It remains a challenge to assess the merger fraction of galaxies at different cosmic epochs in order to probe the evolution of their mass assembly. Using the ILLUSTRIS cosmological simulation ...project, we investigate the relation between the separation of galaxies in a pair, both in velocity and projected spatial separation space, and the probability that these interacting galaxies will merge in the future. From this analysis, we propose a new set of criteria to select close pairs of galaxies along with a new corrective term to be applied to the computation of the galaxy merger fraction. We then probe the evolution of the major and minor merger fraction using the latest Multi-Unit Spectroscopic Explorer (MUSE) deep observations over the Hubble Ultra Deep Field, Hubble Deep Field South, COSMOS-Gr30, and Abell 2744 regions. From a parent sample of 2483 galaxies with spectroscopic redshifts, we identify 366 close pairs spread over a large range of redshifts (0.2 < z < 6) and stellar masses (107 − 1011M⊙). Using the stellar mass ratio between the secondary and primary galaxy as a proxy to split the sample into major, minor, and very minor mergers, we found a total of 183 major, 142 minor, and 47 very minor close pairs corresponding to a mass ratio range of 1:1–1:6, 1:6–1:100, and lower than 1:100, respectively. Due to completeness issues, we do not consider the very minor pairs in the analysis. Overall, the major merger fraction increases up to z ≈ 2−3 reaching 25% for pairs where the most massive galaxy has a stellar mass M⋆ ≥ 109.5 M⊙. Beyond this redshift, the fraction decreases down to ∼5% at z ≈ 6. The major merger fraction for lower-mass primary galaxies with M⋆ ≤ 109.5 M⊙ seems to follow a more constant evolutionary trend with redshift. Thanks to the addition of new MUSE fields and new selection criteria, the increased statistics of the pair samples allow us to significantly shorten the error bars compared to our previous analysis. The evolution of the minor merger fraction is roughly constant with cosmic time, with a fraction of 20% at z < 3 and a slow decrease to 8−13% in the redshift range 3 ≤ z ≤ 6.
The kinematics of distant galaxies from z = 0.1 to z > 2 play a key role in our understanding of galaxy evolution from early times to the present. One of the important parameters is the intrinsic, or ...local, velocity dispersion of a galaxy, which allows one to quantify the degree of non-circular motions such as pressure support. However, this is difficult to measure because the observed dispersion includes the effects of (often severe) beam smearing on the velocity gradient. Here we investigate four methods of measuring the dispersion that have been used in the literature, to assess their effectiveness at recovering the intrinsic dispersion. We discuss the biases inherent in each method, and apply them to model disk galaxies in order to determine which methods yield meaningful quantities and under what conditions. All the mean-weighted dispersion estimators are affected by (residual) beam smearing. In contrast, the dispersion recovered by fitting a spatially and spectrally convolved disk model to the data is unbiased by the beam smearing it is trying to compensate. Because of this, and because the bias it does exhibit depends only on the signal-to-noise ratio (S/N), it can be considered reliable. However, at very low S/N, all methods should be used with caution.
We present the Spectroscopic Imaging survey in the near-infrared (near-IR) with SINFONI (SINS) of high-redshift galaxies. With 80 objects observed and 63 detected in at least one rest-frame optical ...nebular emission line, mainly H Delta *a, SINS represents the largest survey of spatially resolved gas kinematics, morphologies, and physical properties of star-forming galaxies at z ~ 1-3. We describe the selection of the targets, the observations, and the data reduction. We then focus on the 'SINS H Delta *a sample,' consisting of 62 rest-UV/optically selected sources at 1.3 < z < 2.6 for which we targeted primarily the H Delta *a and N II emission lines. Only 30% of this sample had previous near-IR spectroscopic observations. The galaxies were drawn from various imaging surveys with different photometric criteria; as a whole, the SINS H Delta *a sample covers a reasonable representation of massive M 1010 M star-forming galaxies at z 1.5-2.5, with some bias toward bluer systems compared to pure K-selected samples due to the requirement of secure optical redshift. The sample spans 2 orders of magnitude in stellar mass and in absolute and specific star formation rates, with median values 3 X 1010 M, 70 M yr-1, and 3 Gyr-1. The ionized gas distribution and kinematics are spatially resolved on scales ranging from 1.5 kpc for adaptive optics assisted observations to typically 4-5 kpc for seeing-limited data. The H Delta *a morphologies tend to be irregular and/or clumpy. About one-third of the SINS H Delta *a sample galaxies are rotation-dominated yet turbulent disks, another one-third comprises compact and velocity dispersion-dominated objects, and the remaining galaxies are clear interacting/merging systems; the fraction of rotation-dominated systems increases among the more massive part of the sample. The H Delta *a luminosities and equivalent widths suggest on average roughly twice higher dust attenuation toward the H II regions relative to the bulk of the stars, and comparable current and past-averaged star formation rates.
We present ~0.5" resolution near-infrared integral field spectroscopy of the Halpha line emission of 14 z~2 UV-selected BM/BX galaxies, obtained with SINFONI at the ESO Very Large Telescope. The ...average Halpha half-light radius is r1/2~4 h-170 kpc, and line emission is detected over >~20 h-170 kpc in several sources. In nine galaxies, we detect spatially resolved velocity gradients, from 40 to 410 km s-1 over ~10 h-170 kpc. The kinematics of the larger systems are generally consistent with orbital motions. Four galaxies are well described by rotating clumpy disks, and we extracted rotation curves out to radii >~10 h-170 kpc. One or two galaxies exhibit signatures more consistent with mergers. Analyzing all 14 galaxies in the framework of rotating disks, we infer mean inclination- and beam-corrected maximum circular velocities of vc~180 /-90 km s-1 and dynamical masses from ~0.5 to 25×1010 h-170 Msolar within r1/2. The specific angular momenta of our BM/BX galaxies are similar to those of local late-type galaxies. Moreover, the specific angular momenta of their baryons are comparable to those of their dark matter halos. Extrapolating from the average vc at 10 h-170 kpc, the virial mass of the typical halo of a galaxy in our sample is 1011.7 /-0.5 h-170 Msolar. Kinematic modeling of the three best cases implies a ratio of vc to local velocity dispersion vc/sigma~2-4 and, accordingly, a large geometric thickness. We argue that this suggests a mass accretion (alternatively, gas exhaustion) timescale of ~500 Myr. We also argue that if our BM/BX galaxies were initially gas-rich, their clumpy disks would subsequently lose their angular momentum and form compact bulges on a timescale of ~1 Gyr.
We present H alpha integral field spectroscopy of well-resolved, UV/optically selected image star-forming galaxies as part of the SINS survey with SINFONI on the ESO VLT. Our laser guide star ...adaptive optics and good seeing data show the presence of turbulent rotating star-forming outer rings/disks, plus central bulge/inner disk components, whose mass fractions relative to the total dynamical mass appear to scale with the N ii/H alpha flux ratio and the star formation age. We propose that the buildup of the central disks and bulges of massive galaxies at image can be driven by the early secular evolution of gas-rich proto-disks. High-redshift disks exhibit large random motions. This turbulence may in part be stirred up by the release of gravitational energy in the rapid 'cold' accretion flows along the filaments of the cosmic web. As a result, dynamical friction and viscous processes proceed on a timescale of <1 Gyr, at least an order of magnitude faster than in image disk galaxies. Early secular evolution thus drives gas and stars into the central regions and can build up exponential disks and massive bulges, even without major mergers. Secular evolution along with increased efficiency of star formation at high surface densities may also help to account for the short timescales of the stellar buildup observed in massive galaxies at image.
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