ABSTRACT There are very strong observed correlations between the specific star formation rates (sSFRs) of galaxies and their mean surface mass densities, , as well as other aspects of their internal ...structure. These strong correlations have often been taken to argue that the internal structure of a galaxy must play a major physical role, directly or indirectly, in the control of star formation. In this paper we show by means of a very simple toy model that these correlations can arise naturally without any such physical role once the observed evolution of the size-mass relation for star-forming galaxies is taken into account. In particular, the model reproduces the sharp threshold in between galaxies that are star-forming and those that are quenched and the evolution of this threshold with redshift. Similarly, it produces iso-quenched-fraction contours in the fQ(m, Re) plane that are almost exactly parallel to lines of constant for centrals and shallower for satellites. It does so without any dependence on quenching on size or and without invoking any differences between centrals and satellites, beyond the different mass dependences of their quenching laws. The toy model also reproduces several other observations, including the sSFR gradients within galaxies and the appearance of inside-out build-up of passive galaxies. Finally, it is shown that curvature in the main-sequence sSFR-mass relation can produce curvature in the apparent B/T ratios with mass. Our analysis therefore suggests that many of the strong correlations that are observed between galaxy structure and sSFR may well be a consequence of things unrelated to quenching and should not be taken as evidence of the physical processes that drive quenching.
ABSTRACT We characterize the optical variability of quasars in the Palomar Transient Factory and intermediate Palomar Transient Factory (PTF/iPTF) surveys. We re-calibrate the r-band light curves for ...∼28,000 luminous, broad-line active galactic nuclei from the SDSS, producing a total of ∼2.4 million photometric data points. We utilize both the structure function (SF) and power spectrum density (PSD) formalisms to search for links between the optical variability and the physical parameters of the accreting supermassive black holes that power the quasars. The excess variance (SF2) of the quasar sample tends to zero at very short time separations, validating our re-calibration of the time-series data. We find that the the amplitude of variability at a given time-interval, or equivalently the timescale of variability to reach a certain amplitude, is most strongly correlated with luminosity with weak or no dependence on black hole mass and redshift. For a variability level of SF(τ) = 0.07 mag, the timescale has a dependency of . This is broadly consistent with the expectation from a simple Keplerian accretion disk model, which provides . The PSD analysis also reveals that many quasar light curves are steeper than a damped random walk. We find a correlation between the steepness of the PSD slopes, specifically the fraction of slopes steeper than 2.5, and black hole mass, although we cannot exclude the possibility that luminosity or Eddington ratio are the drivers of this effect. This effect is also seen in the SF analysis of the (i)PTF data, and in a PSD analysis of quasars in the SDSS Stripe 82.
We introduce Zurich Atmosphere Purge (ZAP), an approach to sky subtraction based on principal component analysis (PCA) that we have developed for the Multi Unit Spectrographic Explorer (MUSE) ...integral field spectrograph. ZAP employs filtering and data segmentation to enhance the inherent capabilities of PCA for sky subtraction. Extensive testing shows that ZAP reduces sky emission residuals while robustly preserving the flux and line shapes of astronomical sources. The method works in a variety of observational situations from sparse fields with a low density of sources to filled fields in which the target source fills the field of view. With the inclusion of both of these situations, the method is generally applicable to many different science cases and should also be useful for other instrumentation. ZAP is available for download at http://muse-vlt.eu/science/tools.
The gas regulator is defined by the gas consumption timescale ( epsilon super(-1)) and the mass loading lambda of the wind outflow lambda times SFR. The simplest regulator, in which epsilon and ...lambda are constant, sets the sSFR equal to exactly the specific accretion rate of the galaxy; more realistic situations lead to an sSFR that is perturbed from this precise relation. The overall mass-metallicity relation Z(m sub(star)) directly provides the fraction f sub(star)(m sub(star)) of incoming baryons that are being transformed into stars. The observed relation also boosts the sSFR relative to the specific accretion rate and produces a different dependence on mass, both of which are observed. The derived Z(m sub(star), SFR) relation for the regulator system is fit to published Z(m sub(star), SFR) data for the SDSS galaxy population, yielding epsilon and lambda as functions of m sub(star). The fitted epsilon is consistent with observed molecular gas-depletion timescales in galaxies (allowing for the extra atomic gas), while the fitted lambda is also reasonable.
To investigate the variability of the star formation rate (SFR) of galaxies, we define a star formation change parameter, SFR5 Myr/SFR800 Myr, which is the ratio of the SFR averaged within the last 5 ...Myr to the SFR averaged within the last 800 Myr. We show that this parameter can be determined from a combination of H emission and Hδ absorption, plus the 4000 break, with an uncertainty of ∼0.07 dex for star-forming galaxies. We then apply this estimator to MaNGA galaxies, both globally within Re and within radial annuli. We find that the global SFR5 Myr/SFR800 Myr, which indicates by how much a galaxy has changed its specific SFR (sSFR), is nearly independent of its sSFR, i.e., of its position relative to the star formation main sequence (SFMS) as defined by SFR800 Myr. Also, at any sSFR, there are as many galaxies increasing their sSFR as decreasing it, as required if the dispersion in the SFMS is to stay the same. The SFR5 Myr/SFR800 Myr of the overall galaxy population is very close to that expected for the evolving main sequence. Both of these provide a reassuring check on the validity of our calibration of the estimator. We find that galaxies with higher global SFR5 Myr/SFR800 Myr appear to have higher SFR5 Myr/SFR800 Myr at all galactic radii, i.e., that galaxies with a recent temporal enhancement in overall SFR have enhanced star formation at all galactic radii. The dispersion of the SFR5 Myr/SFR800 Myr at a given relative galactic radius and a given stellar mass decreases with the (indirectly inferred) gas depletion time: locations with short gas depletion time appear to undergo bigger variations in their star formation rates on Gyr or less timescales. In Wang et al., we showed that the dispersion in star formation rate surface densities SFR in the galaxy population appears to be inversely correlated with the inferred gas depletion timescale and interpreted this in terms of the dynamical response of a gas-regulator system to changes in the gas inflow rate. In this paper, we can now prove directly with SFR5 Myr/SFR800 Myr that these effects are indeed due to genuine temporal variations in the SFR of individual galaxies on timescales between 107 and 109 yr rather than possibly reflecting intrinsic, non-temporal, differences between different galaxies.
Abstract
We examine the correlations of star formation rate (SFR) and gas-phase metallicity
Z
. We first predict how the SFR, cold gas mass, and
Z
will change with variations in inflow rate or in ...star formation efficiency (SFE) in a simple gas-regulator framework. The changes
SFR and
are found to be negatively (positively) correlated when driving the gas regulator with time-varying inflow rate (SFE). We then study the correlation of
sSFR (specific SFR) and
(O/H) from observations, at both ∼100 pc and galactic scales, based on two two-dimensional spectroscopic surveys with different spatial resolutions, MAD and MaNGA. After taking out the overall mass and radial dependences, which may reflect changes in inflow gas metallicity and/or outflow mass loading, we find that
sSFR and
(O/H) on galactic scales are found to be negatively correlated, but
sSFR and
(O/H) are positively correlated on ∼100 pc scales within galaxies. If we assume that the variations across the population reflect temporal variations in individual objects, we conclude that variations in the SFR are primarily driven by time-varying inflow at galactic scales and driven by time-varying SFE at ∼100 pc scales. We build a theoretical framework to understand the correlation between SFR, gas mass, and metallicity, as well as their variability, which potentially uncovers the relevant physical processes of star formation at different scales.
Abstract
We present emission-line measurements and physical interpretations for a sample of 117 O
iii
emitting galaxies at
z
= 5.33–6.93, using the first deep JWST/NIRCam wide-field slitless ...spectroscopic observations. Our 9.7 hr integration is centered upon the
z
= 6.3 quasar J0100+2802—the first of six fields targeted by the EIGER survey—and covers
λ
= 3–4
μ
m. We detect 133 O
iii
doublets, but close pairs motivated by their small scale clustering excess. The galaxies are characterized by a UV luminosity
M
UV
∼ −19.6 (−17.7 to −22.3), stellar mass ∼10
8
(10
6.8−10.1
)
M
⊙
, H
β
and O
iii
4960+5008
EWs ≈ 850 Å (up to 3000 Å), young ages, a highly excited interstellar medium, and low dust attenuations. These high EWs are very rare in the local universe, but we show they are ubiquitous at
z
∼ 6 based on the measured number densities. The stacked spectrum reveals H
γ
and O
iii
4364
, which shows that the galaxies are typically dust- and metal-poor (E (
B
−
V
) = 0.1,
12
+
log
(
O
/
H
)
=
7.4
) with a high electron temperature (2 × 10
4
K) and a production efficiency of ionizing photons (
ξ
ion
= 10
25.3
Hz erg
−1
). We further show the existence of a strong mass–metallicity relation. The properties of the stars and gas in
z
∼ 6 galaxies conspire to maximize the O
iii
output from galaxies, yielding an O
iii
luminosity density at
z
≈ 6 that is significantly higher than that at
z
≈ 2. Thus, O
iii
emission-line surveys with JWST prove a highly efficient method to trace the galaxy density in the Epoch of Reionization.
Abstract
We investigate the disk formation process in the TNG50 simulation, examining the profiles of SFR surface density (Σ
SFR
), gas inflow and outflow, and the evolution of the angular momentum ...of inflowing gas particles. The TNG50 galaxies tend to have larger star-forming disks, and they also show larger deviations from exponential profiles in Σ
SFR
when compared to real galaxies in the Mapping Nearby Galaxies at APO survey. The stellar surface density of TNG50 galaxies show good exponential profiles, which is found to be the result of strong radial migration of stars over time. However, this strong radial migration of stars in the simulation produces flatter age profiles in TNG50 disks compared to observed galaxies. The star formation in the simulated galaxies is sustained by a net gas inflow, and this gas inflow is the primary driver for the cosmic evolution of star formation, as expected from simple gas-regulator models of galaxies. There is no evidence for any significant loss of angular momentum for the gas particles after they are accreted on to the galaxy, which may account for the large disk sizes in the TNG50 simulation. Adding viscous processes to the disks, such as the magnetic stresses from magnetorotational instability proposed by Wang & Lilly, will likely reduce the sizes of the simulated disks and the tension with the sizes of real galaxies, and this may produce more realistic exponential profiles.
ABSTRACT
We present results from our on-going MusE GAs FLOw and Wind (MEGAFLOW) survey, which consists of 22 quasar lines of sight, each observed with the integral field unit MUSE and the UVES ...spectrograph at the ESO Very Large Telescopes (VLT). The goals of this survey are to study the properties of the circumgalactic medium around z ∼ 1 star-forming galaxies. The absorption-line selected survey consists of 79 strong Mg ii absorbers (with rest-frame equivalent width ≳0.3 Å) and, currently, 86 associated galaxies within 100 projected kpc of the quasar with stellar masses (M⋆) from 109 to 1011 M⊙. We find that the cool halo gas traced by Mg ii is not isotropically distributed around these galaxies from the strong bi-modal distribution in the azimuthal angle of the apparent location of the quasar with respect to the galaxy major axis. This supports a scenario in which outflows are bi-conical in nature and co-exist with a co-planar gaseous structure extending at least up to 60–80 kpc. Assuming that absorbers near the minor axis probe outflows, the current MEGAFLOW sample allowed us to select 26 galaxy–quasar pairs suitable for studying winds. From this sample, using a simple geometrical model, we find that the outflow velocity only exceeds the escape velocity when M⋆ ≲ 4 × 109 M⊙, implying the cool material is likely to fall back except in the smallest haloes. Finally, we find that the mass loading factor η, the ratio between the ejected mass rate and the star formation rate, appears to be roughly constant with respect to the galaxy mass.
In order to relate the observed evolution of the galaxy stellar mass function and the luminosity function of active galactic nuclei (AGNs), we explore a coevolution scenario in which AGNs are ...associated only with the very last phases of the star-forming life of a galaxy. We derive analytically the connections between the parameters of the observed quasar luminosity functions and galaxy mass functions. The (mbh/m*)Qing associated with quenching is given by the ratio of the global black hole accretion rate density (BHARD) and star formation rate density (SFRD) at the epoch in question. Observational data on the SFRD and BHARD suggest (mbh/m*)Qing ∝ (1 + z)1.5 below redshift 2. This evolution reproduces the observed mass-luminosity plane of Sloan Digital Sky Survey quasars, as well as the local mbh/m* relation in passive galaxies. The characteristic Eddington ratio, λ*, is derived from both the BHARD/SFRD ratio and the evolving L* of the AGN population. This increases up to z ∼ 2 as λ* ∝ (1 + z)2.5, but at higher redshifts, λ* stabilizes at the physically interesting Eddington limit, λ* ∼ 1. The new model may be thought of as an opposite extreme to our earlier coevolution scenario in Caplar et al. The main observable difference between the two coevolution scenarios, presented here and in Caplar et al. is in the active fraction of low-mass star-forming galaxies. We compare the predictions with the data from deep multiwavelength surveys and find that the "quenching" scenario developed in the current paper is preferred.
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