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
.
The Cii 158 ...m emission line can arise in all phases of the interstellar medium (ISM), therefore being able to disentangle the different contributions is an important yet unresolved problem when ...undertaking galaxy-wide, integrated Cii observations. We present a new multiphase 3D radiative transfer interface that couples starburst99, a stellar spectrophotometric code, with the photoionization and astrochemistry codes mocassin and 3d-pdr. We model entire star-forming regions, including the ionized, atomic, and molecular phases of the ISM, and apply a Bayesian inference methodology to parametrize how the fraction of the Cii emission originating from molecular regions, f sub( CII,mol), varies as a function of typical integrated properties of galaxies in the local Universe. The main parameters responsible for the variations of f sub( CII,mol) are specific star formation rate (SSFR), gas phase metallicity, H ii region electron number density (n sub( e)), and dust mass fraction. For example, f sub( CII,mol) can increase from 60 to 80 per cent when either n sub( e) increases from 10 super( 1.5) to 10 super( 2.5) cm super( -3), or SSFR decreases from 10 super( -9.6) to 10 super( -10.6) yr super( -1). Our model predicts for the Milky Way that f sub( CII,mol) = 75.8 plus or minus 5.9 per cent, in agreement with the measured value of 75 per cent. When applying the new prescription to a complete sample of galaxies from the Herschel Reference Survey, we find that anywhere from 60 to 80 per cent of the total integrated Cii emission arises from molecular regions. (ProQuest: ... denotes formulae/symbols omitted.)
Nuclear outflows driven by accreting massive black holes are one of the main feedback mechanisms invoked at high-z to reproduce the distinct separation between star-forming disk galaxies and ...quiescent spheroidal systems. Yet our knowledge of feedback at high-z remains limited by the lack of observations of the multiple gas phases in galaxy outflows. In this work, we use new deep, high spatial resolution ALMA CO(3-2) and archival Very Large Telescope/SINFONI H observations to study the molecular and ionized components of the active galactic nucleus (AGN)-driven outflow in zC400528, a massive main-sequence galaxy at z = 2.3 in the process of quenching. We detect a powerful molecular outflow that shows a positive velocity gradient before a turnover and extends for at least ∼10 kpc from the nuclear region, about three times the projected size of the ionized wind. The molecular gas in the outflow does not reach velocities high enough to escape the galaxy and is therefore expected to be reaccreted. Keeping in mind the various assumptions involved in the analysis, we find that the mass and energetics of the outflow are dominated by the molecular phase. The AGN-driven outflow in zC400528 is powerful enough to deplete the molecular gas reservoir on a timescale comparable to that needed to exhaust it by star formation. This suggests that the nuclear outflow is one of the main quenching engines at work in the observed suppression of the central star formation activity in zC400528.
ABSTRACT
We investigate the kinematics of the molecular gas in a sample of seven edge-on (i > 60°) galaxies identified as hosting large-scale outflows of ionized gas, using ALMA CO(1–0) observations ...at ∼1 kpc resolution. We build on Hogarth et al., where we find that molecular gas is more centrally concentrated in galaxies which host winds than in control objects. We perform full three-dimensional kinematic modelling with multiple combinations of kinematic components, allowing us to infer whether these objects share any similarities in their molecular gas structure. We use modelling to pinpoint the kinematic centre of each galaxy, in order to interpret their minor- and major-axis position velocity diagrams (PVDs). From the PVDs, we find that the bulk of the molecular gas in our galaxies is dynamically cold, tracing the rotation curves predicted by our symmetric, rotation-dominated models, but with minor flux asymmetries. Most notably, we find evidence of radial gas motion in a subset of our objects, which demonstrate a characteristic ‘twisting’ in their minor-axis PVDs generally associated with gas flow along the plane of a galaxy. In our highest S/N object, we include bi-symmetric radial flow in our kinematic model, and find (via the Bayesian Information Criterion) that the presence of radial gas motion is strongly favoured. This may provide one mechanism by which molecular gas and star formation are centrally concentrated, enabling the launch of massive ionized gas winds. However, in the remainder of our sample, we do not observe evidence that gas is being driven radially, once again emphasizing the variety of physical processes that may be powering the outflows in these objects, as originally noted in H21.
We study the properties of the cold gas component of the interstellar medium of the Herschel Reference Survey, a complete volume-limited (15 <, ~ D <, ~ 25 Mpc), K-band-selected sample of galaxies ...spanning a wide range in morphological type (from ellipticals to dwarf irregulars) and stellar mass (10 super(9) <, ~ M sub(star) <, ~ 10 super(11) M sub(middot in circle)). The multifrequency data in our hands are used to trace the molecular gas mass distribution and the main scaling relations of the sample, which put strong constraints on galaxy formation simulations. We extend the main scaling relations concerning the total and the molecular gas component determined for massive galaxies (M sub(star) > ~ 10 super(10) M sub(middot in circle)) from the COLD GASS survey down to stellar masses M sub(star) = 10 super(9) M sub(middot in circle). As scaling variables we use the total stellar mass M sub(star), the stellar surface density mu sub(star), the specific star formation rate SSFR, and the metallicity of the target galaxies. By comparing molecular gas masses determined using a constant or a luminosity dependent X sub(CO) conversion factor, we estimate the robustness of these scaling relations on the very uncertain assumptions used to transform CO line intensities into molecular gas masses. The molecular gas distribution of a K-band-selected sample is significantly different from that of a far-infrared-selected sample since it includes a significantly smaller number of objects with M(H sub(2)) <, ~ 6 x 10 super(9) M sub(middot in circle). In spiral galaxies the molecular gas phase is only 25-30% of the atomic gas. The analysis also indicates that the slope of the main scaling relations depends on the adopted conversion factor. Among the sampled relations, all those concerning M(gas)/M sub(star) are statistically significant and show little variation with X sub(CO). We observe a significant correlation between M(H sub(2))/M sub(star) and SSFR, M(H sub(2))/M(HI ) and mu sub(star), M(H sub(2))/M(HI ) and 12 +log (O/H), regardless of the adopted X sub(CO). The total and molecular gas consumption timescales are anticorrelated with the specific star formation rate. The comparison of HRS and COLD GASS data indicates that some of the observed scaling relations are nonlinear.
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ABSTRACT
Outflows form an integral component in regulating the gas cycling in and out of galaxies, although their impact on the galaxy hosts is still poorly understood. Here we present an analysis of ...405 high mass (log M*/M⊙ ≥ 10), star-forming galaxies (excluding AGN) with low inclinations at z ∼ 0, using stacking techniques of the Na D λλ5889, 5895 Å neutral gas tracer in IFU observations from the MaNGA DR15 survey. We detect outflows in the central regions of 78/405 galaxies and determine their extent and power through the construction of stacked annuli. We find outflows are most powerful in central regions and extend out to ∼1Re, with declining mass outflow rates and loading factors as a function of radius. The stacking of spaxels over key galaxy quantities reveals outflow detections in regions of high ΣSFR (≳0.01 M⊙ yr−1 kpc−2) and $\Sigma _{M_{*}}$ (≳107 M⊙ kpc−2) along the resolved main sequence. Clear correlations with ΣSFR suggest it is the main regulator of outflows, with a critical threshold of ∼0.01 M⊙ yr−1 kpc−2 needed to escape the weight of the disc and launch them. Furthermore, measurements of the Hδ and Dn4000 indices reveal virtually identical star formation histories between galaxies with outflows and those without. Finally, through stacking of H i 21 cm observations for a subset of our sample, we find outflow galaxies show reduced H i gas fractions at central velocities compared to their non-detection control counterparts, suggestive of some removal of H i gas, likely in the central regions of the galaxies, but not enough to completely quench the host.
The Herschel Reference Survey is a complete volume-limited, K-band-selected sample of nearby objects including Virgo cluster and isolated objects. Using a recent compilation of HI and CO data for ...this sample we study the effects of the cluster environment on the molecular gas content of spiral galaxies. With the subsample of unperturbed field galaxies, we first identify the stellar mass as the scaling variable that traces the total molecular gas mass of galaxies better. We show that, on average, HI-deficient galaxies are significantly offset (4sigma) from the M(H sub(2)) vs. M sub(star) relation for HI-normal galaxies. We use the M(H sub(2)) vs. M sub(star) scaling relation to define the H sub(2)-deficiency parameter as the difference, on logarithmic scale, between the expected and observed molecular gas mass for a galaxy of given stellar mass. The H sub(2)-deficiency parameter shows a weak and scattered relation with the HI-deficiency parameter, here taken as a proxy for galaxy interactions with the surrounding cluster environment. We also show that, as for the atomic gas, the extent of the molecular disc decreases with increasing HI-deficiency. All together, these results show that cluster galaxies have, on average, a lower molecular gas content than similar objects in the field. Our analysis indicates that ram pressure stripping is the physical process responsible for this molecular gas deficiency. The slope of the H sub(2) - def vs. HI - def relation is less than unity, while the D(HI)/D(i) vs. HI - def relation is steeper than the D(CO)/D(i) vs. HI - def relation, thereby indicating that the molecular gas is removed less efficiently than the atomic gas. This result can be understood if the atomic gas is distributed on a relatively flat disc that is more extended than the stellar disc. It is thus less anchored to the gravitational potential well of the galaxy than the molecular gas phase, which is distributed on an exponential disc with a scalelength r sub(CO) Asymptotically = to 0.2r sub(24.5) (g). There is a clear trend between the NUV-i colour index, which is a proxy for the specific star formation activity, and the H sub(2)-deficiency parameter, which suggests that molecular gas removal quenches the activity of star formation. This causes galaxies migrate from the blue cloud to the green valley and, eventually, to the red sequence. The total gas-consumption timescale of gas deficient cluster galaxies is comparable to that of isolated, unperturbed systems. The total gas depletion timescale determined by considering the recycled fraction is T sub(gas,R) Asymptotically = to 3.0-3.3 Gyr, which is significantly larger than the typical timescale for total gas removal in a ram pressure stripping process, indicated by recent hydrodynamical simulations to be T sub(RP)Asympt otically = to 1.5 Gyr. The comparison of these timescales suggests that ram pressure, rather than a simple stop of the infall of pristine gas from the halo, will be the dominant process driving the future evolution of these cluster galaxies.
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ABSTRACT
Optical spectra contain a wealth of information about the physical properties and formation histories of galaxies. Often though, spectra are too noisy for this information to be accurately ...retrieved. In this study, we explore how machine learning methods can be used to de-noise spectra and increase the amount of information we can gain without having to turn to sample averaging methods such as spectral stacking. Using machine learning methods trained on noise-added spectra – Sloan Digital Sky Survey (SDSS) spectra with Gaussian noise added – we investigate methods of maximizing the information we can gain from these spectra, in particular from emission lines, such that more detailed analysis can be performed. We produce a variational autoencoder (VAE) model, and apply it on a sample of noise-added spectra. Compared to the flux measured in the original SDSS spectra, the model values are accurate within 0.3–0.5 dex, depending on the specific spectral line and signal-to-noise ratio. Overall, the VAE performs better than a principal component analysis method, in terms of reconstruction loss and accuracy of the recovered line fluxes. To demonstrate the applicability and usefulness of the method in the context of large optical spectroscopy surveys, we simulate a population of spectra with noise similar to that in galaxies at z = 0.1 observed by the Dark Energy Spectroscopic Instrument (DESI). We show that we can recover the shape and scatter of the mass–metallicity relation in this ‘DESI-like’ sample, in a way that is not possible without the VAE-assisted de-noising.
ABSTRACT
Scaling laws of dust, H i gas, and metal mass with stellar mass, specific star formation rate, and metallicity are crucial to our understanding of the build-up of galaxies through their ...enrichment with metals and dust. In this work, we analyse how the dust and metal content varies with specific gas mass (MH i/M⋆) across a diverse sample of 423 nearby galaxies. The observed trends are interpreted with a set of Dust and Element evolUtion modelS (DEUS) – including stellar dust production, grain growth, and dust destruction – within a Bayesian framework to enable a rigorous search of the multidimensional parameter space. We find that these scaling laws for galaxies with −1.0 ≲ log MH i/M⋆ ≲ 0 can be reproduced using closed-box models with high fractions (37–89 ${{\ \rm per\ cent}}$) of supernova dust surviving a reverse shock, relatively low grain growth efficiencies (ϵ = 30–40), and long dust lifetimes (1–2 Gyr). The models have present-day dust masses with similar contributions from stellar sources (50–80 ${{\ \rm per\ cent}}$) and grain growth (20–50 ${{\ \rm per\ cent}}$). Over the entire lifetime of these galaxies, the contribution from stardust (>90 ${{\ \rm per\ cent}}$) outweighs the fraction of dust grown in the interstellar medium (<10 ${{\ \rm per\ cent}}$). Our results provide an alternative for the chemical evolution models that require extremely low supernova dust production efficiencies and short grain growth time-scales to reproduce local scaling laws, and could help solving the conundrum on whether or not grains can grow efficiently in the interstellar medium.
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