We revisit the proposed extended Schmidt law, which posits that the star formation efficiency in galaxies depends on the stellar mass surface density, by investigating spatially resolved star ...formation rates (SFRs), gas masses, and stellar masses of star formation regions in a vast range of galactic environments, from the outer disks of dwarf galaxies, to spiral disks and to merging galaxies, as well as individual molecular clouds in M33. We find that these regions are distributed in a tight power law as ∝ , which is also valid for the integrated measurements of disk and merging galaxies at high-z. Interestingly, we show that star formation regions in the outer disks of dwarf galaxies with down to 10−5 yr−1 kpc−2, which are outliers of both the Kennicutt-Schmidt and Silk-Elmegreen laws, also follow the extended Schmidt law. Other outliers in the Kennicutt-Schmidt law, such as extremely metal-poor star formation regions, also show significantly reduced deviation from the extended Schmidt law. These results suggest an important role for existing stars in helping to regulate star formation through the effect of their gravity on the midplane pressure in a wide range of galactic environments.
Abstract It is exceedingly rare to find quiescent low-mass galaxies in the field at low redshift. UGC 5205 is an example of such a quenched field dwarf ( M ⋆ ∼ 3 × 10 8 M ⊙ ). Despite a wealth of ...cold gas ( M HI ∼ 3.5 × 10 8 M ⊙ ) and UV emission that indicates significant star formation in the past few hundred megayears, there is no detection of H α emission—star formation in the last ∼10 Myr—across the face of the galaxy. Meanwhile, the near equal-mass companion of UGC 5205, PGC 027864, is starbursting (which has an H α equivalent width > 1000 Å). In this work, we present new Karl G. Jansky Very Large Array 21 cm line observations of UGC 5205, showing that the lack of star formation is caused by an absence of H i in the main body of the galaxy. The H i of UGC 5205 is highly disturbed; the bulk of the H i resides in several-kiloparsec–long tails, while the H i of PGC 027864 is dominated by ordered rotation. We model the stellar populations of UGC 5205 to show that, as indicated by the UV and H α emission, the galaxy underwent a coordinated quenching event ∼100–300 Myr ago. The asymmetry of outcomes for UGC 5205 and PGC 027864 demonstrate that major mergers can both quench and trigger star formation in dwarfs. However, because the gas remains bound to the system, we suggest that such mergers only temporarily quench star formation. We estimate a total quenched time of ∼560 Myr for UGC 5205, consistent with established upper limits on the quenched fraction of a few percent for dwarfs in the field.
The first galaxies contain stars born out of gas with few or no 'metals' (that is, elements heavier than helium). The lack of metals is expected to inhibit efficient gas cooling and star formation, ...but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun. Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon monoxide emission is unreliable as a tracer of gas at low metallicities, and while dust has been used to trace gas in low-metallicity galaxies, low spatial resolution in the far-infrared has typically led to large uncertainties. Here we report spatially resolved infrared observations of two galaxies with oxygen abundances below ten per cent of the solar value, and show that stars formed very inefficiently in seven star-forming clumps in these galaxies. The efficiencies are less than a tenth of those found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
We present the results of a stacking analysis performed on Spitzer/Infrared Spectrograph high-resolution mid-infrared (mid-IR) spectra of luminous infrared galaxies (LIRGs) in the Great ...Observatories All-Sky LIRG Survey. By binning in relation to mid-IR active galactic nucleus (AGN) fraction and stacking spectra, we detect bright emission lines Ne
ii
and Ne
iii
, which trace star formation, and fainter emission lines Ne
v
and O
iv
, which trace AGN activity, throughout the sample. We find that the Ne
ii
luminosity is fairly constant across all AGN fraction bins, while the O
iv
and Ne
v
luminosities increase by over an order of magnitude. Our measured average line ratios, Ne
v
/Ne
ii
and O
iv
/Ne
ii
, at low AGN fraction are similar to H II galaxies, while the line ratios at high AGN fraction are similar to LINERs and Seyferts. We decompose the O
iv
luminosity into star formation and AGN components by fitting the O
iv
luminosity as a function of the Ne
ii
luminosity and the mid-IR AGN fraction. The O
iv
luminosity in LIRGs is dominated by star formation for mid-IR AGN fractions ≲0.3. With the corrected O
iv
luminosity, we calculate black hole accretion rates (BHARs) ranging from 10
−5
M
⊙
yr
−1
at low AGN fractions to 0.2
M
⊙
yr
−1
at the highest AGN fractions. We find that using the O
iv
luminosity, without correcting for star formation, can lead to overestimation of the BHAR by up to a factor of 30 in starburst-dominated LIRGs. Finally, we show that the BHAR/star formation rate ratio increases by more than three orders of magnitude as a function of mid-IR AGN fraction in LIRGs.
Abstract Low-mass galaxy pair fractions are understudied, and it is unclear whether low-mass pair fractions evolve in the same way as more massive systems over cosmic time. In the era of JWST, Roman, ...and Rubin, selecting galaxy pairs in a self-consistent way will be critical to connect observed pair fractions to cosmological merger rates across all mass scales and redshifts. Utilizing the Illustris TNG100 simulation, we create a sample of physically associated low-mass (10 8 < M * < 5 × 10 9 M ⊙ ) and high-mass (5 × 10 9 < M * < 10 11 M ⊙ ) pairs between z = 0 and 4.2. The low-mass pair fraction increases from z = 0 to 2.5, while the high-mass pair fraction peaks at z = 0 and is constant or slightly decreasing at z > 1. At z = 0 the low-mass major (1:4 mass ratio) pair fraction is 4× lower than high-mass pairs, consistent with findings for cosmological merger rates. We show that separation limits that vary with the mass and redshift of the system, such as scaling by the virial radius of the host halo ( r sep < 1 R vir ), are critical for recovering pair fraction differences between low-mass and high-mass systems. Alternatively, static physical separation limits applied equivalently to all galaxy pairs do not recover the differences between low- and high-mass pair fractions, even up to separations of 300 kpc. Finally, we place isolated mass analogs of Local Group galaxy pairs, i.e., Milky Way (MW)–M31, MW–LMC, LMC–SMC, in a cosmological context, showing that isolated analogs of LMC–SMC-mass pairs and low-separation (<50 kpc) MW–LMC-mass pairs are 2–3× more common at z ≳ 2–3.
We present an extragalactic population model of the cosmic background light to interpret the rich high-quality survey data in the X-ray and IR bands. The model incorporates star formation and ...supermassive black hole (SMBH) accretion in a co-evolution scenario to fit simultaneously 617 data points of number counts, redshift distributions, and local luminosity functions (LFs) with 19 free parameters. The model has four main components, the total IR LF, the SMBH accretion energy fraction in the IR band, the star formation spectral energy distribution (SED), and the unobscured SMBH SED extinguished with a H I column density distribution. As a result of the observational uncertainties about the star formation and SMBH SEDs, we present several variants of the model. The best-fit reduced chi super(2) reaches as small as 2.7-2.9 of which a significant amount (>0.8) is contributed by cosmic variances or caveats associated with data. Compared to previous models, the unique result of this model is to constrain the SMBH energy fraction in the IR band that is found to increase with the IR luminosity but decrease with redshift up to z ~ 1.5; this result is separately verified using aromatic feature equivalent-width data. The joint modeling of X-ray and mid-IR data allows for improved constraints on the obscured active galactic nucleus (AGN), especially the Compton-thick AGN population. All variants of the model require that Compton-thick AGN fractions decrease with the SMBH luminosity but increase with redshift while the type 1 AGN fraction has the reverse trend.
The role of feedback in triggering or quenching star formation and hence driving galaxy evolution can be directly studied with high-resolution integral field observations. The manifestation of ...feedback in shocks is particularly important to examine in galaxy mergers, where violent interactions of gas take place in the interstellar medium during the course of the galactic collision. As part of our effort to systematically study the local population of luminous infrared galaxies within the Great Observatories All-Sky LIRG Survey, we undertook the Keck OSIRIS AO LIRG Analysis observing campaign to study the gas dynamics in the inner kiloparsec regions of these systems at spatial scales of a few tens of pc. With high-resolution near-infrared adaptive optics-assisted integral field observations taken with OSIRIS on the Keck Telescopes, we employ near-infrared diagnostics such as Brγ and the rovibrationally excited H2 lines to quantify the nuclear star formation rate and identify feedback associated with shocked molecular gas seen in 21 nearby luminous infrared galaxies. Shocked molecular gas is preferentially found in the ultraluminous infrared systems but may also be triggered at a lower-luminosity, earlier merging stage. On circumnuclear scales, AGNs have a strong effect on heating the surrounding molecular gas, though their coupling is not simply driven by AGN strength but rather is complicated by orientation, dust shielding, density, and other factors. We find that nuclear star formation correlates with merger class and diminishing projected nuclear separations. These trends are largely consistent with the picture of merger-induced starbursts in the center of galaxy mergers.
Carbon monoxide (CO) is one of the primary coolants of gas and an easily accessible tracer of molecular gas in spiral galaxies, but it is unclear if CO plays a similar role in metal-poor dwarfs. We ...carried out a deep observation with IRAM 30 m to search for CO emission by targeting the brightest far-IR peak in a nearby extremely metal-poor galaxy, Sextans A, with 7% solar metallicity. A marginal signal of CO J = 1-0 emission is seen, which is already faint enough to place a strong constraint on the conversion factor ( alpha sub(CO)) from the CO luminosity to the molecular gas mass that is derived from the spatially resolved dust-mass map. The alpha sub(CO) is at least seven hundred times the Milky Way value. This indicates that CO emission is exceedingly weak in Sextans A, challenging its role as a coolant in extremely metal-poor galaxies.
Mid-infrared molecular hydrogen (H2) emission is a powerful cooling agent in galaxy mergers and in radio galaxies; it is a potential key tracer of gas evolution and energy dissipation associated with ...mergers, star formation, and accretion onto supermassive black holes. We detect mid-IR H2 line emission in at least one rotational transition in 91% of the 214 Luminous Infrared Galaxies (LIRGs) observed with Spitzer as part of the Great Observatories All-sky LIRG Survey. We use H2 excitation diagrams to estimate the range of masses and temperatures of warm molecular gas in these galaxies. We find that LIRGs in which the IR emission originates mostly from the Active Galactic Nuclei (AGN) have about 100 K higher H2 mass-averaged excitation temperatures than LIRGs in which the IR emission originates mostly from star formation. Between 10% and 15% of LIRGs have H2 emission lines that are sufficiently broad to be resolved or partially resolved by the high-resolution modules of Spitzer's Infrared Spectrograph (IRS). Those sources tend to be mergers and contain AGN. This suggests that a significant fraction of the H2 line emission is powered by AGN activity through X-rays, cosmic rays, and turbulence. We find a statistically significant correlation between the kinetic energy in the H2 gas and the H2 to IR luminosity ratio. The sources with the largest warm gas kinetic energies are mergers. We speculate that mergers increase the production of bulk inflows leading to observable broad H2 profiles and possibly denser gas.
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