Interactions and star formation (original) (raw)
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Interactions, Starbursts, and Star Formation
Galaxies, 2015
We study how interactions between galaxies affect star formation within them by considering a sample of almost 1500 of the nearest galaxies, all within a distance of ∼45 Mpc. We use the far-IR emission to define the massive star formation rate (SFR), and then normalise the SFR by the stellar mass of the galaxy to obtain the specific star formation rate (SSFR). We explore the distribution of (S)SFR with morphological type and with stellar mass. We calculate the relative enhancement of SFR and SSFR for each galaxy by normalising them by the median SFR and SSFR values of individual control samples of similar non-interacting galaxies. We find that both the median SFR and SSFR are enhanced in interacting galaxies, and more so as the degree of interaction is higher. The increase is moderate, reaching a maximum of a factor of 1.9 for the highest degree of interaction (mergers). While the SFR and SSFR are enhanced statistically by interactions, in many individual interacting galaxies they are not enhanced at all. Our study is based on a representative sample of nearby galaxies and should be used to place constraints on studies based on samples of galaxies at larger distances.
LESS THAN 10 PERCENT OF STAR FORMATION INz∼ 0.6 MASSIVE GALAXIES IS TRIGGERED BY MAJOR INTERACTIONS
The Astrophysical Journal, 2009
Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses and UV SFRs from COMBO-17, 24µm SFRs from Spitzer and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically-selected very close pairs (separation < 2 ′′) and merger remnants from the HST imaging. Our analysis confirms that the most intensely star-forming systems are indeed interacting or merging. Yet, for massive (M * ≥ 10 10 M ⊙) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios ≤ 1 : 4 and separations ≤ 40 kpc) are only 1.80 ± 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on ∼ 100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only 10% of star formation at 0.4 ≤ z ≤ 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.
Star formation in interacting galaxies
2011
This brief review emphasizes the wide range of environments where interaction induced star formation occurs. In these environments we can study the numerous elaborations of a few basic physical processes, including: gravitational instability, accretion and large-scale shocks.
Galaxy And Mass Assembly (GAMA): the effect of close interactions on star formation in galaxies
Monthly Notices of the Royal Astronomical Society, 2015
The modification of star formation (SF) in galaxy interactions is a complex process, with SF observed to be both enhanced in major mergers and suppressed in minor pair interactions. Such changes likely to arise on short timescales and be directly related to the galaxy-galaxy interaction time. Here we investigate the link between dynamical phase and direct measures of SF on different timescales for pair galaxies, targeting numerous star-formation rate (SFR) indicators and comparing to pair separation, individual galaxy mass and pair mass ratio. We split our sample into the higher (primary) and lower (secondary) mass galaxies in each pair and find that SF is indeed enhanced in all primary galaxies but suppressed in secondaries of minor mergers. We find that changes in SF of primaries are consistent in both major and minor mergers, suggesting that SF in the more massive galaxy is agnostic to pair mass ratio. We also find that SF is enhanced/suppressed more strongly for short-duration SFR indicators (e.g. Hα), highlighting recent changes to SF in these galaxies, which are likely to be induced by the interaction. We propose a scenario where the lower mass galaxy has its SF suppressed by gas heating or stripping, while the higher mass galaxy has its SF enhanced, potentially by tidal gas turbulence and shocks. This is consistent with the seemingly contradictory observations for both SF suppression and enhancement in close pairs.
The Astrophysical Journal, 2012
Using atomic and molecular gas observations from the GASS and COLD GASS surveys and complementary optical/UV data from SDSS and GALEX, we investigate the nature of the variations in the molecular gas depletion time observed across the local massive galaxy population. The large and unbiased COLD GASS sample allows us for the first time to statistically assess the relative importance of galaxy interactions, bar instabilities, morphologies and the presence of AGN in regulating star formation efficiency. We find that both the H 2 mass fraction and depletion time vary as a function of the distance of a galaxy from the main sequence traced by star-forming galaxies in the SFR-M * plane. The longest gas depletion times are found in below-main sequence bulge-dominated galaxies (µ * > 5 × 10 8 M ⊙ kpc −2 , C > 2.6) that are either gas-poor (M H2 /M * <1.5%), or else on average less efficient by a factor of ∼ 2 than disk-dominated galaxy at converting into stars any cold gas they may have. We find no link between the presence of AGN and these long depletion times. In the regime where galaxies are disc-dominated and gas-rich, the galaxies undergoing mergers or showing signs of morphological disruptions have the shortest molecular gas depletion times, while those hosting strong stellar bars have only marginally higher global star formation efficiencies as compared to matched control samples. Our interpretation is that the molecular gas depletion time variations are caused by changes in the ratio between the gas mass traced by the CO(1-0) observations, and the gas mass in high density star-forming cores (as traced by observations of e.g. HCN(1-0)). While interactions, mergers and bar instabilities can locally increase pressure and raise the ratio of efficiently star-forming gas to CO-detected gas (therefore lowering the CO-based depletion time), massive bulges may prevent the formation of dense clumps by stabilizing gas disks against fragmentation, therefore producing the long depletion times. Building a sample representative of the local galaxy population with M * > 10 10 M ⊙ , we derive a global Kennicutt-Schmidt star formation relation of slope 1.18 ± 0.24, and observe structure within the scatter around this relation, with galaxies having low (high) stellar mass surface densities lying systematically above (below) the mean relation, suggesting that Σ H2 is not the only parameter driving the global star formation ability of a galaxy.
Astronomy & Astrophysics, 2012
Aims. We study galaxy pair samples selected from the Sloan Digital Sky Survey (SDSS-DR7) and we perform an analysis of minor and major mergers with the aim of investigating the dependence of galaxy properties on interactions. Methods. We build a galaxy pair catalog requiring r p < 25 kpc h −1 and ∆V < 350 km s −1 within redshift z < 0.1. By visual inspection of SDSS images we removed false identifications and we classify the interactions into three categories: pairs undergoing merging, M; pairs with evident tidal features, T ; and non disturbed, N. We also divide the pair sample into minor and major interactions according to the luminosity ratio of the galaxy members. We study star formation activity through colors, the 4000 Å break, and star formation rates.
History of Galaxy Interactions and Their Impact on Star Formation over the Last 7 Gyr from GEMS
We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z ∼ 0.24-0.80 (lookback time T b ∼ 3-7 Gyr) using ∼ 3600 (M ≥ 1 × 10 9 M ⊙ ) galaxies with GEMS HST , COMBO-17, and Spitzer data. Our results are: (1) Among ∼ 790 high mass (M ≥ 2.5 × 10 10 M ⊙ ) galaxies, the visually-based merger fraction over z ∼ 0.24-0.80, ranges from 9% ± 5% to 8% ± 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1% to 3.5% , and 3.6% to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of ∼ 0.5 Gyr, it follows that over T b ∼ 3-7 Gyr, ∼ 68% of high mass systems have undergone a merger of mass ratio > 1/10, with ∼ 16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous 'major or minor' mergers. The average merger rate is ∼ a few ×10 −4 galaxies Gyr −1 Mpc −3 . Among ∼ 2840 blue cloud galaxies of mass M ≥ 1.0 × 10 9 M ⊙ , similar results hold. (2) We compare the empirical merger fraction and merger rate for high mass galaxies to three ΛCDM-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of five dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ∼ 3698 M ≥ 1.0 × 10 9 M ⊙ galaxies, we find that the mean SFR of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z ∼ 0.24-0.80. Visibly merging systems only account for a small fraction (< 30%) of the cosmic SFR density over T b ∼ 3-7 Gyr. This complements the results of Wolf et al.
Effects of galaxy interactions in different environments
Monthly Notices of the Royal Astronomical Society, 2006
We analyse star formation rates (SFRs) derived from photometric and spectroscopic data of galaxies in pairs in different environments using the 2-degree field galaxy redshift survey (2dFGRS) and the Sloan digital sky survey (SDSS). The two samples comprise several thousand pairs, suitable to explore into detail the dependence of star formation activity in pairs on orbital parameters and global environment. We use the projected galaxy density derived from the fifth brightest neighbour of each galaxy, with a convenient luminosity threshold to characterize environment in both surveys in a consistent way. Star formation activity is derived through the η parameter in 2dFGRS and through the SFR normalized to the total mass in stars, SFR/M * , given by Brinchmann et al. in the SDSS-second data release (SDSS-DR2). For both galaxy pair catalogs, the star formation birth rate parameter is a strong function of the global environment and orbital parameters. Our analysis on SDSS pairs confirms previous results found with the 2dFGRS where suitable thresholds for the star formation activity induced by interactions are estimated at a projected distance r p = 100 h −1 kpc and a relative velocity V = 350 km s −1. We observe that galaxy interactions are more effective at triggering important star formation activity in low-and moderate-density environments with respect to the control sample of galaxies without a close companion. Although close pairs have a larger fraction of actively star-forming galaxies, they also exhibit a greater fraction of red galaxies with respect to those systems without a close companion, an effect that may indicate that dust stirred up during encounters could affect colours and, partially, obscure tidally induced star formation.
Star formation in mergers and interacting galaxies: Gathering the fuel
2006
Selected results from recent studies of star formation in galaxies at different stages of interaction are reviewed. Recent results from the Spitzer Space Telescope are highlighted. Ideas on how large-scale driving of star formation in interacting galaxies might mesh with our understanding of star formation in isolated galaxies and small scale mechanisms within galaxies are considered. In particular, there is evidence that on small scales star formation is determined by the same thermal and turbulent processes in cool compressed clouds as in isolated galaxies. If so, this affirms the notion that the primary role of large-scale dynamics is to gather and compress the gas fuel. In gas-rich interactions this is generally done with increasing efficiency through the merger process.
Physical Galaxy Pairs and Their Effects on Star Formation
We present 776 truly physical galaxy pairs, 569 of them are close pairs and 208 false pairs from Karachentsev (1972) and Reduzzi & Rampazzo (1995) catalogues, which contains 1012 galaxy pairs. Also we carried out star formation activity through the far-infrared emission (FIR) in physical (truly) interacting galaxies in some galaxy pairs and compared them with projection (optical) interacting galaxy pairs. We focused on the triggering of star formation by interactions and analyzed the enhancement of star formation activity in terms of truly physical galaxy pairs. The large fraction of star formation activity is probably due to the activity in the exchange of matter between the truly companions. The star formation rate (SFR) of galaxies in truly galaxy pairs is found to be more enhanced than the apparent pairs.