The star formation history of luminous infrared galaxies (original) (raw)
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Star Formation in Isolated LIRGs: Clues to Star-forming Processes at Higher z
Proceedings of the International Astronomical Union, 2010
Luminous infrared galaxies (LIRGs) are galaxies with LI R > 10 11 L . For a starforming galaxy to emit at a LIRG level, it must have a very high star formation rate (SFR). In the local Universe, the star formation (SF) is primarily triggered by interactions. However, at intermediate redshift, a large fraction of LIRGs are disk galaxies with little sign of recent merger activity. The question arises whether the intermediate redshift LIRGs are "triggered" or experiencing "normal", if elevated, SF. Understanding these SF processes is important since this type of systems may have contributed to 20% or more of the cosmic SFR in the early Universe. In order to address this issue we study similar systems in the Local Universe, that is isolated late-type galaxies displaying LIRG activity. We use different observational techniques in order to trace the star-forming history of these systems. Here we present preliminary results.
The Astronomical Journal, 2008
As part of the Center for Adaptive Optics (AO) Treasury Survey (CATS) we imaged a set of 15 intermediate redshift (z ∼ 0.8) luminous infrared galaxies (LIRGs) with the Keck Laser Guide Star (LGS) AO facility. These galaxies were selected from the Great Observatories Origins Deep Survey (GOODS) southern field, allowing us to combine the high spatial resolution HST optical (B, V , i, and z-bands) images with our near-infrared (K ′-band) images to study the LIRG morphologies and spatially resolved spectral energy distributions (SEDs). Two thirds of the LIRGs are disk galaxies, with only one third showing some evidence for interactions, minor, or major mergers. In contrast with local LIRG disks (which are primarily barred systems), only 10% of the LIRG disks in our sample contain a prominent bar. While the optical bands tend to show significant point-like substructure, indicating distributed star formation, the AO K-band images tend to be smooth. They lack pointlike structures to a K ∼ 23.5 limit This places an upper bound on the number of red super giants per blue-knot at less than 4000. The SEDs of the LIRGs are consistent with distributed dusty star formation, as exhibited by optical to IR colors redder than allowed by old stellar populations alone. This effect is most pronounced in the galaxy cores, possibly indicating central star formation. We also observed a set of 11 intermediate redshift comparison galaxies, selected to be non-ellipticals with apparent K-band magnitudes comparable to the LIRGs. The "normal" (non-LIRG) systems tended to have lower optical luminosity, lower stellar mass, and more irregular morphology than the LIRGs. Half of the "normal" galaxies have SEDs consistent with intermediate aged stellar populations and minimal dust. The other half show evidence for some dusty star formation, usually concentrated in their cores. Our work suggests that the LIRG disk galaxies are similar to large disk systems today, undergoing self regulated star formation, only at 10-20 times higher rates.
2008
We analyze star formation (SF) as a function of stellar mass (M ⋆) and redshift z in the All Wavelength Extended Groth Strip International Survey (AEGIS). For 2905 field galaxies, complete to 10 10 (10 10.8)M ⊙ at z < 0.7(1), with Keck spectroscopic redshifts out to z = 1.1, we compile SF rates (SFR) from emission lines, GALEX, and Spitzer MIPS 24µm photometry, optical-NIR M ⋆ measurements, and HST morphologies. Galaxies with reliable signs of SF form a distinct "main sequence (MS)", with a limited range of SFR at a given M ⋆ and z (1σ ±0.3 dex), and log(SFR) approximately proportional to log(M ⋆). The range of log(SFR) remains constant to z > 1, while the MS as a whole moves to higher SFR as z increases. The range of SFR along the MS constrains the amplitude of episodic variations of SF, and the effect of mergers on SFR. Typical galaxies spend ∼ 67(95)% of their lifetime since z = 1 within a factor of 2(4) of their average SFR at a given M ⋆ and z. The dominant mode of the evolution of SF since z ∼ 1 is apparently a gradual decline of the average SFR in most individual galaxies, not a decreasing frequency of starburst episodes, or a decreasing factor by which SFR are enhanced in starbursts. LIRGs at z ∼ 1 seem to mostly reflect the high SFR typical for massive galaxies at that epoch. The smooth MS may reflect that the same set of few physical processes governs star formation prior to additional quenching processes. A gradual process like gas exhaustion may play a dominant role.
Massive Star Formation in Luminous Infrared Galaxies
2003
We present HST/NICMOS observations of a sample of LIRGs. We show that active star formation appears to be occurring not only in the bright nuclei of these galaxies, but also in luminous super-star clusters and giant HII regions with ages of up to 20-40Myr. This population of bright clusters and HII regions is unprecedented in normal galaxies and emphasizes the effects of the extreme star formation in LIRGs.
Star-formation laws in luminous infrared galaxies
Astronomy & Astrophysics, 2012
Context. The observational study of star-formation relations in galaxies is central for unraveling the related physical processes that are at work on local and global scales. It is still debated whether star formation can be described by a universal law that remains valid in different populations of galaxies. Aims. We aim to expand the sample of extreme starbursts, represented by local luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs), with high-quality observations in the 1-0 line of HCN, which is taken as a proxy for the dense molecular gas content. The new data presented in this work allow us to enlarge in particular the number of LIRGs studied in HCN by a factor 3 compared to previous works. The chosen LIRG sample has a range of HCN luminosities that partly overlaps with that of the normal galaxy population. We study if a universal law can account for the star-formation relations observed for the dense molecular gas in normal star-forming galaxies and extreme starbursts and explore the validity of different theoretical prescriptions of the star-formation law. Methods. We have used the IRAM 30 m telescope to observe a sample of 19 LIRGs in the 1-0 lines of CO, HCN and HCO +. The galaxies were extracted from a sample of local LIRGs with available high-quality and high-resolution images obtained at optical, near and mid IR wavelengths, which probe the star-formation activity. We therefore derived the star-formation rates using different tracers and determined the sizes of the star-forming regions of all targets. Results. The analysis of the new data proves that the efficiency of star formation in the dense molecular gas (SFE dense) of extreme starbursts is a factor 3-4 higher compared to normal galaxies. Kennicutt-Schmidt (KS) power laws were also derived. We find a duality in KS laws that is further reinforced if we account for the likely different conversion factor for HCN (α HCN) in extreme starbursts and for the unobscured star-formation rate in normal galaxies. This result extends the more extreme bimodal behavior of star-formation laws that was derived from CO molecular lines by two recent surveys to the higher molecular densities probed by HCN lines. Conclusions. We compared our observations with the predictions of theoretical models in which the efficiency of star formation is determined by the ratio of a constant star-formation rate per free-fall time (SFR ff) to the local free-fall time (t ff). We find that it is possible to fit the observed differences in the SFE dense between normal galaxies and LIRGs/ULIRGs using a common constant SFR ff and a set of physically acceptable HCN densities, but only if SFR ff ∼ 0.005-0.01 and/or if α HCN is a factor of ∼a few lower than our favored values. Star-formation recipes that explicitly depend on the galaxy global dynamical time scales do not significantly improve the fit to the new HCN data presented in this work.
The 2dF-SDSS LRG and QSO Survey: the star formation histories of luminous red galaxies
Monthly Notices of the Royal Astronomical Society, 2006
We present a detailed investigation into the recent star formation histories of 5,697 Luminous Red Galaxies (LRGs) based on the Hδ(4101Å), and [O ii] (3727Å) lines and the D4000 index. LRGs are luminous (L> 3L*), galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study we utilise LRGs from the recently completed 2dF-SDSS LRG and QSO survey (2SLAQ). Equivalent widths of the Hδ and [O ii] lines are measured and used to define three spectral types, those with only strong Hδ absorption (k+a), those with strong [O ii] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (∼80 per cent), however significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low redshift comparison. While the em and em+a fractions are consistent with the low redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected life time of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted level of k+a and em+a activity is not sufficient to reconcile the predicted mass growth for massive early-types in a hierarchical merging scenario.
Central regions of LIRGs: rings, hidden starbursts, Supernovae and star clusters
Journal of Physics: Conference Series, 2012
We study star formation (SF) in very active environments, in luminous IR galaxies, which are often interacting. A variety of phenomena are detected, such as central starbursts, circumnuclear SF, obscured SNe tracing the history of recent SF, massive super star clusters, and sites of strong off-nuclear SF. All of these can be ultimately used to define the sequence of triggering and propagation of star-formation and interplay with nuclear activity in the lives of gas rich galaxy interactions and mergers. In this paper we present analysis of high-spatial resolution integral field spectroscopy of central regions of two interacting LIRGs. We detect a nuclear 3.3 µm PAH ring around the core of NGC 1614 with thermal-IR IFU observations. The ring's characteristics and relation to the strong star-forming ring detected in recombination lines are presented, as well as a scenario of an outward expanding starburst likely initiated with a (minor) companion detected within a tidal feature. We then present NIR IFU observations of IRAS 19115-2124, aka the Bird, which is an intriguing triple encounter. The third component is a minor one, but, nevertheless, is the source of 3/4 of the SFR of the whole system. Gas inflows and outflows are detected at the locations of the nuclei. Finally, we briefly report on our on-going NIR adaptive optics imaging survey of several dozen LIRGs. We have detected highly obscured core-collapse SNe in the central kpc, and discuss the statistics of "missing SNe" due to dust extinction. We are also determining the characteristics of hundreds of super star clusters in and around the core regions of LIRGs, as a function of host-galaxy properties.
Star-formation histories of local luminous infrared galaxies
Astronomy & Astrophysics, 2015
We present the analysis of the integrated spectral energy distribution (SED) from the ultraviolet (UV) to the farinfrared and Hα of a sample of 29 local systems and individual galaxies with infrared (IR) luminosities between 10 11 L⊙ and 10 11.8 L⊙. We have combined new narrow-band Hα+[N ii] and broad-band g, r optical imaging taken with the Nordic Optical Telescope (NOT), with archival GALEX, 2MASS, Spitzer, and Herschel data. The SEDs (photometry and integrated Hα flux) have been fitted with a modified version of the magphys code using stellar population synthesis models for the UV-near-IR range and thermal emission models for the IR emission taking into account the energy balance between the absorbed and re-emitted radiation. From the SED fits we derive the star-formation histories (SFH) of these galaxies. For nearly half of them the star-formation rate appears to be approximately constant during the last few Gyrs. In the other half, the current star-formation rate seems to be enhanced by a factor of 3-20 with respect to that occured ∼1 Gyr ago. Objects with constant SFH tend to be more massive than starbursts and they are compatible with the expected properties of a main-sequence (M-S) galaxy. Likewise, the derived SFHs show that all our objects were M-S galaxies ∼1 Gyr ago with stellar masses between 10 10.1 and 10 11.5 M⊙. We also derived from our fits the average extinction (Av = 0.6 − 3 mag) and the polycyclic aromatic hydrocarbons (PAH) luminosity to LIR ratio (0.03-0.16). We combined the Av with the total IR and Hα luminosities into a diagram which can be used to identify objects with rapidly changing (increasing or decreasing) SFR during the last 100 Myr.
Star Formation in the [ITAL]Infrared Space Observatory[/ITAL] Atlas of Bright Spiral Galaxies
The Astronomical Journal, 2002
We investigate star formation along the Hubble sequence using the Infrared Space Observatory Atlas of Bright Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc-Scd) spirals than in early-type (Sa-Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in low ionization nuclear emission regions and transition objects compared with H ii galaxies. The mean star formation rate in the sample is 1.4 M yr À1 , based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 Â 10 8 yr, which is $5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.