GOODS- HERSCHEL AND CANDELS: THE MORPHOLOGIES OF ULTRALUMINOUS INFRARED GALAXIES AT z ∼ 2 (original) (raw)
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GOODS-Herschel & CANDELS: The Morphologies of Ultraluminous Infrared Galaxies at z~ 2
Arxiv preprint arXiv: …, 2011
Using deep 100 and 160 µm observations in GOODS-South from GOODS-Herschel, combined with high resolution HST/WFC3 near-infrared imaging from CANDELS, we present the first detailed morphological analysis of a complete, far-infrared (FIR) selected sample of 52 Ultraluminous Infrared Galaxies (ULIRGs: L IR > 10 12 L ⊙ ) at z ∼ 2. By using the Herschel observations in the FIR we are able to identify true ULIRGs, unaffected by the biases of color selections or 24 µm selections that have been problematic for previous studies. We also make use of a comparison sample of galaxies with lower IR luminosities (undetected by Herschel) but with the same redshift and H-band magnitude distribution. Our visual classifications of these two samples indicate that the fractions of objects with disk and spheroid morphologies is roughly the same but that there are significantly more mergers, interactions, and irregular galaxies among the ULIRGs (72 +5 −7 % versus 32 ± 3%). The combination of disk and irregular/interacting morphologies suggests that early stage interactions and minor mergers could play an important role in ULIRGs at z ∼ 2. We compare these fractions with those of a z ∼ 1 sample selected from GOODS-H and COSMOS across a wide luminosity range and find that the fraction of disks decreases systematically with L IR while the fraction of mergers and interactions increases, as has been observed locally. At comparable luminosities, the fraction of ULIRGs with various morphological classifications is similar at z ∼ 2 and z ∼ 1. We investigate the position of the z ∼ 2 ULIRGs, along with 70 z ∼ 2 LIRGs, on the specific star formation rate versus redshift plane, and find 52 systems to be starbursts (i.e., they lie more than a factor of 3 above the main sequence relation). The morphologies of these systems are dominated by interacting and merging systems (∼ 50%). If irregular disks are included as potential minor mergers, then we find that up to ∼ 73% of starbursts are involved in a merger or interaction at some level. Although the final coalescence of a major merger may not be required for the high luminosities of ULIRGs at z ∼ 2 as is the case locally, the large fraction of interactions at all stages and potential minor mergers suggest that the high star formation rates of ULIRGs are still largely externally triggered at z ∼ 2.
The star formation history of luminous infrared galaxies
2006
Aims. We constrain the past star formation histories of a sample of 25 distant (z ∼ 0.7) luminous infrared galaxies (LIRGs) detected with the mid infrared cameras ISOCAM and MIPS onboard the ISO and Spitzer satellites. Methods. We used high-resolution VLT-FORS2 spectroscopy in addition to a comprehensive library of 200 000 model optical spectra to derive Bayesian likelihood estimates of the star formation histories of these galaxies, based on analysis of Balmer absorption lines and the 4000 Å break. Results. The locus of distant LIRGs in the diagram defined by Hδ A and D4000 is roughly comparable to that of local LIRGs observed with IRAS, suggesting that no trend toward an evolution is detected between the local and distant LIRGs. We obtain similar results when using either the H8 or the Hδ A Balmer absorption-line indices in combination with D4000. By computing a birthrate parameter (b = SFR/ SFR) of 4 ± 1, we confirme that the distant LIRGs are currently experiencing a major phase of star formation. The most likely duration of the bursts is 0.10 +0.16 −0.06 Gyr, during which the LIRGs produce ∼5-10% of their current stellar mass. No evidence was found for successive starbursts on the scale of a few times 10 7 yr, such as those predicted by some numerical simulations of major mergers. However, the high number density of those galaxies suggests that they could have experienced between two and four LIRG phases until the present epoch. This scenario is not consistent with the formation of the z ∼ 0.7 LIRGs through the continuous star formation characterizing isolated spiral galaxies as has been independently argued based on their morphology. Instead, minor mergers, tidal interactions, or gas accretion remain plausible triggering mechanisms for more than half of the distant LIRGs that do not harbor the morphology of major mergers.
Ultraluminous Infrared Galaxies: Mergers of Sub-L* Galaxies?
The Astrophysical …, 2001
A sample of 27 low-redshift, mostly cool, ultraluminous infrared galaxies (ULIRGs) has been imaged at 1.6 km with the Hubble Space T elescope (HST ) Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The majority (67%) of the sampleÏs galaxies are multiple-nucleus galaxies with projected separations of up to 17 kpc, and the rest of the sample (33%) are single-nucleus galaxies, as determined by the NICMOS angular resolution limit. The average observed, integrated (host]nucleus) H magnitude of our HST H sample ULIRGs is [24.3, slightly above that of an L* galaxy and 52% of (M H \ [24.2), the sampleÏs galaxies have sub-L* luminosities. The ULIRGs in the HST H sample are not generated as a result of the merging of two luminous (i.e., ºL*) spiral galaxies. Instead, the interactions and mergers occur in general between two, or in some cases more, less massive sub-L* (0.3È0.5L*) galaxies.
Evidence for Multiple Mergers among Ultraluminous Infrared Galaxies: Remnants of Compact Groups?
The Astrophysical Journal, 2000
In a large sample of ULIRGs imaged with HST, we have identified a significant subsample that shows evidence for multiple mergers. The evidence is seen among two classes of ULIRGs: (1) those with multiple remnant nuclei in their core, sometimes accompanied by a complex system of tidal tails; and (2) those that are in fact dense groupings of interacting (soon-to-merge) galaxies. We conservatively estimate that, in the redshift range 0.05<z<0.20, at least 20 (out of 99) ULIRGs satisfy one or both of these criteria. We present several cases and discuss the possibility that the progenitors of ULIRGs may be the more classical weakly interacting compact groups of galaxies (Hickson). An evolutionary progression is consistent with the results: from compact groups to pairs to ULIRGs to elliptical galaxies. The last step follows the blowout of gas and dust from the ULIRG.
THE LESSER ROLE OF STARBURSTS IN STAR FORMATION AT z = 2
The Astrophysical Journal, 2011
Two main modes of star formation are know to control the growth of galaxies: a relatively steady one in disk-like galaxies, defining a tight star formation rate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a sequence which is generally interpreted as driven by merging. Such starburst galaxies are rare but have much higher SFRs, and it is of interest to establish the relative importance of these two modes. PACS/Herschel observations over the whole COSMOS and GOODS-South fields, in conjunction with previous optical/near-IR data, have allowed us to accurately quantify for the first time the relative contribution of the two modes to the global SFR density in the redshift interval 1.5 < z < 2.5, i.e., at the cosmic peak of the star formation activity. The logarithmic distributions of galaxy SFRs at fixed stellar mass are well described by Gaussians, with starburst galaxies representing only a relatively minor deviation that becomes apparent for SFRs more than 4 times higher than on the main sequence. Such starburst galaxies represent only 2% of mass-selected star forming galaxies and account for only 10% of the cosmic SFR density at z ∼ 2. Only when limited to SFR> 1000M ⊙ /yr, off-sequence sources significantly contribute to the SFR density (46 ± 20%). We conclude that merger-driven starbursts play a relatively minor role for the formation of stars in galaxies, whereas they may represent a critical phase towards the quenching of star formation and morphological transformation in galaxies.
THE STAR FORMATION HISTORIES OF z ∼ 2 DUST-OBSCURED GALAXIES AND SUBMILLIMETER-SELECTED GALAXIES
The Astrophysical Journal, 2012
The Spitzer Space Telescope has identified a population of ultraluminous infrared galaxies (ULIRGs) at z ∼ 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses (M * ) of two populations of Spitzer-selected ULIRGs that have extremely red R − [24] colors (dust-obscured galaxies, or DOGs) and compare our results with submillimeter-selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their mid-infrared (mid-IR) spectral energy distribution (SED) at rest frame 1.6 μm associated with stellar emission ("bump DOGs"), while the other set of 51 DOGs have power-law mid-IR SEDs that are typical of obscured active galactic nuclei ("power-law DOGs"). We measure M * by applying Charlot & Bruzual stellar population synthesis models to broadband photometry in the rest-frame ultraviolet, optical, and near-infrared of each of these populations. Assuming a simple stellar population and a Chabrier initial mass function, we find that power-law DOGs and bump DOGs are on average a factor of 2 and 1.5 more massive than SMGs, respectively (median and inter-quartile M * values for SMGs, bump DOGs, and power-law DOGs are log(M * /M ) = 10.42 +0.42 −0.36 , 10.62 +0.36 −0.32 , and 10.71 +0.40 −0.34 , respectively). More realistic star formation histories drawn from two competing theories for the nature of ULIRGs at z ∼ 2 (major merger versus smooth accretion) can increase these mass estimates by up to 0.5 dex. A comparison of our stellar masses with the instantaneous star formation rate (SFR) in these z ∼ 2 ULIRGs provides a preliminary indication supporting high SFRs for a given M * , a situation that arises more naturally in major mergers than in smooth accretion-powered systems.
RESOLVED STAR FORMATION ON SUB-GALACTIC SCALES IN A MERGER AT z = 1.7
The Astrophysical Journal, 2014
We present a detailed analysis of Hubble Space Telescope (HST), Wide Field Camera 3 (WFC3) G141 grism spectroscopy for seven star-forming regions of the highly magnified lensed starburst galaxy RCSGA 032727-132609 at z = 1.704. We measure the spatial variations of the extinction in RCS0327 through the observed Hγ/Hβ emission line ratios, finding a constant average extinction of E(B − V) gas = 0.40 ± 0.07. We infer that the star formation is enhanced as a result of an ongoing interaction, with measured star formation rates derived from demagnified, extinction-corrected Hβ line fluxes for the individual star-forming clumps falling >1-2 dex above the star formation sequence. When combining the HST/WFC3 [OIII]λ5007/Hβ emission line ratio measurements with [NII]/Hα line ratios from Wuyts et al. , we find that the majority of the individual star-forming regions fall along the local "normal" abundance sequence. With the first detections of the He I λ5876Å and He II λ4686Å recombination lines in a distant galaxy, we probe the massive-star content of the star-forming regions in RCS0327. The majority of the star-forming regions have a He I λ5876 to Hβ ratio consistent with the saturated maximum value, which is only possible if they still contain hot O-stars. Two regions have lower ratios, implying that their last burst of new star formation ended ∼ 5 Myr ago. Together, the He I λ5876Å and He II λ4686Å to Hβ line ratios provide indirect evidence for the order in which star formation is stopping in individual star-forming knots of this high redshift merger. We place the spatial variations of the extinction, star formation rate and ionization conditions in the context of the star formation history of RCS0327.
Quasi‐Stellar Objects, Ultraluminous Infrared Galaxies, and Mergers
The Astrophysical Journal, 2001
We test the hypothesis that QSOs are formed via strong tidal interactions or mergers, initially going through an ultraluminous infrared phase. Our approach is to look for traces of this phase in the host galaxies of QSOs. We select a sample of low-redshift bona Ðde QSOs that may be in a transitionary stage between ultraluminous infrared galaxies (ULIGs) and QSOs. These objects, which we call transition QSOs, have an intermediate position in the far-infrared color-color diagram between the regions occupied by the two classes of objects. We carry out a systematic spectroscopic and imaging study of these objects in order to determine their interaction and star-forming histories. By modeling the spectra, we obtain ages for the recent starburst events in the host galaxies and interacting companions. We have discussed the Ðrst Ðve objects in the sample in detail in previous publications ; here we present results for the remaining four objects, and discuss the sample as a whole. We Ðnd that all nine transition QSOs are undergoing tidal interactions and that eight are major mergers. Every object also shows strong recent star-forming activity, and in at least eight cases this activity is directly related to the tidal interaction. The ages we derive for the starburst populations range from currently active star formation in some objects to poststarburst ages Myr in others. There is also a clear connection between interactions, [300 starbursts, and QSO activity. Seven of the QSOs in the sample are also ULIGs ; statistical considerations show that the two phenomena are necessarily physically related in these objects. Our results imply one of two scenarios : (1) at least some ULIGs evolve to become classical QSOs, and the transition stage lasts Myr ; or (2) at least some QSOs are born under the same conditions as ULIGs, and their [300 lifetime as QSOs lasts Myr. We discuss other properties and trends found in the sample, and [300 propose a model that accounts for all of them, as well as the youth of these systems.
The Astrophysical …, 2006
We present results from our Very Large Telescope large program to study the dynamical evolution of local Ultraluminous Infrared Galaxies (ULIRGs) and QSOs. Expanding previous studies by Genzel et al. (2001) and Tacconi et al. (2002), our data now consist of high resolution, long-slit Hand K-band spectra of 54 ULIRGs. This paper mainly presents the kinematics of sources that have coalesced into a single nucleus. The stellar kinematics, extracted from the CO rovibrational bandheads in our spectra, indicate that ULIRG remnants are dynamically heated systems with a mean dispersion of 157 km s −1. The combination of kinematic, structural, and photometric properties of the remnants indicate that they mostly originate from major encounters (in agreement with Dasyra et al. 2006) and that they result in the formation of dispersion-supported systems (elliptical galaxies of the order ∼10 10-10 11 M). Placing ULIRGs on the fundamental plane of early-type galaxies indicates that the end products of ultraluminous mergers are typically less massive and extended than giant ellipticals (in good agreement with Genzel et al. 2001 and Tacconi et al. 2002). Converting the host dispersion into black hole mass with the aid of the M BH − σ relation yields black hole mass estimates of the order 10 7-10 8 M and high accretion rates (of Eddington efficiencies often > 0.5).