Optical spectroscopy of young tidal objects around two interacting galaxy pairs
K. Lee-Waddell J. P. Madrid K. Spekkens C. J. Donzelli B. S. Koribalski P. Serra J. Cannon


firstpage–lastpage
Thermodynamic and information-theoretic description of the Mott transition in the two-dimensional Hubbard model
    A.-M. S. Tremblay
    December 30, 2023
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We present Gemini Multi-Object Spectrograph (GMOS) spectra of Hi-rich tidal features located around the outskirts of two interacting galaxy pairs, NGC 3166/9 and NGC 4725/47.  These follow-up observations are part of a multi-wavelength campaign to study the properties and frequency of tidal dwarf galaxies (TDGs) in group environments.  Based on the calculated gas-phase metallicity and redshift, in addition to the previously measured Hi and stellar properties, we have confirmed the tidal origins of TDG candidate AGC 208457, which has emerged from the tidal debris of an interaction between the NGC 3166/9 galaxy pair.  By comparing Hi and optical recessional velocities, we have also confirmed a physical association of the gaseous knots and star clusters embedded in the metal-rich tidal tail of NGC 4747.





galaxies: dwarf – galaxies: groups: individuals: NGC 3166/9 – galaxies: groups: individual: NGC 4725/47 – galaxies: interactions








§ INTRODUCTION


Tidal interactions dominate the dynamical processing of galaxies in low- to intermediate-density environments <cit.>.  Interactions between gas-rich galaxies can produce prominent tidal features such as transitory tails and knots as well as long-lived tidal dwarf galaxies (TDGs; ).  These features can greatly constrain the interaction history and properties of the host galaxies and enable in depth study of this evolutionary process (e.g. ; ).  

Numerical simulations suggests that TDGs are self-gravitating galaxies that form from the outer disk material of larger galaxies.  Accordingly, they can inherit the chemical abundances of the parent galaxies and are typically more metal-rich than classical dwarfs <cit.>. Various studies using optical spectroscopy, find that stellar clumps located within tidal tails and TDGs generally have oxygen abundance metallicities of 12+log(O/H) > 8.3 and some can be as metal-rich as 12+log(O/H) = 8.8 (, , , ).  Whereas, typical dwarf irregular galaxies have metallicities of 7.3 < 12+log(O/H) < 8.4 (, , ).

While classical dwarf galaxies are generally considered to be dark matter dominated, TDGs are predicted to have little to no associated dark matter <cit.>.  The latter is based on predictions using Lambda cold dark matter (ΛCDM) cosmology, as dark matter is mostly located in outer halo of galaxies rather than in the gaseous disk region where TDGs are formed. However, it is difficult to establish if TDG candidates are actually dark matter free as total mass measurements generally assume dynamical equilibrium and showing that these objects are virialized is quite challenging (see  and  for further details).  Only a handful of TDGs have been detected and spectroscopically confirmed as authentic (e.g. ; ; ), which prevents detailed analysis of this class of objects on a universal scale.  As such, the significance of TDGs and whether they can be used to improve the standard model of cosmology remains a topic of debate (e.g. , ).  

We are carrying out a multi-wavelength investigation of nearby galaxy groups to find TDG candidates and study their properties.  Specifically, we used high-resolution neutral hydrogen (Hi) observations from the Giant Metrewave Radio Telescope (GMRT) to identify gas-rich dwarfs and measure their dynamical properties (; hereafter, ).  Optical imaging, from the Canada-France-Hawaii Telescope (CFHT) MegaCam, detected putative low-surface brightness stellar counterparts of the Hi tidal features and enabled stellar mass measurements (; hereafter, ).  The combination of Hi mapping and optical photometry provided dynamical-to-baryonic mass estimates for the initial classification of tidal objects.  

The most promising TDG candidate identified by our campaign thus far is AGC 208457, located around the NGC 3166/9 galaxy pair.  Our GMRT Hi measurements indicate that AGC 208457 has sufficient mass (M_H_I = 2.3 ± 0.3 × 10^8 M_⊙; ) to become a long-lived TDG <cit.>.  Assuming that this object is self-gravitating and virialized, it has a dynamical-to-gas mass ratio close to unity denoting a lack of dark matter .  Deep optical photometry in  shows two stellar clumps with a combined mass of M_∼ 2 × 10^7 M_⊙ spatially located within the highest Hi column density region of AGC 208457 (see Figure <ref>).  These stars appear to be fairly blue and are bright in the ultraviolet (UV) suggesting that they are quite young (, ).  AGC 208457 therefore has many hallmarks of a TDG.  Confirming a high metallicity for this object would add further evidence for its tidal origin.

 

Two other interesting TDG candidates are found in the eastern tidal tail of NGC 4747 .  Gravitational interactions between neighbouring spiral galaxies, NGC 4725 and NGC 4747, likely resulted in gas-rich tidal tails.  The most prominent tail extends about 8 arcmin (∼30 kpc) to the north-east of NGC 4747 and appears to contain two distinguishable star clusters (SCs) with masses of M_∼ 6 × 10^6 and 9 × 10^6 M_⊙ .  As shown in Figure <ref>, the Hi tail is oriented at a slight angle from a bent stellar tail and contains two Hi peaks (referred to as NE-1 and NE-2) each with a mass of M_H_I≈10^8 M_⊙ .  One of the SCs (i.e. SC-1) approximately coincides spatially with NE-1.  Galaxy Evolution Explorer (GALEX) UV data, from the GR6/7 public data release, reveal a faint arc of recent star-formation that spatially aligns with the two SCs but that is also offset from the Hi .  If the Hi and stellar features have similar recessional velocities, which would imply that they are located at similar distances, then these features would be indicative of the early stages of TDG formation and/or turbulence resulting from a recent interaction event (see ; ).

 

To confirm a physical association between the Hi detections and the stellar components (i.e. verify the distance to each source in order to rule out foreground/background contaminants) of AGC 208457 and the SCs in the tail of NGC 4747 as well as measure metallicities to authenticate tidal origins, we obtained long-slit optical spectroscopy using the Gemini Multi-Object Spectrograph (GMOS).  Our spectroscopic observations are described in Section <ref>.  In Section <ref>, we employ the same method as <cit.>, who confirm a TDG, NGC 5557-E1, with an oxygen abundance of 12+log(O/H) = 8.6, to measure metallicities.  We discuss our findings in Section <ref> and present our final conclusions in Section <ref>.



§ NEW GEMINI OBSERVATIONS



We obtained new spectra of AGC 208457 and the gas-rich tail of NGC 4747 with GMOS on Gemini North. GMOS-North is an e2v CCD detector array built with three CCD chips that produces files with six extensions.  The observations were obtained using GMOS longslit mode with a 1.5-wide slit and the B600_G5307 grating of R = 1700, which has 600 lines per mm ruling density and a blaze wavelength of 461 nm, centered to cover the Hα, Hβ, [Nii] and [Oiii] emission lines.  We use a CCD binning of 2 × 2 pixels resulting in 0.90Å spectral resolution and a spatial resolution of 0.145 arcseconds.

For AGC 208457, the slit was positioned along its major axis, across the two stellar clumps that are clearly visible in the optical images (see Figure <ref>). These features coincide with the highest Hi column density region of AGC 208457 and appear to be the most active sites of star formation in this TDG candidate.  For the tail of NGC 4747, the slit was aligned with the two SCs shown in Figure <ref>.  SC-1 is located within the Hi knot, NE-1, albeit slightly offset from the centre of the gas peak.  

The observations were taken over three nights.  Half of the integration time for each science source was taken at a central wavelength of 5550Å, while other half was at 5600Å.  Additionally, an offset of 15 arcseconds between science exposures was implemented.  These minor shifts mitigate the presence of CCD gaps in the final spectra.  Sky conditions and other observing details are summarized in Table <ref>.  The stellar feature of AGC 208457 has an apparent r'-band magnitude of m_r=18.5 mag  and required significantly more integration time than the SCs within the tail of NGC 4747 (m_r = 17.1 mag and m_r = 17.6 mag for SC-1 and SC-2 respectively) to achieve the estimated signal-to-noise for the weakest emission line (i.e. Hβ; see Section <ref>) required to compute metallicities.  On 24 November, sky conditions worsened during the fourth exposure.  As such, an additional exposure of AGC 208457 was taken in December.  For calibration purposes, observations of the spectrophotometric standard star, EG131, which enable extinction correction, and arcs with the copper-argon CuAr lamp were also carried out during this observing program.





 §.§ Data Processing


All science and calibration data were retrieved from the Gemini archive and reduced using the standard procedure based on the successive application of the following IRAF tasks within the Gemini package. This data reduction method follows the procedure presented in <cit.>. Flatfields were processed with gsflat to create a flat that was then applied to the science data.  Running the task gsreduce enabled bias and overscan subtraction as well as cosmic ray removal. The task gsreduce was also used to flatfield the data and the task gmosaic combined the different file extensions created by detector architecture into a single frame.  Sky subtraction was carried out with gsskysub by defining a region of the CCD frame free of flux from the science spectra to derive the sky level. 

Spectra from the CuAr arcs were processed with gswavelength in order to build the wavelength calibration. The task gstransform was then used to rectify, calibrate, and interpolate the spectra following the solutions derived with gswavelength. The residual calibration error after wavelength correction using the CuAr arcs is ∼0.3Å.  Spectra of the science targets were extracted using the task gsextract without applying any weights (i.e. using a simple sum of pixel values along the spectrum). The width of the extraction in the spatial direction is equivalent to the seeing of the image. Flux calibration and extinction correction of those spectra were carried out using the spectra of EG131 and the tasks gsstandard and gsalibrate.



§ RESULTS



In addition to the properties previously measured from the Hi and optical photometry observations of AGC 208457 and the SCs in the gas-rich tail of NGC 4747, optical velocities as well as gas-phase metallicity measurements are required in order to confirm tidal origins.  Figures <ref> and <ref> show the spectra that were extracted from the brightest optical regions along each slit in our observations.  For AGC 208457, there is one spectrum for the northwest (NW) and two for the southeast (SE) stellar clumps, as indicated by the arrows in Figure <ref>.  We were able to extract four spectra from SC-1 in the NGC 4747 tail (see Figure <ref>); however, SC-2 was too faint to detect emission lines. As evident in the spectra, there appears to be minimal (if any) continuum emission associated with the sources.

 

 

The Hα, Hβ, [Nii], [Oiii] and [Sii] emission line fluxes were measured from each spectrum using the splot routine within IRAF and are presented in Table <ref>.  The dominant source of uncertainty arises from the sensitivity function that is derived from standard stellar spectra.  The residuals in fitting the sensitivity function translate to a ∼10-15% uncertainty in the measured line fluxes.  Note, the spectrum for SC-1c shows a broad emission feature at ∼5250Å, which is likely a background source contaminating the noisier region of the data.



The electron temperature, directly calculated from the flux of [Oiii]_λ 4363, is considered one of the most straight-forward methods to determine the gas-phase metallicity of a source <cit.>.  Although the spectral range of our observations included [Oiii]_λ 4363, this line is too weak to detect in our sources, which is typical of dwarf galaxies (see ).  As such, we follow the method employed by <cit.> and use the empirical N2 and O3N2 line ratios defined as

    = log( []_λ 6583/α)


    = log([]_λ 5007/β×α/[]_λ 6583).

We assume the solar oxygen abundance to be 12 + log(O/H)_⊙ = 8.65 <cit.> and use the calibrations from <cit.> to estimate the chemical abundances from the line ratios:

    12 + log = 8.743 + 0.462 ×


    12 + log = 8.533 - 0.214 ×.


The properties for AGC 208457 and the gas-rich tail of NGC 4747 are presented in Table <ref>.  Radial Hi velocities (v_H_I) and velocity widths (W_20) were measured from the GMRT data ( and ).  Optical velocities (v_) are the average of the radial velocity values for all detectable emission lines along the optical spectra extracted for each source from the GMOS-N observations.  The uncertainty in v_ reflects the standard deviation of the averaged velocity values and the resolution of the spectra.  Due to the faintness of the [Oiii]_λ 5007 emission line, O3N2 ratios could not be computed for all optical sources.  Assuming the group distances for each source, their star-formation rates (SFRs) were estimated using:

    = 7.9 × 10^-42 L_α

where L_α is the luminosity in erg s^-1 <cit.>.

We attempted to use the spectral synthesis code Cloudy v.C17 <cit.> to reproduce the spectra for each source.  However, due to the limited number of detected emission lines, the software was unable to converge on physically possible solutions.  Additionally, since all the sources are quite faint and lack strong continuum emission, deeper analysis of any older stellar populations within these systems is not feasible with the current data (see , ).



Figure <ref> plots the available line ratio values of AGC 208457 and SC-1 in the NGC 4747 tail on a spectral excitation diagnostic diagram.  The data point for SC-1 is an average of the measured values from the various regions of the stellar cluster and the error bars indicate the range of these values.  The grey data points are from the Kitt Peak National Observatory (KPNO) International Spectroscopic Survey (KISS) for starbursting galaxies (SBGs; , , , ).  High metallicity galaxies are found in the lower right while low metallicity galaxies are in the upper left of the plot.  NGC 5557-E1, a dwarf galaxy that was confirmed to be tidal with similar optical spectroscopy observations as employed in this paper (see ), has been included for comparison.

 



§ DISCUSSION



The confirmation of a tidally-formed dwarf galaxy requires a suite of observations to verify longevity and pre-enriched content.  In  and , we identified three Hi objects, AGC 208457 around the NGC 3166/9 pair and two gas-rich knots NE-1 and NE-2 in the tail of NGC 4747, each with sufficient gas mass for long-term survivability and that appear to be in the vicinity of fairly young stellar features.  In this paper, we presented optical spectroscopy of these features that confirm enriched metallicity and verify a physical association between the stars and gas, assuming similar radial velocities of the Hi and stellar components (which already spatially coincide) imply that the line emissions stem from a common astronomical source.  



 §.§ AGC 208457, a TDG candidate


The optical velocities measured for the three components in AGC 208457 are 10-120 km s^-1 lower than the GMRT Hi velocity of the cloud (v_H_I =1343 ± 3 km s^-1).  Stellar clumps NW-a and SE-b (v_ = 1300 ± 50 km s^-1 and 1330 ± 60 km s^-1) agree within the uncertainties and velocity range of the Hi component; whereas, SE-a has a significantly lower value (v_ = 1220 ± 50 km s^-1).  SE-a is possibly stellar debris that is outside of the TDG candidate or indicates an unvirialized system.  Nevertheless, this source is still within the measured  Hi velocity range for NGC 3166/9 pair (see ).

The highest oxygen abundances (8.5 ± 0.2) are found in NW-a and SE-a, while SE-b (8.3 ± 0.2) is lower. These metallicity values are comparable to other tidal features found the literature (i.e. 12+log(O/H) > 8.3; , , ).  Additionally, on the spectral excitation diagram (Fig. <ref>), the NW stellar clump of AGC 208457 has line ratios that are consistent with metal-rich SBGs, which are located in the bottom right portion of the plot.  While the metallicity values alone do not unambiguously distinguish AGC 208457 as a TDG, this object is relatively metal-rich when compared to typical dwarf irregular galaxies (see , , ).

The enhanced metallicity of AGC 208457 is likely inherited from its parent galaxies. Previous spectroscopy observations by <cit.> of NGC 3166 and NGC 3169 indicate that their central regions are metal-rich with above-solar metallicity.  However, the mean metallicity of the outer disk region of NGC 3166 is fairly low <cit.>.  NGC 3166 has a fairly undisturbed stellar disk; whereas, both the Hi and stellar components of NGC 3169 show clear signs of the gravitational interaction occurring between the two galaxies .  Based on the Hi scaling relations from <cit.>, NGC 3169 has a standard gas mass for a galaxy of its size and colour but NGC 3166 has no discernible Hi counterpart and is therefore fairly Hi-deficient .  It appears that AGC 208457 has been formed from material likely drawn out of NGC 3166 and NGC 3169.

The Hα SFR estimated for AGC 208457 is 1-2 orders of magnitude lower than the UV SFR (estimated in ), which is typical for gas-rich dwarf galaxies (see ).  Overall, with an Hi mass of M_H_I = 2.3 ± 0.3 × 10^8 M_⊙, a negligible dark matter component (assuming that the object is self-gravitating; ), fairly young stars (), a high metallicity, and a plausible formation scenario, AGC 208457 has all the properties of a bonafide TDG.



 §.§ Potential TDGs in the gas-rich tail of NGC 4747


The gaseous tail emerging from NGC 4747 is modestly offset from a bent stellar tail located in the same spatial region .  The optical velocities measured for the four stellar components within the tail of NGC 4747 are 20-100 km s^-1 lower than the GMRT Hi velocity of the two Hi peaks (v_H_I =1265 ± 3 km s^-1 and 1258 ± 3 km s^-1).  Nevertheless, the values agree within the uncertainties and Hi velocity range, signifying an actual physical association between the stellar and Hi tails.  SC-1a and SC-1d do have lower central velocities, which may indicate that their stars are projected in front of the Hi tail.  Possible projection effects from the bent tail precludes clear source separation and complicates deeper analysis of the contents of this tidal feature.  Although we could only extract spectra from the region originally identified as SC-1, we presume that these stars are located within the stellar tail of NGC 4747 and their properties are representative of the entire tail.  

The highest oxygen abundances (8.5 ± 0.3) are found in SC-1b and SC-1c, while the other two regions are around 8.4 ± 0.2. We note that both methods (O3N2 and N2) produce consistent results.  As expected, the stars in the NGC 4747 tail are fairly young (< 2.5 Gyr; see ) and metal-rich, as shown in Fig. <ref>, thereby substantiating tidal origins.  This system is in a much earlier evolutionary stage than AGC 208457 and turbulent motions, which can inhibit star-formation in high density Hi regions (see ), are still present within the tail.  Nevertheless, NE-1 and NE-2 appear to have sufficient mass – especially if they continue to accrete content from the tidal tail (see ) – to eventually decouple from their host and become self-gravitating TDGs.



§ CONCLUSIONS



Using GMOS long-slit spectroscopy, we confirm the stellar counterparts and tidal origins of two Hi sources, AGC 208457 and the gas-rich tail of NGC 4747, located around the outskirts of two interacting galaxy pairs (NGC 3166/9 and NGC 4725/47 respectively).  The optical velocities of the stellar components are consistent with the Hi velocities, signifying physical associations between the features.  We measure high oxygen abundances in these objects, indicating that they have formed out of pre-enriched material drawn from their parent galaxies during an interaction event.  In conjunction with the previously measured Hi, dynamical, and stellar properties, AGC 208457 has the characteristics of a genuine TDG.  The Hi knots embedded in the tidal tail of NGC 4747 are in an earlier stage of evolution and they have the potential to evolve into long-lived TDGs.



§ ACKNOWLEDGEMENTS


We thank the reviewer for his/her thoroughly detailed comments and suggestions to improve the clarity of this paper.  The data presented in this paper are based on observations obtained (during program GN-2016B-Q-24) at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil).  The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community.  We are most fortunate to have the opportunity to obtain observations from this mountain. 













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