Large Look-back Times Research Articles

Sp1149. II. Spectroscopy of H ii Regions near the Critical Curve of MACS J1149 and Cluster Lens Models

Galaxy-cluster gravitational lenses enable the study of faint galaxies even at large lookback times, and, recently, time-delay constraints on the Hubble constant. There have been few tests, however, of lens model predictions adjacent to the critical curve (≲8″) where the magnification is greatest. In a companion paper, we use the GLAFIC lens model to constrain the Balmer L–σ relation for H ii regions in a galaxy at redshift z = 1.49 strongly lensed by the MACS J1149 galaxy cluster. Here we perform a detailed comparison between the predictions of 10 cluster lens models that employ multiple modeling assumptions with our measurements of 11 magnified, giant H ii regions. We find that that the models predict magnifications an average factor of 6.2 smaller, a ∼2σ tension, than that inferred from the H ii regions under the assumption that they follow the low-redshift L–σ relation. To evaluate the possibility that the lens model magnifications are strongly biased, we next consider the flux ratios among knots in three images of Sp1149, and find that these are consistent with model predictions. Moreover, while the mass-sheet degeneracy could in principle account for a factor of ∼6 discrepancy in magnification, the value of H 0 inferred from SN Refsdal’s time delay would become implausibly small. We conclude that the lens models are not likely to be highly biased, and that instead the H ii regions in Sp1149 are substantially more luminous than the low-redshift Balmer L–σ relation predicts.

Stellar Dynamical Models for 797 z ∼ 0.8 Galaxies from LEGA-C

Abstract We present spatially resolved stellar kinematics for 797 z = 0.6–1 galaxies selected from the LEGA-C survey and construct axisymmetric Jeans models to quantify their dynamical mass and degree of rotational support. The survey is K s -band selected, irrespective of color or morphological type, and allows for a first assessment of the stellar dynamical structure of the general L* galaxy population at large look-back time. Using light profiles from Hubble Space Telescope imaging as a tracer, our approach corrects for observational effects (seeing convolution and slit geometry), and uses well-informed priors on inclination, anisotropy, and a non-luminous mass component. Tabulated data include total mass estimates in a series of spherical apertures (1, 5, and 10 kpc; 1 × and 2 × R e), as well as rotational velocities, velocity dispersions, and anisotropy. We show that almost all star-forming galaxies and ∼50% of quiescent galaxies are rotation dominated, with deprojected V/σ ∼ 1–2. Revealing the complexity in galaxy evolution, we find that the most massive star-forming galaxies are among the most rotation dominated, and the most massive quiescent galaxies among the least rotation-dominated galaxies. These measurements set a new benchmark for studying galaxy evolution, using stellar dynamical structure for galaxies at large look-back time. Together with the additional information on stellar population properties from the LEGA-C spectra, the dynamical mass and V/σ measurements presented here create new avenues for studying galaxy evolution at large look-back time.

On the scarcity of redshifted OH and millimetre-band molecular absorption

ABSTRACT Despite much searching, redshifted decimetre- and millimetre-band absorption by molecular gas remains very rare, limited to just six systems at zabs ≳ 0.05. Detection of these transitions can yield precise diagnostics of the conditions of the star-forming gas in the earlier Universe, the hydroxyl (OH) radical being of particular interest as in the λ = 18 cm ground state there are four different transitions located close to neutral hydrogen 21- cm and thus detectable with the Square Kilometre Array and its pathfinders. The four transitions of OH have very different dependences on the fundamental constants, thus having much potential in testing for any evolution in these over large look-back times. By collating the photometry in a uniform manner, we confirm our previous hypothesis that the normalized OH absorption strength is correlated with the optical–near-infrared red colour of the sightline. Applying this to the published searches, we find that all, but one (J0414+054), have simply not been searched sufficiently deeply. We suggest that this is due to the standard selection of sources with reliable optical redshifts introducing a bias against those with enough dust with which to shield the molecular gas. For the single source searched to sufficient depth, we have reason to suspect that the high degree of reddening arises from another system along the sightline, thus not being inconsistent with our hypothesis. We also show that the same optical redshift bias can account for the scarcity of millimetre-band absorption.

Dense Gas History of the Universe: From ASPECS to the ngVLA

We review the evolution of the cosmic average molecular gas density to large look-back times, using observations of rotational transitions of CO. Molecular gas is the fuel for star formation in galaxies. Deep searches for CO emission from distant galaxies have delineated the density of molecular gas back to $z \sim 5$, or within 1~Gyr of the Big Bang. The results show a rise and fall in the gas density that parallels, and likely drives, the rise and fall of the cosmic star formation rate density. We present the potential for the next generation Very Large Array to image the distribution and dynamics of the molecular gas in early galaxies, and to make a precise measurement of the dense gas history of the Universe.

Spatially Resolved Stellar Kinematics from LEGA-C: Increased Rotational Support in z ∼ 0.8 Quiescent Galaxies

Abstract We present stellar rotation curves and velocity dispersion profiles for 104 quiescent galaxies at z = 0.6–1 from the Large Early Galaxy Astrophysics Census (LEGA-C) spectroscopic survey. Rotation is typically probed across 10–20 kpc, or to an average of 2.7R e . Combined with central stellar velocity dispersions (σ 0) this provides the first determination of the dynamical state of a sample selected by a lack of star formation activity at large lookback time. The most massive galaxies (M ⋆ > 2 × 1011 M ⊙) generally show no or little rotation measured at 5 kpc ( ∣ V 5 ∣ / σ 0 < 0.2 in eight of ten cases), while ∼64% of less massive galaxies show significant rotation. This is reminiscent of local fast- and slow-rotating ellipticals and implies that low- and high-redshift quiescent galaxies have qualitatively similar dynamical structures. We compare ∣ V 5 ∣ / σ 0 distributions at z ∼ 0.8 and the present day by re-binning and smoothing the kinematic maps of 91 low-redshift quiescent galaxies from the Calar Alto Legacy Integral Field Area (CALIFA) survey and find evidence for a decrease in rotational support since z ∼ 1. This result is especially strong when galaxies are compared at fixed velocity dispersion; if velocity dispersion does not evolve for individual galaxies then the rotational velocity at 5 kpc was an average of 94 ± 22% higher in z ∼ 0.8 quiescent galaxies than today. Considering that the number of quiescent galaxies grows with time and that new additions to the population descend from rotationally supported star-forming galaxies, our results imply that quiescent galaxies must lose angular momentum between z ∼ 1 and the present, presumably through dissipationless merging, and/or that the mechanism that transforms star-forming galaxies also reduces their rotational support.

Stellar Populations of over 1000 z ∼ 0.8 Galaxies from LEGA-C: Ages and Star Formation Histories from Dn4000 and Hδ

Abstract Drawing from the LEGA-C data set, we present the spectroscopic view of the stellar population across a large volume- and mass-selected sample of galaxies at large look-back time. We measure the 4000 Å break (D n 4000) and Balmer absorption line strengths (probed by Hδ) from 1019 high-quality spectra of z = 0.6–1.0 galaxies with M * = 2 × 1010 M ⊙ to 3 × 1011 M ⊙. Our analysis serves as a first illustration of the power of high-resolution, high signal-to-noise ratio continuum spectroscopy at intermediate redshifts as a qualitatively new tool to constrain galaxy formation models. The observed D n 4000–EW(Hδ) distribution of our sample overlaps with the distribution traced by present-day galaxies, but z ∼ 0.8 galaxies populate that locus in a fundamentally different manner. While old galaxies dominate the present-day population at all stellar masses >2 × 1010 M ⊙, we see a bimodal D n 4000–EW(Hδ) distribution at z ∼ 0.8, implying a bimodal light-weighted age distribution. The light-weighted age depends strongly on stellar mass, with the most massive galaxies >1 × 1011 M ⊙ being almost all older than 2 Gyr. At the same time, we estimate that galaxies in this high-mass range are only ∼3 Gyr younger than their z ∼ 0.1 counterparts, at odds with purely passive evolution given a difference in look-back time of >5 Gyr; younger galaxies must grow to >1011 M ⊙ in the meantime, or small amounts of young stars must keep the light-weighted ages young. Star-forming galaxies at z ∼ 0.8 have stronger Hδ absorption than present-day galaxies with the same D n 4000, implying larger short-term variations in star formation activity.

The mid-infrared properties and gas content of active galaxies over large lookback times

Upon an expansion of all of the searches for redshifted HI 21-cm absorption (0.0021 < z 5.19), we update recent results regarding the detection of 21-cm in the non-local Universe. Specifically, we confirm that photo-ionisation of the gas is the mostly likely cause of the low detection rate at high redshift, in addition to finding that at z < 0.1 there may also be a decrease in the detection rate, which we suggest is due to the dilution of the absorption strength by 21-cm emission. By assuming that associated and intervening absorbers have similar cosmological mass densities, we find evidence that the spin temperature of the gas evolves with redshift, consistent with heating by ultra-violet photons. From the near--infrared (3.4, 4.6 and 12 micron) colours, we see that radio galaxies become more quasar-like in their activity with increasing redshift. We also find that the non-detection of 21-cm absorption at high redshift is not likely to be due to the selection of gas-poor ellipticals, in addition to a strong correlation between the ionising photon rate and the [3.4] - [4.6] colour, indicating that the UV photons arise from AGN activity. Like previous studies, we find a correlation between the detection of 21-cm absorption and the [4.6] - [12] colour, which is a tracer of star-forming activity. However, this only applies at the lowest redshifts (z < 0.1), the range considered by the other studies.

The Chemical Abundance Structure of the Inner Milky Way: A Signature of “Upside-down” Disk Formation

Abstract We present a model for the distribution of stars in the inner Galaxy, , measured as a function of vertical distance from the midplane by Hayden et al. (H15). Motivated by an “upside-down” scenario for thick disk formation, in which the thickness of the star-forming gas layer contracts as the stellar mass of the disk grows, we combine one-zone chemical evolution with a simple prescription in which the scale-height of the stellar distribution drops linearly from to over a timescale t c , remaining constant thereafter. We assume a linear-exponential star formation history, . With a star formation efficiency timescale , an outflow mass-loading factor , , and , the model reproduces the observed locus of inner disk stars in and the metallicity distribution functions (MDFs) measured by H15 at , , and . Substantial changes to model parameters lead to disagreement with the H15 data; for example, models with or fail to match the observed MDF at high- and low- , respectively. The inferred scale-height evolution, with z h (t) dropping on a timescale at large lookback times, favors upside-down formation over dynamical heating of an initially thin stellar population as the primary mechanism regulating disk thickness. The failure of our short-t c models suggests that any model in which thick disk formation is a discrete event will not reproduce the continuous dependence of the MDF on found by H15. Our scenario for the evolution of the inner disk can be tested by future measurements of the -distribution and the age–metallicity distribution at .

THE VLT LEGA-C SPECTROSCOPIC SURVEY: THE PHYSICS OF GALAXIES AT A LOOKBACK TIME OF 7 Gyr

ABSTRACT The Large Early Galaxy Census (LEGA-C) is a Public Spectroscopic Survey of ∼3200 K-band selected galaxies at redshifts z = 0.6 − 1.0 with stellar masses M * &gt; 10 10 M ⊙ , conducted with VIMOS on ESO’s Very Large Telescope. The survey is embedded in the COSMOS field (R.A. = 10h00; decl . = + 2 deg ). The 20 hr long integrations produce high-signal-to-noise ratio continuum spectra that reveal ages, metallicities and velocity dispersions of the stellar populations. LEGA-C’s unique combination of sample size and depth will enable us for the first time to map the stellar content at large lookback time, across galaxies of different types and star formation activity. Observations started in 2014 December and are planned to be completed by mid 2018, with early data releases of the spectra and value-added products. In this paper we present the science case, the observing strategy, an overview of the data reduction process and data products, and a first look at the relationship between galaxy structure and spectral properties, as it existed 7 Gyr ago.

CONSTRAINING DARK MATTER HALO PROFILES AND GALAXY FORMATION MODELS USING SPIRAL ARM MORPHOLOGY. II. DARK AND STELLAR MASS CONCENTRATIONS FOR 13 NEARBY FACE-ON GALAXIES

We investigate the use of spiral arm pitch angles as a probe of disk galaxy mass profiles. We confirm our previous result that spiral arm pitch angles (P) are well correlated with the rate of shear (S) in disk galaxy rotation curves. We use this correlation to argue that imaging data alone can provide a powerful probe of galactic mass distributions out to large look-back times. We then use a sample of 13 galaxies, with Spitzer 3.6-$\mu$m imaging data and observed H$\alpha$ rotation curves, to demonstrate how an inferred shear rate coupled with a bulge-disk decomposition model and a Tully-Fisher-derived velocity normalization can be used to place constraints on a galaxy's baryon fraction and dark matter halo profile. Finally we show that there appears to be a trend (albeit a weak correlation) between spiral arm pitch angle and halo concentration. We discuss implications for the suggested link between supermassive black hole (SMBH) mass and dark halo concentration, using pitch angle as a proxy for SMBH mass.

HIGH-RESOLUTION SPECTROSCOPIC IMAGING OF CO IN Az= 4.05 PROTO-CLUSTER

We present a study of the formation of clustered, massive galaxies at large look-back times via spectroscopic imaging of CO in the unique GN20 proto-cluster at z = 4.05. Existing observations show that this is a dense concentration of gas-rich, very active star forming galaxies, including multiple bright submillimeter galaxies (SMGs). Using deep, high-resolution VLA CO(2-1) observations, we image the molecular gas with a resolution of ~1 kpc just 1.6 Gyr after the Big Bang. The SMGs GN20.2a and GN20.2b have deconvolved sizes of ~5 kpc X 3 kpc and ~8 kpc X 5 kpc (Gaussian FWHM) in CO(2-1), respectively, and we measure gas surface densities up to ~12,700/1,700X(sin i) (\alpha_CO/0.8) M_sun/pc^2 for GN20.2a/GN20.2b in the highest-resolution maps. Dynamical mass estimates allow us to constrain the CO-to-H_2 conversion factor to \alpha_CO = 1.7+/-0.8 M_sun (K km s^{-1} pc^2)^-1 for GN20.2a and \alpha_CO = 1.1+/-^{1.5}_{1.1} M_sun (K km/s pc^2)^-1 for GN20.2b. We measure significant offsets (0.5"-1") between the CO and optical emission, indicating either dust obscuration on scales of tens of kpc or that the emission originates from distinct galaxies. CO spectral line energy distributions imply physical conditions comparable to other SMGs and reveal further evidence that GN20.2a and GN20.2b are in different merging stages. We carry out a targeted search for CO emission from the 14 known B-band Lyman break galaxies (LBGs) in the field, tentatively detecting CO in a previously-undetected LBG and placing 3sigma upper limits on the CO luminosities of those that may lie within our bandpass. A blind search for emission line sources down to a 5sigma limiting CO luminosity of L'_CO(2-1) = 8 X 10^9 K km/s pc^2 and covering \Delta z = 0.0273 (~20 comoving Mpc) produces no other strong contenders associated with the proto-cluster.

THE DISPLACED DUSTY INTERSTELLAR MEDIUM OF NGC 3077: TIDAL STRIPPING IN THE M 81 TRIPLET

We present the detection of extended (~30 kpc^2) dust emission in the tidal \hi\ arm near NGC 3077 (member of the M\,81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is typically detected where the \hi\ column densities are >10^21 cm^-2. The SPIRE band ratios show that the dust in the tidal arm is significantly colder (~13 K) than in NGC 3077 itself (~31 K), consistent with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ~1.8 x 10^6 M_sun (assuming beta=2), i.e. substantially larger than the dust mass associated with NGC 3077 (~2 x 10^5 M_sun). Where dust is detected, the dust-to-gas ratio is 6+/-3 x 10^-3, consistent within the uncertainties with what is found in NGC 3077 and nearby spiral galaxies with Galactic metallicities. The faint HII regions in the tidal arm can not be responsible for the detected enriched material and are not the main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic HI, molecules and dust) in this tidal feature was pre-enriched and stripped off NGC 3077 during its recent interaction (~3 x 10^8 yr ago) with M 82 and M 81. This implies that interaction can efficiently remove heavy elements and enriched material (dust, molecular gas) from galaxies. As interactions were more frequent at large lookback times, it is conceivable that they could substantially contribute (along with galactic outflows) to the enrichment of the intergalactic medium.

Indications for 3 Mpc-scale large-scale structure associated with an X-ray luminous cluster of galaxies at z = 0.95

Context. X-ray luminous clusters of galaxies at z ~ 1 are emerging as major cosmological probes and are fundamental tools to study the cosmic large-scale structure and environmental effects of galaxy evolution at large look-back times.

The Physical Properties of Red Supergiants: Comparing Theory and Observations

AbstractRed supergiants (RSGs) are an evolved stage in the life of intermediate massive stars (≤ 25 M⊙). For many years, their location in the H-R diagram was at variance with the evolutionary models. Using the MARCS stellar atmospheres, we have determined new effective temperatures and bolometric luminosities for RSGs in the Milky Way, LMC, and SMC, and our work has resulted in much better agreement with the evolutionary models. We have also found evidence of significant visual extinction due to circumstellar dust. Although in the Milky Way the RSGs contribute only a small fraction (&lt; 1%) of the dust to the interstellar medium (ISM), in starburst galaxies or galaxies at large look-back times, we expect that RSGs may be the main dust source. We are in the process of extending this work now to RSGs of higher and lower metallicities using the galaxies M31 and WLM.

Constraining Dark Matter Halo Profiles and Galaxy Formation Models Using Spiral Arm Morphology. I. Method Outline

We investigate the use of spiral arm pitch angles as a probe of disk galaxy mass profiles. We confirm our previous result that spiral arm pitch angles (P) are well-correlated with the rate of shear (S) in disk galaxy rotation curves, by using a much larger sample (51 galaxies) than used previously (17 galaxies). We use this correlation to argue that imaging data alone can provide a powerful probe of galactic mass distributions out to large lookback times. In contrast to previous work, we show that observed spiral arm pitch angles are similar when measured in the optical (at 0.4 um) and the near-infrared (at 2.1 um) with a mean difference of 2.3+/-2.7 degrees. This is then used to strengthen the known correlation between P and S using B-band images. We then use two example galaxies to demonstrate how an inferred shear rate coupled with a bulge-disk decomposition model and a Tully-Fisher derived velocity normalization can be used to place constraints on a galaxy's baryon fraction and dark matter halo profile. We show that ESO 582-G12, a galaxy with a high shear rate (slightly declining rotation curve) at ~10 kpc, favors an adiabatically contracted halo, with high initial NFW concentration (c_vir > 16) and a high fraction of halo baryons in the form of stars (~15-40%). In contrast, IC 2522 has a low shear rate (rising rotation curve) at ~10 kpc and favors non-adiabatically contracted models with low NFW concentrations (c_vir ~ 2-8) and a low stellar baryon fraction <10%.

Constraints on Changes in Fundamental Constants from a Cosmologically Distant OH Absorber or Emitter

We have detected the four 18 cm OH lines from the z approximaetely 0.765 gravitational lens toward PMN J0134-0931. The 1612 and 1720 MHz lines are in conjugate absorption and emission, providing a laboratory to test the evolution of fundamental constants over a large lookback time. We compare the HI and OH main line absorption redshifts of the different components in the z approximately 0.765 absorber and the z approximately 0.685 lens toward B0218 + 357 to place stringent constraints on changes in F triple-bond g(p)[alpha(2)/mu](1.57). We obtain [DeltaF/F] = (0.44 +/- 0.36(stat) +/- 1.0(sys)t) x 10(-5), consistent with no evolution over the redshift range 0 < z < or = 0.7. The measurements have a 2sigma sensitivity of [Deltaalpha/alpha] < 6.7 x 10(-6) or [Deltamu/mu] < 1.4 x 10(-5) to fractional changes in alpha and mu over a period of approximately 6.5 G yr, half the age of the Universe. These are among the most sensitive constraints on changes in mu.

Atomic carbon at redshift ~2.5

Using the IRAM 30 m telescope we detected the lower fine structure line of neutral carbon (CI( 3 P 1 → 3 P 0 ), ν rest = 492 GHz) towards three high-redshift sources: IRAS FSC 10214 (z = 2.3), sMMJ14011+0252 (z = 2.5) and H1413+117 (Cloverleaf quasar, z = 2.5). SMM J14011+0252 is the first high-redshift, non-AGN source in which Cl has been detected. The C I( 3 P 1 → 3 P 0 ) line from FSC 10214 is almost an order of magnitude weaker than previously claimed, while our detection in the Cloverleaf is in good agreement with earlier observations. The C 1( 3 P 1 → 3 P 0 ) linewidths are similar to the CO widths, indicating that both lines trace similar regions of molecular gas on galactic scales. Derived C I( 3 P 1 → 3 P 0 ) masses for all three objects are of order few ×10 7 M ○. and the implied CI( 3 P 1 → 3 P 0 ) 12 CO(J = 3 → 2) line luminosity ratio is about 0.2. This number is similar to values found in local galaxies. We derive a C abundance of 5 x 10 -5 which implies significant metal enrichment of the cold molecular gas at redshifts 2.5 (age of the universe 2.7 Gyr). We conclude that the physical properties of systems at large lookback times are similar to today's starburst/AGN environments.

Linking the Power Sources of Emission‐Line Galaxy Nuclei from the Highest to the Lowest Redshifts

This dissertation searches for common grounds for the diversity of properties exhibited by the emission-line nuclei of galaxies, from large look-back times to the local universe. I present results of (1) a program of high signal-to-noise spectroscopy for 44 quasars using the MMT and Keck obz 4 servatories; (2) a detailed analysis of the ultraviolet and optical spectral behavior of 22 narrow-line Seyfert 1 (NLS1) galaxies based on archival Hubble Space Telescope (HST) spectra; (3) an in-depth investigation of the proposed link between NLS1s and quasars by means of comparison of composite z 4 spectra and a principal component analysis; (4) a simulation of Seyferts/quasars designed to explore the role of dust in modifying their observed spectral energy distribution; and (5) a sensitive search for accretion signatures in a large sample of nearby emission-line galaxy nuclei, employing a quantitative comparison of the nebular line flux ratios in small (HST) and large (ground-based) apertures. The lowand high-redshift quasars are found to be very similar in their emission characteristics, although differences exist. In particular, the data bolster indications of supersolar metallicities in the luminous, sources, which supports z 4 scenarios that assume substantial star formation concurrent or preceding the quasar phenomena. An in-depth analysis of quasar selection effects is performed, based on comparisons of distributions of equivalent widths for quasar ensembles characterized by different detection techniques. The measurements are consistent with a common parent population, suggesting that the trends identified here are representative of high-z QSOs. The quasars are found to be more metal enriched and z 4 more spectroscopically heterogeneous than the NLS1s, indicating that a close connection between these objects remains doubtful. NLS1s reveal redder UV-blue continua than those measured in other quasars and Seyferts. The sources with UV line absorption are in general less powerful and show redder spectra, suggesting that luminosity-dependent dust absorption may be important in modifying their continua. A recedingtorus–like geometry seems to explain these trends and other observed correlations between quasar luminosity and continuum slope. This relatively simple framework also predicts a number of absorbed AGNs that is consistent with recent modeling and observations of the X-ray background light and extragalactic point source population. Finally, in most of the nearby emission-line nuclei, the expected increased AGN-like behavior at smaller scales is not seen, although the nuclear emission is resolved. This suggests that these sources are not necessarily powered by accretion onto a compact object and that the composite model proposed for the LINER/H ii transition nuclei (which assumes a central accreting-type nucleus surrounded by star-forming regions) is not generally supported. Only about 10% of the narrow-line objects reveal broad Ha features, thus changing their spectral classification from type 2 to type 1.

Properties of the intra cluster medium at high redshift

Abstract We analyze the X–ray properties of the most distant clusters of galaxies (0.3 Z Fe =0.25 +0.04 −0.06 Z ⊙ , and at 〈z〉∼1.2 is Z Fe =0.35 +0.06 −0.05 Z ⊙ (5 clusters at z>1), both measures consistent with the local value Z Fe ≃0.3 Z ⊙ , within 1σconfidence level, thus suggesting no evolution of the mean iron abundance out to z≃1.3. The X–ray scaling properties show hints of negative evolution in the Lbol–T and Mgas–T relations, and a positive evolution in the Entropy–T relation. Together, these results can provide interesting constraints on the thermodynamics of the ICM at large lookback times, pointing towards a redshift z≳2 for the onset of non–gravitational processes.

The Progenitors of Subluminous Type I[CLC]a[/CLC] Supernovae

We find that spectroscopically peculiar subluminous SNe Ia come from an old population. Of the sixteen subluminous SNe Ia known, ten are found in E/S0 galaxies, and the remainder are found in early-type spirals. The probability that this is a chance occurrence is only 0.2%. The finding that subluminous SNe Ia are associated with an older stellar population indicates that for a sufficiently large lookback time (already accessible in current high redshift searches) they will not be found. Due to a scarcity in old populations, hydrogen and helium main sequence stars and He red giant stars that undergo Roche lobe overflow are unlikely to be the progenitors of subluminous SNe Ia. Earlier findings that overluminous SNe Ia (dM15(B) < 0.95) come from a young progenitor population are confirmed. The fact that subluminous SNe Ia and overluminous SNe Ia come from different progenitor populations and also have different properties is a prediction of the CO white dwarf merger progenitor scenario.