Extinction Bump Research Articles

What causes the ultraviolet extinction bump at the cosmic dawn?

Abstract The enigmatic ultraviolet (UV) extinction bump at 2175 Å, the strongest spectroscopic absorption feature superimposed on the interstellar extinction curve, has recently been detected at the cosmic dawn by the James Webb Space Telescope (JWST) in JADES-GS-z6-0, a distant galaxy at redshift z ≈ 6.71, corresponding to a cosmic age of just 800 million years after the Big Bang. Although small graphite grains have historically long been suggested as the carrier of the 2175 Å extinction bump and graphite grains are expected to have already been pervasive in the early Universe, in this work we demonstrate that small graphite grains are not responsible for the UV extinction bump seen at the cosmic dawn in JADES-GS-z6-0, as the extinction bump arising from small graphite grains is too broad and peaks at wavelengths that are too short to be consistent with what is seen in JADES-GS-z6-0.

Polycyclic aromatic hydrocarbon molecules and the 2175Å interstellar extinction bump

ABSTRACT The exact nature of the 2175$\mathring{\rm A}$ extinction bump, the strongest spectroscopic absorption feature superimposed on the interstellar extinction curve, remains unknown ever since its discovery in 1965. Popular candidate carriers for the extinction bump include nano-sized graphitic grains and polycyclic aromatic hydrocarbon (PAH) molecules. To quantitatively evaluate PAHs as a possible carrier, we perform quantum chemical computations for the electronic transitions of 30 compact, pericondensed PAH molecules and their cations as well as anions with a wide range of sizes from 16 to 96 C atoms, and a mean size of 43 C atoms. It is found that a mixture of such PAHs, which individually exhibit sharp absorption features, show a smooth and broad absorption band that resembles the 2175$\mathring{\rm A}$ interstellar extinction bump. Arising from π* ← π transitions, the width and intensity of the absorption bump for otherwise randomly selected and uniformly weighted PAH mixtures, do not vary much with PAH sizes and charge states, whereas the position somewhat shifts to longer wavelengths as PAH size increases. While the computed bump position, with the computational uncertainty taken into account, appears to agree with that of the interstellar extinction bump, the computed width is considerably broader than the interstellar bump if the molecules are uniformly weighted. It appears that, to account for the observed bump width, one has to resort to PAH species of specific sizes and structures.

The 2175 Å interstellar extinction bump: is the wavelength variable?

ABSTRACT The most striking characteristics of the mysterious 2175$\mathring{\rm A}$ extinction bump, the strongest spectroscopic absorption feature seen on the interstellar extinction curve, are the invariant central wavelength and variable bandwidth: its peak position at 2175$\mathring{\rm A}$ is remarkably constant, while its bandwidth varies from one line of sight to another. However, recent studies of the lines of sight towards a number of Herbig Ae/Be stars have revealed that the extinction bump exhibits substantial shifts from the canonical wavelength of 2175$\mathring{\rm A}$. In this work, we revisit these lines of sight and take a physical approach to determine the ultraviolet (UV) extinction curve for each line of sight. It is found that the wavelengths of the derived UV extinction bumps are around 2200$\mathring{\rm A}$ and the scatters are considerably smaller than those of the previous study based on the same set of Herbig Ae/Be stars, consistent with the conventional wisdom of a stable bump position. Nevertheless, the scatters are still appreciably larger than those associated with the nominal bump position of 2175$\mathring{\rm A}$. This is discussed in the context that Herbig Ae/Be stars are not well suited for interstellar extinction studies.

Interstellar 2175 Å extinction bump

<p indent="0mm">The interstellar extinction curve contains important information about the chemical composition and size distribution of interstellar dust. The <sc>2175 Å</sc> extinction bump is the most prominent spectral feature on the interstellar extinction curve. Ever since its first detection in 1965, it is well recognized that although the strength of the <sc>2175 Å</sc> extinction bump varies with astrophysical environments, its peak wavelength is invariant and is independent of environmental conditions. We derived the UV extinction curves for 26 interstellar lines of sight based on data from the International Ultraviolet Explorer (IUE) and the Kurucz stellar model atmospheric spectra. It is found that the central wavelength of the <sc>2175 Å</sc> extinction bump of these interstellar lines of sight is not invariant around <sc>2175 Å.</sc> Instead, it exhibits considerable variation in the <sc>2130–2300 Å</sc> wavelength range, in stark contrast to the over-half-a-century-long, common belief of a stable peak wavelength of <sc>2175 Å</sc> for the extinction bump. The new observational property that the central wavelength of the <sc>2175 Å</sc> extinction bump is not constant imposes a new constraint on identifying the bump carrier.

CRIRES high-resolution near-infrared spectroscopy of diffuse interstellar band profiles

Aims. A high spectral resolution investigation of diffuse interstellar bands (DIBs) in the near-infrared (YJ band) is conducted to test new methods, to confirm and improve existing parameters, and to search for new DIBs. Methods. The CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) on the European Southern Observatory's Very Large Telescope was employed to obtain spectra of four reddened background supergiant stars (HD 183143, HD 165784, HD 92207, HD 111613) and an unreddened comparison star (HD 87737) at the highest resolution of R ≈ 100 000 currently achievable at near-infrared wavelengths, more than twice as high as accomplished in previous near-infrared DIB studies. The correction for telluric absorption was performed by a modelling approach. Non-local thermodynamic equilibrium spectral modelling of available optical and the new near-infrared stellar spectra facilitated a comprehensive characterisation of the atmospheric properties of the background stars. As a consequence, a more precise and accurate determination of the reddening and the reddening law along the respective sight lines could be achieved than feasible before by comparison of the observed and model spectral energy distributions. For DIBs that overlap with stellar lines the DIB profile shapes could be recovered. Results. Seventeen known near-infrared DIBs were confirmed, and 12 previously unknown and generally weaker DIBs were identified in the YJ band. Three DIBs that show uniform profiles along all sight lines were identified, possibly connected to transitions from a common lower state of the same carrier. The divergent extinction curve towards the frequently discussed DIB standard star HD 183143 could be reproduced for the first time, requiring extra absorption by ~3.5 mag due to polycyclic aromatic hydrocarbons (PAHs) to match the ultraviolet extinction bump. This extra absorption probably stems from a circumstellar bubble lying in front of the star which is intersected tangentially by the line of sight, making this particular sight line more peculiar than standard.

Polycyclic aliphatic hydrocarbons: is tetrahedrane present in UIR spectra?

The smallest Platonic hydrocarbon, tetrahedrane, has been subject to frequent theoretical and experimental study for 50 years, but its infrared spectrum and synthetic pathway remain a mystery. The recent partial attribution of the ultraviolet extinction bump observed in the interstellar medium (ISM) of the Milky Way galaxy to hydrogenated T-carbon, a larger tetrahedral cluster formed from tetrahedrane and C4 monomers, has brought renewed interest to the molecule. Similarly, as a polycyclic hydrocarbon, tetrahedrane is similar in structure to the molecules proposed to be responsible for the so-called unidentified infrared bands (UIRs) observed in all kinds of astronomical environments. Furthermore, tetrahedrane's ν2 and ν7 fundamental vibrational frequencies, with values of 3210.6 cm-1 (3.11 μm) and 752.5 cm-1 (13.29 μm) as computed in the present quantum chemical study, have substantial intensities of 59 and 183 km mol-1, respectively. These come tantalizingly close to, but potentially distinct from, the 3.3 and 13.2 μm regions of the infrared spectrum typically included in the UIRs. As such, tetrahedrane or related clusters of these polycyclic aliphatic hydrocarbons may have a role to play in both of these sets of observations and could even help to explain the relation between them. Regardless, if tetrahedrane is present in the ISM, the highly-accurate theoretical data reported herein should help to aid in its identification and may assist in guiding future synthetic experiments as well.

Constraining the Metallicities of Damped Lyα Systems Using Extinction Curves

Abstract In this paper, we present a new method to constrain the metallicities of high-redshift damped Lyα (DLA) absorbers using observed extinction curves. This is the first time such an approach is employed to constrain the metallicities of extragalactic absorbers. To demonstrate our method, we use the spectra of 13 quasars and one gamma-ray burst (GRB) with DLA absorbers detected along their sightlines. By using the Kramers–Kronig (KK) relation, which relates the wavelength-integrated extinction to the total volume occupied by dust per hydrogen nucleon, we set some robust lower limits on the metallicity of the DLAs. The resulting lower limits are all consistent with the DLA metallicities from the literature. The GRB extinction curve exhibits a very strong 2175 Å extinction bump. We try to constrain the metallicity of the GRB DLA by modeling the GRB extinction curve using dust models with two (graphite and silicates) and three (polycyclic aromatic hydrocarbon, hydrogenated amorphous carbon, and silicates) dust components. The two-component model results in a metallicity of Z ∼ −0.45, while the three-component model gives Z ∼ −0.50. However, the lower limit from the KK approach for this DLA is Z ≥ −0.60. Modeling a large sample of extinction curves with a 2175 Å extinction bump and measured DLA metallicities would allow a thorough comparison between the KK and the model-dependent approach. In cases where the precise measurement of the metallicity of a DLA is not possible (e.g., due to the saturation of important absorption lines), the approach presented in this paper can be used to constrain the metallicity.

The MOSDEF Survey: The Variation of the Dust Attenuation Curve with Metallicity

Abstract We derive a UV-optical stellar dust attenuation curve of galaxies at z = 1.4–2.6 as a function of gas-phase metallicity. We use a sample of 218 star-forming galaxies, excluding those with very young or heavily obscured star formation, from the MOSFIRE Deep Evolution Field survey with Hα, Hβ, and [N ii]λ 6585 spectroscopic measurements. We constrain the shape of the attenuation curve by comparing the average flux densities of galaxies sorted into bins of dust obscuration using Balmer decrements, i.e., Hα-to-Hβ luminosities. The average attenuation curve for the high-metallicity sample ( , corresponding to ) has a shallow slope, identical to that of the Calzetti local starburst curve, and a significant UV 2175 Å extinction bump that is ∼0.5× the strength of the Milky Way bump. On the other hand, the average attenuation curve of the low-metallicity sample ( ) has a steeper slope similar to that of the SMC curve, only consistent with the Calzetti slope at the 3σ level. The UV bump is not detected in the low-metallicity curve, indicating the relative lack of the small dust grains causing the bump at low metallicities. Furthermore, we find that on average the nebular reddening (E(B − V)) is a factor of 2 times larger than that of the stellar continuum for galaxies with low metallicities, while the nebular and stellar reddening are similar for galaxies with higher metallicities. The latter is likely due to a high surface density of dusty clouds embedding the star-forming regions but also reddening the continuum in the high-metallicity galaxies.

Towards understanding physical origin of 2175 Å extinction bump in interstellar medium

ABSTRACT The 2175 Å ultraviolet (UV) extinction bump in interstellar medium (ISM) of the Milky Way was discovered in 1965. After intensive exploration of more than a half century, however, its exact origin still remains a big conundrum that is being debated. Here, we propose a mixture model by which the extinction bump in ISM is argued possibly relevant to the clusters of hydrogenated T-carbon (HTC) molecules (C40H16) that have intrinsically a sharp absorption peak at the wavelength 2175 Å. By linearly combining the calculated absorption spectra of HTC mixtures, graphite, MgSiO3, and Fe2SiO4, we show that the UV extinction curves of optional six stars can be nicely fitted. This work poses an alternative explanation towards understanding the physical origin of the 2175 Å extinction bump in ISM of the Milky Way.

UGMRT search for cold gas at z ∼ 1–1.4 towards red quasars

ABSTRACT We present results from our search for H i 21-cm and OH 18-cm absorption at z ∼ 1–1.4 towards red quasars showing strong Mg ii absorption using upgraded Giant Metrewave Radio Telescope. The quasars J1501 + 1822 and J1521 + 5508 show multiple strong associated Mg ii absorption at z ∼ 1.1 and signature of reddening in their optical spectra. We report the detection of H i 21-cm absorption towards J1521 + 5508 at the systemic redshift of the quasar, with $N(\rm{H\,{\small I}}$) = (1.2 ± 0.2) × 1020 cm−2 for spin temperature of 100 K and unit covering factor. The H i 21-cm absorption is offset from the blueshifted strong Mg ii absorbers by ≳1500 km s−1. We do not detect H i 21-cm absorption at the redshift of the associated Mg ii absorption and at the systemic redshift towards J1501 + 1822. We argue that lack of one-to-one correspondence between Mg ii and H i 21-cm absorption could be related with clumpiness of the neutral gas and the radio and optical sightlines probing different volume of the gas. We find that the presence of strong associated Mg ii absorption and reddening in the optical spectra of the quasars lead to an increased detection rate of associated H i 21-cm absorption at z ≥1. We also report non-detection of intervening OH absorption ([OH]/[H i] ≤ (1–4)× 10−8) at z = 1.3 towards two red quasars, J0850 + 5159 and J0852 + 3435, which show strong metal and H i 21-cm absorption and the 2175 Å dust extinction bump.

Red Rain Cells of Kerala as a Possible Carrier of the Diffuse Interstellar Bands and the UV Extinction Bump

Red Rain Cells of Kerala as a Possible Carrier of the Diffuse Interstellar Bands and the UV Extinction Bump

On the dust properties of high-redshift molecular clouds and the connection to the 2175 Å extinction bump

We present a study of the extinction and depletion-derived dust properties of gamma-ray burst (GRB) absorbers at $1<z<3$ showing the presence of neutral carbon (\ion{C}{I}). By modelling their parametric extinction laws, we discover a broad range of dust models characterizing the GRB \ion{C}{I} absorption systems. In addition to the already well-established correlation between the amount of \ion{C}{I} and visual extinction, $A_V$, we also observe a correlation with the total-to-selective reddening, $R_V$. All three quantities are also found to be connected to the presence and strength of the 2175\,{\AA} dust extinction feature. While the amount of \ion{C}{I} is found to be correlated with the SED-derived dust properties, we do not find any evidence for a connection with the depletion-derived dust content as measured from [Zn/Fe] and $N$(Fe)$_{\rm dust}$. To reconcile this, we discuss a scenario where the observed extinction is dominated by the composition of dust particles confined in the molecular gas-phase of the ISM. We argue that since the depletion level trace non-carbonaceous dust in the ISM, the observed extinction in GRB \ion{C}{I} absorbers is primarily produced by carbon-rich dust in the molecular cloud and is therefore only observable in the extinction curves and not in the depletion patterns. This also indicates that the 2175\,{\AA} dust extinction feature is caused by dust and molecules in the cold and molecular gas-phase. This scenario provides a possible resolution to the discrepancy between the depletion- and SED-derived amounts of dust in high-$z$ absorbers.

Gravitationally Lensed Quasar SDSS J1442+4055: Redshifts of Lensing Galaxies, Time Delay, Microlensing Variability, and Intervening Metal System at z ∼ 2

Abstract We present r-band photometric monitoring of the two images, A and B, of the gravitationally lensed quasar SDSS J1442+4055 using the Liverpool Telescope (LT). From the LT light curves between 2015 December and 2018 August, we derive at once a time delay of 25.0 ± 1.5 days (1σ confidence interval; A is leading) and microlensing magnification gradients below 10−4 mag day−1. The delay interval is not expected to be affected by an appreciable microlensing-induced bias, so it can be used to estimate cosmological parameters. This paper also focuses on new Gran Telescopio Canarias (GTC) and LT spectroscopic observations of the lens system. We determine the redshift of two bright galaxies around the doubly imaged quasar using LT spectroscopy, while GTC data lead to low-noise individual spectra of A, B, and the main lensing galaxy, G1. The G1 spectral shape is accurately matched to an early-type galaxy template at z = 0.284, and it has potential for further relevant studies. Additionally, the quasar spectra show absorption by metal-rich gas at z ∼ 2. This dusty absorber is responsible for an extinction bump at a rest-frame wavelength of 2209 ± 2 Å, which has strengths of ∼0.47 and 0.76 mag μm−1 for A and B, respectively. In such an intervening system, the dust-to-gas ratio, gas-phase metallicity indicator [Zn/H], and dust depletion level [Fe/Zn] are relatively high.

Ultradense Hydrogen H(0) as Stable Dark Matter in the Universe: Extended Red Emission Spectra Agree with Rotational Transitions in H(0)

Abstract Studies of ultradense hydrogen H(0) in our laboratory have been reported in around 50 publications. The proton solar wind was shown to agree well with the protons ejected by Coulomb explosions in p(0). H(0) is a quantum material and can have at least two slightly different forms—ultradense protium p(0) and ultradense deuterium D(0)—which are stable even inside many stars. Mixed phases pD(0) have also been studied. These phases are the lowest-energy forms of hydrogen, and H(0) will probably exist everywhere where hydrogen exists in the universe. Rotational spectra from H(0) have been studied in laboratory experiments in emission in the visible range, giving good agreement with observations of ERE (extended red emission) in space. The ERE bands and sharp peaks agree with rotational transitions for a few coupled p–p and p–D pairs in the well studied spin state s = 4 in H(0). Since ERE is observed almost everywhere in space, this proves that H(0) is common in space. The rotational absorption from the ground state in p(0) agrees with the 220 nm extinction bump for three coupled p–p pairs in the most common spin state s = 2 studied. The uneven distribution of deuterium in space may be due to the slightly different properties of D(0), which separate it from p(0). The dark “missing mass” concluded to exist in the halos of rotating galaxies is proposed as being due to accumulation of H(0) there. Other important implications of the superfluid and superconductive phase H(0) in space await discovery.

Quasar 2175 Å dust absorbers – II. Correlation analysis and relationship with other absorption line systems

We present the cold neutral content (H I and C I gas) of 13 quasar 2175 \AA$ $ dust absorbers (2DAs) at $z$ = 1.6 - 2.5 to investigate the correlation between the presence of the UV extinction bump with other physical characteristics. These 2DAs were initially selected from the Sloan Digital Sky Surveys I - III and followed up with the Keck-II telescope and the Multiple Mirror Telescope as detailed in our Paper I. We perform a correlation analysis between metallicity, redshift, depletion level, velocity width, and explore relationships between 2DAs and other absorption line systems. The 2DAs on average have higher metallicity, higher depletion levels, and larger velocity widths than Damped Lyman-$\alpha$ absorbers (DLAs) or subDLAs. The correlation between [Zn/H] and [Fe/Zn] or [Zn/H] and log$\Delta$V$_{90}$ can be used as alternative stellar mass estimators based on the well-established mass-metallicity relation. The estimated stellar masses of the 2DAs in this sample are in the range of $\sim$ 10$^9$ to $\sim$2 $\times$ 10$^{11}$ $M_{\odot}$ with a median value of $\sim$2 $\times$ 10$^{10}$ $M_{\odot}$. The relationship with other quasar absorption line systems can be described as (1) 2DAs are a subset of Mg II and Fe II absorbers, (2) 2DAs are preferentially metal-strong DLAs/subDLAs, (3) More importantly, all of the 2DAs show C I detections with logN(C I) $>$ 14.0 cm$^{-2}$, (4) 2DAs can be used as molecular gas tracers. Their host galaxies are likely to be chemically enriched, evolved, massive (more massive than typical DLA/subDLA galaxies), and presumably star-forming galaxies.

Interstellar Silicon Depletion and the Ultraviolet Extinction

Abstract Spinning small silicate grains were recently invoked to account for the Galactic foreground anomalous microwave emission. These grains, if present, will absorb starlight in the far-ultraviolet (UV). There is also renewed interest in attributing the enigmatic 2175 Å interstellar extinction bump to small silicates. To probe the role of silicon in the UV extinction, we explore the relations between the amount of silicon required to be locked up in silicates [ Si / H ] dust and the 2175 Å bump or the far-UV extinction rise, based on an analysis of the extinction curves along 46 Galactic sightlines for which the gas-phase silicon abundance [ Si / H ] gas is known. We derive [ Si / H ] dust either from [ Si / H ] ISM − [ Si / H ] gas or from the Kramers–Kronig relation, which relates the wavelength-integrated extinction to the total dust volume, where [ Si / H ] ISM is the interstellar silicon reference abundance and taken to be that of proto-Sun or B stars. We also derive [ Si / H ] dust from fitting the observed extinction curves with a mixture of amorphous silicates and graphitic grains. We find that in all three cases [ Si / H ] dust shows no correlation with the 2175 Å bump, while the carbon depletion [ C / H ] dust tends to correlate with the 2175 Å bump. This supports carbon grains instead of silicates as the possible carriers of the 2175 Å bump. We also find that neither [ Si / H ] dust nor [ C / H ] dust alone correlates with the far-UV extinction, suggesting that the far-UV extinction is a combined effect of small carbon grains and silicates.

Steep extinction towards GRB 140506A reconciled from host galaxy observations: Evidence that steep reddening laws are local

We present the spectroscopic and photometric late-time follow-up of the host galaxy of the long-duration Swift gamma-ray burst GRB 140506A at z = 0.889. The optical and near-infrared afterglow of this GRB had a peculiar spectral energy distribution (SED) with a strong flux-drop at 8000 {\AA} (4000 {\AA} rest-frame) suggesting an unusually steep extinction curve. By analyzing the contribution and physical properties of the host galaxy, we here aim at providing additional information on the properties and origin of this steep, non-standard extinction. We find that the strong flux-drop in the GRB afterglow spectrum at < 8000 {\AA} and rise at < 4000 {\AA} is well explained by the combination of a steep extinction curve along the GRB line of sight and contamination by the host galaxy light so that the scenario with an extreme 2175 {\AA} extinction bump can be excluded. We localise the GRB to be at a projected distance of approximately 4 kpc from the centre of the host galaxy. Based on emission-line diagnostics of the four detected nebular lines, Halpha, Hbeta, [O II] and [O III], we find the host to be a modestly star forming (SFR = 1.34 +/- 0.04 Msun yr^-1) and relatively metal poor (Z = 0.35^{+0.15}_{-0.11} Zsun) galaxy with a large dust content, characterized by a measured visual attenuation of A_V = 1.74 +/- 0.41 mag, thus unexceptional in all its physical properties. We model the extinction curve of the host-corrected afterglow and show that the standard dust properties causing the reddening seen in the Local Group are inadequate in describing the steep drop. We conclude that the steep extinction curve seen in the afterglow towards the GRB is of exotic origin, is sightline-dependent only and thus solely a consequence of the circumburst environment.

Interstellar carbon dust

Carbon is the fourth most abundant element in the Universe after hydrogen, helium and oxygen. Synthesized at the late evolutionary stage of stars, carbon is ejected from stars through mass loss of asymptotic giant branch stars and explosion of supernovae and then enters the interstellar medium in the form of ions, atoms, molecules and solid dust particles. Carbon-containing complex organic molecules are essential for the origins of life, and carbonaceous dust is a major component of interstellar grains whose infrared absorption and emission spectra provide a powerful diagnosis of the physical and chemical conditions of astrophysical regions where the dust is found. Carbonaceous dust materials such as graphite, nanodiamonds, polycyclic aromatic hydrocarbon molecules, fullerene, and hydrogenated amorphous carbon are ubiquitously present in the interstellar space and are the most promising candidate carriers of the unidentified 2175 A extinction bump, the unidentified infrared emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, and the mysterious diffuse interstellar absorption bands. Here we focus on the observational properties of interstellar carbonaceous grains and their physical and chemical characteristics. Also discussed are graphene and carbon nanotubes which may be present in the interstellar space.

DIFFUSE INTERSTELLAR BANDS AND THE ULTRAVIOLET EXTINCTION CURVES: THE MISSING LINK REVISITED

A large number of interstellar absorption features at ~ 4000\AA\ -- 1.8 {\mu}m, known as the "diffuse interstellar bands" (DIBs), remains unidentified. Most recent works relate them to large polycyclic aromatic hydrocarbon (PAH) molecules or ultrasmall carbonaceous grains which are also thought to be responsible for the 2175 \AA\ extinction bump and/or the far ultraviolet (UV) extinction rise at $\lambda^{-1} > 5.9\ {\mu}m^{-1}$. Therefore, one might expect some relation between the UV extinction and DIBs. Such a relationship, if established, could put important constraints on the carrier of DIBs. Over the past four decades, whether DIBs are related to the shape of the UV extinction curves has been extensively investigated. However, the results are often inconsistent, partly due to the inconsistencies in characterizing the UV extinction. Here we re-examine the connection between the UV extinction curve and DIBs. We compile the extinction curves and the equivalent widths of 40 DIBs along 97 slightlines. We decompose the extinction curve into three Drude-like functions composed of the visible/near-infrared component, the 2175 \AA\ bump, and the far-UV extinction at $\lambda^{-1} > 5.9\ {\mu}m^{-1}$. We argue that the wavelength-integrated far-UV extinction derived from this decomposition technique best measures the strength of the far-UV extinction. No correlation is found between the far-UV extinction and most (~90\%) of the DIBs. We have also shown that the color excess E(1300-1700), the extinction difference at 1300 \AA\ and 1700 \AA\ often used to measure the strength of the far-UV extinction, does not correlate with DIBs. Finally, we confirm the earlier findings of no correlation between the 2175 \AA\ bump and DIBs or between the 2175 \AA\ bump and the far-UV extinction.

Cold gas and a Milky Way-type 2175-Å bump in a metal-rich and highly depleted absorption system

We report the detection of a strong Milky Way-type 2175 \AA$ $ extinction bump at $z$ = 2.1166 in the quasar spectrum towards SDSS J121143.42+083349.7 from the Sloan Digital Sky Survey (SDSS) Data Release 10. We conduct follow up observations with the Echelle Spectrograph and Imager (ESI) onboard the Keck-II telescope and the Ultraviolet and Visual Echelle Spectrograph (UVES) on the VLT. This 2175 \AA$ $ absorber is remarkable in that we simultaneously detect neutral carbon (C I), neutral chlorine (Cl I), and carbon monoxide (CO). It also qualifies as a damped Lyman alpha system. The J1211+0833 absorber is found to be metal-rich and has a dust depletion pattern resembling that of the Milky Way disk clouds. We use the column densities of the C I fine structure states and the C II/C I ratio (under the assumption of ionization equilibrium) to derive the temperature and volume density in the absorbing gas. A Cloudy photoionization model is constructed, which utilizes additional atoms/ions to constrain the physical conditions. The inferred physical conditions are consistent with a canonical cold (T $\sim$ 100 K) neutral medium with a high density ($n$(H I) $\sim$ 100 cm$^{-3}$) and a slightly higher pressure than the local interstellar medium. Given the simultaneous presence of C I, CO, and the 2175 \AA$ $ bump, combined with the high metallicity, high dust depletion level and overall low ionization state of the gas, the absorber towards J1211+0833 supports the scenario that the presence of the bump requires an evolved stellar population.